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Thread: Max # of circuits in a conduit

  1. #1
    Join Date
    Mar 2006
    Kea'au, Hawaii

    Question Max # of circuits in a conduit

    I'm not clear if code allows you to run multiple general purpose and lighting circuits in a large conduit, and reduce the conduit size as you reach utilization junction boxes that will branch off to supply your room utilization (room outlets) points.

    I have a 40 Ft run along the front side of our ranch house. This run passes a bedroom, living room, another bedroom and out to an outside patio (rain shelter roof only) area. I will need a mix of circuits 12 AWG 20 amp ckts for the general outlets in the bedrooms, living room and patio, plus 14 AWG 15 amp circuits for lighting these areas. I am interested in seperate breakers (power trip isolation) for each room plus seperate lighting breakers. Similar situation for the back side of the house plus the kitchen and heavy utility feeds.

    Can I get a fat enough conduit to run all these wires out of the breaker panel, and down size the conduit between pull point/room feeder junction boxes?

    Or do these things have to be run in seperate home runs to the breaker box?

    I know you should not mix different size wires on the big draw circuits, I'm wondering if the GP outlets and lights will matter. One concern I have is if you want to run an additional circuit, you should kill all the live circuits in the conduit you want to fish the extra wire through (again additional wire pull with existing wires is not recommended due to potential to damage wires already in the conduit).

    Just looking for "the rule" and then peoples feedback/experience of how they view this situation.

  2. #2
    Join Date
    Dec 2003
    Kent, WA


    Code allows as many circuits as you want in a conduit. The problem is derating. As you add more circuits, regardless of the size of the conduit, you have to derate the ampacity of all wires in the conduit because they can't dissipate heat as well and all of those wires are sources of heat. Generally, you should limit a conduit to 4 circuits. Above this you have to start increasing wire to a larger size than normal. If you have high ambient temperatures in this conduit (e.g. in an attic), you already have a derating issue and may not be able to have more than 3 wires without oversizing.

    You can fit a lot of #12 wires in a 3/4" conduit. I'd suggest a few 1/2" conduits or perhaps a few 3/4" conduits. You could have 8 circuits in a conduit (12 wires) if you ran 4 multiwire branch circuits. But I wouldn't recommend that since you have to keep the wires and their associate neutrals straight or you can overload things. You'd definitely want to put tape flags on the neutrals and the associated hots that go with it.
    Kent, WA

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