NEC 705.12(D)(2) 120% rule for a 125A service panel with 100A utility and 40A solar

Question

I had solar installed and I am not sure they placed the breakers right based on an answer in a previous question here.

My main service panel is 125A. I have 100A of utility power and 40A of solar power. They installed the 40A breaker on top, the one in the off position in the picture below since I'm waiting for permit.

My inverter is SolarEdge SE7600H-US. The Maximum Continuous Output Current @240V is 32A according to the datasheet.

I had trouble understanding NEC 705.12(D)(2). I found https://www.purepower.com/blog/120-rule-explained-nec-705-12d2 and https://www.purepower.com/blog/2014-nec-705-12d2-a-new-120-rule that somewhat help.

So based on the new 2014 NEC 705.12(D)(2) from the above blog:

Option A not feasible

1.25 * inverter FLA + Main OCPD <= Bus Rating

1.25 * 32A + 100A <= 125A

140A <= 125A not true

Option B

1.25 * inverter FLA + Main OCPD <= Bus Rating * 1.2

1.25 * 32A + 100A <= 125A * 1.2

140A <= 150A

But for option B the code says:

"Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125% of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120% of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back fed breaker from the inverter that displays the following or equivalent wording: WARNING: INVERTER OUTPUT CONNECTION; DO NOT RELOCATE THIS OVERCURRENT DEVICE"

I mentioned the above to the installer on the day of the installation but he said first time he heard this rule and he was just following the design plans he received. I don't think he was an electrician. But then how are they allowed to do electrical work? I thought only the homeowner could do electrical work without a license.

Questions:

1. Do I have to relocate the breakers so that the load is between main and solar breaker? I have the feeder length to relocate the 100A load but not the 15A load see photo without cover. What options do I have for the 15A load?

2. Do I also need to relocate the main breaker in the center all the way to the bottom? I don't think I have feeder length for that. Earlier I got that this is granted an exception.

3. How can I convince the installer to come and fix it before inspection to avoid further delays? Or is this something that can be fixed on the spot and the city inspector could wait a few minutes for this to be addressed? Or is this something I can fix on my own? By the way, is a new cover needed or the empty slots could be covered in some other way?

Answer 1

Your situation is neither of the above

The panel you have is what's called a center fed panel, and as a result, the Code text that you quoted simply doesn't apply to it as that text specifies that the utility and solar (in your case) sources need to be at opposite ends of the panel, and your utility source is in the middle.

However, current Code provides for your case in NEC 705.12(B)(3) point 4:

(4) A connection at either end of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar.

As a result, applying this rule, you get 120% of 125 = 150A to split between the main breaker and the solar breaker, and 100A main + 40A solar makes 140A, so you're good to go from a Code standpoint. I wouldn't be concerned about the potential for a busbar overload here, either -- while you don't necessarily have the load diversity that is envisioned by the Code authors, your current situation can't overload a 125a busbar without tripping something. Diverting any new loads to the upper section of the panel, between the solar and utility feed breakers, would be wise, though.

Answer 2

The solar is a continuous load, so it gets provisioned at 125% of nameplate of the inverter. If you have a 32A inverter that derates 125% to 40A, you put it on a 40A breaker and treat it as a 40A source for the 120% rule. Do not apply this 125% twice (common error).

Thus 125A panel bus x 120% = 150A max input.
100A utility + 40A solar = 140A and you're fine.

This is the normal rule, not sure why they're calling it a new rule. It does not permit overloading the bus. It permits a configuration where most of the loads are between the two supply breakers, recognizing that current will be flowing from opposite directions into the loads, and therefore, no part of the panel will see 120% of current.

After this first cut of the rule was made, the issue was raised that many "all-in-one" panels have the main breaker in the middle. The rule was amended to allow this, as long as the loads are still in between the supply breakers.

Solar is a "Gold Rush" market full of incompetent snakes

If the US government offered every American $10,000 a year for college, the next day every college would raise their tuition by $10,000 :) The solar subsidies have resulted in a "gold rush" scenario where solar is largely a sales business - the best salesmen out there are hawking solar HARD, while the best market people on Wall Street figure out how to rig packages so they harvest all the profits to be made.

With such a sales push, there is a crush of activity where supply of technically qualified personnel cannot keep up with demand. As such, the installer who shows up at your home simply takes notes and sends them up to an actual expert in an office, who writes up how your house should be done and sends it back. That expert probably does 50 houses a day LOL. That installer is an expendable contractor who doesn't want to be fired, so they do Exactly What The Office Tells Them.

We've seen homeowners go round-and-round with solar companies due to the indirectness of communication and "Chinese Whispers"* between the field installer and the expert. In one case the field installer copy/pasted from another house and missed the detail that the "main subpanel" did not have a main breaker, so the expert proscribed an illegal installation. Expert was right but was given bad data.

