AC Combiner Busbar, NEC rules

[carultch]

In the past, our practice would be to size a panelboard's amps to only 1.25* the total maximum continuous current of the inverters. The panelboard gets treated as a "No Loads Panel", dedicated to the PV System, with a label on it saying not to add loads.

Now, the 2014 NEC 690.12 and 690.13, requires solutions which often imply auxiliary circuits for the PV system that consume power instead of produce power. Usually power supplies that operate contactors and AFCI circuitry in the combiners, or similar. This could also include revenue grade meter power supplies. This is negligible in current compared to the total inverters, but this is a load.

Is it acceptable to connect 160 operating Amperes of inverters (i.e. 200A total of OCPD) to a dedicated 200A panelboard with a 200A main breaker, and still have these small loads connected at 15A breaker(s)? The sum of breakers (excluding the main supply) will exceed the rating of the busbar. But only the inverter breakers will load the panel anywhere near its ampere rating.

[Mike90250]

I'm not a code guru, but to my eye, a single 15A breaker as a load for the utility gear, should be OK. I don't know why it can't run off a 5A breaker which would breeze past the inspector . But, if the panel is labeled NO LOADS, you can't quibble that. Maybe time for No Domestic Loads, which should permit the utility gear on a small breaker.

[inetdog]

In the past, our practice would be to size a panelboard's amps to only 1.25* the total maximum continuous current of the inverters. The panelboard gets treated as a "No Loads Panel", dedicated to the PV System, with a label on it saying not to add loads.

Now, the 2014 NEC 690.12 and 690.13, requires solutions which often imply auxiliary circuits for the PV system that consume power instead of produce power. Usually power supplies that operate contactors and AFCI circuitry in the combiners, or similar. This could also include revenue grade meter power supplies. This is negligible in current compared to the total inverters, but this is a load.

Is it acceptable to connect 160 operating Amperes of inverters (i.e. 200A total of OCPD) to a dedicated 200A panelboard with a 200A main breaker, and still have these small loads connected at 15A breaker(s)? The sum of breakers (excluding the main supply) will exceed the rating of the busbar. But only the inverter breakers will load the panel anywhere near its ampere rating.

The simplest solution that no inspector could find fault with would be to run a feeder to the combiner panels and along the same route a 20A branch circuit that will feed all of the auxiliary loads and maybe some work lights, etc.
A second route would be to connect the loads to the load side of one of the PV backfeed breakers, perhaps with its own small breaker or fuse. The limiting factor there would be that you would have to have that inverter's breaker closed to get the aux equipment to work. Probably not a great design.
The problem with throwing in a 15A or even a 5A aux load breaker in the panel is that at that point you will technically be required to apply the 120% rule to the panel bus, which could cut the allowed PV amps of the panel nearly in half.

[PVAndy]

I'm not a code guru, but to my eye, a single 15A breaker as a load for the utility gear, should be OK. I don't know why it can't run off a 5A breaker which would breeze past the inspector . But, if the panel is labeled NO LOADS, you can't quibble that. Maybe time for No Domestic Loads, which should permit the utility gear on a small breaker.

We label our combiner panels "Do Not Add Load"

Regarding the original question, I'm not quite clear what is being asked. If it is how much total feed breakers you can feed a subpanel with the 125% rule is already taken into account when the breakers are sized for the inverters so you can feed 100%. Any misc breakers you have for monitoring etc don't come into play since they are load not source.

Andy

[inetdog]

We label our combiner panels "Do Not Add Load"

Regarding the original question, I'm not quite clear what is being asked. If it is how much total feed breakers you can feed a subpanel with the 125% rule is already taken into account when the breakers are sized for the inverters so you can feed 100%. Any misc breakers you have for monitoring etc don't come into play since they are load not source.

Andy

I think the point, Andy, is that although a lot of us feel that a combiner panel should not be subject to the 120% rule, because there are not loads, some inspectors will still apply the 120% rule both to a combiner panel and to the feeder wires leading to it.

Suppose, for example, you have a general purpose sub-panel with a 100A main breaker and some unspecified amount of load circuits. If the bus rating is 125A, the maximum PV backfeed you are allowed is 125 x 1.2 -100 = 150 - 100 = 50A.
Now if you have no loads, most inspectors will allow PV backfeed up to the value of the main breaker, or 100A in the example case.

What is being asked is that if the difference between the two cases is only that there is no load on the second panel, what happens when you add one or more 20A circuits for miscellaneous loads associated with operating the PV.
It seems logical that as long as the total of the added loads (and for this you have to add up the breaker ratings, because the loads may short or fault in some other way) is less than the bus rating it does not matter where the power comes from. But the NEC does not recognize that argument.

A lot of people do not recognize the difference between the rules for possible overload and continuous load (both 125%) and the rule for backfeeding a bus which also has loads (120%), and confuse the three.