electrical manhole standards

8/10/2019 Electrical Manhole Standards

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DEPARTMENT OF THE AIR FORCE

HEADQUARTERS AIR FORCE CIVIL ENGINEER SUPPORT AGENCY

19 JAN 2011

APPROVED FOR PUBLIC RELEASE: DISTRIBUTION UNLIMITED

FROM: HQ AFCESA/CEO139 Barnes Drive Suite 1Tyndall AFB FL 32403-5319

SUBJECT: Engineering Technical Letter (ETL) 11-10 (Change 1): ElectricalManhole Design Considerations

1. Purpose. This ETL provides technical guidance and criteria for the design ofelectrical manholes, with electrical safety and arc flash considerations as a design input.

Associated underground distribution design criteria are also addressed. This ETL doesnot apply to manholes associated with airfield lighting circuits.

2. Application. Recommendations in this ETL are optional. The recommendationsprovided here apply to new designs.

2.1.Authority: Air Force instruction (AFI) 32-1063, Electric Power Systems

2.2.Effective Date: Immediately

2.3. Intended Users:

Major command (MAJCOM) engineers

Base civil engineers (BCE)

2.4. Coordination:

MAJCOM electrical engineers

3. Referenced Publications.

3.1.Air Force (available at http://www.e-publishing.af.mil ):

AFI 32-1063, Electric Power Systems

AFI 32-1064, Electrical Safe Practices

Air Force Occupational Safety and Health (AFOSH) standard 91-25, ConfinedSpaces

ETL 11-9, Electrical Manhole Entry and Work Procedures,

http://www.wbdg.org/ccb/browse_cat.php?o=33&c=125

3.2.Joint (available at http://www.wbdg.org/ccb/browse_cat.php?o=29&c=4 ):

Unified Facilities Criteria (UFC) 3-501-01, Electrical Engineering

UFC 3-550-01, Exterior Electrical Power Distribution

UFC 3-560-01, Electrical Safety, O&M

Unified Facilities Guide Specification (UFGS) 33 71 02.00 20, UndergroundElectrical Distribution, http://www.wbdg.org/ccb/browse_org.php?o=70


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3.3. Industry:

Institute of Electrical and Electronics Engineers (IEEE) 835, IEEE StandardPower Cable Ampacity Tables, http://www.ieee.org/index.html

IEEE C2, National Electrical Safety Code (NESC),

http://www.ieee.org/index.html National Fire Protection Association (NFPA) 70, National Electrical Code

(NEC), http://www.nfpa.org/index.asp

4. Background.

4.1. AFI 32-1064, Electrical Safe Practices, states: Work on or near energizeddistribution lines is prohibited except in rare circumstances and then only whenapproved by the BCE. Work on or near energized electrical equipment in manholesis extremely dangerous and requires all circuits inside the manhole to bedeenergized. UFC 3-560-01,Electrical Safety, O&M, states that all equipment inside

a manhole, including insulated conductors, should be deenergized before allowingentry into a manhole. ETL 11-9, Electrical Manhole Entry and Work Procedures,expands on these requirements and provides guidance for work activities that canbe performed inside electrical manholes containing energized circuits.

4.2. Because of the concerns regarding entry into electrical manholes containingenergized circuits, underground distribution system designs should address accessrestrictions as a specific design consideration. UFC 3-550-01, Exterior ElectricalPower Distribution, provides tri-Service design requirements for undergrounddistribution systems, including electrical manholes. This ETL expands on the UFCsrequirements and provides additional recommendations for work inside electrical

manholes containing energized electrical equipment with arc flash as a concern.Underground distribution system design is also addressed.

5. Defini tions.

5.1. Duct bank. Two or more conduits (or ducts) routed together in a commonexcavation with or without concrete encasement.

5.2.Handhole. An opening in an underground system containing cable, equipment,or both, into which personnel reach but do not enter, for the purpose of installing,operating, or maintaining cable, equipment, or both.

5.3.High voltage. For the purpose of this ETL, a class of nominal system voltagesgreater than 34,500 volts.

5.4.Low voltage. For the purpose of this ETL, a class of nominal system voltagesless than or equal to 600 volts.


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5.5.Manhole. A subsurface enclosure that personnel may enter, used for installing,operating, and maintaining cable.

5.6. Medium voltage. For the purpose of this ETL, a class of nominal systemvoltages above 600 volts and less than or equal to 34,500 volts.