I'm sure in this case the expert was told you have an "all-in-one" with a center main breaker, and envisioned a panel choc-a-bloc full of branch circuit breakers, and gave correct advice for that circumstance not yours.

So yes. For the 120% rule to work, the load breakers must be between main and solar breaker. Fortunately that looks easy, and better yet, you can do it without disturbing their work, which means, you don't void their warranty.

Move the breakers between

Do I have to relocate the breakers so that the load is between main and solar breaker? I have the feeder length to relocate the 100A load but not the 15A load see photo without cover. What options do I have for the 15A load?

That would be one way to solve it, yes. Move the 100A load and 15A load between solar and utility breaker.

As far as extending the 15A circuit, that's gonna be the hard part. You're gonna have to hunker down and spend big on costly Polaris connec--- wait a minute, did you say 15A circuit? 2 cent wire nut!

Do I also need to relocate the main breaker in the center all the way to the bottom? I don't think I have feeder length for that. Earlier I got that this is granted an exception.

You can't. In these "all-in-ones" the main breaker is immovable.

How can I convince the installer to come and fix it before inspection to avoid further delays? Or is this something that can be fixed on the spot and the city inspector could wait a few minutes for this to be addressed? Or is this something I can fix on my own? By the way, is a new cover needed or the empty slots could be covered in some other way?

LOL good luck getting the solar contractor to fix it. The field installer will swear on a stack of bibles "we did everything right". The expert will swear on a stack of bibles their spec is correct for the drawing they were shown. Both are correct. An asker with a similar problem took 2 months to get it sorted, and it only got corrected because the asker was such a pest that solar company did it as a concession, not because they understood it was wrong. That seems like the hard way; it's the fallacy of sunk costs (in time); "20 more minutes on this will fix it!" NO lol.

As I said your best option is to move your load breakers between the main and solar breakers. That doesn't touch their stuff so it doesn't void your warranty. In other words, I'm advising you to change your panel to be what they already think it is, as a path of least resistance. I advised the same to the other person but they had some principle about it, and it would have involved buying a $150 breaker.

How do you fill the holes? Normally I'd say "any random breaker" but in this case that would confuse. So they make blank cover plates.

Speaking of blank cover plates, the surge suppressor doesn't need to move, but did you realize a primary source of surges is your own appliances?

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* I mean the children's game of passing messages around a circle and watching them get distorted, not flimsy, bargain priced whispers found on Amazon Marketplace :)

Answer 3

Let's look at all of the options under 705.12(B)(3). (That is where this section lives in NEC 2017 and newer, apparently having been moved a little bit since NEC 2014). I've looked at the 2017 and 2020 version of NEC, which are basically the same here, except 2020 adds one more option that is not applicable, and 2020 uses numbers instead of lowercase letters for these options.

Option (a) won't work as you have described.

Option (b) is not strictly applicable, as the sources are not on each end of the busbar. (You would still be missing the required label).

Option (c) won't work, because the sum of the breakers (excluding the one protecting the busbar) exceeds the rating of the busbar. (This one would also need a label.)

Option (d)... Let's come back to that one.

Option (e) would require an engineer to sign on the current panel arraignment, after having run fault current analyses to ensure that under normal operation, the bubar will not be overloaded, and that under fault conditions, the busbar wont be dangerously overloaded. Technically, using that could be possible, but then all future changes to this panel would require an engineer to rerun the calculations and recertify the design. Which makes this impractical.

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So let's look at option (d) a bit more closely.

It says (NEC 2017):

A connection at either end, but not both ends, of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar.

or (NEC 2020):

A connection at either end of a center-fed panelboard in dwellings shall be permitted where the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar.

The wording from "the sum of" onwards is identical to the wording in option (b). This is a dwelling, and it is a center fed panelboard, so we are good there. This option oddly lacks any labeling requirements.

Therefore, the current setup does appear to technically meet the letter of the code as installed. However, despite technically meeting code, it does not meet the spirit, and should be considered unsafe.

The intention of option (d) was that there be significant load on that side of the panel board such that the current on any portion of the busbar in practice would not exceed its rating, even in a fault condition. Here that is not true, and the busbar could be overloaded in the middle, if the subpanel is drawing its full 100A plus the 15 A breaker drawing it's load. If more circuits were added to the bottom (regardless of whether they are legal under a load calculation or not), it would become even easier to overload the busbar.

Moving the subpanel breaker to the top fixes this (since that panel serves most of your loads), and the 15A breaker on the bottom can remain where it is. With the moved subpanel breaker, it is not really conceivable that the busbar could be overloaded in practice.

I would also strongly advise adding the warning label required under option (b) to the solar backfeed breaker, even though it is not technically a code requirement under option (d). This is because the warning would still be applicable, and having it avoids a less knowable electrician (or homeowner) moving the breaker and causing a code violation in the future.