Generator current & power


Old 12-26-10, 10:58 AM
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Join Date: Dec 2010
Location: USA
Posts: 4
Generator current & power

I'll be getting a whole-house backup generator in the near future, and I'm trying to understand the sizing more properly.

The units I've looked at all show amps fitting their power rating at 240V (and a pf of 1.0).

A/C is still a bit of voodoo to me, so I want to make sure I understand things correctly.

My house has 200A single-phase service. This means I could have four 50A electric ranges going at full capacity for 200A @240V, or 20 fully-loaded 20A circuits (10 on each bus) for 400A @120V, right?

So when I see a 22kW generator described as providing 92A, that's 92A @240V, or 184A @120V, assuming the latter is distributed evenly across the buses, correct?

So if I add up all the 120V equipment and come up with 100A, then add in the 240V electric dryer's 28A, I have 78A of load at 240V, or 156A at 120V, all assuming the 120V load is evenly distributed. Is that correct?

My goal is to pick a generator size that will run everything, including the dryer, all air conditioners, etc.

I do have a professional installer coming, but I want to be reasonably well-informed going in.
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Old 12-26-10, 12:18 PM
Tolyn Ironhand's Avatar
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Location: Twin Cities, MN
Posts: 11,984
It is better to compare in watts. Electric ranges and most all other appliances are wired on a circuit that is greater than what the appliances really draw in amps. A range with everything on will draw aprox 42 amps. (This will depend on the model) on your 50 amp circuit. However, this is only with all burners going and both oven elements on. This pretty much never happens since most cases the elements will cycle during cooking.

So, the 22kw generator is rated to run 22,000 watts of stuff at one time. The generator might also have a peak rating which is handy for items like motors which will draw up to 3 times normal running current. (IE: A/C condensers) Although at this size generator it might not have a peak rating. To convert amps to watts take the nameplate rating (NOT the breaker size) and multiply it by the voltage. For example your range nameplate is 36 amps. 36 X 240volts = 8,640 watts. Again, this is at full running of all burners and oven on high.

My suggestion to you is to do a load calculation (Google "load calculation") and it will give you the watts (same as VA) or amps (orboth depending on the program). If you want amps from watts divide watts by 240 volts.

Here is a copy of my house load calculation: (You can see how it will take a demand factor in to account)

General Light and Receptacle Load
4600sqft x 3VA = 13800VA Small Appliance Circuits Load
4 x 1500VA = 6000VA Laundry Branch Circuit(s) Load
1 x 1500VA = 1500VA Total General Load Less Demand
sum of above less T220.11 demand
sum of above = 21300VA
first 3000 at 100 % = 3000
next 117,000 at 35% = 6405
remainder at 25% = 0
sum less demand = 9405VA Fastened In Place Appliances Load
freezer = 750VA
fridge = 750VA
small fridge = 400VA
2 = 400VA
sum less 75% demand = 1725VA Clothes Dryer Circuit(s)Load
Total dryer circuit(s) load = 5000VA Cooking Appliance(s) Load
range 8kVA
T220.19 Column A = 0kVA
T220.19 Column B = 0kVA
T220.19 Column C = 6.4kVA
Total calc.d range load = 6.4kVA Heating or Air-Conditioning Load
Air Conditioner = 4800VA Largest Motor Load
25% of largest motor = 200VA Total Computed Load
27530 Volt-Amps Computed Amperage
27530 divided by 240 = 115Amps

Bottom line: I would need a generator that would put out at least 28kw to run my whole house. This is not likely since there is only two of us.

Last edited by Tolyn Ironhand; 12-27-10 at 02:00 PM.
Old 12-26-10, 01:26 PM
lawrosa's Avatar
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Join Date: Dec 2010
Location: USA
Posts: 17,607
I did mine by watts. I wanted to spend as least as possible to run emergency power. Heat, Water, Refridge, lights,TV,ceiling fans (I dont have central A/C. Window units and A/C is not important)

This is what I came up with so after this I went out to look for biggest/cheapest gen for what I need. first # running watts. If second # start up watts

Accessories Running Watts Surge Watts
A Lights 200
B Ryans TV ceiling fan 132
C Well pump 1164 1920
D Refridge 540 1080
E Heat/circ/zone valves 96
F TV Living/comp. Outlets, 300
ceiling fan

Total 2432 3000 5432

5432 watts if I ran all on with surge, so you would think at least 6000 watt start up. My thinking was if we all need to shower turn off all circuits but lights and well pump. Then when done turn off well and turn refridge back on, tv, heat.... You know controling your wattage use.

With that I bought a 3250 watt 4000 watt max gen for $399 on sale. Was $499 Briggs 6 hp 4 gal tank. Colman powermate
Looks like this.

Also went bigger on tranfer switch to 7500 watt max incase I get A/c and need a bigger gen. Came with all wireing and outside plug. 1 wire from gen to house. $350

31406CRK Pro/Tran Kit

Labor? Myself.

Total around $800. I did buy the wire from outside plug to transfer switch. About 20 feet.

Remember its emergency power. But if you have the cash go for it. I have seen 15kw self starting, auto transfer switch, and all the bells and whistles install for $20,000 plus

I could not justify the cost. We dont loss power much. A day or two max. 8 hours here and there.

Hope this helps.

Mike NJ
Old 12-27-10, 06:47 AM
Join Date: Dec 2010
Location: USA
Posts: 12
IMO...only generate what you need. Heat, fridge and freezers, some lights...that sort of stuff.
Also ,check with local building office, there are a few villages near me that you have to apply for a special variance to use the automatic whole house generators. They are set to cycle and run a test every so often and apparently there have been a lot of neighbors complaining about the noise.

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