Several weeks ago, I was reading a thread here about electrical circuits carried in conduit. One of the things mentioned was that when multiple circuits are carried in the same conduit, if there are too many, you have to derate the current carrying capacity of each circuit. One poster mentioned that for all practical purposes, this applied when you have more than 9 conductors in a conduit. I had never heard of this before and it piqued my curiosity, as many of the circuits in my home are in emt conduit. Naturally I had to take off the panel cover and check. Most of the conduits carried one or two circuits, but the conduit going to the kitchen carries 5 circuits (11 wires – 5 hot, 5 neutral, 1 ground). These are all #12 AWG wires and all circuits are protected by 20A breakers. I don’t think any of these circuits comes close to carrying 20 amps; the biggest load is the refrigerator (on it’s own circuit). My question is, does 9 conductors mean 9 wires, or 9 hot wires? Is my situation safe, or do I need to replace my breakers with 15A models? I hope not, as 2 of them are GFIC breakers, and they tend to be pricey.
Thanks, Conrad
Replies
At >9 the derating crosses the point where the 90c column saves you. Your pipe is 2 over that so it should be derated. In a dwelling I doubt you would ever actually have any trouble at all but that is the rule.
You count the neutral as a current carrying conductor, as it carries the unbalanced load back to the grounding bus bar. Also, you are derating the potential of the conductor, not the conductor size, which governs the size of your breaker. In other words, you don't need a new breaker, you need bigger conduit, smaller wire (for more allowable conductors), or less wire in the conduit. Don't go pulling new wires just yet, that conduit with the 10 current carrying conductors is probably already sized properly, is the connector in the panel for the kitchen bigger than the other connectors? The bigger the conduit, the more conductors it can safely hold.
Basically, if it aint broke, don't fix it.
Arc, don't confuse "fill" with "derating". I could put 10 current carrying #14s in a 6" pipe and legally it still needs to be derated the same as it would in a 1".
I understand what you mean, your derating as a result of extra resistance caused by extra heat for a full conduit. If the kitchen circuits are working, he will be doing more harm than good by changing out breakers.
Thanks, guys. I've had no trouble with these circuits whatsoever. Sounds like I got myself worried over nothing. I'll leave things as they are.
Conrad