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Technical Data TD00405018E

Effective August 2009

Generator Sizing Guide

EATON CORPORATION www.eaton.com

Onsite Estimating Sheet

Generator Sizing Instructions:

There is not a single correct sizing solution. Following are several methods

that, when mixed with good judgement, should result in an appropriately sized

generator. Remember to consider load growth, seasonality, and effects of

starting motors.

As municipalities and states adopt the new 2008 NEC Electrical Code, there

may be new sizing requirements, spelled out in the code book, which the

installation technician must follow. Always check with the local inspection

department to confirm which code cycle will affect your install.

Never add Amps when sizing a generator. Convert Amps to kW and add kW to

determine the required generator size. Power factors for various motor loads

vary widely. Adding Amps without properly accounting for the power factor

and/or mixing voltages will result in improperly sizing the generator.

When motors start, they create a current surge that step loads the generator

and creates a voltage dip. After selecting a generator, reference the

generator’s surge capability using table 3. Verify that voltage dip is adequate

for the application. Most commercial applications should be limited to about

15% voltage dip and residential applications should be limited to a 30%

voltage dip. Some applications utilize an uninterruptible power supply (UPS) to

back up critical loads. Please read sizing guide for this load type.

Measurement Method

Use a clamp-on Amp meter or power analyzer to measure facility load levels.

Clamp each leg separately and take the measurement during peak usage

levels.

240V 1ø Applications: To determine peak usage in kW, add the highest Amp

readings from the two legs, multiply by 120 and divide by 1,000.

(L1 + L2)120 / 1000

Size the generator 10 to 20% larger than the peak measured load.

3ø Applications: Add the peak Amp readings from all three legs and divide by

3 to determine peak Amps. Multiply peak Amps by volts, multiply the result

by 1.732 (square root of 3), then divide by 1000 to convert Amps to kW.

Peak Amps = (L1 + L2 + L3) / 3

kW = [(Peak Amps x Volts) x 1.732] / 1000*

*Assumes power factor of 1.0

Size the generator 20 to 25% larger than the peak measured load.

Peak Amps = _____________ Peak kW= _____________

Billing History Method Commercial

Many commercial customers have a utility rate structure that has a peak

demand charge. Using a year’s worth of electric bills, size the generator 25%

larger than the largest peak demand.

Verify motor and UPS load compatibility: Peak Demand = _______

Load Summation Method

•

Enter running kW for all motor loads (except the largest) expected to run

during peak load levels into table 6. Refer to table 1 for typical motor load

sizes and electrical requirements.

•

Enter kW for all non-motor loads expected to run during peak load lev-

els into table 7. Refer to table 2 for typical residential loads and rules of

thumb.

•

Add the running motor load kW, non-motor load kW, and the starting kW

of the largest motor load.

Motor load running total (minus largest motor): _______________ kW

(ref. table 6)

Non-motor load total: (ref. table 7) _______________ kW

Starting load from largest cycling motor: _______________ kW

(ref. table 6)

Total electrical loads: = _______________ kW

Select generator: Commercial (add 20 to 25% to total kW)

Residential (add 10 to 20% to total kW)

•

Confirm that voltage dip is within acceptable limits by comparing motor

LRA to generator surge capability (see table #3).

•

Confirm UPS compatibility (see page 6).

System Capacity – Load Calculation

If the local municipality or state you are in has adopted the 2008 NEC Code,

you may be required to use this step. Article 702 of the 2008 NEC includes a

new requirement for sizing (702.5B). If no other method for sizing is accept-

able, sizing of the generator shall be made in accordance with Article 220 of

the NEC. The system capacity estimating sheet will guide you through this

process.

Project Layout

Ball Park Estimates (Do not use for final sizing)

Estimate based on 60% service size: (commercial)

240 Volts, 1 Ø: __________ Amps x .15 = __________ kW

208 Volts, 3 Ø: __________ Amps x .22 = __________ kW

240 Volts, 3 Ø: __________ Amps x .25 = __________ kW

480 Volts, 3 Ø: __________ Amps x .50 = __________ kW

Estimate based on 40% service size: (residential)

240 Volts, 1 Ø: __________ Amps x .10 = __________ kW

208 Volts, 3 Ø: __________ Amps x .15 = __________ kW

240 Volts, 3 Ø: __________ Amps x .17 = __________ kW

480 Volts, 3 Ø: __________ Amps x .34 = __________ kW

Estimate based on square footage

Fast food, convenience stores, kW = 50 kW + 10 watts/sq. ft.

restaurants, grocery stores

Other commercial applications kW = 30 kW + 5 watts/sq. ft.

Square footage = __________ Estimated kW = __________

Amps to kW Rule of Thumb (assumes .8 pf)

For 480 volt systems Amps = kW x 1.5

For 208 volt systems Amps = kW x 3.5

For 240 volt 3 Ø systems Amps = kW x 3

For 240 volt 1 Ø systems Amps = kW x 4

Eaton Generator Manual de usuario Pagina 10

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