6. Acronyms.

AFI - Air Force instructionAFOSH - Air Force Occupational Safety and HealthBCE - base civil engineer (or equivalent)ft - footIEEE - Institute of Electrical and Electronics Engineersm - meterMAJCOM - major commandNFPA - National Fire Protection Association

UFC - Unified Facilities CriteriaUFGS - Unified Facilities Guide Specification

7. Requirements. UFC 3-501-01, Electrical Engineering, provides minimumrequirements for design analyses and calculations. UFC 3-550-01 provides minimumrequirements for underground distribution system design. The following subparagraphsclarify these requirements and provide additional recommendations.

7.1. Formal Design.

7.1.1. Provide a formal design and design analysis in accordance withUFC 3-501-01 for underground distribution systems.

7.1.2. Specify key features of the underground distribution system design andinstallation. UFGS 33 71 02.00 20, Underground Electrical Distribution, isrecommended for use; edit the specification as necessary for base-specificdesign features. UFGS 33 71 02.00 20 also addresses personnel qualificationsfor performing conductor installation and splicingimportant for ensuring areliable installation.

7.2. Electrical Distribution Systems Without Manholes.

7.2.1. The installation of electrical manholes is not a standard designrequirement; underground distribution systems can be installed without the use ofelectrical manholes. An underground distribution system without electricalmanholes is viable for installations involving a few circuits; however, electricalmanholes will usually be required when many circuits are involved.

7.2.2.UFC 3-550-01 allows the use of pad-mounted sectionalizing terminationcabinets only when switching, isolation, or electrical protection for the


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downstream circuit is not required or anticipated. Sectionalizing terminationcabinets can be used instead of in-line splices in manholes or for minor loadsthat do not warrant the expense of pad-mounted switchgear.

7.2.3. For a design that uses pad-mounted sectionalizing termination cabinets

instead of electrical manholes, specify the maximum allowed distance betweencabinets. UFC 3-550-01 requires the following for electrical manholes:Separation on straight runs must not exceed 400 ft (120 m). In situations wheregreater separation is desired and this greater separation is not prohibited byeither excessive pulling tension or site requirements, separation of up to 600 ft(180 m) is permitted. Unlike a straight cable pull between two electricalmanholes, a cable pull between two pad-mounted sectionalizing terminationcabinets involves a minimum of two 90-degree bends. Confirm that the specifiedpull distance 1) satisfies the required maximum pulling tension; and 2) is withinthe ability of electrical shop personnel to replace, if necessary.

7.2.4. Install pad-mounted switchgear rather than pad-mounted sectionalizingtermination cabinets at all locations where load break switching and isolationmight be necessary.

7.3. Electrical Distribution System Configuration Preferences.

7.3.1. Minimize the number of circuits inside an electrical manhole. Installingmany circuits inside an electrical manhole will make it more difficult to deenergizeall affected circuits when working inside the manhole. It is preferable to havemore manholes with fewer circuits per manhole rather than fewer manholes withmore circuits per manhole.

7.3.2. Typically, the circuits installed inside an electrical manhole will share acommon duct bank. As more sets of conductors are installed in a common ductbank, derate the ampacity of each set of conductors to account for the additionalheat-loading of multiple sets of conductors. NFPA 70, National Electrical Code(NEC), provides a basis for conductor ampacity, and NFPA 70, Figure 310.60,provides basic underground duct configurations to consider. IEEE 835, IEEEStandard Power Cable Ampacity Tables, provides similar information and canalso be used as a guide.

7.3.3. Evaluate the cross-connect capability between circuits. Avoid installingredundant circuits or circuits that provide alternate supply capability inside thesame electrical manhole or duct bank.

7.3.4.Consider 500-kcmil (thousand circular mils) conductors for the main run fornew or upgraded circuits. The intent is to improve cross-connect capability duringperiods that circuits inside an electrical manhole must be deenergized.


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7.3.5. Directional boring for underground distribution system installation isauthorized only to the extent allowed by UFC 3-550-01. Directional boring is notauthorized as a standard installation practice for an entire circuit.

7.4. Electrical Manhole Design.

7.4.1. Handholes are prohibited by UFC 3-550-01 for medium-voltage primarydistribution circuits.

7.4.2.Per UFC 3-550-01, the following equipment is prohibited inside electricalmanholes:

Load junctionsthis includes load junctions with either load-break ordead-break elbows;

Separable splices used to supply lateral circuits (bolt-T connections);

T-splices, Y-splices, and similar types of splices used to supply lateralcircuits;

Power distribution equipment, including transformers and switches.

