I recently got a couple of estimates for getting solar panels installed on our roof. (I am in Southern California.)

The first installer recommended that we get 8kW of solar, and he said it would be no problem to install.

The second installer said that the maximum amount of solar we could get is 4kW. He said that if we wanted more than 4kW of solar, this would require upgrading the wires from our house to the electric company (from 100 Amp to 200 Amp) and that doing so would be expensive and would require digging up the street to lay the new wires.

Any idea what the second installer is talking about? I don't understand why that would be needed, because if electricity is flowing from the power company to us, the amperage shouldn't be any more than it is currently. And if electricity is flowing from us to the power company, the amperage shouldn't be more than 33.3 Amps. (8kW at 240V.)

Additional information:

  • Our utility is Southern California Edison. (I called them to ask about this, but they said they know nothing about solar, and that we should consult with the installer.)

  • Both installers said that the breaker panel would need to be upgraded from 100A to 200A. But only the second installer said that we would need to upgrade the connection to the utility (and thus dig up the street). The first installer made it sound like our panel could have 200A internal capacity, and still just have a 100A connection to the utility.

Here is a picture of our panel. (The breaker labeled "SPA" is actually the air conditioner. The previous owners had a spa, but they removed it. This breaker was unused, so when we had central air conditioning installed, we used the breaker for the air conditioning instead.)

breaker panel

Here is the label on the inside of the cover of the breaker panel:

breaker panel label

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    I have not heard of this but I don’t live in Ca any more. You are correct on your calculation I would find someone else to get a quote a modest home usually support at least 5kw but things like trees even power poles affect your setup with micro grids shadows can be canceled out but do affect the overall system performance. Putting a larger service in is nuts.
    – Ed Beal
    Commented Sep 9, 2020 at 22:37
  • Bovine fesces. It has nothing to do with outside residence. Injecting current reduces the actual current outside when it is loaded. AWG 10 might be the lightest gauge, is 1 mohm/ft per wire say for a 1% loss in voltage for 33A at 240V. e.g. for a length of pair, L=2.4V/(0.001 ohm* 33A) / 2wires= 36 ft. Dropping 3 gauges always cuts the voltage loss in half. Cost of wire vs 80W x % power losses may be computed for payback period tradeoff. Commented Sep 9, 2020 at 22:52
  • There is no technical limit to the size of your solar panels. Even 24kW supply peak as long as you load does not exceed the service amp rating which ought to be controllable in any decent controller such as Wuawei’s GTI. Commented Sep 9, 2020 at 23:05
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    Looks like 125A to me. Commented Sep 10, 2020 at 23:48
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    @manassehkatz because OP just updated with a pic of the label. Yeah it's clear none of the solar people bothered to remove the deadfront and Read The Fine Label. tearing up the street, facepalm... Commented Sep 11, 2020 at 14:54

3 Answers 3


You have 125A busses, so you don't need to replace your meter-main just to fit solar

As your loadcenter's labeling states, the busbars in the loadcenter are rated for 125A. Applying the 120% rule from NEC 705.12(B)(3)(d) to your situation as your meter-main uses center-fed bussing instead of the normal end-fed configuration, we can feed 150A of power into the bus from both sources. Given that you have a 100A main breaker to go with your 100A service from SCE, you have room for 80% of 50A, aka 40A or 9.6kW of solar, without having to make major hardware changes.

All the solar installer needs to do in your case, then, is replace the existing oven breaker with a BQC240250, landing the oven wires on the two outer poles and the solar feed-in on the two inner poles.

There are other things that you might wish to have done while someone's working in there

However, there are a few things that would be good to get out of the way while you have someone working in your panel. Mostly, this consists of swapping the existing Challenger breakers for their Eaton BR counterparts (Challenger was tangled in several messes involving substandard electrical hardware, which leads people to cast aspersions on anything with their name on it, but the type C busbars are fine since they're a clone of the Bryant/Westinghouse design).

