continuous load factor apply to feeders too ?

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RustyShackleford

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Here's the situation: House has subpanel for its pair of storage-tank water-heaters. Wattages are 3500 and 4500, and they are protected by 20- and 30-amp breakers, respectively (at 240v); thus each easily complies with the 80% factor for continuous loads. Question: is it code-compliant to protect the subpanel feeder with a 40-amp breaker in the main panel ? Since 40-amps*240-vac is 9600 watts, and the sum of the water heater loads is 8000 watts, the 80% factor is missed.


I believe the applicable code is "“Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.” So the question becomes, is a subpanel feeder also considered a branch circuit (from the point of view of the main panel) ?


Assuming this is compliant, what about practical issues ? As far as safety, surely there's no overheating danger in protecting the feeder with a smaller breaker (40-amp instead of 50-amp). How likely are nuisance trips of the 40-amp breaker ? The 40-amp breaker provides a margin of 120% instead of 125%; that's pretty close Given a water-heater rarely runs continuously for 3 hours, never mind two doing it simultaneously, I wouldn't expect problems.
 
A feeder is not a branch circuit, but if you look in the article on feeders you will find exactly the same language as you quoted for branch circuits.

You're going to need 8awg regardless, which is usually allowed to be protected by a 50A breaker, so this is also not a big deal. I don't know how likely nuisance trips would be a with a 40A, but I can pretty much guarantee less likely with a 50A.
 
I agree with jagged unless your feeder is NM cable, then you're stuck with a 40 amp OCPD for the #8 conductors.
 
2014 NEC said:
422.13 Storage-Type Water Heaters. A fixed storage type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load for the purposes of sizing branch circuits.
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This means it is not necessary to consider them as continuous loads when sizing feeders.

Cheers, Wayne
 
wwhitney said:
You quote from NEC "422.13 Storage-Type Water Heaters. A fixed storage type water heater that has a capacity of 450 L (120 gal) or less shall be considered a continuous load for the purposes of sizing branch circuits.

And conclude: this means it is not necessary to consider them as continuous loads when sizing feeders.
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Sorry, but I am not following at all. Your NEC citation says the subpanel must consider the water heaters as continuous loads, but how does that imply that the feeder need not consider them as continuous loads ?

BTW, this is wired with 6/2 NM-B. But it's running for 3ft in a conduit with another big 2-conductor cable (#4 or #6 SE). So I believe the 80% de-rating (310.15(B)(3)(a)) reduces it's ampacity to 44 amps. Or does it ? 334.80 is most confusing, saying the 90-degree column of 310.15(B)(16) can be used when adjusting, as long as the adjusted value doesn't exceed the 60-degree column, but then goes on to suggest that's not the case under certain conditions.

Simplest thing would just be to use a 40-amp instead of 50-amp breaker, if the continuous load thing is ok.
 
infinity said:
I agree with jagged unless your feeder is NM cable, then you're stuck with a 40 amp OCPD for the #8 conductors.
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Feeder is 6/2 NM. I prefer to use 40amp OCPD, except it isn't 125% of the sum of the continuous loads on the two branch circuits of the subpanel, hence the question of my OP.

Could be simpler just to not run the feeder in the piece of conduit (but less neat in the cluttered area between bottom of load center and floor), then no de-rating issue and I'd be content to leave 50amp OCPD.
 
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RustyShackleford said:
Sorry, but I am not following at all. Your NEC citation says the subpanel must consider the water heaters as continuous loads, but how does that imply that the feeder need not consider them as continuous loads ?
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Basic principle of legalese: if the intent was to consider them as continuous loads for all purposes, the sentence would just have been ". . . shall be considered a continuous load." The presence of the modifier "for the purposes of sizing branch circuits" means that for all other purposes, they need not be considered a continuous load.

There's even a fancy latin name for this principle, but I forget it.

RustyShackleford said:
BTW, this is wired with 6/2 NM-B. But it's running for 3ft in a conduit with another big 2-conductor cable (#4 or #6 SE). So I believe the 80% de-rating (310.15(B)(3)(a)) reduces it's ampacity to 44 amps. Or does it ? 334.80 is most confusing, saying the 90-degree column of 310.15(B)(16) can be used when adjusting, as long as the adjusted value doesn't exceed the 60-degree column, but then goes on to suggest that's not the case under certain conditions.
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The second and third paragraphs of (2014) 334.80 are not limitations on the first paragraph but additional requirements. In the situation you describe, the ampacity of the 6/2 NM is the lesser of 55 amps (60C ampacity) and 80% * 75 amps (90C ampacity), or still 55 amps.

Cheers,
Wayne
 
wwhitney said:
BThe presence of the modifier "for the purposes of sizing branch circuits" means that for all other purposes, they need not be considered a continuous load.

In the situation you describe, the ampacity of the 6/2 NM is the lesser of 55 amps (60C ampacity) and 80% * 75 amps (90C ampacity), or still 55 amps.
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Ok, thanks. So it is compliant to use a 50-amp breaker (because the ampacity of the feeder wiring is 55 amps) or to use a 40-amp breaker (because it's not required to protect the feeder with 125% * (4500+3500) watts ).

As far as nuisance trips, though water heaters are deemed to be a "continuous load" by the NEC, the definition of that says "3 hours under normal use" which doesn't apply to a storage-tank (or any type) water heater AFAIC.
 
RustyShackleford said:
Ok, thanks. So it is compliant to use a 50-amp breaker (because the ampacity of the feeder wiring is 55 amps) or to use a 40-amp breaker (because it's not required to protect the feeder with 125% * (4500+3500) watts ).

As far as nuisance trips, though water heaters are deemed to be a "continuous load" by the NEC, the definition of that says "3 hours under normal use" which doesn't apply to a storage-tank (or any type) water heater AFAIC.
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I would use the largest OCPD permitted by the NEC which is 60 amps.
 
Little Bill said:
OP said they used 6-2 NM cable for the feeder so he is limited to 55A. Unless you mean the "next size up rule" but I don't see a 55A breaker or fuse. So 55A is the limit for NM.
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Yes, I stated that I would use the largest OCPD permitted by the NEC, a 60 OCPD is permitted by the next size up rule.
 
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