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Posted By: Niko size of the feeder from generator - 11/16/09 04:41 AM
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.

Posted By: sparkyinak Re: size of the feeder from generator - 11/16/09 08:29 AM
Is it manual or auto xfer switch?
Posted By: gfretwell Re: size of the feeder from generator - 11/16/09 09:17 AM
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.
Posted By: pdh Re: size of the feeder from generator - 11/16/09 12:59 PM
Originally Posted by gfretwell
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.
Posted By: Niko Re: size of the feeder from generator - 11/16/09 06:39 PM
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
Posted By: George Little Re: size of the feeder from generator - 11/16/09 09:36 PM
Okay so what is the full load? If the panel has a 200a. main, are we calling that the full load? I hope not.
Posted By: gfretwell Re: size of the feeder from generator - 11/16/09 10:27 PM
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.
Posted By: renosteinke Re: size of the feeder from generator - 11/16/09 11:12 PM
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.
Posted By: Niko Re: size of the feeder from generator - 11/16/09 11:31 PM
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.

Posted By: renosteinke Re: size of the feeder from generator - 11/17/09 12:38 AM
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.
Posted By: gfretwell Re: size of the feeder from generator - 11/17/09 07:52 AM
Reno I agree your method may sound like common sense but I am not sure it satisfies 702.5(b)(2)(a).
The IAEI guys were really banging this around a while ago. I will go see what they came up with.
I do remember they said you can allow the user to do load management on a manually switched genny but when it was auto, they were more leaning toward total attached load. That would be the load calc for the house if this switched at the service disconnect ... I think.
That is how I remember it. I will see if that is right.
Posted By: gfretwell Re: size of the feeder from generator - 11/17/09 08:02 AM
This was the last word on the discussion and the participants seemed to agree.

Quote
Seems to me the first question to ask the customer is do they want to have their generator start automatically or are they wanting to transfer it manually. If they select a manual transfer then we should be able to connect any size generator to the "whole house" and the customer can then micro manages their loads.

If they select an automatic system (and most will) then the generator should at least be able to handle any loads that are on the system when no one is home, the A/C, refrigerator, pumps, etc. So a whole house back up may not include the washer, dryer, range, oven, etc.

Typically when sizing a generator for a customer I would add the starting kW of the largest motor together with the running load of any other motors to be backed up and then add the kW of any non-motor loads, and then adding 20% at the end.

Educating the consumer is the key, if they are going to own their own power supply then there should be some responsibility on their part to understand the system.

To me the issue is not so much the customer having a professionally installed system but the one buying a portable at the last minute and backing up his house through the dryer receptacle.
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Posted By: renosteinke Re: size of the feeder from generator - 11/17/09 04:59 PM
Greg, I must admit that even I'm not sure my method complies with the code as written. In this instance, I will fault the code, rather than the method.

After all, these are optional systems - that is, 'nice to have' systems. So what if the system fails? Just as important, what's the worst that can happen? The genny stall and quit running? That's not that big of a deal with an optional system.

I can see where you might want to be more thorough in the design of a genny for backing up critical systems, but that's not what we're talking about here.

Just as important -as I've already explained- is that an ordinary load calc will result in your having too large of a generator, and that's worse than if the generator were too small. Our 3 watts/sq. ft. figure is a lot of power alone, an amount that's rarely fully used.

I really hope the 2011 cycle addresses this section; it's bad code.
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