Siemens Standby Generators manual Generator Sizing Guide

Generator Sizing Guide

Onsite Estimating Sheet

Generator Sizing Instructions:

There is not a single correct sizing solution. The instructions below identify multiple methods that, when mixed with good judgment, should result in a moderately sized generator. Remember to consider load growth, seasonality, and effects of starting motors.

When motors start, they create a current surge that step loads the generator. As a result of this step loading, the generator

2)Enter all non-motor loads expected to run during peak load levels into Table 7. Reference Table 2 for typical residential loads and rules of thumb.

3)Sum the running motor load data but do not include the largest motor that is cycling. Add to this value the non-motor load data and the starting kW for the largest cycling motor.

will experience a voltage dip. After selecting a generator, reference the generator's surge capability using Table 3. Verify that the generator voltage dip is adequate for the application. Most commercial applications should be limited to 15% voltage dip and residential applications should be limited to 30% voltage dip.

Some commercial applications utilize one or multiple uninterruptible power supplies (UPS) to backup critical loads.

Motor running load total (minus largest cycling motor): Motor starting load from largest cycling motor: Non-motor load total:

Total (above items):

Select generator (Total x 1.25)

______ kW (Ref. Table 6)

+______ kW (Ref. Table 6)

+______ kW (Ref. Table 7) = ______ kW

______ kW

Please read sizing guidelines for this load type.

Measurement Method

Use a clamp-on amp meter or power analyzer to measure facil- ity load levels. The measurement should be made at peak load levels. Size the generator 25% larger than the peak measured load. Verify motor and UPS load compatibility.

Measured Amps = _______

Billing History Method

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

1)Enter all motors loads expected to run during peak load levels into Table 6. Reference Table 1 for typical motor sizes and electrical requirements.

4)Verify voltage dip compatibility using generator Surge Capability Table 3. Verify UPS compatibility using sizing guidelines provided.

Initial Estimate and Cross Check Methods

These methods are for initial estimates and cross checks only. Size the generator using one of the above methods.

Estimate based on 60% service size:

240 Volts, 1Ø: ________ amps x .15 = ________ kW

208 Volts, 3Ø: ________ amps x .22 = ________ kW

480 Volts, 3Ø: ________ amps x .50 = ________ kW

Estimate Based on Square Footage

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

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

Square footage	= ____________
Estimated kW	= ____________
		LP
		LPG: 8.55 ft.3/lb., 4.24 lbs./gal.,
		2500 btu/ft.3
		LPG: 36 ft.3 = 1 gal.
		Natural Gas
		1 cubic foot = 1,000 BTUs
		1 therm = 100,000 BTUs
		Gas consumption = 13,000-16,000 btu
		per kW/hr.
		Pressure
		1 inch mercury = 13.61 inches
		Water Column
		1 inch Water Column = 0.036 psi
		5-14 inches water column = 0.18 psi
		to 0.50 psi
		Air Conditioning
		1 hp per 1 ton
		1 ton = 12,000 btu
		Rule of Thumb
		For 480 volt systems kW x 1.5 = Amps
		For 208 volt systems kW x 3.5 = Amps
		For 240 volt single phase systems
		kW x 4 = Amps

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