120% Rule In MSP & Sub Panel

[JimmysLimeade]

I have a client that is installing a new PV system on a home that has an existing system. The existing system is fed off of a 20A breaker in the 200/200A MSP, they want to install a new 225/200A sub panel, on feed thru lugs in the MSP, with a new 40A PV breaker. I told them that would break the 120% rule of the MSP, even though it meets the 120% rule in the sub panel. They are saying that since there is a main OCPD in the sub panel, that the sub panel busbar is separate from the MSP busbar and that you dont have to comply with the 120% rule in the MSP for the solar connected in the sub panel.

I know that isn't true but when I try to explain that to them, they arent listening, is there a relevant code section that I can use to explain this? They are on the 2020 NEC

Picture attached of how they want to connect

 

[ggunn]

Your client is mistaken. 705.12 rules must be complied with in every panel between the PV and the service. The system already has 20A of its allotted 40A of headroom in the MSP taken up by the existing system and can only accept 20A more (16A of max inverter current) no matter how it comes in.

 

[JimmysLimeade]

Your client is mistaken. 705.12 rules must be complied with in every panel between the PV and the service. The system already has 20A of its allotted 40A of headroom taken up by the existing system and can only accept 20A more (16A of max inverter current) no matter how it comes in.

That is my understanding as well, I just dont know how to relay that info to my client in a way they understand. Our EE said that they wont stamp it as is, and they still doubled down.

 

[wwhitney]

Maybe something like this: "Suppose those feed-thru lugs were replaced with a 40A breaker with the new PV connected to it. Do you agree that configuration doesn't comply with the 120% rule in the MSP? Moving that breaker to a subpanel and connecting instead to feed-thru lugs doesn't change anything electrically as as far as what sources of current can supply the bus. So the diagram also does not comply."

Cheers, Wayne

 

[solarken]

That is my understanding as well, I just dont know how to relay that info to my client in a way they understand. Our EE said that they wont stamp it as is, and they still doubled down.

An alternative would be to replace their main load center with one with a 225A bus (assuming their current one has a 200A bus and not 225A) and move the solaredge backfed breaker and the new 40A enphase backfed breaker to a small combiner panel that backfeeds a 60A breaker in the new loadcenter. If their current load center has a 225A bus, then just add the small combiner panel and replace the 20A breaker with 60A.

 

[JimmysLimeade]

Maybe something like this: "Suppose those feed-thru lugs were replaced with a 40A breaker with the new PV connected to it. Do you agree that configuration doesn't comply with the 120% rule in the MSP? Moving that breaker to a subpanel and connecting instead to feed-thru lugs doesn't change anything electrically as as far as what sources of current can supply the bus. So the diagram also does not comply."

Cheers, Wayne

Their response is: "If the connection were made at the feed through lugs yes, I would agree. However when a main breaker is placed in the sub-panel, the current on the passthrough lugs would be limited to the rating of the main breaker. This means that if we place the PV breaker on the opposite end of the sub-panel from the main breaker, there would be no way for more current than the main breaker in the sub-panel is rated for to pass through those conductors. So the meter main would be protected at it's 120% and the sub-panel would be protected the same. The most that either busbar would be subject to would be the sum of the main breaker and the PV breaker ratings which is in compliance with all subsections of 705.12."

Im gonna see if a phone call could resolve this better than email, and if not then ill figure something else out

 

[ggunn]

Their response is: "If the connection were made at the feed through lugs yes, I would agree. However when a main breaker is placed in the sub-panel, the current on the passthrough lugs would be limited to the rating of the main breaker. This means that if we place the PV breaker on the opposite end of the sub-panel from the main breaker, there would be no way for more current than the main breaker in the sub-panel is rated for to pass through those conductors. So the meter main would be protected at it's 120% and the sub-panel would be protected the same. The most that either busbar would be subject to would be the sum of the main breaker and the PV breaker ratings which is in compliance with all subsections of 705.12."

Im gonna see if a phone call could resolve this better than email, and if not then ill figure something else out

The amount of current passing through the main breaker to the service is irrelevant; the 120% rule is designed to prevent hot spots on the main panel bus. Your EE is correct; I wouldn't stamp it either.

Your client's options are: downsize the main breaker to make room for the added solar, replace the main panel with a higher rated busbar (leaving the MCB at 200A or making the panel big enough so that the 120% rule works as it is), move the interconnection to the supply side of the main breaker, or downsize the PV. It doesn't make any difference if it makes "logical" sense to him or not, those are the rules.

