I don't see the 83% rule as changing the conductor ampacity...just a permission to protect the conductor with an OCPD that has a rating higher than the ampacity of the conductor...just like 240.4(B) does not change the ampacity, but only permits the conductor to have overcurrent protection higher than its rated ampacity. The words below tell me that the 83% rule does not actually change the conductor ampacity.
I agree. Under the old 310.15(B)(7) table a few cycles back it was much more ambiguous what the actual 'ampacity' of the conductors was. But the way 310.12 is worded nowadays makes it as Don says.
You cannot use the 83% rule on the feeders because neither feeder by itself supplies the whole dwelling and they are smaller than the service conductors. That's the end of that discussion, taps or not.
I really don't know what code intends. Seems to not be absolutely clear whether you could use that 310.15(B)(7) for a feeder tap. You certainly can supply exact same load with a service conductor sized via this method though. Is one that much more of a hazard than the other?
I just figured that since they don’t let you go up to next standard breaker size that the 83% would not apply either, I believe they should add this into code. 83% not permitted on taps where it’s says cannot round up to next size standard breaker for taps. To cut down on the confusion. Just trying to figure out right from wrong.
I'm struggling to think of a possible way that a tap would ever qualify for the 83% rule in the first place. If it's protected by the service disconnect OCPD it's not a tap. If it's small enough to be a tap then it doesn't qaulify for the 83% rule for one or more reasons. So it's a moot question.
As in:
Ah, okay. Multiple dwelling units on a service. I think that's the only way.
Can you expand on that I'm not seeing it, that you can't use the 83% rule for a feeder that carries the entire load of a dwelling unit if it is rated smaller than the service.
I was thinking of a service that only supplies a single dwelling unit, and is not oversized. You're correct it's not true in every last case. So much for being precise.
Why not just a 200A service disconnect, 100A conductors connected to the load terminals of the 200A device, to 100A MB panel, all serving a single dwelling unit?
Only that that would be weird and I've never seen such a thing. Pardon my lack of imagination.
This service I mentioned is controversial depending on how you interpret the code book. Codes not clear in this particular scenario. My interpretation of the code is 83% is ok from meter to 400 breaker disco, Then the feeder conductors from disco to the 2 200 amp panels with main breakers located in garage on interior wall should be sized for 200 amps (250 al or 3/0 cu) installed within a raceway. I failed this site and was overridden by a super, contractor ran 2 sets of 4/0 al ser cable from disco to panels total length 25’ up walls and about 10’ through the attic without a raceway. I did not agree with this install.
Thanks, So you would allow the 83% on this install on the feeders even though the 200 amp breakers are not at the point of supply (tap conductors)
Well I wouldn’t call it parallel, Because each cable runs to separate 200 amp panels. So it’s a 400 amp main breaker disco, with 2 4/0 al ser cables under each lug on load side of breaker going to 2 separate panels
Yes but how large is the equipment grounding conductor in the 4/0 ser? It must be large enough for the 400 amp breaker which would be at least #1 al.
4/0 4/0 4/0 2/0 Ser cable, So equipment ground is good, Each cable does not supply the whole load, and per 240.21 you can’t cannot go up to next size breaker size. So in my opinion I think that they should use 175 amp mains Inside Panels or install 250 al in a raceway to comply
