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I am going to be upgrading a full, very small 6-space panel in my attached garage to a much larger (40+ space) panel soon. I will have a number of circuits to run from this panel to a planned addition, then I will take advantage of the additional spaces to start bringing other wiring in the house up to (modern) code. To get from the panel, which will be surface mounted, in the garage, I will need to run the cable down the wall then through hole(s) punched in the brick foundation and into the basement/crawlspace.

My plan is to run as many pieces of conduit as I can get out of the bottom of the panel, and of as large a diameter as possible during the panel install. Only a few cables will be run on day one, but I want to have as much space available as possible for when I do additional work in the future, and I don't want to break out the sledge and chisel any more often than necessary.

At this point, I do not have a panel picked out, so I don't know what size knockouts may be available, however, I will beg/buy/borrow whatever tools may be necessary to make as large a knockout as possible to give myself plenty of space for the future expansion.

Assuming I get/can make holes large enough for 2" PVC conduit, how many 12/2 NM-B cables can I run through each one?

  • Yes, I know pulling NM through conduit is difficult. However, these will be a fairly short runs (under 36", I'd imagine) straight down, then around one 90° elbow and through a short straight piece through the wall and into the basement, so the pain will be minimal. Since I'm thinking PVC, I don't need any sort of nipple on the inside for protection from the conduit, correct?
  • Yes, I know that individual THHN would be easier to pull, but that would mean I would need a collection of junction boxen just inside the basement in a space that is already very cramped and/or to continue runs in conduit to a place where I can put junction boxen, which I don't particularly want to do.
  • I would be OK with going with metal conduit if I can get a larger number of cables into each one. PVC is easier (for a non-electrician without the proper tools), but I can cut steel pipe, I just don't have any threading tools, so non-threaded fittings would be easier/cheaper (fewer tools to invest in).

Note that I have checked a couple of on-line fill calculators, but they don't give NM-B as an option for cable type because the general recommendation is to not pull cable in conduit.

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    I'm not sure of the rules (which is why this is just a comment) but in addition to the conduit fill limitation, I believe there are limits (technically, derating) based on the number of conductors, temperature rating of the wires/cables, etc., such that the end result is if you are using conduit beyond a certain length (24" comes to mind, but it could be that's only if it is between two panels), you have some serious limitations as to how many cables you can put into a conduit. Apr 14 at 15:00
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    ummm... Interesting point, @manassehkatz-Moving2Codidact - one I hadn't thought about. I've considered simply building a chase with a removable cover to run the wiring down, but somehow, I've got to get it through a brick foundation wall without destroying the foundation. I'm sure someone will have some suggestions. Beware: question creep!
    – FreeMan
    Apr 14 at 15:10
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    @manassehkatz-Moving2Codidact Good point (310.15(C)(1)), to keep 70% rating would require less than 10 current carriers, so 4 12/2 nm-b could fit in 1.5" pvc40. If stuck on a single 2" he could use smaller greater dimension of 10/3 for 50% derating (even abandoning a conductor if MWBC's aren't practical) will fit 9 cables in a 2". Apr 14 at 15:34
  • Can you make it so the distance between the top of the LB into the basement wall and the bottom of the panel is <24"? Also is there room for a single big box either below the panel or at the entry point into the basement? Apr 15 at 1:05
  • If you have more than 3 current carrying conductors in a conduit you have to derate the wires and embiggen them as well. Derating factors are significant. 10 current carrying wires in a conduit require 50% derating. Derating factor for 4-6 wires is 80%, so if you want 15A circuits, you might want to figure out what size conduit you need to pull two 3 wire+ground split neutral circuits and what size to pull one and run a rack of small conduits instead. Add one pipe of sufficient size to pass the main breaker current to a sub panel.
    – K H
    Apr 15 at 8:30
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The dimension of Encore 12/2 nm-b is .16x.45 inches. Belden Cable has a calculator, it says you can fit only 8 in 2".

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