Exceeding home standby generator rating

brn3aMarch 24, 2010


Sorry for cross-posting.

We are considering purchasing a home standby generator. When the house was built, we had them rough in a generator transfer switch so we could install the actual generator in the future. At the time, we did not know there were different options for transfer switches. Ours is a whole house transfer switch, meaning it is wired into the main circuit breaker box with a single switch. All circuits that are on in the main circuit breaker box will be carried by the generator.

Well now we want to buy a generator, but do not want or need one that will power the entire house. My thoughts are to get one that will power only the circuits we absolutely need and to control that by flipping off circuits on the main circuit breaker box after the generator comes on.

Here's the potential problem. Since most of the standby genrators are auto-on, when there is a power failure the generator will come on automatically. I then have to run downstairs and flip the circuits I do not want. I'm thinking it might take me 5-10 minutes to get down there to shut everything off, or longer (ie. never) if for instance no one is home.

What if there is a lot running at the time and these circuits are more than the generator can handle (for instance the central AC)? What will happen to the generator? Will it shut off to protect itself? Will it be harmed? Will it hurt the AC or other circuits in the house?



PS. We are looking at the Eaton generators. Has anyone had experience with these? Thoughts?

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Isn't your transfer switch in the panel a manual switch? So won't you still have to 'run down to the basement' to switch it over?

Otherwise you would need to rewire a subpanel to support just those circuits that the generator can supply if you chose to have automatic swichover.

    Bookmark   March 24, 2010 at 9:45AM
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Of course, your best move would be to have the your panel rewired so only the desired circuits are switched.
If the generator is too heavily loaded, the voltage output will drop and the voltage monitor relay will turn off the generator. This protects both the generator and loads from damage by overheating.

    Bookmark   March 24, 2010 at 9:53AM
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The transfer switch is automatic (and is outside). It was installed to support a large generator (one that would supply the whole house). This is not what we want to do. The cost of taking out the current transfer switch and/or re-wiring the panel in a finished room would likely be close to the same price as buying a generator that could support the entire load.

It's good to know that it will just shut off and not damage anything.

    Bookmark   March 24, 2010 at 3:01PM
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An undersized generator probably won't "just shut off and not damage anything". Picture this- The power goes off and your transfer switch sends a start command to the generator - the generator starts and as soon as the voltage and frequency are close to normal the transfer swich will load it with your entire house - your lights, TV, VCR, and everything else will see voltage for a brief period (perhaps a few seconds) until the generator bogs down under the overload.

Maybe the generator breaker will trip before the transfer switch disconnects, but not likely. If you are lucky, all that might happen is that the generator will run uselessly with its breaker open.

Or the transfer switch could disconnect the generator, which will then speed back up to normal. The transfer switch will then connect it back to your house. Until the set bogs down again. The overloaded set still won't power the house so, repeat the cycle over and over until either the generator dies a sudden death, the transfer switch actuator(s) die, or enough equipment in your house is damaged to cause a load reduction. Your AC compressor will be a likely candidate.


You have 3 reasonable options (1) get a properly sized generator set to power your entire house. (2) rewire with smaller generator, probably a different transfer switch and subpanel. (3) decide you can't afford proper standby power and forget it.

You would be well advised to seek on site input from an experienced generator installer. Situations such as your own are exactly what prompted the latest NEC revisions concerning residential standby systems.

    Bookmark   March 24, 2010 at 6:29PM
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Ron Natalie

If it's an automatic transfer switch the only code way to do it is to size the generator to accept the full load.

In addition to the scheme that wayne describes, the other thing that can happen other than the breaker tripping or the transfer switch cycling is that you'll stall the generator.

    Bookmark   March 25, 2010 at 8:20AM
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The generator should have a breaker on the output. Exceed the output rating and the breaker will trip.
What type of electric loads do you have in your house?
What size of generator were you thinking of buying?

    Bookmark   March 25, 2010 at 8:41AM
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When an automatic transfer switch with even basic voltage/frequency monitoring is involved the breaker on the genset output will usually NOT trip if a short or overload occurs on the load side of said switch. Small generators cannot develop the fault current needed to quicky trip a breaker without going well under voltage/frequency, so the transfer switch logic will disconnect long before the bimetallic or magnetic element(s) will trip the breaker.

The same V/F sensing will also prevent stalling unless the engine has very poor transient response.

