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#183250 - 01/01/09 10:23 AM Sizeing a whole house generator
leland Offline
Member

Registered: 08/20/07
Posts: 856
Loc: Lowell area, Ma. USA
I have never done a whole house generator from the beginning.Always been an engineerd situation.
Thinking of 1 for my own house.

200A main.
Would it have to be for the full 200A or can you size them down? Not sure where to start.
Thanx, Lee

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#183252 - 01/01/09 10:38 AM Re: Sizeing a whole house generator [Re: leland]
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
Unless your house is total electric, you likely don't need a 48 kw generator. A generator that will handle the full 200 amps and the transfer switch is going to be realy pricey.

One of my customers has a 7000 sq ft house & the generator is 12kw if I remember correctly. We have it wired to a 200 amp panel & it is up to him which loads he wants to run during a power outage. Instead of using a 200 amp transfer switch, we used an interlock kit that allowed him to feed the panel with a smaller two pole breaker.

To size your generator, decide what loads you feel must absolutely be on and then if that number idoesn't breeak the bank add the loads that it would be nice to have & that would be the size generator you need.
_________________________
Few things are harder to put up with than the annoyance of a good example.

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#183255 - 01/01/09 10:58 AM Re: Sizeing a whole house generator [Re: Tom]
leland Offline
Member

Registered: 08/20/07
Posts: 856
Loc: Lowell area, Ma. USA
Gotcha, Like I said the engineers have always done the work. I can size the portables comfortably(http://www.cumminsonan.com/residential/select/)
with this as a guide.
I'm at the infant stage right now so no particulars.
The idea is a transfer switch at the panel location.200 in to panel from the poco,then perhaps a 20KW (150A) Gen set.

Just not sure about the coralation between the 200A panel and the smaller Emerg side.
I would assume the T switch would need to be rated for the poco service size.
?

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#183268 - 01/01/09 07:01 PM Re: Sizeing a whole house generator [Re: leland]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Generators are sized to the actual loads that they are expected to power. With an existing house, the easiest way is to measure the amps you're actually drawing with "everything on," and base your sizing on that.

You don't want to oversize. With generators, "bigger" is NOT better. Talk to your genny rep, but typically you want the generator tom operate at about 85% of its' rated capacity. For the 'normal' house, we're usually talking about a draw of about 70 amps.

Having a larger transfer switch is a good idea; but, again, there's no sense in overdoing things.

The most important thing about a generator instal is that the generator operate reliably. In practice, this means running the generator periodically, under load. There are two ways to do this.

One way is to have the generator turn on automatically, and have the transfer switch let it power the house for, say, an hour each month.

The other way is to add a "load bank" to the generator. A "load bank" is simply a very, very big toaster. The generator powers this load when it is 'exercising,' and does not use the transfer switch to power the house during this period.

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#183270 - 01/01/09 07:10 PM Re: Sizeing a whole house generator [Re: renosteinke]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
The transfer switch needs to be large enough on the mains side to handle the potential maximum mains load, which in theory is as large as the main OCD level. I've never seen asymmetric transfer switches, though.

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#183274 - 01/01/09 10:54 PM Re: Sizeing a whole house generator [Re: pdh]
Rewired Offline
Member

Registered: 01/01/06
Posts: 567
Loc: Hamilton, Ontario, Canada
Instead of having the generator supply the entire residence and therefore would require a transfer switch equal in size to the mains OCD, you could supply all the loads that would be considered "essential" during an outage from a separate smaller panel and therefore a smaller transfer switch. This would also help eliminate the possibility of overloading the generator as non essential / high wattage loads can not ever be supplied by the generator.

A.D


Edited by Rewired (01/01/09 10:56 PM)
Edit Reason: did not take "keyboarding" in school.

