pccel001 - instrumentation electrical requirements

December 1999

Process Industry PracticesProcess Control

PIP PCCEL001Instrumentation Electrical Requirements

PURPOSE AND USE OF PROCESS INDUSTRY PRACTICES

In an effort to minimize the cost of process industry facilities, this Practice hasbeen prepared from the technical requirements in the existing standards of majorindustrial users, contractors, or standards organizations. By harmonizing these technicalrequirements into a single set of Practices, administrative, application, and engineeringcosts to both the purchaser and the manufacturer should be reduced. While this Practiceis expected to incorporate the majority of requirements of most users, individualapplications may involve requirements that will be appended to and take precedence overthis Practice. Determinations concerning fitness for purpose and particular matters orapplication of the Practice to particular project or engineering situations should not bemade solely on information contained in these materials. The use of trade names fromtime to time should not be viewed as an expression of preference but rather recognized asnormal usage in the trade. Other brands having the same specifications are equallycorrect and may be substituted for those named. All Practices or guidelines are intendedto be consistent with applicable laws and regulations including OSHA requirements. Tothe extent these Practices or guidelines should conflict with OSHA or other applicablelaws or regulations, such laws or regulations must be followed. Consult an appropriateprofessional before applying or acting on any material contained in or suggested by thePractice.

© Process Industry Practices (PIP), Construction Industry Institute, TheUniversity of Texas at Austin, 3208 Red River Street, Suite 300, Austin,Texas 78705. PIP member companies and subscribers may copy this Practicefor their internal use.

Not printed with State funds

December 1999

Process Industry Practices Page 1 of 20

Process Industry PracticesProcess Control

PIP PCCEL001Instrumentation Electrical Requirements

Table of Contents

1. Introduction ..................................21.1 Purpose ............................................21.2 Scope ...............................................2

2. References....................................22.1 Process Industry Practices................22.2 Industry Codes and Standards ..........22.3 Government Regulations ..................4

3. Definitions.....................................4

4. General..........................................5

5. Environmental ..............................6

6. Instrument SignalCompatibility ...............................6

7. Wire and Cable Systems..............77.1 General.............................................77.2 Instrument Wiring .............................77.3 Segregation/Separation

Requirements .................................117.4 Cable Tray ......................................117.5 Conduit Systems.............................117.6 Junction Boxes ...............................12

8. Terminations ..............................158.1 General .......................................... 158.2 Milliamp Signals - Typically 4 to

20 mA ............................................ 158.3 Voltage Signals - 100 mV or

Greater........................................... 158.4 Voltage Signals - Less than

100 mV........................................... 168.5 Thermocouple Signals.................... 16

9. Instrument Power Systems .......169.1 General .......................................... 169.2 Branch Circuits Design ................... 17

10.Grounding .................................18

11. Intrinsically Safe InstrumentSystems.....................................19

12.Non-Incendive Systems ...........19

13.Control Panel and CabinetWiring ........................................20

14. Installation.................................20

Figure 1:Typical Instrument SystemsPower/Grounding Requirements

PIP PCCEL001 Instrumentation Electrical Requirements December 1999

Page 2 of 20 Process Industry Practices

1. Introduction

1.1 Purpose

The purpose of this Practice is to provide electrical requirements for processmeasurement and control systems.

1.2 Scope

This Practice provides the requirements for the equipment selection, system design,and installation of electrical supply and wiring to support process measurement andcontrol systems.

2. References

Applicable requirements in the latest edition (or the edition indicated) of the following industrycodes, standards, and references shall be considered an integral part of this Practice. Shorttitles will be used herein when appropriate.

2.1 Process Industry Practices (PIP)

– PIP ELSWC03 - 600 Volt Power and Control Cable

– PIP ELSWC03D - Data Sheet for 600 Volt Power and Control Cable

– PIP ELSWC05 - 300 Volt Instrumentation Tray Cable

– PIP ELSWC05D - Data Sheet for 300 Volt Instrumentation Tray Cable

– PIP PCCGN001 - General Instrument Design Checklist

– PIP PCCGN002 - General Instrument Installation Criteria

– PIP PCESS001 - Safety Systems Guidelines

– PIP PCIEF000 - Instrumentation Fabrication Details

– PIP PCIEL000 - Instrumentation Electrical Details

– PIP PCSCB001 - Control Building Considerations Specification

– PIP PCSEL003 - Instrument Junction Boxes Specifications

2.2 Industry Codes and Standards

• American Petroleum Institute (API)

– API RP500 - Recommended Practice for Classification of Locations forElectrical Installations at Petroleum Facilities Classified as Class 1,Division 1 and Division 2

– API RP505 - Recommended Practice for the Classification of Locations forElectrical Installations at Petroleum Facilities Classified as Class 1, Zone 0,Zone 1, and Zone 2

– API RP552 - Transmission Systems

– API RP554 - Process Instrumentation and Control

PIP PCCEL001December 1999 Instrumentation Electrical Requirements


• Institute of Electrical and Electronic Engineers (IEEE)

– IEEE 518 - IEEE Guide for the Installation of Electrical Equipment toMinimize Electrical Noise Inputs to Controllers from External Sources

– IEEE 1100 - Recommended Practice for Powering and Grounding SensitiveElectronic Equipment, The Emerald Book