In my 15+ years in solar I have had this argument many, many times. It makes no difference how the current gets to a panel busbar; 705.12 applies.

 

[wwhitney]

Their response is: "If the connection were made at the feed through lugs yes, I would agree."

To which I respond: "The 40A PV breaker is connected to the MSP via the feed thru lugs. Additional intervening breakers do not affect any of the comparisons required by 705.12."

So the meter main would be protected at it's 120% and the sub-panel would be protected the same. The most that either busbar would be subject to would be the sum of the main breaker and the PV breaker ratings which is in compliance with all subsections of 705.12."

To which I respond: "This part is incorrect for the MSP. It may see 200A from the service, and assuming maximum sized inverters, 16A from the 20A PV breaker plus 32A from the feed thru lugs, all simultaneously. 125% * (16+32) + 200A = 260A > 240A = 120% * 200A."

You haven't actually specified the inverter sizes, but I assume their output currents add up to in excess of 32A, as if not, there would be no problem.

Cheers, Wayne

 

[solarken]

Maybe say this to them: Picture adding a 40A branch Breaker to the Meter-main Panel and feeding a Main Lug only panel with 225A bus that has a single 40A breaker that is backfed by the new Enphase system, and a sign that says NO OTHER BREAKERS ALLOWED on the new MLO panel. There is no electrical difference between this fictional arrangement and the dwg. Now you have a 20A SolarEdge breaker and a 40A Enphase breaker, totalling 60A, backfeeding a 200A bus Meter-main that is permitted 240A under the 120% rule. With a 200A main breaker, 200A + 60A = 260A exceeds what is allowed by 20A.

 

[jaggedben]

The opening paragraph of 705.12 says "Where distribution equipment or feeders are fed simultaneously by a primary source of electricity and one or more other power sources and are capable of supplying multiple branch circuits or feeders, or both, the interconnecting equipment shall comply with 705.12(A) through (E)." All panelboards between the primary source and the other source are interconnecting equipment and meet that description. Plus, obviously, there is no important physical difference between the way current can flow through them from the different sources just because one is nearer the interconnected power source.

 

[jaggedben]

Their response is: "If the connection were made at the feed through lugs yes, I would agree. However when a main breaker is placed in the sub-panel, the current on the passthrough lugs would be limited to the rating of the main breaker. This means that if we place the PV breaker on the opposite end of the sub-panel from the main breaker, there would be no way for more current than the main breaker in the sub-panel is rated for to pass through those conductors. So the meter main would be protected at it's 120% and the sub-panel would be protected the same. The most that either busbar would be subject to would be the sum of the main breaker and the PV breaker ratings which is in compliance with all subsections of 705.12."

That's simply illogical and incorrect for the ratings of equipment in the drawing.

 

[electrofelon]

Not really my business, but who is your client and why are they dictating details of the installation to you? Why don't they install it themselves if they have it all figured out?

 

[pv_n00b]

I think you have made a reasonable attempt to explain this to you client. Now say to them, okay if you are convinced that this is NEC compliant then you stamp the drawings. Oh, you are not a licensed engineer? Then I guess you have to accept what the licensed engineer says.

 

[JimmysLimeade]

Not really my business, but who is your client and why are they dictating details of the installation to you? Why don't they install it themselves if they have it all figured out?

My company does 3rd party design and engineering for contractors that install the systems. I basically told them to kick rocks, and that we wouldn't design/stamp the plans how they wanted them. We ended up derating the main breaker to comply with the 120% rule.

Thanks for the help on this from everyone!

 

[jaggedben]

Now it makes sense. The client is a smart ass installer who is actually weak on electrical theory.

 

[JimmysLimeade]

lol yup! worst part is that they said theyve installed this configuration before with their previous partners

 

[ggunn]

lol yup! worst part is that they said theyve installed this configuration before with their previous partners

Either they were lying or whoever designed and inspected their project(s) were as clueless as they are.

 

[jaggedben]

lol yup! worst part is that they said theyve installed this configuration before with their previous partners

Oh, I believe it...

 

[pv_n00b]

lol yup! worst part is that they said theyve installed this configuration before with their previous partners

Ah the famous, "But our other engineer let us do this all the time."