    Bookmark   March 25, 2010 at 6:49PM
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Here's the rundown on what we have and want to run and what we feel is optional:

well pump (1hp)
some lights (CFLs)and ceiling fans
receptacles for box fans
circulating pumps for radiant heat (300 watts max)

TV (46 lcd)
Microwave (1200 watt)

No need:
Heat pump

The generator installer/electrician feels that a 14kw or 17kw will pretty much run everything in the house without a problem. I'd prefer to spend less and get a 8kw standby generator. He's ok with that, and says if need be, he can install a load shedding circuit for the heatpump

    Bookmark   March 26, 2010 at 10:22AM
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Ron Natalie

If you put in something to automatically shed the load than you can use a smaller generator, but as I mentioned earlier, if you have an ATS you legally need to have a big enough generator to take the rated load.

And yes, mine is way oversized. Most of the heavy burners are gas (dryer, water heater) and the heat is ground-source heat pump. The resting load with not much turned on (other than the normal house lighting and heat) is about 6 KVA (last time we had a power failure I went out and read the readings off the generator).

Don't have any pictures of the generator handy, but here's the disconnect and the transfer switch:

    Bookmark   March 26, 2010 at 2:18PM
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IIRC, a load shedding device combined with a smaller set can also be NEC compliant.

    Bookmark   March 26, 2010 at 3:44PM
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Hi Mr. Brn3a,

I have a 20kw home generator that we put in after being without power 4 years ago. After the recent hammering that New York got, we are mighty glad to have it.

The basic answer to your question is that you should not hook up a generator to circuits that will overload it. I think there as been a recent code change that no longers allows you to have such a situation. The way it is usually done is to have a sub panel, and have the generator run all the ciruits on that one sub panel only.

At my basement, I have a 2x 200 AMP service feeding into two 200 Amp panels (400 amp total). The generator only powers up one of the panels. The other panel is not backed up. Note although a 20kw genset, a 200amp panel is in the strict sense too big for it. Math: 20,000 / 120 = 166 amps. Since 200 Amp is larger than 166, this should be a no go for code. My one was put in I think before the code change.

Assuming you get a 15kw genset, then the amount of current you can get out of it is 15,000 / 120 = 125 amps. So if you have a 100 amp sub panel, then you are in code.

Most generators will have a breaker trip. If you try and draw mower power from the genset than it can deliver, the genset will trip and go offline. This is a safety that as far as I know pretty much all gensets. So in general you should be O.K. even if you try and power a huge panel. The worst that should happen is the genset breaker trips. Most breakers will not reset untill you reset it manually. Another poster is however correct in that for a few seconds prior to the trip, your genset is suffering a bit, but probablly will survive. Note you probablly will only use your genset once a year if that, so the odd overload will hurt, ouch, but that is about it.

As far as transfer switches go, the higher cost gensets have dedicated transfer switches. My Cummings Onan one does. Also in the transfer switch are a buch of wires in addition to the main power ones, e.g. sump heater for cold start, battery maintainer power, and other control lines.
My guess is your transfer switch you have already there should be fine for a simple, i.e. manual start generator, but for what you are looking at 15kw and up, most gensets that size come with a dedicated "smart" transfer switch that does the auto switch over etc. So the transfer switch and generator come as a set.

As for things to put on the genset, I think your list is fine. I would add if you have any, sump pumps, ejector pumps. Also do you have a oil / gas heater boiler. Most need electrical power to turn on and run. (Are you in Florida?)

I had a look at the EATON gen sets at they I think are OK. I have no experience with these. Most people think Cummins Onan and Kohler are the best. Generac at Lowes and Home Depot avoid.

I have posted a bit in the past on Garden web about gensets. The largest ETON one is a 3,600 RPM, 1 liter engine. A better unit would run only at 1,800 rpm, but have instead 4 copper windings instead of two to give you 60Hz.
In general lower rpm is better. Heavier is better (why? more copper, more inertia able to handle surges more easily, and can start big stuff e.g. Air Con).

Tyically I think for a 10kw, auto start propane powered genset with installation you are probally looking about $US 10,000 all in. This would include rewiring all your panels, propane tank, trenching for gas lines etc. a propane tank Gensets recently have become much more expensive due to the rise in copper prices.

If you go for the big gensets, with tuck / car engines then you are lookign at $US 20,000 and up. Note for these sorts of geneset, you need heavy lift like a crane to install.

