Conduit fill size when using multiple cables of different sizes

Question

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!

Answer 1

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).

Answer 2

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.)