Electrical – Conduit Fill and Derating for THHN Cables (outdoor run)

electrical

Project: Run a 15Amp and a 20Amp circuit to bathroom from breaker panel outside the house.

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After much reading, I have determined that NEC requires adherence to the following. Any suggestions for improvement are most welcome.

  • Exterior run from panel to the spot where it enters the attic will be done in a conduit (Cantex PVC SCH 40).

  • Because outdoors is considered damp/wet area, only THHN/THWN-2 wire should be used at least until the wire enters the attic.

  • For a 3/4" ID conduit, the max fill area for 2 or more wires (or circular-equivalent cables) is 40% of the available or 0.213 sq inch.

  • I will have a total of 6 wires, 3 per circuit, half of which will be 14 Gauge and the remaining 12 gauge. Alternatively I could also run all wires at 12 gauge.

  • If I went with 12AWG (3) and 14AWG (3), the total occupied area will be 0.0097*3=0.0291 sq inch for 14 AWG, and 0.0133*3=0.0399 sq inch for 12 AWG for a grand total of 0.07 sq in or 11.40% fill, which is well within the limits allowed under NEC 2017. I did this calculation assuming 3 conductors per circuit, but presumably the grounding wire is not considered a conductor, so the actual numbers are lower by 1/3.

  • As for derating the conductors, it is my understanding based on this post that for 15/20 Amp circuits running less than 9 wires, the derating really is not a concern. Someone correct me if I am wrong.

So in nutshell, I should be good to run either six 12 gauge wires or three each of 12 and 14 gauge wires through a single conduit from panel to attic entry point. Here are my specific questions:

  1. Since I will be converting the THHN/THWN-2 wire to Romex 12/2 or 14/2 cable, is it okay to join the green sheathed grounding wire of the former type with the bare copper one from the romex?

  2. If I use 12 gauge THHN wire on both circuits, am I limited to using only 12 gauge romex cable inside the house? This is what I am assuming, but just want to make sure.

  3. Is there a benefit to using stranded vs solid THHN?

Best Answer

  • On the Schedule 40, don't use it by default. Use Schedule 80 (or EMT) by default unless Code says it's alright to use Schedule 40. For instance anywhere at risk of physical damage must be Sched 80 (or EMT).
  • Wet area technically requires THWN or XHHW, however almost all THHN is cross-listed THWN, so you're surely fine. All outdoor conduit is presumed to to be full of water, the wire is the defense.
  • Conduit fill: Rule of thumb: Up to nine #12 wires in 1/2" EMT or 1/2" Sched 40 PVC before you even have to start to think about a conduit fill calc. This is a coincidence and has nothing to do with derate. Watch out for Sched 80, it is smaller than you think. For conduit fill purposes, all wires and wire-like things (fiber-optic) count, grounds too.
  • In derate, 9 is correct, but as your link discusses, grounds and certain neutrals are exempt. It boils down to (in split-phase service) up to 4 circuits of any kind before you need to start to derate #12 or #14 wire. In 3-phase service, 3 circuits.
  • The reason to run all 12AWG is so you don't have to buy two different sizes of wire. I stock 10 colors of 12AWG and zero 14AWG, because distinguishing circuits is a heck of a lot more important.
  • Both circuits can share 1 ground wire in conduit. It must be large enough for the largest circuit. Remember current only flows on ground wires during fault conditions, and you are unlikely to have two high-current ground faults at once, especially on GFCI.
  • Wire colors: you must distinguish wires, and it also helps to group them by binding hot-neutral pairs with tape. Preferably, use alternate colors, e.g. black-white for one hot-neutral pair, and brown-gray for the other. Gray is the other allowed neutral color.
  • On individual wires in conduit, remarking white wires to be "hot" is not allowed, which means any tape marking on wires is only a marking. Feel free to mark a pair black-yellow and white-yellow, and the other pair black-blue and white-blue.
  1. Yes, that's how you do it. But combine both Romex ground wires to one THHN ground wire.
  2. You are always allowed to use larger wire than is required for the circuit. However you must breaker for the smallest limitation.
    • For instance if you ran #12 and #14 wire in a circuit, that must be breakered 15A.
    • If you used #10 and #12 wire to a 15A single dedicated socket, that must be 15A because of the socket.
  3. Stranded, stranded, stranded! In this case one end goes to a breaker, and the other goes to a wire nut. Those are both easy to attach to stranded wire. If you were a novice attaching stranded wire to a screw terminal for the first time, I'd be more cautious, because that's easy to mess up. Not a problem here. You'll love it and will never want to go back to solid.

Those 10 colors of #12 wire I own are all stranded.

To mix solid and stranded on a wire-nut, don't pre-twist. Line them all up ends-square and even (if anything, stranded 1/16" proud) and twist like the dickens. Let twisting the nut do the work. Never tape wire-nuts to hold it together; if it needs that, it will arc and start a fire; improve your technique until it can survive a pull test on all wires.

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