However, there is one more subtle swap I'd do, simply due to the congestion evident in your panel. There is a BD1520 (Garage Front Lights/Outdoor Outlet Below Panel) and a BR120 (Laundry) near the top of your panel, and with a bit of swapping and consolidation, we can get a quadruplex breaker in there instead as well to provide a place to attach a future subpanel for more loads. In particular, we'd need to replace one of the two A1515s (either Garage/F.A.U. or Plugs+Lights/Alarm) with a BD1520, swapping the Laundry circuit down to the new 20A pole in place of one of the 15A circuits. Then, a wirenut and pigtail can be used to combine the serviceperson's receptacle at the panel with the Garage Front Lights circuit, putting them on one of the 15A poles of a BQC2502115 that replaces the BD1520 and BR120 near the top; the other 15A pole gets used for the 15A circuit that was displaced when we replaced one of the A1515s with the BD1520.

That way, you can have another 50A spare for a future feeder to a subpanel. (You could do more, but you'd have to move circuits to the new subpanel to make room for the feeder breaker. Barring that, you'd need to replace the existing local A/C disconnect with a 50A spa disconnect in order to preserve GFCI protection at the A/C as per the 2020 NEC, then remove the GFCI from the main panel, move the A/C feed to the 50A poles on the new quadruplex, and put a bigger feeder breaker in the hole the GFCI breaker left behind. Anyone see why too-small panels are such a pain in the rear end?)

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    CA hasn't signed up for NEC 2020 yet, so OP could eliminate GFCI protection on the A/C unit, if done right now. Once NEC 2020 lands, won't be possible. Commented Sep 11, 2020 at 14:48

This is based on what's called the "120% rule" in the National Electric Code (article 705.12(B)(2)(3)(b)). It's complicated, and don't ask me to explain the "why" questions, it just is what it is...

The rule says that if you have two sources of power, i.e. the utility and a solar inverter, the sum TOTAL of the breakers in your panel cannot add up to more than 120% of the rating of the BUS BARS in that panel. So if you have a 100A service, it's possible that you have 100A bus. So you would add together all of the breakers, INCLUDING the breaker that would feed your solar inverter and it cannot come to more than 120A.

For example, if you have 100A bus bars to a typical home with typical appliances, you can probably only add a 20A 2 pole breaker, which would limit you to a 3800W inverter (breakers can only be loaded to 80%). If however you have a panel with a 100A main, but 125A bus, which is often the case, then you might be able to use up to a 50A inverter breaker and get a max. 9600W inverter. In the case of your contractors, one probably knew enough to check, the other was going to tell you later and up-charge you, or downgrade your solar system by reducing the price, but keeping some of the profits. Or, the first guy did investigate and saw that he could add a 50A breaker, the 2nd guy just guessed when he saw the 100A Main.

  • The first guy did say we would need to upgrade the panel to more than 100A, and that part makes sense to me. But the first guy didn't say anything about digging up the street. Why would the wires to the utility need to be upgraded to 200A just because the panel is upgraded to 200A?
    – user31708
    Commented Sep 10, 2020 at 0:56
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    @user31708 -- most 200A panels can be downbreakered to use a 100A (backfed, if worse comes to worse) main breaker, so you're right, there is no reason to upgrade the service lateral to 200A just to get fatter panel busses Commented Sep 10, 2020 at 2:39
  • If true, this rule does not compute. The DT can supply a lot more than what the 100A service panel is limited to and any external GTI current reduces any current from the DT, not add to it. GTI’s programmed to pf=1 by default so the current never adds to DT supplied (+) current, it’s always generating so that excess current is -ve and panel is limited by design , so it doesn’t change the load capacity. I suspect the 120% rule means you can supply up to 120% to the grid yet only up to 100% to the residence limited by the internal busbars and breakers. Commented Sep 10, 2020 at 4:20
  • As I said, don't get into the why's, it ot going to be satisfactory...
    – JRaef
    Commented Sep 10, 2020 at 5:17
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    @TonyStewartSunnyskyguyEE75 -- the 120% rule has nothing to do with protecting the utility wiring and everything to do with protecting the panel's busbars from getting overloaded, because the main breaker can no longer be the "gatekeeper" of all electricity that gets to the busbars Commented Sep 10, 2020 at 23:22

Thanks to your additional info, it's plain you have 125A busing. It's written all over the panel label. Your supply breaker + solar is capped at 120% of 125A, or 150A, for reasons I discuss in my legacy answer. Given your 100A main breaker, you can support up to 50A of solar. Already.