• ISA

– ISA RP12.1 - Definitions and Information Pertaining to Electrical Instrumentsin Hazardous (Classified) Locations

– ISA RP12.6 - Wiring Practices for Hazardous (Classified) LocationsInstrumentation Part 1: Intrinsic Safety

– ISA RP60.8 - Electrical Guide for Control Centers

– ISA S12.1 - Definitions and Information Pertaining to Electrical Instrumentsin Hazardous (Classified) Locations

– ISA S12.6 - Installation of Intrinsic Safe Systems for Hazardous (Classified)Locations

– ISA S12.10 - Area Classification in Hazardous (Classified) Dust Locations

– ISA S12.12 - Nonincendive Electrical Equipment for Use in Class I and IIDivision 2 and Class III Divisions 1 and 2 Hazardous (Classified) Locations

– ISA S50.1 - Compatibility of Analog Signals for Electronic Industrial ProcessInstruments

– ISA S84.01 - Application of Safety Instrumented Systems for the ProcessIndustries

– ISA Comprehensive Dictionary of Measurement and Control

• National Electric Manufacturers Association (NEMA)

– NEMA 250 - Enclosures for Electrical Equipment (1000 Volts Maximum)

– NEMA ICS6 - Enclosures for Industrial Control and Systems

– NEMA VE2 - Metal Cable Tray Installation Guidelines

• National Fire Protection Association (NFPA)

– NFPA 70 - National Electrical Code (NEC)

– NFPA 496 - Purged and Pressurized Enclosures for Electrical Equipment

– NFPA 497 - Recommended Practice for the Classification of FlammableLiquids, Gases, or Vapors and of Hazardous (Classified) Locations forElectrical Installations in Chemical Process Areas

– NFPA 499 - Recommended Practice for the Classification of CombustibleDusts and of Hazardous (Classified) Locations for Electrical Installations inChemical Process Areas



2.3 Government Regulations

Federal Standards and Instructions of the Occupational Safety and HealthAdministration, including any requirements by state or local agencies with jurisdiction,shall apply.

• United States Department of Labor, Occupational Safety and HealthAdministration (OSHA)

– OSHA 29 CFR 1910 - Occupational Safety and Health Standards

3. Definitions

AC Safety Ground: The grounding system required by NEC Article 250 to provide protectionfor personnel and electrical equipment. The Instrument Ground Bus is connected to the SafetyGround per the NEC requirements.

Control Room: The location where direct operation of the unit or plant is performed and whereoperations personnel are in constant attendance

Field Instrument Enclosure: The term used in the generic sense to indicate a cabinet orbuilding that houses instruments and/or wiring termination external to Instrument Rooms orControl Buildings

Field Junction Box (JB): See Junction Box

Field-Powered Devices: Instruments located in the field that are powered from sources otherthan the Basic Process Control System (BPCS) and/or Safety Instrumented System (SIS).These are sometimes referred to as 4-wire devices.

Interface Box or Marshalling Cabinet: See Junction Box

Instrument Rooms: The term used in the generic sense for a walk-in type structure, includingrack rooms, remote instrument enclosures, or any completely enclosed structure that housescontrol equipment

Instrument Ground Bus (IGB): Grounding system connected to a high-quality earth ground,independent from the AC Safety Ground, for all instrumentation signals. The IGB is tied to theAC Safety Ground at only one point, near a high-quality earth ground, per NEC and Figure 1of this Practice. The IGB is isolated from all other grounds.

Junction Box (JB): A protective enclosure around connections between electric wires or cables(ISA Comprehensive Dictionary). The junction box may be a Field Junction Box, usuallyreferred to as JB, or it may be within a control building or instrument enclosure, where it isreferred to as an Interface Box (IB). The IB is sometimes called a Marshalling Cabinet orTermination Cabinet.

Owner: Principal end user

Uninterruptible Power Supply (UPS): A power supply system that provides uninterruptedpower to the connected equipment for a specified period of time, even during failure of the



primary power source. The UPS is typically a rectifier with batteries and an inverter system toprovide alternating current (AC). A UPS is often used to provide reliable power to theinstrumentation power distribution system. (Refer to NEC Article 645-11.)

4. General

4.1 All process measurement and control equipment shall be suitable for the electrical areaclassification in which they are installed.

4.2 All techniques used to comply with area classification requirements shall comply withNEC Chapter 5 or the authority having jurisdiction.

4.3 Equipment and enclosures that require purging to comply with area class requirementsshall be purged in accordance with NFPA 496. Use of purging to reduce areaclassification shall require Owner’s approval. For environmental purges, see Section5.4.

4.4 Instruments shall not be installed in Zone 0, Zone 1, or Division 1 areas unlessapproved by Owner.

4.5 Instrument electrical equipment shall be rated for continuous energized duty.

4.6 Instrument cabinets or panels that contain more than one power source shall have acaution sign identifying the separate power sources.

4.7 Redundant instruments shall be supplied from separate circuits and from separatebranches where practical for each instrument. Separate I/O cards should beconsidered.

