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I have a main panel in my garage and I would like to install a subpanel in my basement. My plan is to run a 60A circuit to the subpanel (using 6/3 NM) as well as 2x 12/3 NM homeruns; one to feed 2 cirucits of lights and a second that would not be connected to my main panel yet, but would be available for convenience if I later wanted to add 2 more circuits in the basement. I was planning to use Schedule 40 PVC (inspector told me this would be fine), and the total run of conduit within the garage to the junction box in the basement will be approx 40' in length.

What size of PVC conduit would I need to satisfy conduit fill requirements? Would I be able to use smaller conduit sizes if I stripped the sheathing from the 14/3 and 12/3 cables?

In my research, I've come across this post (What type of wire for subpanel in attached garage?), and it seems that my cross-sectional areas would be:

6/3     650 mils    0.3316625
12/3    347 mils    0.094521
12/3    347 mils    0.094521

Resulting in a total area of

0.3316625 + 0.094521 + 0.094521 = 0.5207045.

In this post (How do I determine the fill rating of a conduit?), there's a table in NEC 2014 Chapter 9 Table 4 for PVC Schedule 40 indicating Over 2 wires requires 40% fill area; so it seems like 1 1/4" PVC would allow for .581" size, granting enough size for the .521" area that I believe I would have wih those 3 cables. Am I deriving this correctly?

Thanks for the help!

  • I did not check your calculations, but you have to be careful mixing square inches / square mils / circular mils. My guess is 1.25" is adequate size. – batsplatsterson Nov 3 '15 at 10:00
  • Look around for "conduit fill calculators" on the interwebs. Some suck, some are pretty good, all tend to be a lot less work than sorting it out by hand. – Ecnerwal Nov 3 '15 at 13:49
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Stripping the jacket off the cables is not an option. The wires inside are only rated as part of a cable assembly, and cannot be used as individual wires.

I didn't check your numbers, but the process you followed is correct.

If you're dead set on installing cables, you might want to bump the conduit up a size. The extra space will make the pull a bit easier. Also make sure you use plenty of lube. Though since you're installing conduit anyway, consider installing individual wires instead of cables.

It also might make more sense to bump up the size of the feeder a bit, and eliminate the two branch circuits. Increase the size of the secondary panel, and supply the 20 ampere branch circuits from there instead. This might save money on materials, make the pull easier, and reduce the size of the conduit (not sure about this, as I haven't run any numbers).

  • Thanks for the suggestions. The reason I decided to go with 60A subpanel was so that I could run 6/3 romex and only run conduit within the garage (was planning to run it outside the existing drywall). Then within the basement, I was going to run it through the soffits & floor joists, where it wouldn't need conduit for that portion. If I used THHN (which I think you're saying is wires instead of cable), then it would need to be in conduit point-to-point. I like the upsizing conduit size - will do. I appreciate your feedback if anything about that sounds off! – mallio Nov 5 '15 at 3:06
  • Btw - the 2 branch circuits are just for the lighting and the 60A feeder is for outlets, as I was planning to have 5 circuits on the 60A subpanel. Having 90A pumped to the basement felt conservative while saving myself some installation hassle, being my first DIY electrical project. – mallio Nov 5 '15 at 3:08
  • You can run individual wires in conduit, then transition to NM cable in a box or conduit body. But if you're only pulling through conduit for a short distance, it might not be so bad pulling the cable. – Tester101 Nov 5 '15 at 3:31
  • Thanks for the tip - I'll have 30-40 feet of conduit to run within the garage. I'll consider using individual wires if it'll make the job easier. – mallio Nov 5 '15 at 3:38
  • You still might want to consider a larger second panel, and eliminating the extra circuits. I'm still confused as to why you're planning to run those circuits in addition to the feeder? It just doesn't make sense. – Tester101 Nov 5 '15 at 4:01
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I did not check your calculations, but you have to be careful mixing units, square inches / square mils / circular mils. My guess (just a guess) is 1.25" is adequate size. However, there are a few other considerations before figuring that out.

One, is it correct that there is no service in the building whose basement you're putting the subpanel in? Just seems strange, usually you think of the garage being fed from the main building, if you wanted another panel in the main building, you'd run it from the main panel in the main building.

Two, why run some circuits back to the garage, rather from the subpanel you're installing? Again just seems odd, normally you'd just size the subpanel to feed those circuits as well. That could be dangerous, someone may think that if the power is turned off at the subpanel, or at the 60 amp feed from the garage, that the other building is off, but some circuits in the other building are still hot.

Three, why use NM cable in conduit? Generally that's a bad idea, hard to pull, reduce the ampacity / derate, more expensive, may be a code violation depending.

Finally - if you're going to do this, I'd certainly run two conduits. By far the most work and expense of the feed from building to building is digging the trench and backfilling it, not the cost and labor of the PVC and a few fittings. I'd definitely run two conduits for what you have in mind, one for the main feed, one for the additional circuits. It's more flexible in the future. (In fact I'd install one empty conduit just in case, maybe for phone / internet / cable TV / alarm / etc. in the future, but that's me.)

  • +1 for more conduit. Trenches are expensive, conduit is cheap. +2 for why NM in conduit, but I can't actually give +2. – Ecnerwal Nov 3 '15 at 13:47
  • I read the question as having an attached garage, and the panel is in the garage. That's the way my house is set up - the main and only panel for the house is in the garage on an exterior wall. – JPhi1618 Nov 3 '15 at 16:54
  • Thanks for pointer on mixing units, I'll have to recheck my numbers given that. @JPh1618 has it right, my garage is attached and the main panel is on the exterior wall of that garage. I'm planning to install a subpanel in my basement, and the conduit would be for the 30-40' run within the garage, as I'm planning to run the NM outside of the drywall. The idea of running circuits back to the main panel in addition to the feeder to the 60A subpanel is to have plenty of capacity in the basement, where the homeruns could be for lighting and 60A sub for outlets. – mallio Nov 5 '15 at 3:18
  • The 60A subpanel would supply small workshop (20A), electric fireplace (20A), bathroom (20A), and 2 15A circuits that would service a wet bar and treadmill. To be on the safe side, I thought having 2 15A homeruns in addition to the 60A feeder would provide plenty of power & wouldn't require conduit in the 50' run inside the basement. – mallio Nov 5 '15 at 3:22

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