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I've just added a 100A subpanel to a house with a 200A main panel, and would like to know if I did everything correctly and up to code before getting it inspected (I'm in WA state USA -- 2020 NEC).

What I installed

photograph of subpanel

It's a Square D HOM2448L125PQGC 125A 24-48 panel with main lugs (but no main breaker). IIUC, you don't need an explicit disconnect (whether main breaker or otherwise) on a subpanel if it's right next to the main panel that feeds it.

Per #3 / #5 THHN and 1-1/4" metal flex conduit for feeding 100A subpanel?, I used 1-1-1-3 AL SER for the feeder.

Wiring the subpanel

I drilled 1 1/4" holes through the studs (1.5" from the edge) for the SER to go to the correct stud bay. This part of the garage is unfinished and it's unlikely that drywall will be added in the future.

I used 1 1/4" knockouts and NM cable clamps + bushings on both panels for the feeder. I didn't staple the SER cable since it wasn't close to any of the vertical studs, but I did tighten the clamps enough to provide strain relief. I used Noalox on the lugs and the tips of the stripped Al conductors before tightening.

Most importantly, I used a torque screwdriver to tighten all lugs to the spec (50 in-lb) per the breaker and bus bars.

Grounding

I installed one of the provided ground bars on the right of the subpanel. Since this is a subpanel, I didn't attach the screw that bonds the neutral bus to the panel / grounding system.

I didn't install any additional ground rods since it's right next to the main panel, so the subpanel's ground bar is only connected to the main panel's neutral / ground bars (which are bonded there).

subpanel closeup photograph

Moving circuits

I also moved two 15A 120V AFCI circuits to the subpanel to make space for the 100A subpanel breaker. I used 2x 14/2 runs of Romex, with 1/2" knockouts and NM clamps on both panels but no bushing (there wasn't enough thread after adding the locknut to securely attach the bushing). IIUC, bushings are optional for that small size.

Load calculation

I did a load calculation using the Mike Holt app (which uses the NEC optional method), and got a total load of 122A (requiring a 125A breaker). The house currently has 200A service. Happy to share details if relevant.

I'm not sure if the 100A subpanel counts towards the actual load calculation, since it's not really adding any load itself. I used a 100A breaker because it could take 1 AWG wires and seemed like a nice round number here. But let me know if I'm mistaken and need to change the subpanel breaker size.

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  • You don't need new load calcs for what you've done so far. You need them for your envisioned end state, once you fill up the new panel with whatever you plan for it. One calc for the new panel and one for the entire service including the new panel. Do that now so you can balance things correctly as you install or move them. You''ll avoid putting too many large new loads in the subpanel, which will cause it to reach capacity without taking up many spaces. Then you've go no usable space in either panel.
    – jay613
    Jul 15 at 21:40
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    While it may be unlikely that drywall will be added soon, someone will want to add some at some point. It would be an added bonus to put protective plates on the studs in front of every stud penetration (both electrical and plumbing). As cheap as those things are, it's just not worth it (IMHO) not to put them on.
    – FreeMan
    Jul 15 at 21:41
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    Too late now, but you could have just put the sub-panel lugs on the bottom and saved the whole trip around the inside of the panel to get to the top. But no point in changing it now. The inverted "Line" text confirms that this panel can be top or bottom fed.
    – Ecnerwal
    Jul 15 at 21:43

1 Answer 1

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TLDR: Looks great. The only thing I'm not a fan of is the 100A breaker being directly opposite another large 240V breaker. I understand the temptation to crowd the biggest breakers near the main so they have the least distance to travel on the bus, but buses are overbuilt, and the bigger risk is overloading the thin little bus stabs. If the opposite breaker is 20A I wouldn't worry about it, but larger and I would maybe swap that 240V breaker with two breakers below it.

Now if that breaker to the right is a generator interlock, or a surge suppressor, disregard what I just said - the surge doesn't take any amps (for any amount of time long enough to matter thermally).

IIUC, you don't need an explicit disconnect (whether main breaker or otherwise) on a subpanel if it's right next to the main panel that feeds it.... I used 1-1-1-3 AL SER for the feeder.

Correct... and correct! Common error avoided!

The subpanel wiring looks fine however I'd put a panel over it to provide physical protection.

I used a torque screwdriver to tighten all lugs to the spec (50 in-lb) per the breaker and bus bars.

GOOD! (though this won't be inspected). Make sure you actually checked the spec on each thing (neutral bar, lugs, breaker) as they might differ.

I installed one of the provided ground bars on the right of the subpanel.

Wait, HOMeline provided accessory ground bars for free?? Must be a counterfeit LOL... no just kidding. That's fine. Yes, either ground bar will carry ground to the other via the panel steel.

I also moved two 15A 120V AFCI circuits to the subpanel to make space for the 100A subpanel breaker.

OK. The circuit ground can land on the ground bar in the main panel. Only hot and neutral need to come through. Using an AFCI is a clever trick, since it "keeps you honest" - there's nothing really to inspect there.

I did a load calculation using the Mike Holt app (which uses the NEC optional method), and got a total load of 122A (requiring a 125A breaker).

There are many horrible sources of info for Load Calculations (largely: clickbait) but Mike Holt is excellent. Nice job sorting wheat from chaff!

122A is unsurprising for a "typical" all-electric house with gas furnace. Once you bring in electric heat, pool, hot tub, or "novice, range-anxiety-driven" sized EV charging, all bets are off. Of course modern extended range heat pumps that don't need emergency heat, tend to disappear from the Load Calc since they overlap with A/C.

Oh, and EV charging doesn't need to take any space out of the Load Calc at all, since it's an inherently 21st century load, and they have several clever methods to make it invisible to the Load Calc.

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  • Thanks for the quick feedback! I put the 100A breaker at the top mainly to leave enough slack for future installs without having to bend the conductors in the already cramped panel. If I were to move it lower, are there any tips and tricks on how to leave slack without having to bend thick conductors?
    – peter
    Jul 15 at 20:54
  • Also, do you have any thoughts on the load calc part? Is this relevant at all if I'm adding a subpanel with no new loads?
    – peter
    Jul 15 at 20:55
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    Panel spaces are 100% irrelevant to the load calculation. Only actual loads count.
    – nobody
    Jul 15 at 20:57
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    @peter I would just move the smaller circuit opposite it downward, and move 15/20A circuits up. Yeah I forgot about the Load Calc.
    – Harper - Reinstate Monica
    Jul 15 at 21:19

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