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I'm trying to do some improvements on an existing structure that leaves me little room for maneuvering. I'll break it down as simply as possible. I have two large PVC junction boxes (completely nonmetallic) separated by a standard 2x4 stud. It's a total distance of 4 inches. I need to get 120/240 200A from one junction box to another. That's two 3/0 THWN-2 (ungrounded), one 2/0 THWN-2 (grounded), and one 2AWG THWN-2 (grounding).

I really don't want to put a 2-inch conduit between the boxes. That requires drilling a hole 2 1/2 inches in the stud, leaving only 1/2 of stud on either side. Basically, there's no stud left there (and it's a load-bearing wall).

I had a thought that I could run four 3/4" 4-inch-long PVC conduits (completely nonmetallic) tightly grouped together (just a few millimeters from each other) from one junction box, through the stud, to the other junction box and put one conductor in each conduit. Electrically, this should work perfectly. As there are no metallic components anywhere in the boxes or conduit, there should be no inducted voltages or inductive heating. But there's electrical theory, and then there's the code.

In general, the code would seem to prohibit this. I think I may have found something in the code that will support it, but it's not very clear, and I could use some help interpreting. First there's this:

300.3(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

300.3(B)(1) is only for parallel installations and doesn't apply. 300.3(B)(2) is only for grounding and bonding conductors and doesn't apply. 300.3(B)(4) is a special rule for enclosures, pull boxes, and neutrals, and doesn't apply.

But 300.3(B)(3) is interesting, and may provide me the out I need. Maybe.

300.3(B)(3) Nonferrous Wiring Methods. Conductors in wiring methods with a nonmetallic or other nonmagnetic sheath, where run in different raceways, auxiliary gutters, cable trays, trenches, cables, or cords, shall comply with the provisions of 300.20(B). Conductors in single-conductor Type MI cable with a nonmagnetic sheath shall comply with the provisions of 332.31. Conductors of single-conductor Type MC cable with a nonmagnetic sheath shall comply with the provisions of 330.31, 330.116, and 300.20(B).

This isn't Type MI or Type MC, so the last two sentences don't apply. But the first sentence seems to leave things wide-open as long as everything is nonmetallic. It doesn't read very well, but it seems to say that the conductors can be in separate conduits as long as everything is nonmetallic. It's restricted by 300.20(B), but that doesn't seem to hinder me any:

300.20(B) Individual conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through with the individual conductors pass or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit.

Since everything here is non-metallic, I would be in compliance with 300.20(B). So everything here hinges on the reading of the first sentence of 300.3(B)(3). 300.3(B)(3) is listed as an exception to the requirement in 300.3(B) that all conductors be in the same conduit, and the first sentence of 300.3(B)(3) appears to permit my proposed solution, and my proposed solution complies with 300.20(B) since it never passes through anything metal.

Thoughts? Again, we're talking about 4 inches of conduit here, and I'm struggling to comply with both the electric code and the building code (can't structurally compromise the load-bearing wall).

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    Are these before or after your first overcurrent protection device? I.e. Are they between the meter and the main breaker? – Harper Nov 11 '18 at 18:14
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    These are after the first overcurrent protection device. They are between the main breaker (400A) and one of two load centers (200A). – Nick Williams Nov 11 '18 at 18:30
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    Relevant thing in my mind is that a 400A breaker is ill equipped to protect 3/0 wire. The 200A breaker protects it, but it's in the wrong place to do so. I would at least go metal with those boxes and conduit, notching be damned, it gives you more of a fighting chance to provoke a trip out of that big 400A and avert a fire. It's also much better at dissipating heat. – Harper Nov 11 '18 at 18:40
  • @NickWilliams -- what make and model is your service-entrance device? – ThreePhaseEel Nov 11 '18 at 18:46
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    @NickWilliams -- also, is this a single stud or a doubled (sistered) stud? – ThreePhaseEel Nov 11 '18 at 18:47
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You don't need to make your single conduit nipple that fat

A 2" nominal nipple is overkill here -- a 1.25" nominal Schedule 40 PVC nipple provides 561mm2 of usable fill since the 60% nipple fill rule (NEC Chapter 9, Note 4) applies to your circumstance as your nipple is far shorter than the 24" limit on this rule, while your four conductors add up to just under 533mm2 of area using Table 5 dimensions for the THHNs and a bare 2AWG copper wire for the ground (this saves a bit on fill compared to using an insulated THHN for the ground wire).

Given that the OD of 1.25" nominal schedule 40 PVC is 1.66" as per Cresline's specifications, this fits within the 2.1" maximum bore allowed for a doubled load-bearing 2x4 (3.5" wide) stud by the 60% rule in IRC R602.6. If doubling the stud is not an option, then the alternative is to use a high-strength (tensile capable/wraparound) stud shoe such as a Simpson HSS2-SDS1.5 with the full complement of specified fasteners to reinforce the bored area.

If you want to use RMC instead

If, for one reason or another, you want to use RMC or EMT with bonding bushings instead of PVC here, a 1.25" nipple will suffice as well, given that you only need a bit under 500mm2 of fill as the nipple becomes the grounding conductor in that case, whereas a RMC or EMT nipple provides over 590mm2.

  • Oh, thank you! I knew about the 24-inch exception to the number of conductors derating, but I totally missed the 24-inch exception that lets you apply 60% fill instead of 40% fill. – Nick Williams Nov 11 '18 at 22:45
  • On that subject, are threaded PVC nipples even a thing? I've never seen them before, and a cursory search only turns up metallic nipples. I'll use metallic and bond it if I must, but I was hoping to stick to PVC. (Adding male adapters to a cut length of PVC would, in my specific situation, mean needing to bore a bigger hole in the stud.) – Nick Williams Nov 11 '18 at 23:06
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    @NickWilliams -- I don't believe they are, but you could extend the nipple into the box and have the male adapters on the inside :P – ThreePhaseEel Nov 11 '18 at 23:09
  • A fascinating idea. I'm sure the inspector would raise an eyebrow, but I don't see how he could actually fault it. OTOH, I wonder if I could just thread the ends of a length of PVC conduit with a threading machine in the same manner I would thread the ends of a rigid conduit. (It seems odd to me that PVC water pipe nipples are a thing, but not PVC conduit nipples.) – Nick Williams Nov 11 '18 at 23:15
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    @NickWilliams -- threading PVC is a bit of a no-mans-land (it's allowed by the manufacturers for Sch80, but that would bump you to 1.5" nominal conduit in order to get sufficient fill) – ThreePhaseEel Nov 11 '18 at 23:56

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