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#183275 - 01/01/09 11:20 PM Re: Sizeing a whole house generator [Re: Rewired]
Ann Brush Offline
Member

Registered: 12/27/05
Posts: 155
Loc: Ohia
IMHO sizing the genny for 200A is a total waste of money. In an emergency situation you should not need more than 50A at most for a 2200 sq ft house with gas heat. We live in a rural area (frequent power outages) and have a PTO 13.5kW generator that is cord connected to an whole-house inlet. That inlet back feeds a mechanical transfer CB (50A) and main switch inter-lock in position 2 on the main panel. In this manner we can operate any circuit we need without having to define it beforehand. We have enough power to operate the house plus one "big" appliance and I just dont try to run the dryer and the oven at the same time. General house load is 20A on each line. Remnants of hurricane Ike saw it getting 16h of use a day for 6 days without a hitch. Costs of generators above 12-13kW start getting up-there. The cost of the interlock (Square-D) and 50A CB was less than $50 3 years ago and beat the pants off related charges for any transfer switch system with pre-defined "critical" circuits, not to mention the simplicity of installation. I have yet to trip the breaker but a 50A at the panel and a 50A pop out breaker on the genny should prevent an undervoltage situation. I also had hubby wire up a frequency meter that plugs (two separate cords and caps) into two outlets (supplied by L1 and L2) that I identified and the display scrolls through voltage, frequency and rpm so we can keep an eye on the genny performance under different loads from in the house. The idiot gauge on the PTO generator is HOPELESS, and we use frequency to set the throttle on the tractor. I am very happy with the set-up and it was modestly priced for what we can do with it. Sure a auto-transfer and start would be nice but we would never get the utility out of it for what those units cost. Thats my 0.02.

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#183276 - 01/02/09 12:10 AM Re: Sizeing a whole house generator [Re: Ann Brush]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
I was always under the impression that an "Alternator" was for the most essential loads in a house, during times of PoCo system failure?
Things like lights, freezer and 1 recept.

Running a whole house on an Alternator,is just stupid,like you seriously need all them things under Emergency power conditions.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#183280 - 01/02/09 09:54 AM Re: Sizeing a whole house generator [Re: Trumpy]
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Just a few idle thoughts ....

I recall an ice storm when I was a kid, and the power was out for a week. It wasn't a particularly difficult time. A fireplace, a couple candles, the Coleman lantern and BBQ on the patio for cooking.

These days, folks seem to be asking for service drops that, in the past, would power an entire village. 800 amps? Even today, with the latest McMansions, I haven't seen an actual current draw of more than 70 amps.

What is with this dependence upon electricity? Is this fear a carryover of the Y2K silliness?

Looking back on that ice storm ....
If we had a tiny generator - one you could carry with one hand - we would have had normal heat in our home. Our primary heat was natural gas; electricity just powered the blower and controls.
A few lights would have been nice.
Some folks have wells, and, yes, you do need to keep the toilets flushing. Water storage is a moot issue; if you store it, you need to keep it from freezing.

Looking at this from a trade perspective .... perhaps we ought to consider this when we desigh homes, and feed these minimal (but essential) loads from a sub-panel, just to make the addition of a genny easier. After all, the latest NEC changes almost demand we have more than one panel in a home; how we split up the circuits is something to think about.

There is a contradiction in our goals. With LEED design, CFL bulbs, "Energy Star" appliances, etc .... our necessary service sizes ought to be declining, not growing. I'm sorry, I'm still shocked by all the 400+ amp household services I've encountered ... having grown up is a perfectly nice, full size house that had a 60 amp service! For that matter, we did it with 8-120v circuits and 1-240v circuit ..... as compared to the 40 circuits commonly found today.

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#183283 - 01/02/09 10:14 AM Re: Sizeing a whole house generator [Re: renosteinke]
pdh Offline
Member

Registered: 01/20/05
Posts: 354
Originally Posted By: renosteinke
These days, folks seem to be asking for service drops that, in the past, would power an entire village. 800 amps? Even today, with the latest McMansions, I haven't seen an actual current draw of more than 70 amps.

What is with this dependence upon electricity? Is this fear a carryover of the Y2K silliness?

Let me add a new word (well, 3 word combination) to the lexicon: service drop envy

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