Conduit, wires, breakers.

Reply Sun 4 Aug, 2013 11:07 am
I was reading today on the web that for a 3/4" conduit, 20 amp circuit, 12 gauge wire, you can have a total of six wires, three hot, three separate neutrals. I basically learned that the conduit fill allowance of sixteen wires for 3/4" pipe is just telling you how many wires can fit into the conduit, but after you figure out the derating of the conductors, the safe amount would be six wires.

I get that, but I am confused on why conduit size is not an issue. If you had 1/2" conduit, I would imagine that six conductors would be to many, and if you used 1" conduit, I would imagine that you can put more than six conductors in there safely.

Could some one please tell how many #12 wires a 1/2" conduit can handle safely on a 20mp circuit? Also the same question for 1" conduit

Thank you,

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Reply Sat 10 Aug, 2013 04:20 pm
Conduit size does not matter when it comes to de-rating. First you must calculate, based on circuit amperage size, what size wire you need. You can have three current carrying conductors in a conduit before you nee to de-rate. When you de-rate for 14,12, and 10 gauge you are allowed to use a higher starting current for your calculations. Example, #12 wire you can use 25 amps for your calculations. ( although you can never put #12 wire on larger than a 20A breaker) When you have up to and including 6 #12 current carrying conductors, you have to de-rate 80%. 80% of 25 amps is 20 amps, so, you can have up to 6 #12 current carrying conductors in a raceway before you have to up-size the wiring accordingly. Once you determine the amount of conductors in the conduit, and, whether they need to be up-sized and to which size, then you base conduit size on conductors contained therein. Whether or not a conductor is considered a current carrying conductor is a whole new topic for debate, but, nowadays with almost everything being electronic, it is wise to consider both ungrounded and grounded conductors as current carrying.
Placid Carcass
Reply Sun 18 Aug, 2013 10:58 am
You have no idea what you are talking about. When determining how many wires can fill a conduit, conduit size absolutely matters, as well as de-rating value.
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