Based on this post on installing an EV charger, I decided to research installing a sub-panel next to an existing surface mounted indoor sub (Square D HOM2040L125PRB), which is used for Powerwall backup loads. The current sub-panel has all the loads connected through cables feeding into a 2.5" conduit nipple on the back of both the sub and main (Eaton BR) panels. I did a conduit fill calc and adding 3 new #2/0 wires and 1 #1 wire would bring it to 29%. For the angle pull the distance between the raceways in the current sub is about 20", the box depth of the main panel is 4".


  • Square D HOM3060L225PC (Homeline, 1 phase, 30 spaces, 60 circuits, 225A convertible main lugs, PoN)
  • 10' Southwire Aluminum SER cable 2/0-2/0-2/0-1
  • 125 AMP Eaton BR breaker
  • Bushings, clamps, fittings, etc.

My plan is to surface mount the panel next to the current sub and use a 2" nipple to connect them together. Strip the jacket off the SER cable and run each wire into the main panel through the Powerwall sub-panel. Terminate the wires and torque according to spec.

Was wondering if this was a solid plan and what "gotchas" or other complexities I can prepare for. The Powerwall sub has a jacketed ground wire from the main panel into the Gateway and then back to the sub-panel. From there another ground wire is terminated at my home's Ufer ground.


  • The main panel is pretty crowded with all the splices from the Powerwall install. Getting to the ground bar basically requires pulling out all the wires so it can be accessed.
  • If I'm using metal conduit would I still need to run a wire for ground? The ground wire in the SER cable is bare so I'm not certain if I can use that, just need to confirm it.
  • Any experience with using a Tesla installed (or other solar battery provider) sub and having issues with using it as a feed-through? I'm not adding any load to the sub so I don't expect any issues, if anything I'll be removing some loads to the new sub-panel.

Any advice appreciated. Thanks!

Conduit view from sub-panel

View from main

Main splice crowding

Diagram showing a side and front view of the panels

  • 1
    You'll need to redo the fill calcs for the existing nipple... Also, can you provide a diagram of your proposed setup? It's complex enough the text isn't quite doing it justice... Commented Sep 1, 2023 at 1:47
  • 1
    You can't "strip the jacket off the SER cable" and use the conductors alone, as the conductors are (in every cable I've ever seen, even though it COULD be done differently) not, themselves, properly marked. You need to buy THHN or XHHW type WIRES that are marked, or you need cable jacket (where the markings are) around the conductors at least 6mm (IIRC) into each box. Realistically I think you need enough jacket for the markings to be intact, somewhere.
    – Ecnerwal
    Commented Sep 1, 2023 at 3:56
  • @ThreePhaseEel, I added a quick diagram of front and side views of the panels. Sorry yes, the fill calc I mention in my post is from the existing conduit, which I did from scratch by manually counting up the wires going through it. Commented Sep 1, 2023 at 5:50
  • @Ecnerwal, my main driver for attempting to use Aluminum SER cable is that I can get it cut by the foot; other types of cable including the ones you mention are not available or accessible so easily where I'm located. Copper seems to be more readily available but the price jump is pretty drastic. It's a short enough run I might just have to bite the bullet. Commented Sep 1, 2023 at 5:55
  • 2
    So, leave the SER jacket in place. Your nipples are less than 24" long, so they can use the nipple fill rules, and you can pass through the existing sub-panel with the jacket (and markings) fully intact, stripping the jacket only in the main and new sub.
    – Ecnerwal
    Commented Sep 1, 2023 at 13:31

1 Answer 1


Answering my own question with details on the installation such as changes I made and pictures with the different steps. Some changes from the original plan were to use 2/0 XHHW wire instead of SER and instead of going right of the battery backup panel I went left since it would give me more room for future circuit runs.

To get the sub mounted on a sturdy surface I put up some 3/4" RTD sheeting with a coat of primer on it. Once the board was up getting the panel mounted and level was quick. Sub Mounted on plywood Side view of sub

For running the wires I shutdown the solar equipment and flipped the main breaker. In the main panel I put the 125A breaker higher up the panel so it wouldn't be next to the 90A solar breaker, this gave me more room to work with the 0/2 wire and I was also worried about part of the bus bar running extremely hot. It was difficult working with the spaghetti mess that was left by the solar installers. I basically had to pull it all out then bend and push-in the new wires and put all of the messy splices back.

Bundle of wires and breakers Wires wedged in

There was no room for the neutral on the main ground bus so I tapped a lug onto the back of the panel above the current ground bar. Neutral terminated on lug

Then finally got the wires cut and terminated to the new panel. New wired sub

For cutting the wires I bought a ratcheting cable cutter I found on Amazon, which worked really well. I used a torque wrench to secure the wires to breakers, lugs, etc. to the various specifications and also applied Noalox. I plan on opening up the panels soon to check the work and see if there are any glaring issues.

Additional Parts List:

  • 30' 2/0 XHHW
  • 10' 1 XHHW - Ground
  • 2" rigid offset nipple + (lock-nuts, bushing)
  • Eaton neutral lug (for main panel)
  • Ratcheting cable cutter
  • Noalox

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