Hot Topics: Legal Solar Installation

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Original Post: Is my solar install legal?

drjrt - Member

Thanks to help on this site I have been learning about the effects of multiple input sources to a load center, specifically solar. I had solar put on my home about 6 months ago (a licensed electrician was employed) but I'm now wondering if it is according to code.

On a 200 amp panel I have 4 strings of 330watt panels, 13 to a string. Each string feeds the panel on DP 20 amp breakers. This theoretically could result in 71.5 amps being dumped on the bus of this panel (52 x 330watts / 240v = 71.5 amps).

I also have breakers on this panel that in total exceed 150 amps. Therefore, my understanding is if all my service breakers were pulling at maximum and the solar was putting out at maximum this would exceed the bus rating of the panel. Am correct?

If so, shouldn't "we" have moved circuits off this panel (I have another 200 amp panel right next to it) to stay below that theoretical maximum? Or am I missing something?

PJmax - Group Moderator

Is this a grid tie system?

Remember..... your panel loads are loads coming out of the panel while your solar is power going into the panel. They will offset each other.

drjrt - Thread Starter

Yes, I'm grid tied. This is the NEC that I'm worried about:

NEC in the requirements in 705.12(B)(2):

(3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.

(a) The sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.

Informational Note: This general rule assumes no limitation in the number of the loads or sources applied to busbars or their locations.

(b) Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent 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 power source that displays the following or equivalent wording:


The warning sign(s) or label(s) shall comply with 110.21(B).

drjrt - Thread Starter

The way I'm reading this is if I have a busbar rated at 200 amp (not sure as the panels are old) then they can handle up to 120% or 240 amps. If my solar is maximally generating 72 amp and have service breakers potentially pulling 150 amps (222 amps * 125%) is 270 amps which exceeds the 240 amps maximum on the busbar.

Again, maybe I'm completely misunderstanding this rule.

ibpooks - Forum Topic Moderator

I think you're understanding the rule correctly, but applying it more rigidly than is typical in residential situation. Usually for residential services we use a process called "demand load calculation" to make a reasonable estimate of actual load on the service. This comes out much less than the sum of the breaker handles. We touched on this a bit in your cabin thread, but the gist of it that there are no reasonable cases where every circuit in a residential panel is operating at maximum capacity simultaneously. That actual load figure is used most every where for subsequent calculations of the service.

Even still I think your home panel is probably OK. Panels rated at 200A usually have bus bars rated at 225A, which makes your total input power of 270A.

joed - Member

You are confusing the solar INPUTTING power to the panel and the breaker USING that power. They don't add up.

The original problem was solar inputting power to the panel and the grid inputting power to the panel. Those do add up.

drjrt - Thread Starter

Yes, my example above was not accurate.

I checked and I have 350 amps of breakers on my 200 amp panel with up to 72 amps of solar input.

Some of these circuits supply high-amp devices like HVAC, sump pumps, commercial grade dehumidifier.

While I'm not so worried about my cabin install, this seems like a situation where it's not inconceivable that these devices could draw 200 amps (although unlikely as they've never tripped the breaker before) and they still wouldn't exceed 270 amps or it would trip the main breaker, correct?

joed - Member

Not everything runs at the same time. Nothing runs at the full rating of the breaker. Many breakers like receptacle and light circuits have almost nothing on them. Adding up breaker values is a useless effort.

Norm201 - Member

Long and short story...if it passes local inspection, it's legal. My daughter recently had solar panels installed on the house (bad idea if I was asked, but that's another story). Town inspection did not like they way the panels were installed or mounter at the circuit box. Company had to redo it, buy nothing was unsafe.

Geochurchi - Member

Hi, was the install designed by a Solar co., was it also inspected?

drjrt - Thread Starter

Yes it was installed by a Solar Install company.

They use one electrician on all their projects.

I'm not sure how thorough the inspection was....

lawrosa - Super Moderator

I don't think its clear what you have there.

If you have a grid tie system you need an inverter. The strings attach to the inverter. From the inverter it ties into the main panel with one double pole breaker. 40 - 60 amp typically i believe.

Or you can have micro inverters..

Not sure what you got there.

a solar system diagram

a solar power system diagram

drjrt - Thread Starter

My home installation is four (4) strings consisting of thirteen (13) 330 watt panels utilizing Enphase microinverters.

Each string terminates in a 20amp DP breaker in (one of my) 200 amp house breakers.

Like your top picture although there is no disconnect between the string and the breaker.

engr3000 - Member

According to 705.12 (D)(2)(3)(b), the sum of 125% of the maximum inverter output current and the panel's main breaker rating cannot exceed 120% of the panel busbar rating.

125% of your 71.5A inverter output is 89.4A.

120% of your 200A panel rating is 240A.

So 240A - 89.4A = 150.6A is the maximum main breaker size allowable for the panel. If you can get a 150A main breaker for your particular panel then it would be code compliant.

By the way, the backfed breakers feeding in the inverter power must be located on the opposite end of the busbar from the main breaker to take advantage of the 120% of busbar rating rule. Otherwise you are limited to the actual busbar rating.

​As mentioned above, it's unlikely that you will trip the main breaker of the panel with your loads, even if you replace the main breaker with a 150A one. Also, when the solar panels are operating they will reduce the amount of current drawn by your loads through the main breaker.

drjrt - Thread Starter

Thanks. Looks like I need to get with my solar installer and review the wiring I currently have.