Most home owners go the route you are going, 10kw to 12 kw, propane, auto start, two cylinder, lawn mower engine based, installaing without heavy lift. After all you are only going to need this puppy for maybe 10 days out of 5 years ....

Good luck and all the best.

Warmest regards, Mike.

Here is a link that might be useful: Costco Cummins Onan

    Bookmark   March 26, 2010 at 5:51PM
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Where do you live?

If FL, then think about AC, either your current HP or a window unit used in emergency (hurricane).

Also, the washer isn't that much.

If prone to icy weather and gas heat, backing up the blower would be good.

    Bookmark   March 27, 2010 at 2:42AM
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Ron Natalie

Yes, if their is an automatic switch, the automatic loadshedding is allowed as well so as not to exceed the generator capacity.

If the transfer is manual, then the generator has to only be sized to the "intended" loads. You're expected to be smart enough to not use more than intended :)

    Bookmark   March 27, 2010 at 10:57AM
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"The generator should have a breaker on the output. Exceed the output rating and the breaker will trip."

Anything electronic will blow much faster than a breaker can trip.

    Bookmark   April 1, 2010 at 8:34PM
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"Anything electronic will blow much faster than a breaker can trip."

My Yamaha receiver has been through all manner of hell, including having an entire can of Mt Dew spilled in it while it was on, and being thrown out a second-story window into the driveway - and we did live on a very flaky generator/inverter-from-the-car setup for a while. Other than needing a shower, no problems. Any electrical anomalies just trip the protection circuit and shut it off.

"electronic" doesn't automatically mean "fragile" or "sensitive".

I do agree, anything 'sensitive' (which I read as 'poorly-engineered to begin with') will certainly fry before the breaker trips.

    Bookmark   April 1, 2010 at 10:38PM
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Eaton recently changed their supplier to Generac. Generac redesigned many (all?) of their units and Eaton says they are having good experiences with them. I haven't looked at smaller air cooled units but all of the liquid cooled I looked at have new specs.

    Bookmark   April 2, 2010 at 11:00AM
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"I do agree, anything 'sensitive' (which I read as 'poorly-engineered to begin with') will certainly fry before the breaker trips."

Solid state devices fail in microseconds if their rating is exceeded.

The fastest a mechanical breaker (or a fuse) can operate is milliseconds.

The electronics will fail over 1000 times faster.

The fact that the breaker tripped without any electronics failing means the electronics did not get overloaded.

    Bookmark   April 2, 2010 at 3:54PM
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There's no argument there, brickeyee.

In context, the breaker on the OP's generator *IS* likely to trip before undervoltage or oddball waveforms damage MOST consumer electronics.

Computer PSUs typically operate *PROPERLY* down to about 90V or so, and shut themselves off (protection circuit) below that.

Devices with linear power supplies aren't going to give a darn about undervoltage - and the waveform issue is never going to be seen by the secondary side - the transformer won't be happy, but that usually just blows the input fuse.

I AGREE that, if the problem is overcurrent, everything electronic is long gone before the breaker trips. Overcurrent through any one particular electronic device isn't the issue here, it's undervoltage before the generator's breaker trips.

Overcurrent through one particular electronic device, I might suggest, probably means that said device is already fried anyway.

    Bookmark   April 2, 2010 at 4:28PM
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I think this is a non-issue if you plan to get the Eaton 8kW unit. You can set the time delay you want before it kicks in during a power outage. So, you will have plenty of time to go to the basement to flip the breakers you want off.

    Bookmark   January 9, 2011 at 11:01PM
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And if no one is home?

    Bookmark   January 10, 2011 at 5:01PM
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what brand and model number is your Automatic tranfer switch? What did it cost? I want a whole house transfer switch, not just certain circuits...

    Bookmark   January 11, 2011 at 8:23PM
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Mine is an Asco 300. They are not inexpensive, mostly because the switch is rated for "emergency" service use.

Asco also offers less costly 165 and 185 series switches that are rated for optional standby service and as such are generally OK for residential use.

    Bookmark   January 11, 2011 at 10:27PM
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rorser, did you notice the date of the posts you are replying to?

WHY do folks make replies to really old threads as if they were just asked???
I think it is safe to say this project is done already.

    Bookmark   January 12, 2011 at 1:46PM
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