4.8 Indicator lights (non-annunciator lights used to visually communicate messages) oncontrol panels shall be uniformly color-coded within each operating unit and,preferably, across each site. A recommended color code for panel indicating lights is:

4.8.1 Green - Normal state, operating, usually energized

4.8.2 Red - Abnormal state, out of service, usually de-energized, or potentiallydangerous

Comment: This convention is opposite to electrical power indicating lightconvention and is not allowed on UL- (Underwriters Laboratories,Inc.) labeled devices.

4.9 Each device of 50 V (AC or DC) or greater shall have its own individual lockabledisconnect switch with appropriate labeling. Either local or remote lockout ispermissible.

4.10 All electrical equipment including instrumentation shall be designed and installed inaccordance with NEC and OSHA 1910 Subpart S.

4.11 Any box, including junction boxes, containing voltages higher than 50 V (AC or DC)shall have a caution sign on the outside of the box.

4.12 Voltages over 120 VAC in instrument enclosures require Owner’s approval.



4.13 Field instruments directly connected to flammable or explosive process fluids shall bedesigned with multiple barriers to prevent entry of vapors into the cable/conduitsystem. If the additional seal is required, a drain, vent, or other device is required sothat the failure of the primary seal is obvious. Refer to NEC Article 501-5(f)(3).

Comment: The use of a sealing fitting with a drain, provided that the drain is onthe instrument side, will satisfy the above-mentioned NEC requirement.

4.14 Minimum Design Capacity Allowance shall be provided as follows:

4.14.1 Design capacity allowance is the capacity available at the completion ofdesign.

4.14.2 Control building cabinets and consoles shall be designed to accommodate aminimum of 20% additional equipment.

4.14.3 Field junction boxes and field cables shall be designed and installed toaccommodate a minimum of 20% additional field devices.

4.14.4 Field wire raceways, exclusive of conduits, shall be designed for a minimumof 20% future additions.

4.14.5 Cable tray from the process area into the control room and instrumentbuildings shall be sized for a minimum of 40% future additions.

4.14.6 Power panels shall have at least 20% spare circuit breakers or fuses installedat design completion (refer to Section 9.2.5).

4.15 Input and output circuits of controllers that are not current limited shall be protectedby fuses.

5. Environmental Considerations

5.1 All instrument equipment shall be enclosed in environmentally appropriate housings.

5.2 NEMA 12 rated enclosures shall be used as a minimum for dry, indoor locations. Forindoor areas that contain oil or dust, NEMA 13 is the minimum rating (refer to NEMAICS6 and NEMA 250).

5.3 NEMA 4 shall be the minimum outdoor rating for enclosures, and NEMA 4X shall beused in wet locations or where corrosion is an issue. Explosion-proof and dust-ignition-proof enclosures shall be rated for the environment in which they are installed.If NEMA 7 enclosures are required outdoors, dual-rated enclosures (NEMA 7 andNEMA 4 or 4X) shall be used (refer to NEMA ICS6 and NEMA 250). See Section 4.3on the use of purging for reduction of the electrical area classification.

5.4 With the Owner’s approval, enclosure purging for environmental purposes shall beconsidered where the ambient atmosphere contains corrosive contaminates or whereother severe conditions exist. Use of purge media other than instrument air shallrequire specific Owner’s approval.

6. Instrument Signal Compatibility

6.1 All analog transmitted instrumentation signals shall be a current signal having a range



of 4 mA to 20 mA DC in accordance with ISA S50.1. Other analog signals for existinginstrumentation systems may be allowed with Owner’s approval.

6.2 Digital communications between smart transmitters and control systems shall adhereto vendor recommendations.

6.3 Power supply voltage shall nominally be 24 VDC.

7. Wire and Cable Systems

7.1 General

The installation of all wiring and cables, including low-voltage instrument cables, shallcomply with NEC requirements for the type of cable or wiring being used and thelocation in which it is being installed.

7.1.1 The routing for cables from field areas to the control room and instrumentroom in new plant areas is in overhead cable trays. Cable trays should not berouted in fire hazardous areas (e.g., over pumps, under process air fans, etc.).

7.1.2 For fire protection of instrument cables, refer to PIP PCCGN002 or toOwner-supplied procedure.

7.1.3 Cables that are laid directly under raised floors shall be specifically approvedfor this method of installation by NEC Article 354. AC power wiring (120VAC and higher) routed in the raised floor space shall utilize conduit, metal-enclosed raceway, metal clad cable, or cable tray, with adequate separationfrom the signal wiring.

7.1.4 The use of the area under a raised floor for an HVAC plenum shall requireOwner’s approval. The wiring shall be installed in accordance with NECArticle 300-222. Any wiring in the HVAC plenum shall be “Plenum Rated”and non-toxic, unless installed in an enclosed conduit system.

7.1.5 For Safety Instrumented Systems (SIS) wiring, refer to ISA S84.01 andPIP PCESS001.

7.1.6 If the room is classified as an information technology equipment room, it shallmeet all the requirements of NEC Article 645.

Comment: Process control electronic equipment alone does not require theroom to be classified as a computer/data-processing room.

7.2 Instrument Wiring

All cables for general instrument use, both single pair and multiple wire, shall meet therequirements of PIP ELSWC03 and PIP ELSWC05. All instrument wiring shall belisted for the specific application for which it is being used.

7.2.1 Wiring

7.2.1.1 Each wire within a field cable, including spares entering a JB orinterface cabinet (marshalling panel), shall be terminated.



