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I wired two circuits for dryer and water heater. I installed an electrical box on the wall right above the electrical panel, fished NM 10/3 and NM 10/2 through the wall, and run 5 THHN #10 wires in a 1/2'' EMT.

Because the wall is between the house and the garage I wanted to avoid making a big opening, installing the box flush to the wall and having a box extension for the EMT conduit. So I decided to install the box on top of the wall, run the NM cables in two holes and fire block just the two holes.

But I forgot to do a box fill calculation so I had to install a box extension after all... Having 10 conducting wires of gauge 10, NM connectors (this which I think counts as internal and this which should count as external), and 2 grounding wires requires 30 cu in. In retrospect I should had installed a deep square box of 30.5 cu in instead of the current one that is 21 cu in. To satisfy the minimum box fill volume I added an extension box and now I'm at 41 cu in so I'm good. But it looks weird and ugly. Am I violating any code with this setup? I'm in California. Are there any other smaller box extensions I could use that would give me the 9 cu in I'm missing?

If you notice any code violations from the pictures below please let me know.

Box above the electrical panel with two NM cables fished behind the wall and EMT conduit going to the next box:

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Box above existing dryer outlet with NM cable fished behind the wall going to the outlet below and with liquidtight whip attached on the left for the water heater:

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Dryer outlet that replaced a NEMA 10-30R:

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Changed the wiring on the dryer to 4 wires:

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Electrical sub panel: I capped the 2 hots of the previous dryer wiring with a wire nut, installed two Raco the insider NM connectors for the two orange NM cables, replaced the old handle-tied 30A breakers for the dryer with a new GFCI 30A breaker, and replaced two 15A breakers with a 4-pole quad circuit breaker to get 30A for the water heater.

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Electrical sub panel photos before I did any work. I had to move some breakers around to move the tandem breakers at the bottom and also make space for the water heater breaker.

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For completeness, main panel from top to bottom: solar, EVSE, air handler, heat pump, service disconnect, surge protector, power meter for EVSE load management, subpanel, driveway lights and plug.

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  • Hard to tell for certain... in the first pic with the box open, is there a connector in the back of the box to protect the wires as they enter the box?
    – FreeMan
    Jun 30, 2023 at 11:59
  • @FreeMan looks to me like there are bushings in both knockouts on the back (although one looks like it was inserted in the opposite direction, not sure what's up with that).
    – Huesmann
    Jun 30, 2023 at 13:29
  • It is hard to tell, @Huesmann, that's why I asked. If it's there, then all is good, but I wanted to call it to attention.
    – FreeMan
    Jun 30, 2023 at 15:17
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    Yes I installed connectors in the back of the box for the cables entering the box. See comment on the answer below which ones I used. The 3/4'' is inserted from the back while the 1/2'' is inserted from inside. Both of the plastic clamps are in the correct direction.
    – user162793
    Jun 30, 2023 at 16:40
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    As noted, I was just checking. Glad to hear you got it right. TBH, looks like a pretty good install to me (I'm no electrician, but I play one at home).
    – FreeMan
    Jun 30, 2023 at 17:25

2 Answers 2

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But it looks weird and ugly. Am I violating any code with this setup? I'm in California. Are there any other smaller box extensions I could use that would give me the 9 cu in I'm missing?

Nope, that's legit. You can do that a couple more boxes tall if you want to. (there's a limit before you violate 110.12 "Neat and Workmanlike", but it's a judgment call.)

Anyway, your cubic inch problem is why 4-11/16" boxes are my automatic go-to anytime I have the slightest concern with cubic inches. At 42 cubes they can't be beat. Unfortunately it's not really compatible with the big-box store life, since (oddly) they don't specialize in big boxes, and you'll go broke paying their obscene prices. (this is true on a lot of things actually - "Save Big Money" is a shameless lie). I get my 4-11/16" at a friendly electrical supply. And if that's too awkward a name, try "120mm".

If you notice any code violations from the pictures below please let me know.

Right off the bat I see red wires on single-pole breakers in the tandem groups. These are probably Multi-Wire Branch Circuits aka shared-neutral circuits. 2 hots 1 neutral, altogether counts as 1 circuit. The hot wires must be phased very carefully or neutral will be overloaded and tandems encourage tragic mistake. Further, modern code requires MWBCs be handle-tied so that a maintainer is forced to turn off the entire circuit. (the handle-tie requirement tends to enforce correct phasing, since you can't stick a handle tie between the two handles on a tandem).

So I would go ahead and trace those red wires to their cable, identify their partner black wire and trace them back... untangle and tape them together to make it more obvious they are associated, and make sure those two are on a pair of breakers that are handle-tied. A quadplex is an acceptable form of handle-tie (it provides common trip which is overkill for a MWBC with only 120V loads*, but that's fine). Siemens sells appropriate handle-ties to go between tandems.

