RMA (Renewable Meter Adapter)

[cleantech92058]

The local utility offers a utiltiy/renewable meter adapter to tie-in the solar system lineside (installed by utility). The ratings are 48A continuous - up 60A solar breaker (125% rule)
We have designed a solar system that includes 2 inverters - one has a Max AC Output of 21A, the other 25A. They go through an AC combiner and then 1 string to the meter adapter.
In our opinion the continuous value here is 46A (21A + 25A) - the AJH claims it is the breaker value in the AC combiner (21A x 125% = 26.25 = 30A and 25A x 125% = 31.25A = 40A).
Can you clarify?

 

[ggunn]

The local utility offers a utiltiy/renewable meter adapter to tie-in the solar system lineside (installed by utility). The ratings are 48A continuous - up 60A solar breaker (125% rule)
We have designed a solar system that includes 2 inverters - one has a Max AC Output of 21A, the other 25A. They go through an AC combiner and then 1 string to the meter adapter.
In our opinion the continuous value here is 46A (21A + 25A) - the AJH claims it is the breaker value in the AC combiner (21A x 125% = 26.25 = 30A and 25A x 125% = 31.25A = 40A).
Can you clarify?

If I understand you correctly and if you are operating under NEC2014 the inspector is mistaken. 125% of the inverter nameplate current is the number you use for all calculations from the inverter to the service.

An exception to this is 705.12(D)(2)(3)(c) where the OPCD ratings are used instead, but it doesn't look to me like this is the situation you are dealing with.

 

[cleantech92058]

If I understand you correctly and if you are operating under NEC2014 the inspector is mistaken. 125% of the inverter nameplate current is the number you use for all calculations from the inverter to the service.

An exception to this is 705.12(D)(2)(3)(c) where the OPCD ratings are used instead, but it doesn't look to me like this is the situation you are dealing with.

Thank you for your reply.
So - just to be clear - we can use (2) 30A breakers (1 for the 21A and 1 for the 25A input circuits) in the combiner and apply the 125% rule (60A breaker) at the RMA (21 +25 = 46 *125% = 57.5).
The RMA provides an internal breaker limiting solar input to 48 amps (continuous) maximum and has a 60-A circuit breaker for overcurrent protection.

 

[ggunn]

Thank you for your reply.
So - just to be clear - we can use (2) 30A breakers (1 for the 21A and 1 for the 25A input circuits) in the combiner and apply the 125% rule (60A breaker) at the RMA (21 +25 = 46 *125% = 57.5).
The RMA provides an internal breaker limiting solar input to 48 amps (continuous) maximum and has a 60-A circuit breaker for overcurrent protection.

I'm not familiar with the RMA; what sort of interconnection does it make with the service? Is it like a standard MDP with a main breaker, or does it make a supply side connection (no main breaker)? If it's the former then the 120% rule applies (assuming NEC 2014), i.e. 125% of the nameplate maximum inverter current (Imax) plus the rating of the main breaker cannot exceed 120% of the rating of the busbar. You have to know the main breaker rating and the rating of the busbar to make the calculation. If it's a supply side connection you are only limited by the size of the service.

If your AHJ is telling you that the rating of the interconnection breaker has to be the sum of the two inverter breakers in the combiner panel, they are mistaken. It only has to be equal to or greater than 125% of the sum of the two inverter nameplate ratings, the same as if it were a single inverter. Incidentally, by code 125% of Imax is the minimum size OCPD you must use; you can make it as big as you want as long as you size the conductors so that they are protected.

 

[jaggedben]

It is a supply side connection.

You should be able to put a 60A breaker in the RMA to a solar subpanel, and then a 30A and 35A breaker for the inverters in the sub. (The AHJ seems to be demonstrating ignorance that 35A breakers exist, although it's a moot point.)

The language is clearer in the 2014 and 2017 codes, but I think the 2011 code would still allow that, too.

 

[SolarAlternativesDesign]

I think that the limitation on the allowable inverter ratings are related to the input rating of the Renewable Meter Adapter. Does the RMA manufacturer provide its requirements - continuous or not, breaker size or inverter current rating? I think the NEC would just require complying with the listing of the RMA equipment.
The 120% rule in 2014 NEC 705.12(D)(2) applies to the busbar in a load side connection, which isn't being used in your case.
The subpanel breaker ratings aren't related to protection of the service panel busbar but are related to the protection of the feeder conductors from the inverters, so you need to size those conductors accordingly.
Our local electrical supply company says they can order Murray Siemens 35A breakers, which they say are mainly for solar jobs.
Good luck!