7.2.1.2 Splitting multiple conductor cables among multiple devices/boxes inthe field is not permitted.

7.2.1.3 Protection against back electromotive force (EMF) shall beprovided for inductive loads such as relays, solenoids, etc. This maybe accomplished within the equipment card or by one of themethods below:

a. For DC loads, a diode may be installed electrically across thecoil.

b. For AC inductive loads, a metal oxide varistor (MOV) maybe installed across the coil.

7.2.1.4 Lightning protective devices shall be used when any of thefollowing conditions exist (see API RP552, Section 16 and NEC,Article 280):

a. Electronic systems are located in large, open areas, such astank farms.

b. Instrument wire and cable runs exceed 1500 feet.

c. Instrument cables run aerially on poles.

7.2.1.5 Each pair or triad shall be permanently tagged and identified at bothends. Wire markers, tubular heat shrink, or other permanentlyaffixed markers shall be used to ensure permanence of the marking.Machine printing is preferred for clarity.

7.2.1.6 In general, cables shall meet the design criteria listed below andshall be selected in accordance with Table 1.

a. Single Pair Instrument Signal Cable with an Overall Shield(SPISCO)

Application: This type covers the minimum requirements forsingle circuit cable for analog (e.g., 4 - 20 mA DC, 1 - 5 VDC,0 - 100 mVDC) or low-voltage (50 volts DC or less), discretesignals for instrumentation and control signal transmission.

Type: The pairs shall be 2 copper conductors, minimum 16AWG, with an overall shield.

b. Single Pair Thermocouple Extension Cable with an OverallShield (SPTECO)

Application: This type covers the minimum requirements forsingle pair cable made up of individually shielded, thermocoupleextension wires twisted into pairs.

Type: Each pair shall be 2 solid alloy, thermocouple extensionwires per ISA MC96.1 with an overall shield. The individualwires and outer jackets shall be color coded perANSI/ISA MC96.1.



c. Single Triad Instrument Signal Cable with an Overall Shield(STISCO)

Application: This type covers the minimum requirements forsingle circuit cable where three wires are required (e.g., RTDsand vibration) for instrumentation and control signaltransmission.

Type: The triads shall be 3 copper conductors, minimum16 AWG, with an overall shield.

d. Multi-Pair Instrument Signal Cable with an Overall Shield(MPISCO)

Application: This type covers the minimum requirements formultiple circuit cable for high-level analog (e.g.,4 - 20 mA DC, 1 - 5 VDC) or low-voltage digital (50 volts DCor less) signals used for instrumentation and control signaltransmission. This type of cable is not to be used for signalsbelow 100 millivolts.

Type: The cable shall be 12 or 24 pairs with an overall shield.Each pair shall be 2 copper conductors, minimum 20 AWG.

e. Multi-Pair Thermocouple Extension Cable, Individually Shieldedwith Overall Shield (MPTECI)

Application: This type covers the minimum requirements formultiple pair cable made up of individually shielded,thermocouple extension wires twisted into pairs.

Type: The cable shall be 12 or 24 pairs with an overall shield.Each pair shall be 2 solid alloy, thermocouple extension wiresper ISA MC96.1 with an individual shield. Each pair shall havean identifying number and shall be insulated from other pairshields. Conductors shall be minimum 20 AWG. The individualwires and outer jackets shall be color coded per ANSI/ISAMC96.1.

f. Multi-Pair Instrument Signal Cable Individually Shielded Pairswith Overall Shield (MPISCI)

Application: This type covers the minimum requirements formultiple individually shielded, circuit cable for low-level analog(e.g., 0 - 100 mVDC). With Owner’s approval, it may be usedfor high-level analog (e.g., 4 - 20 mA DC, 1 - 5 VDC) or forlow-voltage digital (50 volts DC or less) signals used forinstrumentation and control signal transmission.

Type: The cable shall be 12 or 24 pairs with an overall shield.Each pair shall be 2 copper conductors, minimum 20 AWG,with an individual shield insulated from other pair’s shields.



g. Multi Triad Instrument Signal Cable with an Overall Shield(MTISCO)

Application: This type covers the minimum requirements fortriads (three wire) multiple circuit cable for high-level analog(e.g., 4 - 20 mA DC, 1 - 5 VDC) or low-voltage digital(50 volts DC or less) signals used for instrumentation andcontrol signal transmission. This type of cable is not to be usedfor signals below 100 Millivolts.

Type: The cable shall be 12 or 24 pairs with an overall shield.Each pair shall be 2 copper conductors, minimum 20 AWG.

h. Multi-Triad Instrument Signal Cable Individually ShieldedTriads with Overall Shield (MTISCI)

Application: This type covers the minimum requirements formultiple individually shielded circuit cable for low-level analog(e.g., 0 - 100 mVDC). With Owner’s approval, it may be usedfor high-level analog (e.g., 4 - 20 mA DC, 1 - 5 VDC) or forlow-voltage digital (50 volts DC or less) signals used forinstrumentation and control signal transmission.

Type: The cable shall be 12 or 24 triads with an overall shield.Each triad shall be 2 copper conductors minimum 20 AWG,with an individual shield insulated from other pair’s shields.

