Solar PV Project with 3 Inverters feeding a Dedicated PV AC Combiner Panel with 3 bre

[SolarGP]

Scenario: We are feeding the AC output from 3 x SolarEdge 33 kW 3-phase inverters into a dedicated AC Combiner Panel. Each Inverter outputs 40 Amps and will directly connect to a 3-Pole 50 Amp breaker upsized by 1.25 for Continuous load. That gives a total Amperage of the sum of these breakers of 150 Amps. It is a dedicated AC solar panel and the output of the panel are a set of 4/0's running down the building to a Main Disconnect and Main Panel.

Question: Given the sum of the breakers being 150 Amps. According to NEC code 705 .12(D)(2), the 120% rule, couldn't the main breaker could also be 150 Amps? So what would the minimum Busbar rating need to be?

 

[GoldDigger]

It is not really clear in earlier code cycles and subject to wild interpretation by AHJ.
Under [2014] and [2017] almost all AHJs will not try to apply the 120% rule to a combiner panel with no loads.
The alternative is to apply the 100% rule to the sum of all branch breakers, which should be fine in your case as long as the bus is rated as high as the main and sum of the branches is also less than or equal to the main.

 

[jaggedben]

According to the 2014 or 2017 NEC the main breaker and panel can be rated 150A. You're required to put a sign on it that the total sum of breakers can't exceed the buscar rating.

If you are on the 2011 code or earlier it's a bit more complicated.

 

[PWDickerson]

I see that the OP is based in New York, and according to the NFPA's NEC Adoption Page, New York is under the 2014 NEC. If the 2014 NEC is in force for this job, you will want to qualify the bus bar of the AC combiner panel using 705.12(D)(2)(3)(c), which states:


"The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Permanent warning labels shall be applied to distribution equipment that displays the following or equivalent wording:

WARNING: THIS EQUIPMENT FED BY MULTIPLE SOURCES. TOTAL RATING OF ALL OVERCURRENT DEVICES, EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE, SHALL NOT EXCEED AMPACITY OF BUSBAR."


This will allow your AC combiner bus to be rated at 150A.

If you qualify the AC combiner panel under 705.12(D)(2)(3)(b) (the 120% rule), you will need a minimum bus rating of 250A ( (150+50+50+50)/1.2 ), which is, of course, overkill.

 

[ggunn]

According to the 2014 or 2017 NEC the main breaker and panel can be rated 150A. You're required to put a sign on it that the total sum of breakers can't exceed the buscar rating.

If you are on the 2011 code or earlier it's a bit more complicated.

FWIW, I was building solar inverter aggregation panels to what became 705.12(D)(2)(3)(c) standards in the 2014 NEC as long ago as 2011. I never had an AHJ try to hold my feet to the fire over it. Common sense prevailed, I guess.

 

[electrofelon]

I'm not sure if y'all answered his question. The question was about the "main breaker" but I guess it's not clear if this main breaker is in the combiner, or he is referring to the service disconnect. My first thought was the latter.

 

[ggunn]

Scenario: We are feeding the AC output from 3 x SolarEdge 33 kW 3-phase inverters into a dedicated AC Combiner Panel. Each Inverter outputs 40 Amps and will directly connect to a 3-Pole 50 Amp breaker upsized by 1.25 for Continuous load. That gives a total Amperage of the sum of these breakers of 150 Amps. It is a dedicated AC solar panel and the output of the panel are a set of 4/0's running down the building to a Main Disconnect and Main Panel.

Question: Given the sum of the breakers being 150 Amps. According to NEC code 705 .12(D)(2), the 120% rule, couldn't the main breaker could also be 150 Amps? So what would the minimum Busbar rating need to be?

If the panel containing the solar inverters is protected by a load breaker in the main panel (and it should be), it can be an MLO panel (no main breaker).

That said, keep in mind that the 120% rule applies in the main panel, i.e., 125% of your total inverter nameplate current plus the OCPD on the main panel busbars must be less than 120% of the main panel busbar rating, and the load breaker in the main panel protecting the sub panel must be at the opposite end of the main panel busbar from the main breaker if the sum is more than 100% of the main busbar rating..