Of course now you'll need a new solar company; those guys are dumb as rocks.

The big deal

The internal busing on the service panel must have enough ampacity to support the loads. Normally that is controlled by limiting the main breaker size, but solar puts a monkey-wrench into that. As JRaef discusses, there is a 120% favorable derate; they give you a mulligan for 20% of panel bus capacity.

So the 4kw question is: Does your panel have a 100A bus (allowing 120A) or a 125A bus (allowing 150A)? Typically this style of panel is built with a 125A bus and an option for either 100A or 125A main breaker. So you need to find that out. Taking a picture of the panel front was interesting, but we need a picture of the panel labeling. It will discuss busing.

I don't know what's happening. Maybe they kneejerked "100A main, 100A bus". The other company definitely does not understand the concept that busing can be different from main breaker. They think if you have 200A busing you must have a 200A main breaker and must dig up the street for 200A service wires. Totally unnecessary.

Most of your breakers need replacing anyway

Mind you, breakers are $5-10.

Challenger is a difficult panel. The panel structure and busing are perfectly OK, same as the successful BR line. But the breakers themselves are notorious firestarters and need to be replaced with BR.

Now the white-handled breaker at the top provides GFCI protection to the former spa panel. It is probably a correct BR breaker, but might also be a GE THQL type breaker. Check the breaker labeling - if it's BR that's fine.

But an additional note on that GFCI breaker: spas need GFCI protection; air conditioners don't -- so the GFCI is superfluous right now. However NEC 2020 is coming, which will require GFCI protection on 240V circuits. Once it lands, you will not be able to eliminate the GFCI protection. Do it now if you have a mind to, or you'll soon be married to it. Note that 50A is way too big for an A/C unit, so you must have another breaker or fuses downline at the A/C disconnect. If it's a fuseless disconnect, you have a problem here.

The Westinghouse BR breaker is safe, but it has the statutory glitch of not being cross-labeled for Type C (your bus). Least of our worries.

The main breaker is an odd exception since it's labeled Challenger, but is not Challenger's style, and is cross-listed BR/C. I suspect this is a Bryant breaker branded Challenger, and is OK. Do you feel lucky?

All the rest are Challenger firestarters and need to be replaced with equivalent Eaton BR/C type (BR type are cross-listed Type C). Each will be $10-20.

ThreePhaseEel has some more advice on panel cleanup.

Plan A: you do have 125A busing

Both gangs are wrong, you can handle up to 50A of solar. Happy dappy.

Plan B: You have 100A busing and want 8kw of solar

Replace the main breaker with 80A Eaton BR. Let's do the math now:

100A busing + 20% derate = 120A bus capacity.

80A main breaker + 40A solar = 120A load. Copacetic!

You have a mildly increased chance of main breaker trips, but most people don't use nearly as much power as they think they do.

  • "we need a picture of the panel labeling" - Where do I find this labeling?
    – user31708
    Commented Sep 10, 2020 at 19:07
  • "You cannot connect solar to the spa panel." - What does this mean, exactly? There is only one panel, the one I took a picture of. The breaker marked "SPA" is actually the air conditioner. (Previous owner had a spa, but they took it out. Breaker was unused, so we reused it when we had air conditioning installed.)
    – user31708
    Commented Sep 10, 2020 at 19:10
  • @user31708 There should be a big label that shows the model number, panel schematic, list of compatible accessories, screw torques, and all that jazz. Might be inside the cover aka deadfront. I assumed there was a spa subpanel that powered both the spa and the A/C. Because the labeling said both, and that's the only way it could power both. Commented Sep 10, 2020 at 19:56
  • Thanks, I found the label (inside the cover) and added a picture to my post.
    – user31708
    Commented Sep 10, 2020 at 23:24
  • I think the white-handled GFCI breaker at the top may actually be a C-H/Eaton GFCB -- it matches the photos on Eaton's site, at least.... Commented Sep 10, 2020 at 23:57

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