7.2.2 Circuit Impedance

7.2.2.1 Wire resistance of single device, 4 - 20 mA instrument loops canusually be neglected for one-way distances less than 1000 feet.

Where multiple devices are used in applications where the one-wayloop distance is over 1000 feet and are wired in series in a4 - 20 mA loop, the total circuit impedance shall be evaluated forproper operation.

7.2.2.2 120 VAC wiring to solenoid valves, relays, and other electro-mechanical devices shall be sized to have a voltage drop of less than5% from the voltage source transformer to the control device at therated holding current.

7.2.2.3 For long runs where wiring distributed capacitance is in parallelwith the control device, designer shall perform calculations toensure correct operability of each circuit.

Comment: AC input as discussed above may not de-energizedue to the distributed capacitance effect. The use ofrectifiers and DC relays should be considered.



7.2.2.4 DC wiring to solenoid valves, relays, and other electromechanicaldevices should be sized to have a voltage drop of less than 10%.

Comment: Voltage drop calculations are not required for low-wattage (1.4 watts) DC solenoid valves that usemultiple conductor cables, 20 AWG twisted pairs,and have lengths less than 700 feet. For distancesover 700 feet, the use of higher voltage devices orlarger wire size should be considered.

7.2.2.5 When low-energy devices such as pilot-operated solenoids, pilotlights, etc., are driven by an electronic amplifier, the “off position”leakage current must be low enough to allow the load to be “turned-off” when de-energized.

7.3 Segregation/Separation Requirements

7.3.1 Instrument wiring shall be segregated according to wiring class and circuittype in accordance with Table 1 and separation distances in accordance withTable 2.

7.3.2 Generally, each circuit type shall be run in a separate tray/conduit, except asnoted in Table 2.

7.3.3 Intrinsically safe systems and fire detection systems shall be segregated fromother wiring and shall have dedicated junction boxes and marshalling panels.

7.3.4 If different circuit types have to be terminated in the same junction box, eachcircuit type shall enter the junction box in a separate cable or conduit and shallbe terminated on terminal strips that are physically separated according tocircuit type. Plastic or metal barriers that are labeled with the circuit type orpower level shall have separate terminal strips.

7.4 Cable Tray

7.4.1 Cable tray systems shall be utilized to route multi-pair and multi-wire cablesfrom field junction boxes or remote I/O enclosures to instrument buildings (seeNEMA VE2).

7.4.2 Cable tray systems shall be grounded in accordance withNEC Table 318-7(b)(2).

7.4.3 Cable tray fill shall be based on NEC Tables 318-9, 318-9(c), and 318-10.

7.5 Conduit Systems

7.5.1 Instruments installed in NEC Class I, Division 2, or Division 1 with Owner’sapproval that by design rely on a single compression seal diaphragm or tube,shall require an additional seal that meets the conditions of the process fluid.(See 4.13).

7.5.2 Terminations of the conduit shall be in accordance with PIP PCIEL000.



7.5.2.1 Field instruments shall be wired from the field junction box to thefield instrument by a dedicated, individual cable. Single pair cablesin conduit or trays shall be the preferred wiring method.

7.5.2.2 Termination of conduit with flex conduit in Division 2 or approvedflex for Division 1 area shall be used to provide isolation of theconduit system from vibration to protect against thermal expansionand for maintenance. The length shall be limited to 24 inches unlessotherwise approved by Owner.

7.5.2.3 Conduit wiring capacity shall be sized in accordance with NEC.

Comment: When performing this sizing calculation, care mustbe taken to allow for the reduced area of theconduit fitting.

7.5.2.4 In highly corrosive environments, aluminum, PVC, or PVC-coatedconduit may be used where approved by Owner. When PVC isused, the use of a grounding wire and additional supports arerequired in accordance with NEC Article 347.

7.5.2.5 Fiber and data highways shall meet equipment and cable vendor’sinstallation requirements and NEC Article 770.

7.5.2.6 Redundant data highways shall have separate routings withmaximum practical physical separation to minimize the possibilityof a single event causing the simultaneous loss of both wiringsystems. Redundant data highway routings shall be approved byOwner.

7.6 Junction Boxes

Field junction boxes shall meet the requirements of PIP PCSEL000.



Table 1Wire and Cable Requirements for Instrument Circuits

(For control panel wiring, see PIP PCSCP001. For 300 volt instrument tray cable, see PIP ELSWC02.For 600 volt power and control cable, see PIP ELSWC03.)

A. NEC Class 1 Circuits (Notes 1 and 3)

Instrument Class Circuit Example Conduit Wiring Tray Cable

A. 120 VAC or Less Switches, Solenoids, Relays 600 V THWN, Insulated,14 AWG Minimum

600 V TC, Insulated

B. NEC Article 727 Circuits, Conduits, and Cable Tray Installations (Notes 1 and 11)

B1. InstrumentCircuit Class andType

Circuit Example Single Pairs or Triads Multiple Pair or Triad Cable

B2. 24 VDC or Less(Notes 4 and 8)

4 - 20 mA DC RTDs, WeighCells, Solenoids, Alarms,Switches, Relays,Secondary Motor Control,Digital Process Transmitter

300 V Shielded Twisted Pairs orTriads

CABLE TYPES:SPISCO, STISCO

(Notes 2, 3, 5, 9, and 10)

