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How many 14/2 romex cables can I, according to NEC (or safely), put in a 1/2 pvc conduit? For background: the conduit will be buried until surfacing in a junction box; using burial-rated "romex"black but want the additional security of conduit.

  • When you say "burial rated Romex" are you talking about UF? Also, is this a schedule 40 or a schedule 80 conduit, and is this conduit run for the full length, or just the stub-up? – ThreePhaseEel Jun 3 '18 at 3:53
  • (1) yea i guess uf, idk its grey and it says rated for underground use. (2) schedule 80. (3) and full length I guess? I mean the wire is either in a junction box or a conduit (never exposed to air). – Alex Jun 3 '18 at 5:05
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Use individual THWNs here instead, or else prepare to face the full wrath of the fill rules (and the electrician that comes over to rescue your pull job!)

A 1/2" schedule 80 PVC conduit has 56mm2 of usable fill for individual wires, or 75mm2 of usable fill for a single wire or cable. A 14AWG THWN takes up about 6.3mm2 of fill; however, a multiconductor cable's fill is computed as if it were a round wire with a diameter equal to the major diameter (long dimension) of the cable (Note 9 from chapter 9 of the NEC):

(9) A multiconductor cable, optical fiber cable, or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter.

As a result of this, given that a 14/2 UF cable has dimensions of 0.185" by 0.385", as per Encore Wire's specsheet, or 4.7mm by 9.8mm in handier units, we then get an effective fill area for a single 14/2 UF cable of just over 75mm2!

Given that a single 14AWG bare copper conductor for the EGC counts for 2.1mm2 of fill, you can get 4 hots and 4 neutrals (i.e. 4 circuits) + 1 EGC (which all 4 circuits can share) into your conduit without busting the 56mm2 fill limit and allowing the conductors to carry their full permitted 15A still due to the way the derate rules work. Compare that to the 75mm2 UF cable not quite fitting down the conduit by Code, making you emit profanity all the way through the pull, while only providing a single circuit to wherever this is going.

(Of course, this conduit will let you run a bigger feeder there if you want multiple circuits at the destination, too -- you could get 3 10AWG THWNs + a bare 10AWG ground down there and power your 4 15A circuits from a subpanel that way, instead of stuffing more 14AWG wires down the conduit.)

  • very informative answer! when looking at the 1/2 i was fairly certain only one could reasonably fit but wasn't sure. i wasn't too concerned about pulling as the distance is super small and mostly for landscape lighting. thanks! ill keep it to 1 per 14/2. – Alex Jun 3 '18 at 6:25
  • @Alex -- why are you using UF in conduit in first place instead of individual THWNs? – ThreePhaseEel Jun 3 '18 at 13:04
  • @Alex You miss the point. Even 14/2 is too large for 1/2 conduit. He said so, it's over 75mm2! He was just using that as an example of why that idea can't work. – Harper - Reinstate Monica Jun 3 '18 at 13:25
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    @Harper it works out to be about 75.11 (.385" => 9.779mm and 1/4*π*9.78^2)... .11mm is negligible but I'll still check w local inspector. In case you haven't figure out by now, I'm dead set on using Romex mostly because I bought a custom length and can't return it :) – Alex Jun 4 '18 at 7:00
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    Beware the fallacy of sunk costs. UF is also legal anywhere you'd use NM so it can be put to good use many places. – Harper - Reinstate Monica Jun 4 '18 at 14:15
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At the first junction box at the start of the conduit, you terminate the Romex cable. When there are several things of different colors, that is cable. Individual things are wire.

Through the conduit, you run special stuff called THWN. This is not a cable. This is sold as individual wires and you need 3: black white and green or bare. They make this in solid or stranded, I recommend stranded. These will go very easily down the conduit. You will not believe how flexible it is. You cannot make THWN by tearing the jacket off Romex.

Use wirenuts to attach each color to the corresponding romex wire.

At the other end of the conduit, do the same thing to transition back to Romex for your last bit of distance.

Code requires you fully assemble the conduit before pulling wires through it. You can't lay the wire out and slide sticks of conduit over it. That's why Romex is almost impossible to pull.

  • Yeah -- sliding conduit over wire would tear up the wire insulation like no tomorrow (aka why Code prohibits that) -- pulling wire messes it up badly enough as is! – ThreePhaseEel Jun 4 '18 at 1:46
  • I really appreciate the additional info! I know what a cable and wire is though haha but I appreciate dumbing it down just in case. I'm still going to try and use romex if my local inspector allows it for reasons mentioned in my other comment, but I'll keep THWN in for any future conduit usage (I should have asked before buying). Also don't be too concerned about me not being able to pull the romex - it's only a short distance! One question: why do you recommend stranded THWN over non? flexibility? – Alex Jun 4 '18 at 7:04
  • @Alex -- flexibility indeed – ThreePhaseEel Jun 4 '18 at 11:46
  • @Alex Yup. Stranded because it's so easy to work with, flexibility (non-snake-wrestling) being a big part of that. I've pulled 12 wires of #10 and #12 over 100' and it was totally DIYable. – Harper - Reinstate Monica Jun 4 '18 at 14:12

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