I'm getting solar installed with some powerwalls. As it stands I'm going to have my entire house backed up.

However, I plan to add two EV chargers in the future and I don't want these backed up.

The solar contractor told me this:

Main has to have no other breaker except Gateway supply breaker and main breaker to comply with 705.12.(D)(2)(3)(c)

After doing my own research, that makes sense to me.

They told me I would have to upgrade to a 400A main panel so I can feed the existing 200A gateway/sub and then a second 200A sub for my EV chargers.

After running some numbers (and just looking at what is here today) I don't think I need anywhere near 400A of power for my property. That's A LOT of power.

It's not a massive 4000sqft mansion with 10 AC units, it's 2000 sqft with a single 5ton AC and a pool and a few gas applicances.

We currently have a main with a 100A breaker and all of our loads work fine.

Even if we do decide to add 2 EV chargers we would be at 180A max, but we wouldn't have everything on while the 2 EV chargers are in use.

In my mind 200A should be just fine, am I missing something?

This was quoted as an additional $2-3k

It may even just be cheaper to just get a second meter installed for just the cars, the SCE site estimates that at $1500.

My question is, do I need to upgrade to 400A service to add my EV chargers?

I'm located in Southern California on SCE.

Link to proposed electrical plans

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Niall C.
    Apr 2, 2018 at 15:00
  • 1
    Can you talk to your AHJ about how they apply 705.12(D)(2)(3) and edit your post with that information?
    – Hari
    Apr 2, 2018 at 18:18
  • Can you fix the contradiction in your plans please? Right now, the notation by the new main panel's busbar says it has a 200A bus, but the note above and to the right of it says it has a 225A bus, and the difference matters here... Apr 27, 2018 at 22:47

2 Answers 2


Short answer is "no"

There are a couple ways of fixing this short of going to 400A busbars in the main panel, which are not exactly the cheapest thing in the world. Read on for options, although you'll need to discuss both with your AHJ and the latter option (using the secondary disconnect on a suitably equipped meter-main for the EVs) with your utility as well.

One other caveat with most of this is you're best off with 32A EV chargers (vs 40A) to avoid starving the house for power too much -- luckily, 32A@240VAC is a standard circuit size in most of the rest of the world, so you can get EV chargers that size fairly readily.

Easy fix -- downgrade the 200A feeder breaker in the main panel to 125A

Given the rules of NEC 705.12(D)(2)(3)(c) and that the total overcurrent protection for the planned EV chargers won't exceed 100A, the easiest solution to this problem is to downgrade the main panel's feeder breaker to 125A, which brings you back into compliance with the rules. This does mean that if both the Powerwalls are charging at their full 30A rate, your solar system isn't helping at all, and you're pulling significant load (more than 65A) on the house side, you could trip the feeder breaker.

If you are OK with starving the EV chargers for current somewhat, or your AHJ is willing to apply 240.4(B) rules to this situation, then you can use a 150A feeder breaker, but that limits you to 70A (without 240.4(B)) or 80A (with 240.4(B)) maximum breakering for the EV chargers, taken together. (If you could put a 75A breaker in, that'd work, but 75A is not a standard breaker size.)

If that's not an acceptable state of affairs...

If the above situation is not serviceable, then there is another option, but it involves replacing your meter socket with a specific meter main (or meter breaker), which may or may not be more costly than the 400A panel. In particular, the Eaton/Cooper 2M2RP provides a 200A continuous socket with two load taps: one for a 200A maximum main breaker, and a second provision for an auxiliary service disconnect. However, the lugs on the meter socket are limited to 125% of 200A (their continuous rating), or a 250A maximum, so the disconnects must sum to that (vs. the 225A maximum rating on your proposed main panelboard's busbars).

This means that you can use a maximum of a 175A main breaker in the unit (downbreakering it from the shipped 200A), and the auxiliary disconnect paired with that can be a maximum of 75A as a result; however, 75A isn't a standard breaker rating, so we can use an 80A breaker instead as per 240.4(B) (it's a feeder, we're protecting a set of lugs, and we're below the 800A maximum for the rule). Given that the 2M2RP takes Siemens breakers (for historical reasons), and that Siemens has replaced the QJ line with the backwards-compatible QR, we need a QR22B175 for the main, and the provision for the secondary main can be left unpopulated for now. When the EV chargers are installed in this scenario, a Q280 is inserted in that secondary main slot, and then a feeder is run from it to a main lug panel with the EV-charger branch breakers in it.