300 V ITC PVC InsulatedTwisted Pairs or Triads, OverallShield

CABLE TYPES:MPISCO, MTISCO

(Notes 2, 3, 4, 9, and 10)

B3. Frequency orPulse Train DigitalCommunication(Note 6)

Speed, Vibration, TurbineMeter

300 V Shielded Twisted Pairs orTriads

CABLE TYPES:SPISCO, STISCO

(Notes 2, 3, 9, and 10)

Individually Shielded Pairs TrayCable or Triads

CABLE TYPES:MPISCO, MTISCO

(Notes 2, 3, 5, 9, and 10)

B4. ThermocoupleMeasurement(Note 8)

Thermocouples 300 V ITC, 16 AWG, IndividuallyShielded Twisted Pair

CABLE TYPES:SPTECO

(Notes 2, 3, 9, and 10)

300 V ITC, 20 AWG, IndividuallyShielded Twisted Pairs, OverallShield

CABLE TYPES:MPTECO

(Notes 2, 3, 9, and 10)

C. Data Highways and Other High-Speed Circuits

Data Highways(Notes 7 and 10)

EIA-422A Data Highways/High-Speed Communication

Follow Cable and SystemManufacturer’sRecommendation

Follow Cable and SystemManufacturer’sRecommendation

Notes:1. Type A circuits shall be separated from all other types of instrument circuits by either conduit tray dividers or separate trays.2. 20 AWG minimum wire size for multi-pair cable; 16 AWG minimum wire size for single pair cable. A larger wire size shall be used if

required by the calculations.3. Wire sizes listed in the table and notes are minimum requirements. Actual wire size shall be based on load current and voltage drop

requirements. The use of parallel wires to meet the current requirements is not allowed.4. Higher DC levels require Owner’s approval.5. Type ITC cable shall not be installed on either non-power-limited circuits or powered-limited circuits operating at more than 150 volts

or more than 5 amperes.6. Vibration signals shall be run in galvanized steel conduit, flexible conduit, or armored cables from the probe to the local transducer.7. Maximum separation between redundant highways shall be obtained within the operating plant. The use of a single cable tray for

primary and redundant highways is not acceptable.8. Type B1 and B3 circuits may be routed in the same conduits and trays.9. For a description of the six-letter designators, see Section 6.2.1.7 and refer to the listings in

PIP ELSWC05 and PIP ELSWC05D.10. Differences in the manufacturer-recommended cable and these requirements shall be resolved with the Owner.11. Physical barriers are also required to separate intrinsically safe wiring from other wiring.



Table 2Segregation and Minimum Spacing Requirements for Parallel Runs

(For wiring level definitions, see Note 1. For units of measurement in inches, see Note 3.)

Steel-Conduit-to-Steel-Conduit Spacing

Wiring Levels 1 2 3 4 5

1 0 1 3 12 12

2 1 0 3 9 12

3 3 3 0 0 0

4 12 9 0 0 0

5 12 12 0 0 0

Tray-to-Tray Spacing or Tray-to-Conduit Spacing

Wiring Levels 1 2 3 4 5

1 0 1 6 26 26

2 1 0 6 18 26

3 6 6 0 6 12

4 26 18 6 0 0

5 26 26 12 0 0Notes:1. Wiring Levels:

Level 1 - High Noise Susceptibility Application 1. 4 - 20 mA, 24 VDC instrument signals, other analog < 50 VDC and digital process transmitters 2. Discrete, 24 VDC instrument signals 3. +/- 50 VDC and common buses feeding analog instrument signals 4. +/- 24 VDC and common buses feeding digital instrument signals 5. Thermocouple and RTD circuits 6. Telephone circuitsLevel 2 - Medium Noise Susceptibility Application 1. Light and switching circuits 24 VDC or less 2. Analog signals > 50 VDC < 28 VAC rippleLevel 3 - Low Noise Susceptibility Application 1. 120/240 VAC feeders < 20 amps 2. Light and switching circuits 24 VDC or greater 3. 120/240 VDC relay, contactor and circuit breaker coils < 20 amps 4. Analog signals > 50 VDC < 28 VAC rippleLevel 4 - Medium Power Application 1. Primaries and secondary of transformers > 5 KVA 2. AC and DC buses 0 - 800 volts with currents > 20 ampsLevel 5 - High Power Application 1. AC and DC buses > 1 kW or currents > 800 amps, or both

2. This table is based on IEEE 518.3. This table does not apply to the use of non-metallic conduit.4. Cable tray spacing is defined as the minimum distance (in inches) between the top of one tray and the bottom of the tray above or

between the sides of adjacent trays. This spacing also applies to the distance between trays and power equipment less than 100KVA.Conduit spacing is defined as the minimum distance (in inches) between the outside surfaces of conduits. This spacing alsoapplies to the distance between conduits and power equipment less than 100 KVA.

5. When unlike signal levels must cross, in trays they shall cross at 90-degree angles. Where it is not possible to maintain spacing,a grounded metal barrier should be placed between unlike levels at the crossover point.

6. Levels 3 and 4 may be run in a common tray if separated by a barrier.7. When separate trays are impractical, Levels 1 and 2 may be combined in a common tray, provided a grounded metal barrier

separates the levels.8. Trays for all levels were based on metal, solidly grounded, with good ground continuity.9. Only cables having the same voltage class (Table 1) may be run together.

