Whole house standby generators and 310.12

[jaggedben]

...

Now suppose the load calc comes out to over 100A, but the generator has some automatic load shedding so that the load on the generator will be just under 100A. Does 310.12(B) apply to that feeder?

I'd say yes.

Does it matter whether the loads shed are loads that are optional for a dwelling unit, vs load mandatory for a dwelling unit?

What does that mean? Like, whether they are not required by a load calc such as 220.82?
I'm inclined to say in principle it doesn't (or shouldn't) matter.

 

[wwhitney]

What does that mean? Like, whether they are not required by a load calc such as 220.82?
I'm inclined to say in principle it doesn't (or shouldn't) matter.

Well, say I remove (load shed or just put in the non-backed up loads panel) just the pottery kiln in the garage. The resulting set of loads is a set of loads that could be the complete set of loads of a dwelling unit, so there's no reason to expect any non-conservativeness from allowing the use of 310.12(B) for that set of loads.

But say instead I remove all the kitchen circuits except the refrigerator circuit. Now the set of loads on the generator is a set of loads that would never match the complete set of loads of a dwelling unit, it has no SABCs. So I can see the argument that 310.12(B) does not apply to the generator feeder in this case.

Cheers, Wayne

 

[jaggedben]

Okay but what I'm getting at is do the the actual connected loads matter so long as I have a valid load calc?

In your example a fridge wouldn't need a 100A feeder anyway. But supposed I wanted a 100A feeder because I might add more stuff to the critical loads panel in the future, so I do a load calc that includes the 4500VA and sqft VA required by 220.82 (or whichever, and that load calc comes in under 100A. Why shouldn't I get to use 310.12? I submit that as long as my load calc for 'entire load associated with [the] individual dwelling unit' is under 100A I can use 4awg cu, regardless of what's actually connected.

One interesting thing about the way 310.12(C) is written is that the actual size of the service does not explicitly matter. The 'entire load associated with an individual dwelling unit' should be considered to be the result of a load calc. Thus if the load calc is under 100A, but the service was sized at 125A, my feeder should still not 'be required to have an ampacity greater than that specified in ... 310.12(B)' i.e. 100A. Can't say I've noticed this before but it's an interesting argument. The relation to the discussion, again, is that load calc trumps whether any particular loads are connected.

There's another question you're raising (i.e. applicable beyond load shedding), which is what is included in the set of 'the entire load associated with an individual dwelling unit'. You seem to be saying a kiln does not have to be included because it's not a necessary part of a dwelling unit (or a dwelling unit load calc) but it seems to me that if it's powered from within the dwelling (and not, say, a separate outbuilding) then it's 'associated with' that dwelling unit. Your apparent interpretation (or question, anyway) is not one I would probably not try to raise with an AHJ.

 

[wwhitney]

I submit that as long as my load calc for 'entire load associated with [the] individual dwelling unit' is under 100A I can use 4awg cu, regardless of what's actually connected.

I agree that if the entire load associated with the dwelling unit calcs out to 100A or less, then it doesn't matter what load is backed up, 310.12(C) applies to the generator feeder. That was the second case (first interesting case) in my post #19.

The question about what loads are shed is for the 3rd case, where the service load calc comes out to over 100A, and so I arrange for some loads to be in a non-backed up panel, so that the backed up load calc comes out under 100A.

In which case the plausible answers are (1) No, you can never use 310.12 for that 100A feeder, or possibly (2) If you look at just the loads that are backed up, do those plausibly match the complete load of some other dwelling unit? If so, why shouldn't you be able to use 310.12(B) in that case for the generator 100A feeder?

I.e. the argument for (2) is "Suppose I have dwelling unit A with a certain set of loads whose load calc comes out to under 100A. Then I can run it off a 100A generator breaker using 310.12(B). Likewise if I have dwelling unit B with all the same loads as A plus a few more; its load calc comes out to between 100A and 125A. So I can run dwelling unit B off a 125A service using 310.12(A). Now my actual dwelling unit will run in one of two modes: on-grid, like dwelling unit B, or off-grid, like dwelling unit A. So clearly I should be able to use 310.12(B) for the 100A generator feeder and 310.12(A) for the service."

Cheers, Wayne

 

[wwhitney]

BTW, 310.12 is just a kludge. I feel that it was originally included due to the fact that 2 CCCs create heat at a lower rate than 3 CCCs, in fact at 2/3 the rate for equal conductors and currents. The inclusion of 120/208V 3-wire "single" phase feeders in 310.12 lost sight of that reason, however, and makes it look like it has to do with residential load diversity.

Seems like we should just delete 310.12, and change the ampacity adjustment table to read "1-2 conductors 1.2" and "3 conductors 1.0". Note that conductor temperature rise varies as the square of the current through the conductor, and so with 2/3 the heat generated, you could have a current that is sqrt(3/2) = 1.2 times as great and still have the same conductor temperature rise.

That does assume the conductors are in proximity and their insulation temperature is determined by the ability of the immediate vicinity of the conductors to reject heat to the greater world, as opposed to the ability of an individual conductor to reject heat to the immediate environment. E.g. in a cable wiring method, that you are comparing a 2 conductor cable to a 3 conductor cable, and the two cables can reject heat equally well.

Note that sqrt(2/3) = 0.816, quite close to the 0.83 value in 310.12.

Cheers,
Wayne

 

[Fred B]

Better check 445.13(A)......"The ampacity of the conductors from the generator output terminals to the first distribution device(s) containing overcurrent protection shall not be less than 115 percent of the nameplate current rating of the generator. "
"Exception: Where the design and operation of the generator prevent overloading, the ampacity of the conductors shall not be less than 100 percent of the nameplate current rating of the generator."

No mention of a reduction for "entire load of a dwelling unit" so from this a #2AL would not be to code on a 100A. It doesn't mean anything that we see installations with less size, see same on "subpanels" all the time.