# Thread: Generator sizing and Heat Pumps

1. Senior Member
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## Generator sizing and Heat Pumps

I need to size a generator. Here are a couple of questions. The generator literature refers to AC units as being X number of Tons. Looking at the name plate of the outdoor unit I see no reference to TONS.
Also, looking at the name plate, what figure states the actual usage in watts or amps?
What I see is minimum circuit ampacity, min and maximum breaker etc.

2. Greg, RLA (Running Load Amps) for running, but starting current must be considered

3. Senior Member
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The tons is usually buried in the model number. A ton is 12,000 BTU. So a 2 ton unit will probably have an 024 in the model number. A 4 ton would probably have 048 in the model number, and so on.

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Originally Posted by Greg1707
I need to size a generator. Here are a couple of questions. The generator literature refers to AC units as being X number of Tons. Looking at the name plate of the outdoor unit I see no reference to TONS.
Also, looking at the name plate, what figure states the actual usage in watts or amps?
What I see is minimum circuit ampacity, min and maximum breaker etc.
It's sometimes difficult to get running load out of HVAC manufactures. Your right, they give you a minimum circuit ampacity and that's it until the manuals are available. Even then it takes carefull reading to determine the running load. .

I use the SEER rating to get close.

SEER stands for Season Energy Efficiency Ratio. SEER can be from 5-13 I use 12 when I don't know

SEER is defined as the cooling capacity, measured in BTU/Hr, of a continuously operating air conditioner by the electric power input, measured in WATTs.

I Ton of AC= 12,000 BTUs/HR

SEER= (BTU/hr)/Watts (definition of SEER )

OR Watts=( BTU/hr)/SEER

Or KVA= (BTUs/Hr)/(SEER*1000)*0.8 Assuming 80% Power factor.

Comments and correction to this are welcome. I have ask mechinical engineers to check this but I don't think they did in great detail. :-?

5. Junior Member
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When I size generators for A/C loads I do it by the RLA printed on the nameplate. For starting current, I make sure that the LRA rating will not cause a 30% dip in voltage (for residential, 15% commercial).

The manufacturer of the generator generally provides its dealers with the max starting capabilities of it's generators. I have the tables for Generac if that's what you're using. Just for instance, the 14kW unit can only handle a maximum spike of 102A without a 30% drop in voltage. Soft start equipment is a good idea if the starting current is what's making your generator size very large.

I can also sell you generators tax free if you live out of state

6. Originally Posted by IllinoisElectrician
I have the tables for Generac if that's what you're using.
That would be great info to have.

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