10. Trays and conduits containing Levels 1 and 2 shall not be routed parallel to high-power equipment enclosures of 100 KVA andlarger at a spacing of less than 5 feet for trays and 2-1/2 feet for conduit.

11. Where the spacing listed in Table 2 is difficult to maintain, parallel runs should be minimized and in no case be run parallel toeach other for a distance greater than 5 feet.



8. Terminations

8.1 General

8.1.1 All field instruments wiring shall be landed on terminal strips in field junctionboxes. See NEC Article 110-14. Devices with attached pigtails may be splicedin conduit fittings with Owner’s approval. SeeNEC Article 370-16.

8.1.2 Individual circuit wires shall not be spliced.

8.1.3 Connections to terminal strips shall be limited to two wires per screw. If thewires are solid or ferruled, then only one wire shall be connected per screw.

8.1.4 When terminating lugs are used on stranded wire, they shall be of the lockingtype and limited to their design capability (i.e., two wires maximum per lug).

8.1.5 Terminal shall be tightened to the torque required by manufacturer.

8.1.6 All wires (including spares and shield drain wires) of each cable shall belanded on both ends in continuous order on terminals, dressed out neatly, andlabeled.

8.1.7 Overall cable shield drain wires and individual pair shields shall be groundedto the IGB in the instrument building in which the cables terminate. Groundedthermocouples shall be installed perSection 8.5 with shield wire grounded in thermocouple head.

8.1.8 All wire pairs, triads, and cables (including spares) used to connect instrumentcomponents and systems shall have uniform identification labels on each wireand cable at each point of termination.

8.2 Milliamp Signals - Typically 4 - 20 mA

8.2.1 Each signal pair shield shall be connected to the IGB and shall be isolatedfrom ground elsewhere.

8.2.2 Signal (power) isolation shall be required for all devices powered from sourcesother than the Basic Process Control System (BPCS) and SIS that are locatedin the instrument building.

8.2.3 The black wire shall be positive at the power source and the white wire shallbe negative.

8.2.4 Individual drain wires shall be isolated from ground at the field device andwhere exposed in conduit or cable seal fittings by use of electrical tape or heatshrink tubing.

8.3 Voltage Signals - 100 mV or Greater

The cabling for these measurement signals, sometimes called high-level signals, shallmeet the same requirements as mA signals. (See Section 8.2.)



8.4 Voltage Signals - Less than 100 mV

These measurement signals, sometimes called low-level signals, shall meet the samerequirements as thermocouples. (See Section 8.5.)

8.5 Thermocouple Signals

8.5.1 Thermocouple extension wire shall be used between the thermocouple and thetransmitter, multiplexer, or other monitoring device.

8.5.2 Thermocouple extension wire shall be the same type of material as thethermocouple. Specifically designed extension wire may be used instead ofexotic materials with Owner’s approval.

8.5.3 The number of thermocouple extension cable junctions shall be minimized.

8.5.4 For grounded thermocouples, individual shield wires shall be grounded only atthe thermocouple connection in the head.

Comment: Shielding of thermocouple signals is extremely important toprevent noise.

8.5.5 If the ambient temperature is above 190º F (in rotating equipment or nearfurnaces), thermocouple wires shall have high-temperature insulation such asextended polytetra-fluorethylene.

9. Instrument Power Systems

9.1 General

9.1.1 Control system instrument power requirements generally shall be 120 VACand/or 24 VDC. Use of other voltages requires Owner’s approval.

9.1.2 AC power for instrument systems and supervisory control computers shall besupplied from UPS. The UPS shall be supplied from both a primary and asecondary power source.

9.1.3 For a typical power distribution system for the BPCS and SIS that requires aUPS, see Figure 1.

9.1.4 Alarms shall be installed in the control room to indicate:

a. The loss of the secondary source

b. A transfer to the secondary source

c. UPS trouble

d. UPS on batteries

e. Low-battery alarm

9.1.5 A dedicated isolation instrument power transformer shall be provided for UPSbypass power.

9.1.6 The instrument power system shall be separate and distinct from non-criticalcircuits such as heating and lighting.



9.1.7 Operating time capability (power from batteries of UPS) required after thetotal outage of AC supply power shall be set by the Owner considering unitresponse time and needs of units served.

Comment: A minimum of 30 minutes is a typical time for instrumentsystems power backup. Where additional time is required,consideration should be given to a backup generator as asecondary source for UPS.

9.1.8 Provisions for testing the UPS with process running shall be provided.

9.2 Branch Circuits Design

9.2.1 Circuit breakers or fuses shall be used to safeguard principal branches fromsubordinate branch faults. Each instrument supply shall be protected by anindividual fuse or a current-limiting device.

9.2.2 Circuit protective devices shall be coordinated to ensure that the device nearestthe overload or fault will open first, minimizing the possibility of initiating aUPS transfer and isolating the fault from the rest of the system. Circuits andloops shall be protected in related process systems.

Comment: UPS power distribution panels usually require either fast-actingelectronic circuit breakers or fuses to allow circuit protectivedevice coordination.

9.2.3 Power connections to redundant control system components and modules (e.g.,DC power supplies, BPCS control modules, etc.) shall be taken from separatebranch circuits and separate supplies (minimum of different breakers ordistribution panels) to reduce common mode failures.