 

[PWDickerson]

That said, keep in mind that the 120% rule applies in the main panel, i.e., 125% of your total inverter nameplate current plus the OCPD on the main panel busbars must be less than 120% of the main panel busbar rating, and the load breaker in the main panel protecting the sub panel must be at the opposite end of the main panel busbar from the main breaker if the sum is more than 100% of the main busbar rating..

More broadly, you can qualify the main panel using 705.12(D)(2)(3)(a) (the 100% rule), 705.12(D)(2)(3)(b) (the 120% rule), or 705.12(D)(2)(3)(c) (the sum of the breakers rule). Any one will work. Most the time, the sum of the breakers in the main panel exceeds its rating, so the 120% rule is the easiest of the three to meet.

 

[ggunn]

More broadly, you can qualify the main panel using 705.12(D)(2)(3)(a) (the 100% rule), 705.12(D)(2)(3)(b) (the 120% rule), or 705.12(D)(2)(3)(c) (the sum of the breakers rule). Any one will work. Most the time, the sum of the breakers in the main panel exceeds its rating, so the 120% rule is the easiest of the three to meet.

For many/most residential services I have dealt with the main breaker rating and the busbar rating for the MDP were the same, so (a) and (c) were ruled out.

If 1.25 X inverter Imax + the bus rating > 1.2 X the bus rating you can upsize the panel and keep the same main breaker, which is expensive, or you can downsize the main breaker and run the risk of nuisance trips. Another alternative is a supply (line) side interconnection, but many AHJ's have their own rules governing them, so it's best to peruse their published interconnection guidelines, if there are any, or failing that, have a preconstruction meeting with them so that you won't get surprised at your inspection.

 

[ggunn]

For many/most residential services I have dealt with the main breaker rating and the busbar rating for the MDP were the same, so (a) and (c) were ruled out.

If 1.25 X inverter Imax + the bus rating > 1.2 X the bus rating you can upsize the panel and keep the same main breaker, which is expensive, or you can downsize the main breaker and run the risk of nuisance trips. Another alternative is a supply (line) side interconnection, but many AHJ's have their own rules governing them, so it's best to peruse their published interconnection guidelines, if there are any, or failing that, have a preconstruction meeting with them so that you won't get surprised at your inspection.

Correction: If the main breaker and busbar ratings are the same, only (a) is automatically ruled out since the main breaker rating is not part of the (c) calculation. Sorry for posting before thinking.

 

[jaggedben]

For many/most residential services I have dealt with the main breaker rating and the busbar rating for the MDP were the same, so (a) and (c) were ruled out.

Just a reminder, the OP explicitly asked about an AC combiner panel, so (c) is the most relevant to the thread.

 

[ggunn]

Just a reminder, the OP explicitly asked about an AC combiner panel, so (c) is the most relevant to the thread.

I understand, but he needs to comply with 705.12(2)(3) in the MDP as well.

 

[SolarGP]

Response to

Response to

Hi All
Thank you for taking the time to respond and with the very useful responses. Our engineer consultants actually called out a 400 Amp Bus with 150 Amp main breaker which I knew was far too large and just plainly incorrect. So I conclude from your collective posts that I could go with a 100 Amp Bus with perhaps a little risk that an inspection may want to apply the 120% rule or I could go with a 250 Amp Bus and definitely be covered. Thank you all again for your comments.

 

[jaggedben]

Hi All
Thank you for taking the time to respond and with the very useful responses. Our engineer consultants actually called out a 400 Amp Bus with 150 Amp main breaker which I knew was far too large and just plainly incorrect. So I conclude from your collective posts that I could go with a 100 Amp Bus with perhaps a little risk that an inspection may want to apply the 120% rule or I could go with a 250 Amp Bus and definitely be covered. Thank you all again for your comments.

Um, typo? Per your original post you need 150A miminum, I'd assume that's what you meant. I agree with 250 meeting the 120% rule, but that really shouldn't be necessary if you've moved beyond the 2011 code. BTW, I've never seen a 250A rated bus so that's probably why someone spec'd 400A, it being the next size up that would be readily available. 150A is pretty rare too, I'd guess you'll end up with 200.

 

[PWDickerson]

I think Siemens makes a 250 A rated panelboard. However, I agree that the minimum required is 150 A, and a 200 A panel will likely be the most cost effective and readily available.