Where is your main breaker? Because first, I'm really weirded out that a whole house will fit on a 16-space panel, but second, maybe it's a trick of the light, but this panel really, really looks like a Split Bus / Rule of Six panel where the "Main Breaker" is actually the "up to six" breakers in the Main Breaker area. On a 16-space split-bus, those are done as 8 main breaker area spaces and 8 "Lighting Area" spaces (which are fed from a breaker in the Main Breaker area, presumably the 40A). These panels have two problems. First, nothing except the NEC Article 220 Load Calculation prevents overload and nobody ever re-does their 220.82 Load Calculation when they add stuff. And second, if the 40A breaker feeds the lower half, that's way too much stuff for a 40A breaker to support. Maybe I'm mistaken.

Changed the wiring on the dryer to 4 wires:

Make sure you followed the procedure in the manual. What I'm particularly concerned with is removing the neutral-ground bonding jumper. Unplug the dryer and measure resistance between neutral and ground. If it's infinity, you're all set.

I capped the 2 hots of the previous dryer wiring with a wire nut

If the wiring at the dryer end is in a state where it can be reused for a load that doesn't require neutral (e.g. EV charging, heat pump dryer, ??? future load) then that's fine. However if it's destroyed beyond use at the dryer end, it must be destroyed beyond use at the panel end also.




* Am I implying MWBCs can also support 240V loads? Yeah.

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  • Thanks for pointing out that the Multi-Wire Branch Circuits aren't up to modern code. Since I didn't touch them (well I did have to move them a bit around but I don't think this counts) the inspector cannot ask me to bring them up to code right? Regardless I'll research more and independently address it. This is my subpanel. I added more photos of the labels before my reshuffling and I also added photos of my main panel.
    – user162793
    Jun 30, 2023 at 22:26
  • Regarding the dryer wiring, I did remove the neutral-ground bonding jumper per instructions. Thanks for the tip measuring resistance. The previous dryer circuit was abandoned with the dryer end taped inside the wall behind the dryer outlet. I cannot reuse that circuit because of a hidden junction box, see more details at diy.stackexchange.com/questions/272955/… where you helped me before. How do you recommend destroying it at the panel end?
    – user162793
    Jun 30, 2023 at 22:28
  • I realized my photo angle is bad and it's confusing which breaker each wire connects to. My MWBCs are 20A for microwave/fridge and 20A for dishwasher/disposal. They are all on a triplex breaker. Phasing seems fine. Dishwasher/disposal are in the inner/common trip part, while microwave/fridge are in the outer part of the triplex breaker. I only need a handle tie for the outer part of the triplex breaker. I need something like the following i.stack.imgur.com/zoH5K.png but the only ones I can find are common trip. What's the part number I need?
    – user162793
    Jul 2, 2023 at 9:56
  • "(well I did have to move them a bit around but I don't think this counts)" It does if you messed up their phasing!!! But it sounds like you're on top of that. To get legal I'd move the triplex above the 15/20 so you have 20 and 20 adjacent, and get a handle-tie. Alternately talk to a Siemens dealer about the non-common-trip version of the Q220220. They would be the ones for a handle-tie anyway. Trying to shop on the web for electrical equipment is a waste of time. Jul 2, 2023 at 17:15
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The extension box to meet fill volume is code legal. Since you seem to have the clamps outside the box, your fill is only 27.5" there - up to 4 grounds count as one, before you need to start adding 1/4 of wire fill for the 5th and beyond. (NEC 2020 and beyond.) So you have 10 conductors + 2 grounds that count as 1, for a total fill count of 11, or 27.5". If you had internal clamps (which I don't see) you'd need another count for those and be at 30 c.i. If you didn't clamp the NM cables, that would be a violation and need to be corrected.

If you don't like the extension box you can choose a larger box, such as a 4-11/16 x 4-11/16" (at 29.5 c.i. for a 1.5 inch deep box, 42 c.i. for a 2-1/8) or a 4 x 4 x 2-1/8 for 30.5 c.i. - or you could use a blank exposed work cover of at least 6 c.i. capacity (so, a raised cover) to bring a 4 x 4 x 1.5 from 21.5 c.i. up to 27.5 c.i. or more. Most 1/2" raised 4x4 covers are rated 6.5 c.i. so those would get you to 28 c.i. for the box+cover.

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    I had never seen a blank raised cover, but $4 on Amazon for a RACO 804C I think is the winner! Jun 30, 2023 at 14:57
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    They also look a tiny bit nicer than a deeper box and a flat cover, IMHO.
    – Ecnerwal
    Jun 30, 2023 at 14:59
  • For the NM 10/3 I have this lowes.com/pd/… which is considered internal right? For the NM 10/2 I have RACO the insider which is external. Yes for the ground wires I only calculated them as one. By the way, I didn't install a ground pig tail. Instead I attached one of the two 6 inches ground wires with a green screw before twisting them together with a wire nut. Is that acceptable?
    – user162793
    Jun 30, 2023 at 16:21
  • Great to hear the extension box to meet fill volume is code legal. I find it ugly but not ugly enough to redo it with a deeper box. I'm looking for nicer alternatives that don't involve removing the box.
    – user162793
    Jun 30, 2023 at 16:30

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