Emergency Generator Sizing

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marcelo

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Port Chester, NY
Does NEC provides any guidelines for sizing an emergency generator? I need to size a whole house generator, the house has a 400A service. Can you help me please?

Much appreciated,

Marcelo
 
Yes, see 702.5 of the NEC. For most jurisdictions in the State of Florida, the generator must be sized to serve all the loads intended to be operated at the same time based on an Article 220 load calculation. Consult you local AHJ for exact requirements.

You will likely not be required to provide a generator rated at 400A. The calculated demand on the service / generator will likely be well below 400A using the optional method in Article 220.
 
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Bryan, here in Broward County we base the size on the largest demand for the previous year as per FPL (POCO). This is PE stamped. Bear in mind I do large facilities but I have seen this on resi. plans also.
 
I would also approve that method Chris. We (City of North Port) modeled our generator permitting guidelines based on the one written by Tarry Baker for the Broward County Board of Rules and Appeals.
 
Make sure you allow for any and all desired starting loads. This is where our customers are not aware and generators bought from supply house or box stores do not carry the load. ACs need to account for LRA, not OCP. (Forgive me if I tell you what you know, but this is the biggest oversight we see on customers seeking our help.):smile: Also, remember the transfer switch is sized for the service, not the generator. As correctly stated...

bphgravity said:
You will likely not be required to provide a generator rated at 400A. The calculated demand on the service / generator will likely be well below 400A using the optional method in Article 220.
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The only other caution I may have is if the customer has an elevator. The generator generally has to be oversized to account for an elevator because for some reason the elevator controls are super-sensitive to voltage fluctuations and the motor load drops out and will not operate. We sold one to a homeowner through the electrician or GC, one. The EC and homeowner handpicked the desired circuits (this was a 400a service with generator and 200a ATS taking on one panel). When I showed up to do the start-up during the final it pained me to tell the HO that the generator would probably not handle the elevator, even though the motor data plate was less than 20a. The 16kw generator, with the other circuits already on there, was not stable enough for the elevator controls. Not the first time we encountered this, but the most recent.
 
All great advice so far. I didn't know about the elevators being so sensitive to voltage variations.. I've found that most customers want things to be "business as usual" during a power failure. However, trying to run the entire house on the generator can become quite an expensive proposition not to mention the physical size of the generator can grow to the size of your truck. For example, a Guardian 60KW generator is about 7.5'L x 3'w x 4'h The next thing you have to consider is what the fuel source for the generator will be. The same unit will consume 500 cu ft/hr using NG and 5.5 gal/hr using LP. Finally, the generator will cost you about $12K and the 400 amp transfer switch about $1.8K and you can't forget all the misc. wire, parts and accessories you will need to do the job.

My opinion, for what it's worth, I would try and convince my customer that they do not need to run the 3 AC units, pool motor, hot tub, Jacuzzi, double wall oven, electric range, microwave, dishwasher, clothes dryer, etc. during a power outage. I would pick approx. 1/3 of the loads deemed absolutely necessary and transfer those. If that doesn't work then sell them whatever you can. I just think you'll be spinning your wheels for nothing.
 
goldstar said:
My opinion, for what it's worth, I would try and convince my customer that they do not need to run the 3 AC units, pool motor, hot tub, Jacuzzi, double wall oven, electric range, microwave, dishwasher, clothes dryer, etc. during a power outage. I would pick approx. 1/3 of the loads deemed absolutely necessary and transfer those. If that doesn't work then sell them whatever you can. I just think you'll be spinning your wheels for nothing.
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I agree. Most of our sales on 400a service homes are a 200a switch taking on one of the panels, whether it is a new home with chosen circuits going to appropriate panel or retro jobs that takes rewiring both panels and swapping circuits (what a pain, btw. I don't think I have ever charged enough to do that exasperating job. I did one the other day and the original EC took 2 circuits at a time and twisted the grounds together before putting them under the lug. I finally broke down and took the whole panel apart.:mad: )
Most of our HVAC around here are heat pumps and air handlers, so if you have similar set up, and you are only taking on a portion of the house, remember the AC they want in the MBRM won't work without the air handler. Some people forget that the generator had better handle those 7.5-10kw heat strips should they lose power in the winter.:wink:
 
360Youth said:
Some people forget that the generator had better handle those 7.5-10kw heat strips should they lose power in the winter.:wink:
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Not necessarily:

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An emergency generator would be an Art 700 application. An optionally required standby system, as installed in a dwelling unit, is a 702 application. Take a look at the scope of each article and note the difference.
Flordia has some rules that require sizing of 702 generators, otherwise you can run a 400 amp service on a 5 KW generator. It may not work well but its an optionally required system, and the generator is protected against overload by its main breaker.
 
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