7.4.3. Design electrical manholes with personnel access and adequate interiorworking space. Attachment 1 provides an example of an electrical manholedesign that provides a large access door for improved personnel access andretrieval. The interior space also allows adequate working space. An electricalmanhole does not require an oversized design if a single circuit passes throughit.

Note: An oversized electrical manhole with a large entrance from above isintended to improve personnel safety from an arc flash event. However, active

work inside an electrical manhole containing energized circuits will still bedangerous. AFI 32-1064, AFOSH Standard 91-25, Confined Spaces, and ETL11-9 still apply.

7.4.4.Individually fireproof medium-voltage cables in all underground structures.

7.4.5. Label each conductor with the associated equipment identification, theelectrical phase, and the conductor size.

7.4.6. Install all conductors on cable racks. UFGS 33 71 02.00 20 providesrequirements.

7.4.7. UFC 3-550-01 states: Route cable installations inside manholes alongthose walls providing the longest route and the maximum spare cable lengths.One complete loop of conductors is recommended.


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8. Point of Contact. The authority having jurisdiction on all matters discussed withinthis ETL is the Chief Electrical Engineer, HQ AFCESA/CEOA. To reach the ChiefElectrical Engineer, e-mail [email protected], call DSN 523-6995 or commercial (850) 283-6995, or mail to 139 Barnes Drive, Suite 1, Tyndall AFB,FL 32408-5319.

CLIFFORD C. FETTER, P.E., GS-15, DAF 2 AtchsActing Chief, Operations and Programs Support 1. Electrical Manhole Design

Examples2. Distribution List


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ELECTRICAL MANHOLE DESIGN EXAMPLES

A1.1. Electrical Manhole Size. If multiple circuits are routed through a single electricalmanhole, the manhole should be large enough to have all conductors secured on cableracks with adequate working room near the conductors. Figures A1.1 and A1.2 show an

example of a manhole with large, hinged access doors for easier entry to and from themanhole. Galvanized-steel, spring-assisted access doors are available in standardsizes as large as 48 inches by 72 inches (1.21 meters by 1.82 meters). Figures A1.3and A1.4 provide photographs of an installation.

Figure A1.1. Electrical Manhole Configuration


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Figure A1.2. Electrical Manhole Access Doors

Figure A1.3. Electrical Manhole Installed with Large Entrance


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Figure A1.4. Electrical Manhole Interior View

A1.2. Cable Racks. Cable racks are an important design and installation requirement.Safe entry into an electrical manhole containing energized circuits is not possible if theconductors are not neatly racked and out of the way. Figure A1.5 shows an electricalmanhole where safe entry is not possible. Furthermore, the conductors shown in Figure

A1.5 are not labeled in any way; there is no indication regarding which circuit is which.Figure A1.6 shows a typical cable rack. The routing of conductors inside a manhole alsoshould be specified. Otherwise, there might not be any slack provided, as shown inFigure A1.7; repair activities will be more difficult in this location.

Figure A1.5. Unsecured Conductors Inside an Electrical Manhole


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Figure A1.6. Typical Cable Rack Detail

Figure A1.7. Inadequate Conductor Slack/Loop Inside Manhole


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A1.3. Conduit Entrance. The conduit entrance design and installation should bespecified. Keeping the manhole clean and dry can be achieved if close attention is paidto sealing all entrances.

Figure A1.8. Typical Conduit Entrance Detail

A1.4. Conductor Routing Details . The design should specify, and the as-builtdocumentation should confirm, the conductors installed inside each conduit. Labeling isnecessary to facilitate conductor identification. Figure A1.9 shows an example drawingthat details the location of each circuit.


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Figure A1.9. Duct Configuration Detail


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DISTRIBUTION LIST

DEPARTMENT OF DEFENSE

Defense Commissary Agency (1) AAFES (1)Design and Construction Division ATTN: RE-C2250 Foulois St., Suite 2 PO Box 660202Lackland AFB, TX 78236 Dallas, TX 75266-0202

SPECIAL INTEREST ORGANIZATIONS

Information Handling Services (1) Construction Criteria Base (1)15 Inverness Way East National Institute of Bldg Sciences

Englewood, CO 80150 1201 L Street NW, Suite 400Washington, DC 20005

electrical manhole standards

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