ECN Forum Forums Code Related Discussion NEC & other Code issues size of the feeder from generator

I am planning to install a 12KW standby generator for a residential, but instead of choosing certain circuits to be on the transfer switch i have recommended to have a transfer switch without a load center so the whole house will be served by the generator. The generator will be protected by a 50A main disconnect and the customer knows that are limited to only 50Amps

But am i correct to say that 702.5(b)(2)(a) will prohibit this installation that i have proposed?

(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.

Is it manual or auto xfer switch?

That is a new code rule and yes it says if you do automatic switching the generator must be able to supply the whole transferred load.

If it is manually switched the user can do manual load management. If you think about it, that does make sense.
They do not want a 12kva generator trying to supply a 15 KVA load and only depending on a breaker to save it.

And with good reason, too. 62.5 amps is not likely to trip a 50 amp breaker for a little while (see your breaker's trip curves for specifics). Yet, that kind of load is quite likely to stall the generator, or cause other bad behavior if an automatic shutdown doesn't happen. Overheating the wiring is the least of the worries in this case.

It will be a auto transfer switch.

Thank you for your input. Now i think about it, it does make sense. I should have checked first before I recommended that setup.

Thanks again

Okay so what is the full load? If the panel has a 200a. main, are we calling that the full load? I hope not.

I think the full load certainly would have to include all the unattended loads, HVAC, water heater and a certain amount of the general lighting load, maybe the range and dryer. The first 2 are the killer. If you could put a smart box in front of them similar to the PoCo energy saver that required manual restart if on generator power you would go a long way towards making me happy about 702.5(b)(2)(a).
If you are doing a load in the washer (water heater going, 23a), a load in the dryer (23a or so) and have something on the stove that alone would overload a 12KVA generator and when the A/C kicks on you got trouble right here in River City.
I do think a simple interlock that trips out these big, discretionary loads on the transfer would be a solution and you can let the user do load management by manually turning them back on.

I'm really not all that worried about overloading the generator; too much demand, and it stalls.

"Sizing" can also be a challenge, in that generators have a 'surge' as well as a 'constant' demand rating. Think of it as a circuit breaker curve - it can handle an overload of short duration. With that in mind, I'd size the feeder and OCPD at, or just over, the 'constant demand' rating.

Sizing a generator is also very much a matter of professional judgement, in that 'more' is most certainly not 'better.' Bad things happen when a generator is run with too little load; most manufacturers want the generator to run at about 85% of the continuous load rating.

Since the typical house micht very well have a 200-amp service, it's pretty hard to ever measure more than 60-amps actually being used at any one time.

Does that statement sound outrageous? It did to me- until I went around in the midst of a desert summer, actually measuring the power being used, even with the AC running.

I can understand the need for a 200-amp transfer switch on a 200-amp service, but I have no problem feeding that same switch from a 60-amp (15KW) generator; I just want a 60-amp OCPD on that genny, and not rely upon the 200-amp main breaker at the panel.

Let the genny stall a few times, and the customer will learn all about load management.

The customer knows that they can't turn on too many appliances and they are aware of the electrical load concept and demands. However, just because the customer knows what to do it still does not satisfy the 702.5(b)(2)(a).

Since the sub panel is rated and fed by a 200Amp feeder but the total load is not 200Amps, would you say that this is a judgment call left to AHJ, or you would interpret 702.5(b)(2) as the 200Amp panel being the total load.

I would say that first you measure what's actually being used. That is, you go around, turn everything on, and set you amp-clamp on the feed. What are you actually drawing?

You can do this either by artificially loading the system up, or by measuring when circumstances would have the place operating full-out.

I am troubled by this new section of the NEC, in that it seems (dare I say it) written not only by an engineer, but one completely unfamiliar with generators - especially ones used in 'optional' circumstances.

The issue of accurately anticipating the actual loading is critical. You don't just install one, and let it sit there like a fire extinguisher, to be used only as needed. Rather, you have to reqularly run the generator (exercise it) under load. When you do that, you can either use the generator to supply your household loads of the moment, or have a "load bank" (glorified toaster) installed.

I think you can see where I'm going with this: if the generator is too large, it will not operate reliably. If you use service load calculation methods, you will end up with a far larger generator than you will ever use; when the time comes that you need it, you might as well have a boat anchor sitting on the pad.`

So, to repeat myself: to establish the load, measure the existing load and use that figure - rather than the service size.

If any AHJ want to be concientious about generator instals, I suggest that he focus instead on the exercising protocols and noise management.