Sizing Stand Alone Home Generator

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chevyx92

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VA BCH, VA
Got a call from a customer who wants a stand alone natural gas fed "Whole House" generator. He wants everything to work when there's a power outage. He's got a 400 amp meter can feeding (2) 200 A circuit breaker enclosures on the outside of house which feed (2) 200 amp panels inside the garage. My question to you guys is do I size it for the 400 amp service which would be a 96KW generator or maybe just put a transfer switch on (1) of the 200 amp panels if I can talk him into that? Just curious what you guys do when you come across 400 amp services where the customer wants a "Whole house generator". Any input would be greatly appreciated. Thanks.
 
NEC in Art 702 requires generator sized to the calculated load, per Art 220. You often can do some load shedding to get the load reduced.
 
Just because there is 2-200 amp mains doesn't mean you will ever see a peak demand of 400 amps, you may not even see a peak demand of 200 amps for many dwellings with 400 amps service gear - exception being those with a lot of electric heat, and you will want a big generator if they want to run all that heat, otherwise set up to shed the heat or some of the heat and size generator for the rest of the load
 
Nominal service size means a little, but not much. Actual house peak load is important of course, and you should note how big the A/C unit is. If the utility meter is "smart" it will capture peak load info and you can perhaps get it from the POCO (best) or read it off the scrolling meter display. 200 amps x 240 volts = 48 kva of course, but that would be overkill in every case I've seen. My observation over the years is that for a "typical" house (+/- 2,000 sq ft, 200 amp service, 4 ton A/C), a 16 kW will do the trick in just about every case. Don't make the mistake I did and locate the generator on the other side of your bedroom wall. A 16 kW air cooled Generac makes a racket.
 
I would use two se rated ats,s, one for each 200 amp panel, and use the load shed feature to cut out all heavy loads. To size the generator do a load calc like its a service but don't include heavy loads you can live without. I bet you will find a 22 kW will do the job. Most of those heavy loads will be usable during an outage but just not all at the same time. I would guess most homes operate at less than 3 kW over 90% of the time.
 
You folks pushing load shedding ..

chevyx92 said:
He wants everything to work when there's a power outage.
Click to expand...

...how is that going to work?


I would do an article 220 load calculation, size the unit on that and use a single transfer switch.

Price it up and see if they bite, if the have a problem with the price then you can discuss price reduction options like load shedding or leaving some items off the generator all together.
 
iwire said:
You folks pushing load shedding ..



...how is that going to work?


I would do an article 220 load calculation, size the unit on that and use a single transfer switch.

Price it up and see if they bite, if the have a problem with the price then you can discuss price reduction options like load shedding or leaving some items off the generator all together.
Click to expand...
They always want everything to work, until you tell them the price difference between being able to handle everything vs. shedding certain loads that may not be that necessary.:happyyes:

If they get the big generator - they later complain about how much fuel it uses:huh:
 
kwired said:
They always want everything to work, until you tell them the price difference between being able to handle everything vs. shedding certain loads that may not be that necessary.:happyyes:

If they get the big generator - they later complain about how much fuel it uses:huh:
Click to expand...

Regardless of that it does not make business sense (IMO) to start with less than they asked for.

Price it up as they asked with a nice profit and see if it sticks, if they have sticker shock you have room to reduce the costs without cutting into profits.

To each their own. :)
 
I agree with the others. In the end it depends on how deep his pockets are and your salesmanship.
A majority of the "whole house-no load shed" jobs I hear about change when priced.
It seems to me with the new load shed capability there is little reason to design "whole house", but if your customer wants that and can write the check.. great
 
You didn't mention the age of the house and if the original plans or drawings are available, this alone can help you determine if and when the price determines load shedding consideration and will aid you in determining what's not required. Of course
this will with existing plans also help you quicker than re-documenting the whole house. If you went over in the evening and just walked around the house and noted whats plugged in where, and what lights are used this alone will help the owner understand that in fact they don't need every receptacle circuit.

You also didn't mention if it's going to be Public Gas supply or Diesel or Gas.
It would be costly to sink a tank for the last two.

You also didn't mention any past records of what the owner is using monthly power wise, and that they can supply or that you can retrieve from Dominion Power.