Three-phase UPS output is desirable and allows redundant components to beconnected to separate phases, minimizing the impact of voltage depression onfaulted phases before clearing.

9.2.4 Fast-acting single pole breakers or fuses shall be used for the 120 VAC,single-phase 2-wire circuits. Each circuit shall have an individual, unswitchedneutral wire. Where only one single-phase inverter is supplied, fuses may berequired to clear faults in adequate time to prevent loss of power suppliesduring a fault.



9.2.5 Design and installation of the neutral wire shall satisfy the followingrequirements:

Grounding: The main bonding jumper at the derived source or the first paneldownstream of the derived source establishes the AC neutralground point. All circuit neutral wiring shall be insulated.

Color Coding: Neutral wires shall be white except when there is more thanone, in which case, each shall be color coded to satisfyNEC Article 210.5.

Splicing: Splicing of the neutral wire is prohibited except when necessaryat a field device.

Connection: The neutral wire connected to a device shall always be the oneassigned to the source that serves that device, never one fromsome other circuit source.

9.2.6 Assignment of computer circuits shall meet the following requirements:

a. Each computer system or subsystem requiring power shall be on aseparate circuit. The circuit may be wired with type “SO” service cord(NEC Table 400-4). Computer cabinets and devices shall be wired eitherto the computer power panel or connected to a receptacle using a “twist-lock” plug. UPS receptacles shall be distinguishable from otherreceptacles.

b. High-power peripheral devices, such as line printers, shall be on aseparate circuit and shall be connected via a wall- or floor-mountedreceptacle, providing an isolated AC Safety Ground per NECArticle 250-74, Exception 4.

c. Peripheral devices (e.g., CRTs, gateways, etc.) and other equipmentessential to normal process control shall be assigned to the highestreliability power available (such as UPS power).

d. Devices that are not essential for normal process control may be assignedto lower reliability power.

e. No spare wall or floor outlets are to be installed in the computer powersystem unless clearly marked as to the service.

10. Grounding

10.1 Grounding electronic systems (e.g., BPCS, SIS, and Programmable Logic Controller(PLC)) shall follow the criteria below and the manufacturer’s recommendations. Anydeviation shall be reviewed by the manufacturer’s technical representative andapproved by Owner. See Figure 1 (INSTGND001).

10.2 Two grounding systems shall be required in the control room or instrument building:

a. AC Safety Grounding for personnel safety

b. IGB for signal reference



The IGB and AC Safety Grounds shall join electrically at a single point near a high-quality earth ground (IEEE 1100, API RP 552, and Figure 1).

10.3 All electrical equipment shall be grounded to the AC Safety Ground for personnelprotection to satisfy NEC Article 250.

10.4 The IGB wiring shall be installed to provide a stable voltage reference. The groundingsystem(s) should provide ground resistance level(s) consistent with supplierrecommendations and should meet the NEC requirements.

10.5 Ground systems shall be installed to provide a ground system that meets vendorrequirements. Since various suppliers have different grounding requirements, caremust be taken to develop a scheme that meets those requirements, meets the NECrequirements, has testing capabilities, and has supplier approval. (See IEEE 1100.)

Comment: A removable link may be provided between the AC Safety Ground andthe IGB to allow testing of the IGB.

10.6 All IGB bars, plates, and wire shall be isolated from building AC Safety Ground pathexcept at the one point specified in Section 10.2 above.

10.7 The following signal and equipment connections shall be made to the IGB:

a. Instrument power supplies (negative leads)

b. BPCS, PLC, SIS, and other control modules (signal common, logic ground,network ground, etc.)

c. Instrument (individual and overall cable) shields

10.8 Signals originating from instruments powered by sources external to the controlbuilding (e.g., a chromatograph in an analyzer building) often are electricallyreferenced to another ground potential. These instruments shall have isolated outputs,or the output signals shall be electrically isolated when brought into the BPCS, PLC,SIS, and other control modules.

10.9 Ground loops shall be avoided.

Comment: Ground loops exist when extraneous, unwanted currents flow in a wirecausing an offset in the instrument reading. Care must be taken toavoid multiple grounds.

Comment: When instrumentation wiring is of the shielded type, shields should begrounded at only one place.

11. Intrinsically Safe Instrument Systems

11.1 Use of intrinsically safe systems shall require Owner’s approval.

11.2 Refer to NEC Article 500-4(e) and ISA RP12.6 for requirements of intrinsically safesystems.

12. Non-Incendive Systems

12.1 Use of non-incendive systems shall require Owner’s approval.



12.2 All non-incendive wiring shall be installed in accordance with NEC Article 500-4(f)and ISA RP 12.12.

13. Control Panel and Cabinet Wiring

All control panels, consoles, and equipment cabinets shall be wired in accordance withPIP PCSCP001.

14. Installation

All instrumentation wiring installations shall be in accordance with installation details inPIP PCIEL000, unless otherwise approved by Owner. All Type B circuits shall use InstrumentTray Cable (ITC).

15. Data Highways

Primary and backup data highway cables shall follow different routing and preferentiallyenters buildings and/or enclosures from opposite sides.

pccel001 - instrumentation electrical requirements

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