two conventional water heaters on a subpanel

[RustyShackleford]

I have a subpanel with two branch circuits, driving two water heaters, rated at 3500 and 4500 watts, respectively. What ampacity is required for the feeder to the sub panel ? Is it necessary to apply the 25% continuous-load margin to both loads, or just the largest load ? If the former, then 125% * (4500+3500) / 240 or about 42 amps, if the latter then (4500*125% + 3500)/240 = 38 amps.

I believe 422.11(E) gives the requirements for each water heater, but this doesn't seem to apply to the feeder.

 

[wwhitney]

I take "conventional" to mean "storage-type," so 422.13 controls. As it specifies a continuous load "for the purposes of sizing branch circuits," it is implicit that for sizing the feeder, you have no continuous loads. Your feeder load is 8000 VA.

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Cheers, Wayne

 

[augie47]

The 125% requirement {422.11(F)in '17} applies to branch circuits. Your feeder can be sized base on the individual wattages. 33 amps


)Typed as wwhitney posted)

 

[jaggedben]

So in this particular case it turns out to make no practical difference. You need 8awg wire regardless, and then why would you bother looking for a 35A breaker when you are allowed to use a 40 (or depending on your wiring method, a 50).

 

[RustyShackleford]

So in this particular case it turns out to make no practical difference. You need 8awg wire regardless, and then why would you bother looking for a 35A breaker when you are allowed to use a 40 (or depending on your wiring method, a 50).

Well, if I had to add the 125% up-rated ampacities of the two water heaters I'd need 6awg (if NM-B) and a 45- or 50-amp breaker.

But I don't need to, so thanks for the replies.

Yes, by "conventional" I mean storage-tank style.