IMO you'll need to supply power for all lights, all kitchen, H2O heater, All HVAC (I'll assume multiply split systems) and a few receptacle circuits sprinkled around the house; Master bed room, Bath(s), Living Room(All-because of Wi-Fi and Cable), other bed rooms(because they might share a circuit). And where ever their Phone Docking is
done. A outdoor receptacle, and or outside lighting.

People don't understand this cost difference and are so Americanize, as to "I got have it all" mentality.

If in fact they don't want any hands on (Auto transfers switch), cost is not a problem, and no load shedding is not desired, sell them the primo package.

Just remind them that the Auto Transfer will take off when their on vacation, or on the other side of town. That now in fact they might need to put Mini UPS to support
24 hour CPU's, or again Cable Power or Wi-Fi to keep things running, and remind them this is because it is not in fact instant power but instant start.

This is a serious handing holding type deal and sale, you need all the numbers to qualify anything your going to present! You could with the numbers proof to them the cost of whole house verses load shedding, and present a one week cost of both.

Good luck with the sales pitch.
 
iwire said:
Regardless of that it does not make business sense (IMO) to start with less than they asked for.

Price it up as they asked with a nice profit and see if it sticks, if they have sticker shock you have room to reduce the costs without cutting into profits.

To each their own. :)
Click to expand...
You don't have same customers I have, even those I have that can afford the higher cost unit, still are often stringent with their money on items that are not as easily shown off, if you want to sell them a car they are easier to talk into something with more bells and whistles, but the generator that sits out behind the house only becomes important during a power outage, and if you don't sell them one within a year or so after that power outage they forget how important it was until the next power outage.
 
kwired said:
You don't have same customers I have,
Click to expand...

And neither does the OP.


even those I have that can afford the higher cost unit, still are often stringent with their money on items that are not as easily shown off, if you want to sell them a car they are easier to talk into something with more bells and whistles, but the generator that sits out behind the house only becomes important during a power outage, and if you don't sell them one within a year or so after that power outage they forget how important it was until the next power outage
Click to expand...

Damn my friend give it a rest. :)
 
iwire said:
You folks pushing load shedding ..



...how is that going to work?


I would do an article 220 load calculation, size the unit on that and use a single transfer switch.

Price it up and see if they bite, if the have a problem with the price then you can discuss price reduction options like load shedding or leaving some items off the generator all together.
Click to expand...

So would you use 220.83(A) to calculate the load?
 
iwire said:
That is not NEC acceptable and would likely result in a larger generator than needed.
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Is a larger generator then needed a violation? I agree the method is not the method NEC allows for calculation, but as you said end result is still likely a larger generator then what it could have been.
 
kwired said:
Is a larger generator then needed a violation? I agree the method is not the method NEC allows for calculation, but as you said end result is still likely a larger generator then what it could have been.
Click to expand...



The method of determining exsiting load that K8 suggested is a violtion (IMO)

You can put in a 1MW generator for a 1W load if you want.

But surely you know that.
 
iwire said:
You folks pushing load shedding ..



...how is that going to work?


I would do an article 220 load calculation, size the unit on that and use a single transfer switch.

Price it up and see if they bite, if the have a problem with the price then you can discuss price reduction options like load shedding or leaving some items off the generator all together.
Click to expand...


But that would leave an oversized generator. Most home around here with a 200amp service get a 15 or 20kw generator with 80 or 100amp feeders going into a 200amp whole home ATS. Unless the home is all electric heat the generator runs everything during an outage, even an electric range.

Of course this might not be to code, but if it is why not do it? BTW, is there anything in code allowing this?
 
mbrooke said:
But that would leave an oversized generator. Most home around here with a 200amp service get a 15 or 20kw generator with 80 or 100amp feeders going into a 200amp whole home ATS. Unless the home is all electric heat the generator runs everything during an outage, even an electric range.

Of course this might not be to code, but if it is why not do it? BTW, is there anything in code allowing this?
Click to expand...

If that is true with the use of the 15-20KW generators with the 80-100 amp feeders going into the whole house ATS then the calculated load must be within the ampacity of the feeder wires.
 
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