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Merry Christmas!

After lengthy research, still looking for some input.

  • New sub panel in basement. Thinking Square D QO Plug-on neutral. Neutral and ground isolated, adding 2 ground bus bars - bonded to case. (HOM2448M100PC)
  • Basement sub panel fed from and directly below 150a sub panel in garage, about 15 feet. Garage is dry-walled but not finished yet and would be easy to cut, basement not yet dry-walled.
  • Thinking to fuse it at 90a @ garage sub-panel
  • Looking at 310.15(b)(16) @ 75 degree Celsius column, given that all terminals (and wiring) are rated for 75'C (is the basement panel? are the breakers?)

Question 1: Am I correct by looking at that table and column. And if so, can I use 2-2-2-4 AL SER with no conduit from garage panel to basement panel?

Question 2: Can I place basement panel in framing between a finished and unfinished space? (i.e. inside a utility room between wall of living space). 2x4's and back of panel would butt up to drywall of living space.This wall is not insulated.

I am in Colorado.

  • What make and model is the existing panel? Also, I take it the wall the existing panel is in is a 2x4 wall as well? – ThreePhaseEel Dec 24 '19 at 18:40
  • Hello, and welcome to Home Improvement. It is preferred if you can post separate questions instead of combining your questions into one. That way, it helps the people answering your question and also others hunting for at least one of your questions. Thanks! – Daniel Griscom Dec 24 '19 at 18:54
  • See 110.14.c.1 below 1 awg or 100 amp the 60 degree table is used, if your load is non continuous #2 would be fine. I am not sure if I understand the finished and unfinished description but the new panel can be in an unfinished area if the wires enter in conduit, or the areas the wires are run is boxed in with 1/2” plywood or Sheetrock Covering the exposed wires below 8’ – Ed Beal Dec 24 '19 at 19:11
  • @EdBeal -- 110.14(C) point 3 means that the listed/labeled termination rating governs for 99.9% of the gear we see in The Real World – ThreePhaseEel Dec 24 '19 at 19:12
  • Does the existing subpanel have thru lugs, by any chance? A feed breaker may not be necessary. – Harper - Reinstate Monica Dec 24 '19 at 19:28
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I'd upsize to 125A if I were in your shoes

Given the short length of your wire run (20-odd feet of SER should be sufficient), there's really not much reason to penny-pinch by using an odd-sized feeder; as a result, I'd go up to 125A as larger two-pole breakers require 4 spaces in the feeding panel instead of 2, and you won't be able to make full use of a 225A feeder and subpanel anyway, which is the next size larger as far as main lug panels go.

This means that you'll be using 1/0-1/0-1/0-2 aluminum SER cable instead of the 2-2-2-4 you were originally planning on, and a 125A feeder breaker instead of the (admittedly odd sized) 90A feeder breaker in your original plan. If conduit is called for (which isn't likely, given that the chances of something between studs being considered subject to physical damage are rather low), then you can use a 1.5" EMT with 3 1/0AWG aluminum wires inside it instead. In any case, the 75° column in the ampacity tables is what governs for feeders in realistic work, see 110.14(C) point 3 for details.

If you want to use a larger conduit, by the way, that is fine; IRC R602.6.1 would require strapping for a bore larger than 1.75" (50% of the 3.5" actual width of a 2x4 top plate) through the top plate of a load-bearing wall, but even that is not needed for a non-load-bearing partition.

You have the wrong panel picked out anyway

Given that your subpanel is in the same building as the panel feeding it, there is no need for your new subpanel to have a main breaker; furthermore, it's a Homeline, not a QO, so you'll have to keep that in mind when populating it with breakers since the two breaker types are very much not interchangeable!

With all of the above taken into account, and going for as many spaces as we can so you don't have to rip it out and replace it later, we can use a HOM3060L125PGC if you can find it, or a HOM2448L125PGC if the former is not available. Both of these panels support the full 125A capacity of the feeder I am suggesting, have main lugs (since a main breaker is not needed here), and come with factory-fitted ground bars (obviating the need to buy ground bar kits and install them).

Having a panel flush-mounted into an unfinished wall is fine, even if it looks odd

There is no Code issue with flush-mounting a panel into an unfinished or partially-finished wall; just keep in mind that it may be necessary to unmount and remount the panel when the wall is finished so that the outer portion of the panel cover fits correctly over the drywall. (This weighs against using conduit for the feeder, BTW.)

Last but not least: TORQUE ALL LUGS TO SPEC

There is one other thing you'll need to take into account, and that is that the set-screw type mechanical lugs used on circuit breakers and loadcenters need to be torqued to the values labeled on the breaker or loadcenter for that size wire in order to perform reliably over the long-haul. This was added to Code as a requirement with 110.4(D) in the 2017 NEC; even if your AHJ has not adopted the 2017 edition, using an inch-pound torque wrench and/or torque screwdriver to tighten the lugs to their specification torques is a good idea anyway, lest your electrical system lose you the race!

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  • Thank you for pointing out the panel. I will probably upsize and get the HOM3060L125PC (readily available). The panel in the garage is HOM3060L225PGC so that works even better. I could fuse that new panel with 4 slot 125a and use 1/0-1/0-1/0-2 AL SER, but still confused with the amp rating - Celsius columns. The 75'C states that 1/0 AL conductors are rated for 120a and 100a in the 60'C column? – NoCO DYI Dec 24 '19 at 19:51
  • I have torque wrenches, when installing I will make sure this is according to specs. – NoCO DYI Dec 24 '19 at 19:53
  • @NoCODYI -- 240.4(B) lets you round up to the next higher standard breaker size from the feeder's actual ampacity, so a 125A breaker is the correct breaker to use with 1/0 Al wire running at 75degC – ThreePhaseEel Dec 25 '19 at 0:15
  • @NoCODYI also, make sure the torque tools you are planning to use are calibrated in inch-pounds (most foot-pound torque wrenches are rather inaccurate at the low end of their range, and go up far too high for electrical work) – ThreePhaseEel Dec 25 '19 at 0:16
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I'd bump to 150A so you can skip both breakers

Your current plan is paying for two expensive breakers you do not need - the 90A supply breaker and the 100A "main" in the subpanel.

My suggestion is the existing subpanel is already protected by a 150A breaker at wherever it's supplied from, yes? Therefore, that breaker will protect anything we do that is 150A rated. By bumping wire and panel to 150A, we are shaded by that breaker's protection and don't need any new breakers at all.

So simply continue onward with the same type of cable that's coming into this subpanel, 4-wire of course, presumably 2/0 Al.

I'd pick a main lug panel with a 150A+ bus, like a HOM3060L225PGPVP, S3040L1200P if you foresee a future generator, or a TLM3220CCU1K if you really like bonus breakers.

And we simply tap or chain off the feed to the existing subpanel.

  • If the existing subpanel happens to have feed-thru-lugs, we tap off those.
  • Or, use normal 3-void Polaris connectors to "tee" the power fed coming in so that you can add the additional wires.

  • Or, get a double-lugging kit for the existing panel (Square D calls these "sub-feed lugs"), which converts the single supply lug to a double. (again the main breaker isn't needed in the existing subpanel, so the double-lug kit can drop in its place; this in reverse).

  • Or, fit a "circuit breaker mainlug kit" which fits in like 4 breakers, but gives a set of lugs instead of actual breakers. (these are prohibitive on HOM, if your existing panel is HOM).

Remember, the existing subpanel doesn't need a main breaker either, because there is another breaker upstream of it, so you tap before the existing subpanel's main breaker.

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