rerickson
experienced amateur
When over three wires from the same branch circuit are run in a conduit, NEC appears to require them to be derated just as if they were on separate circuits, all capable of carrying full current at the same time (310.15B2a). Is there a Code paragraph I’m missing?
Because all these #12’s are on one 20A breaker, the maximum I2R heat comes when a single wire carries the full current, in which case the other wires are just passive conduit fill. Spreading the 20A among the wires actually cuts down on the total heat production. I would expect the derating computation to be based on the maximum possible heating, but haven’t found provision for this, and the exceptions of 310.15B2 don’t seem to apply. This can't be such an unusual situation!
The formula of 310.15C is plainly not going to be engineered for a residential application, unless there’s a much simpler calculation that an inspector’s likely to accept. Perhaps Annex B.310.11 can treat this as a diversity computation, but I'm not clear how that would apply for <10 cond, and it's all rather terse... Is there a simple solution?
My actual situation is slightly more complex than the simple case above. There’ll be a multiwire circuit pair with shared neutral, so max total current through the conduit is 40A, with max heating in two wires (either H-N or opposite-phase hots). Possibly another 20A circuit shares the conduit. Nine total conductors (including neutrals) will be split up as needed. The environment is a warm 50C, which is why derating becomes such an issue; .82 times .8 puts #12 THHN below 20A.
P.S. I hope I’m not wearing out my welcome with all these questions, but you all are a very helpful resource! Thank you so much.
Because all these #12’s are on one 20A breaker, the maximum I2R heat comes when a single wire carries the full current, in which case the other wires are just passive conduit fill. Spreading the 20A among the wires actually cuts down on the total heat production. I would expect the derating computation to be based on the maximum possible heating, but haven’t found provision for this, and the exceptions of 310.15B2 don’t seem to apply. This can't be such an unusual situation!
The formula of 310.15C is plainly not going to be engineered for a residential application, unless there’s a much simpler calculation that an inspector’s likely to accept. Perhaps Annex B.310.11 can treat this as a diversity computation, but I'm not clear how that would apply for <10 cond, and it's all rather terse... Is there a simple solution?
My actual situation is slightly more complex than the simple case above. There’ll be a multiwire circuit pair with shared neutral, so max total current through the conduit is 40A, with max heating in two wires (either H-N or opposite-phase hots). Possibly another 20A circuit shares the conduit. Nine total conductors (including neutrals) will be split up as needed. The environment is a warm 50C, which is why derating becomes such an issue; .82 times .8 puts #12 THHN below 20A.
P.S. I hope I’m not wearing out my welcome with all these questions, but you all are a very helpful resource! Thank you so much.