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Sizing generator #168580 09/09/07 08:25 PM
Joined: Apr 2003
Posts: 362
Obsaleet Offline OP
Hi All,
I just looked at a job and the gentleman would like to put a small portable gen. in. I have decided to use a homline interlock with a 12cir homeline loadcenter. He would like the following loads:

sump pump
Oil fired hot water furnace
front den area and master bed room( happen to be on same circuit)
panel receptacle
kitchen circuit

Was thinking a 3500w would do, but after doing some math maybe a 5000w unit.

Thanks Ob(Phil)

Choose your customers, don't let them choose you.
Work Gear for Electricians and the Trades
Re: Sizing generator [Re: Obsaleet] #168611 09/10/07 09:16 AM
Joined: Dec 2002
Posts: 317
sabrown Offline
You don't really give enough load information, but here are some other considerations you need to consider.

Elevation: Many generators start derating at 500 feet above sea level.

Fuel Type: Fuels like propane affect the output of generators.

Ambient air temperature and installation location air flow.

Expected usage and life expecations of the generator.

Load diversity and peak step KVA.

User mentatility.

Until this is all thrown into the equation you are just hoping to be lucky. For my own power outage use, knowing limitations would help me as a user limit my loading.

I would talk to a reputable generator (distributor) dealer of one of the well known brands and then go with what they recommend. If they are not aware of the above considerations you may be better served by looking elsewhere.

To use their information to size a cheaper brand will leave you wishing you had listened. For example, a Honda motor on a generator is not a Honda generator, carboration, or muffler and can be a huge dissapiontment.


Re: Sizing generator [Re: sabrown] #168617 09/10/07 11:31 AM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline
A 3500 will likely work, but at 15A, won't give much leeway in the kitchen, especially if he wants to make a pot of coffee or use the microwave without unplugging his fridge and freezer. If he can afford it (seems like a small difference at this point) 5000W would be safer, or even 7000W. A lot depends on which phase everything is on.

Anecdotally, I can easily power every receptacle and lighting in my house with my cheap 3500, with all TVs, fridges, etc, on... and ironically, is too underloaded and puts out too high a voltage until we turn all the lights in the house on to inrease the load and get the droop down. (My computer UPS beeps incessantly about it!) It's not enough for the heat pumps, range or water heater, though.

Re: Sizing generator [Re: SteveFehr] #168619 09/10/07 12:39 PM
Joined: Jun 2007
Posts: 301
JValdes Offline
Those recept loads, TV's (more than two?) and FRIDGES ect... must be very small. A 3500 watt generator will not operate one household clothes dryer. And I think it would be a stretch if you could power more than one fridge and two or more TV's plus your lighting with 3500 watts.

3500 watt will not do the job. Remember the sump pump, freezer and fridge are motor loads with high starting current. Plus you must account for the lighting loads. Thats why I use CFL's everywhere I can in my house.
Go to any generator web site and you will see a wattage calculator. You just plug in the loads and it will give you the size recommended. Of course it will be a little bigger than actually required.

Also, you really don't need a seperate panel for the generator loads unless you just want the extra work. You can use a mechanical interlock and use the main panel for generator distribution.

Trust me....Get the right size the first time. If not, I would save the receipt for the generator.

Re: Sizing generator [Re: JValdes] #168636 09/11/07 05:34 AM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline
Yes, you're correct- a 3500W generator will not operate a clothes dryer. It cannot operate full nameplate rating of a range or 2.5 ton heat pump, either. However, that said, have you ever tried clamping these appliances? Despite being powered by a 60A and 30A 240V circuit, my AC load last july was a mere 3120W- low enough that a 3500W genny could power it. Likewise, turning on the oven, or one element on the range, doesn't draw 40A, just 10 or 12A. (Makes sense that a 1200W range element will only draw 1200W!) My fridge at work draws 8A in-rush, and 1.3A while running; I haven't clamped my fridge at home, but I imagine it's larger. My microwave draws 12.5A. Sufficed to say, all these appliances can be easily run by a small generator- just not all at once.

With the heat pump, range, dryer and water heater breakers open, my whole house load, IIRC, was 3A on one conductor and 5A on the other. This reflected our typical condition, with lights on in most rooms, 2TVs on, my computer, my wife's laptop, two refrigerators, and every clock, VCR, etc, in the house- roughly 1000W of load. Makes sense- my average electric bill points to less than 3000W of average load, and most of that is for heating and hot water, things I wouldn't expect (or try) to power from a generator like this.

Re: Sizing generator [Re: SteveFehr] #168650 09/11/07 05:22 PM
Joined: Aug 2002
Posts: 402
jdevlin Offline
Your AC might run at 3150 watts but a 3500 watt genny will not run it. It would trip out at startup of the AC. Start up draw is 2-3 times the run current.

Re: Sizing generator [Re: jdevlin] #168672 09/12/07 01:23 PM
Joined: Jun 2007
Posts: 301
JValdes Offline
I am in total agreement that I cannot calculate at nameplate. My clothes dryer on high with a full load of clothes pulls 22.5 amps.
I am glad this subject came up as I purchased a 8kw generator last fall. (Have not used it yet)
I have been testing the loads around here and have found that they all draw less current than nameplate.
I just had a hard time believing all the loads you mentioned would work at 3500 watt.
I must appologize for questioning your statement. I should have realized that what you have in your house would be the same as mine.
I now feel much better about my purchase......Sincerely John

Last edited by JValdes; 09/12/07 01:23 PM.
Re: Sizing generator [Re: JValdes] #168777 09/14/07 07:05 PM
Joined: Apr 2003
Posts: 362
Obsaleet Offline OP
Thanks for the infoguys. You confirmed my thinking that a larger than 3500 will be the best bet. Most of the above loads are on the light side except for the kitchen. He is a pretty saavy guy and I beleive he undrstands the load concept. I like to use the sub panels as, this keeps the the tendency to try and run everything at once. Which I already know is not possible(200a service).


Choose your customers, don't let them choose you.
Re: Sizing generator [Re: Obsaleet] #168830 09/17/07 04:07 PM
Joined: Dec 2005
Posts: 152
Ann Brush Offline
I bought a 240v electric kettle overseas and wired a plug in the kitchen with a 240 / 110 recep. The kettle alone draws 3400W (nameplate). Wouldn't want to run it on your set-up but I LOVE getting my tea ready in less than a minute.

Re: Sizing generator [Re: Ann Brush] #168844 09/17/07 11:11 PM
Joined: Jul 2004
Posts: 9,571
gfretwell Offline
I bet that is a 220v rated kettle so you may really be getting 3780w or so.

Greg Fretwell
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