If your AHJ or utility rejects the application of 240.4(B) here, one can use a QR22B150 for the main and a Q2100 for the secondary, instead -- this does mean that pulling full power from the house while the Powerwalls are charging at full rate and the solar system is outputting nothing puts you in main-breaker-trip land, but that is quite unlikely. Another option would be to use a QR22B175 for the main and a Q270 for the secondary, but that starves the EV chargers for current ever-so-slightly.

  • I may be poking Eaton support here in the near future anyway, so if I do, I'll ask about the use of the 2M2RP here (whether there's an amp limit on the secondary main beyond the 125% rule and the frame size, as well as whether tandem/quadruplex breakers can be used in that slot) Apr 23, 2018 at 11:48
  • 2
    I contacted Eaton support -- it is OK to go over 50A in the secondary disconnect spot on the 2M2RP, but they'll have to get back to me on using a quadplex in the secondary disconnect space. May 29, 2018 at 22:36
  • Just got a reply back from Eaton support on the quadplex -- and nope! This simply means you need to use a feeder-and-subpanel for the EV chargers, though, which isn't that big a deal. May 31, 2018 at 18:51

400A houses are very commonly wired simply with dual 200A main panels, 2x your basic $150 panel from Lowes. Dual-lugged 400A meter bases are specifically made for that purpose. There's not even a requirement the panels be of equal size, the panels could be 225A and 150A respectively. There is a requirement both panels be very close to the meter.

Just for reference, the typical setup is a snowbelt development estate with no gas lines even laid. Ranch house, 2500sf, lots of surface area, 1970s tier insulation, heat pump, with dual 70A of emergency heat, which runs very hard whenever winter temps are below heat pump minimums. The emergency heat alone occupies 175A of one panel's capacity due to derate. They are both main panels. Main breakers; neutral-ground bond.

The only issue is that news of this configuration may not have reached the ears of Southern California Edison, most definitely not being in the Snowbelt, so not having a lot of call for 140 amps of emergency heat.

Anyway, back to your problem. Imagine if you already had the reasonable, safe and proven dual 200A setup as above. And imagine you remove one 200A main panel and replace it with a 200A breaker unit. From that continue with your drawing as drawn starting at the backup gateway. Totally separate from your drawing, the other 200A panel would still be there. Entirely outside the gateway. Entirely outside the scope of the hybrid system.

There'd be no question of this being legal if you had 2 meters each feeding a 200A panel, as might be in a duplex. The fact of a common meter doesn't change anything.

As such I am disappointed in your solar provider "pooh-pooh"ing your 400A idea and telling you that you need expensive 400A exotica. This works fine.

  • Transfer for a hybrid solar system like the OP's is single-throw not double-throw...(either integral to a solar/battery inverter/charger such as with Outback Radians, or in its own box as in the Backup Gateway depicted in the OP's diagram) May 30, 2018 at 0:37
  • @ThreePhaseEel I am betting on scope, saying the scope of the hybrid system is only the one panel, and the other panel is mains only and not in need of an interlock. Does that help? And really my answer is to address all the handwringing about oh-so-expensive 400A panels. Since you want two panels anyway... May 30, 2018 at 0:53
  • Take another look at the diagram linked in the question -- the Backup Gateway is the interlock (transfer means). (It is closed when there is mains on the upstream end and open otherwise, which is all it needs to do given the topology of a Powerwall based system.) May 30, 2018 at 1:08
  • @ThreePhaseEel I see what you're saying. The drawing would substitute for one of the main panels I mentioned. By the way, is it me, or is that drawing really kind of ... overwrought? Is there a reason the 2 "subs" shouldn't be combined into one sub? May 30, 2018 at 1:36
  • Correction -- the reason they used the extra subpanel there is to avoid weirdness involving 705.12(D)(2)(3)(a) and/or (D)(2)(3)(c) -- instead, 705.12(D)(2)(1)(b) can be applied to the situation with the feeder tap+second subpanel (I suspect 705.12(D)(2)(2) is inapplicable because the tap conductors are feeding the inverter(s)) May 30, 2018 at 22:44

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