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Pardon my uneducated question, new to the codes...in short I want to add a subpanel, along an existing one. Here is the setup:

  • There is a 200A service with a main service breaker and branch-breakers panel on the outside of the house. It is located on the garage external wall.
  • A single 100A subpanel is feed across the garage by a multi-conductor cable (looks like SE cable AWG-2 3+1 AL). It's protected via a 100A breaker in the main panel.
  • There is no main breaker in the subpanel, just branch brakers.
  • The feeder cable runs across the garage ceiling to the subpanel through a drywalled "duct" ~3ftx1ft on the ceiling of the garage (there are other cables and AC air ducts there). About 35ft run.

I want to add a second 100A sub panel, in exactly the same location, side by side w/ the first one.

  1. What cable/wiring type can I use to feed it?
  2. Do I need a main breaker in the subpanel if I have a breaker in the main panel? It's all one "building".
  3. I would have a back entry to the main panel in a 2x4 wall - how to make the "turn"? (the existing feeder cable is just bent and enters via 1" adaptor).
  4. I have a 1/0-1/0-1/0-2 Gray Stranded AL SER cable I could use (prob overkill for 100A but I have it). Is it up to code to run it along existing feeder via this walled ceiling "duct"? On a 100A breaker?

Thanks, Michal

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    Why do you want another subpanel in the first place? If it's because the first subpanel is full, why not just get a bigger 100 amp panel? – whatsisname May 9 '20 at 3:34
  • Good question :-) Yea, out of space (need to add 3x 20A for heat pumps) and I think the feeder cable is not adequate - when A/C turns on (2x60A breakers) the lights in whole house dim substantially. Also - seems that's a lot of work to rewire it and since I already have 200A service...? I thought installing the panel and the feeder cable would be easier. This is still ok with NEC, I hope? – Michal May 9 '20 at 23:12
  • Did you learn your lesson about too-small panels? :) Also, what other markings are on the SE-R cable besides SE-R? – Harper - Reinstate Monica May 10 '20 at 14:23
  • Cable Markings: ALUMAFLEX AA8178 TYPE SE STYLE SER TYPE XHHW-2 CDR S 600V – Michal May 11 '20 at 15:47
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Garages are just the kind of place where you go through panel spaces like, as ThreePhaseEel says, a teenager through Mountain Dew. 240V power tool, boom, 2 spaces. EVSE, boom, 2 spaces. Double-stuff breakers aren't even an option because everything needs GFCI and/or AFCI (as of NEC 2020). I presume you are once bitten twice shy, and are eyeing a nice 30-space panel so you'll never run out of spaces again.

We're all for saving money. But panels are the worst place to chintz out.

Cable/wiring -- you can use NM-B cable, UF-B cable (under NM-B rules), individual conductors in conduit (cable in conduit is legal if your conduit is oversize enough, but usually so hard to pull that you end up calling an electrician). You cannot use USE cable, but most USE is multi-listed as other types that are legal indoors, so you're all set.

Do you need a main breaker in the sub if the main panel has a breaker? Indoors. Well first, the breaker in the main panel is mandatory to protect the cable run. But yes, since you're in the same building, no main disconnect is required at the sub. (in outbuildings, you don't need a main breaker, but you do need a main disconnect, and that's the cheapest way to do that).

Can I use 1/0 Al for 100A? Yeah, you can use it for 125A actually. The only gotcha with 125A fusing is that most panels have 125A ratings on their bus stabs, which includes both this space, and the space across from it. So if you have a 100A across from a 15-15 duplex, that's 130A and that's too much. A 100A can only be across from 25A breakers max. A 125A can be across from nothing, so you need empty spaces there.

What you call 2 breakers is actually a 2-pole breaker with handle ties. It occupies 2 spaces in 2 rows. So you are concerned with the loads on each row.

Do they care how many subs I have? Nope. They care about the ampacity of the loads you have provisioned into each subpanel, as determined by relevant formulas. They also care about the ampacity of all the provisioned loads being served by the main panel. But I trust you won't be exceeding that.

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  • Thanks for hints! So there should be "main disconnect" in the subpanel or is the breaker in main panel considered as such? The orinigal panel does not have anything there but it's prob 20+ years old. >> A 100A can only be across from 25A breakers max. A 125A can be across from nothing, so you need empty spaces there. That interesting point. The main panel has only one-side breakers (there is no "across") with planty of spaces left (there is really only one breaker of 100A there for the existing subpanel). Not sure what's the main rated at. – Michal May 11 '20 at 15:59
  • You don't need a sub panel disconnect because it is in the same building. A main lug sub will suffice. If it's a single row panel, then search the panel labeling for anything about stab limits, otherwise I would assume 125A. Yes you can have a 100A and 125A and even some more breakers in a 200A panel :) – Harper - Reinstate Monica May 11 '20 at 16:06
  • I am reading there is 6-to-one-hand rule for subpanels. That is if the subpanel has 6 or less breakers then it does not need a disconnect. Although, I can find only main breakers rather then just disconnect switches, but that's fine at $60 for QO panels. – Michal May 11 '20 at 17:04
  • @Michal I have a subpanel with 15 breakers and no disconnect; it's fine because it's in the same building. My understanding is the "Rule of Six" is obsoleted for consumer panels, though I'm not 110% sure on that: if it was in an outbuilding, and had 2 breakers, it would need a disconnect. – Harper - Reinstate Monica May 11 '20 at 17:44

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