MEP FundamentalsPower

MEP FundamentalsPower

MEP FundamentalsPower

September, 2007

1. What is AIC?

2. If 3-phase is better, why not 4 or 5?

3. Do all tenants have a transformer? Why or why not?

Examples of Questions we will answer:

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September, 2007

Electricity is created by:Nature - lightningBattery - chemicalGenerator- mechanical Creates alternating

current as it rotates Why three phases, why

not four?

Motor in reverse

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September, 2007

Generation:Heat > steam > turbine > generatorVoltage boosted for distanceSubstations step down to 7,200-15,000vOn-site transformer steps down to 480 or 208v

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September, 2007

Ground:

The earth safely absorbs electricityNot just one of those NEC things

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September, 2007

Grounding and bonding:Ground is a connection to the earth that only carries current when there is a short (green wire)Bond is connection between components to the groundGrounding is required: At service initiation At low voltage side of transformer

Initiation of new voltage (480v to 208v)

Grounds need to be where inspectors can find them

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September, 2007

Transformers:

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September, 2007

Fault Current:How much current will flow when a fault (short) occurs?What limits the current?Common panel/breaker rating is 10,000 ampsHow do you know? Utility company transformer

available fault current Fuses in landlord or tenants main

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September, 2007

Transformers Typically 480/277 to 208/120

30 KVA to 83 Amps (290 Lbs.) $ 2,300

45 KVA to 125 Amps (400 Lbs.) $ 2,800

75 KVA to 208 Amps (500 Lbs.) $ 4,000

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September, 2007

Breakers vs. Fuses

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September, 2007

Types of Landlord distribution equipment:Utility metered Wireway Switchgear Meter Center

Landlord redistribution, no meters Distribution Panelboard / Switchboard Buss duct

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September, 2007

Wireway (Utility Metered)

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September, 2007

Switchgear (Utility Metered)

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September, 2007

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Meter Center (Utility Metered)

September, 2007

DistributionPanelboard/Switchboard

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(Landlord Redistribution, no meters)

September, 2007

Buss duct

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(Landlord Redistribution, no meters)

September, 2007

CT’s / CT Cabinet

September, 2007

Load Center Residential Not bolt-on breakers Breakers not switch duty rated

Panelboards Grouping of 20A breakers

Distribution Panelboards Larger breakers, 60A – 400A

Switchboards Distribution panelboard with fused switches

Switchgear Large utility rated equipment

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September, 2007

Breakers:

Rating (maximum continuous current)Some appliances create short duration surges at start-up(ex. electric motor)

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September, 2007

Ground fault:

Hot conductor connected to groundCreate arc/sparkCaution: intermittent fault will not trip breakerGround fault interrupters

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September, 2007

Common voltages:Single phase: 120v – household outlet

Three phase: 240v = 120v x 2 wires for larger load (dryer) 208v/120v/3ph/4w 480v/277v/3ph/4w

What’s the last number? Three wires are power Four wire – neutral Ground isn’t counted, it’s assumed

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September, 2007

Neutral Another path to ground

(white wire) One hot (black) wire plus

neutral = single phase 277v or 120v

Why not 277v x 3 or 120v x 3 for three phase? Power triangle, really is

geometry.

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September, 2007

Load calculation:NEC rules

No consideration for diversity permitted. Actual load rarely exceeds 65% of service size

Actual rating of equipment/lighting is required Equipment nameplates Lighting ballast inefficiencies must be considered

Continuous loads (operating for three + hours) Rated load x 1.25 Lights What about AC and heat?

Track lighting considered 75 watts per foot for service size, but not branching size

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September, 2007

Loadcalculation:

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September, 2007

Wire size:Sized like pipe (velocity, diameter and pressure)Continuous loads de-rate wire capacity

Heat Defined as 3+ hours (lights)

Multiple conductors de-rate wire capacity Heat (more wires = more heat)

Voltage drops Wire acts as resistor based on

length of wire Long distance with continuous

load = larger diameter

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September, 2007

Wire size:

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September, 2007

Wire size / Voltage Drop:How long? Wire can run as far as voltage (ex. 480v = 480 feet)

Single phase in out and back Three phase is out only Don’t ask why

Conduit sizing Never consume more than 40% of cross-sectional

area Heat Pulling wire through conduit 40%

Max

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September, 2007

Electrical formulas:Energy = power for a period of time Energy = usage, watts x time Electric meters use this measurement Units are watt hours

Power = voltage x amperage Similar to auto horsepower Power = demand, kilowatts (1 kw = one thousand

watts) Voltage = the pressure (pushing) on the current Amperage = current, the flow rate

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September, 2007

Electrical formulas continuedExamples of calculations Single Phase: 2.4 kw = 120v x 20 amperes

What is 115v or 110v? Three phase: 16.6 kw = 480v x 20 amperes x 1.73

1.73 is the square root of three (for three phase)

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September, 2007

Rules of thumb:20 watts per square foot for retail tenants with their own rooftop units. 15 watts per square foot for retail tenants with landlord central air conditioning. 40 watts per square foot for sit-down restaurants. 100 watts per square foot for food court type tenants.

MEP TrainingPower

September, 2007

1. What is AIC?

2. If 3-phase is better, why not 4 or 5?

3. Do all tenants have a transformer? Why or why not?

MEP TrainingPower