Interconnected Electric Power Sources - NEC 705

[erickench]

I'm looking at this month's edition of EC&M magazine. There's an article written by Mike Holt on Interconnected Electric Power sources and I noticed a couple of things. The illustration Fig.2 state's that two 30 CBs feeding a panelboard from inverters is a violation because the sum total i.e. 2X30=60 exceeds 40A. Also, the article text itself on page 34 state's that a 225A bus panelboard protected by 225A main circuit breaker could have four 50A inverter OCPDs, if those breakers fill all slots in the panelboard. I still have my 2011 NEC but I do not see where in the code that the first case above is a violation or how 705.12(D)(2) applies to the second case. Does anyone have a 2014 NEC? Were there any changes made to the rule?

 

[GoldDigger]

I'm looking at this month's edition of EC&M magazine. There's an article written by Mike Holt on Interconnected Electric Power sources and I noticed a couple of things. The illustration Fig.2 state's that two 30 CBs feeding a panelboard from inverters is a violation because the sum total i.e. 2X30=60 exceeds 40A. Also, the article text itself on page 34 state's that a 225A bus panelboard protected by 225A main circuit breaker could have four 50A inverter OCPDs, if those breakers fill all slots in the panelboard. I still have my 2011 NEC but I do not see where in the code that the first case above is a violation or how 705.12(D)(2) applies to the second case. Does anyone have a 2014 NEC? Were there any changes made to the rule?

In the first case, the PV is being backfed into a panel that also feeds loads. That is what provides the possibility for the panel bus itself to be overloaded.
In the second case, the panel is an AC combiner panel, because there are no loads and in fact there is no way to attach a load to it (all slots full). That logically removes the reason for the 120% rule. But as far as clarity in the code, I still do not see just where the distinction is made between a load-serving panel and an inverter-combiner panel. Because of that, you may be dependent on the inspector and AHJ making the same interpretation of the code that I described.

 

[erickench]

After looking at the article text again I see now why it's a violation. Mike Holt used a 200A OCPD for a 200A bus panel for his example.

(200X1.2)-200=240-200=40A

Figure 2 shows two 30A CBs which added together exceeds the 40A allowed. But I'm still puzzled as to why in the second case Mike Holt did not mention it as an Inverter-Combiner Panel. Figure 3 on page 36 show's just a basic panel.

 

[GoldDigger]

In the first case, the PV is being backfed into a panel that also feeds loads. That is what provides the possibility for the panel bus itself to be overloaded.
In the second case, the panel is an AC combiner panel, because there are no loads and in fact there is no way to attach a load to it (all slots full). That logically removes the reason for the 120% rule. But as far as clarity in the code, I still do not see just where the distinction is made between a load-serving panel and an inverter-combiner panel. Because of that, you may be dependent on the inspector and AHJ making the same interpretation of the code that I described.

If I had to find some wording to justify my interpretation, I would say this:

The text of 705.12(D)(2) talks about the sum of the ratings of all of the breakers supplying the panel.

..circuits supplying power to a busbar or conductor...

In the first case, the presence of loads tells us that the main breaker can also supply (deliver) power to the panel if the load exceeds the PV output power.
In the second case, not counting the conveniently ignored trivial amount of monitoring power going to the GTIs when the sun is down, there are no loads, so the main breaker or feeder wires of the panel will never be supplying power. So that leaves (D)(2) only requiring that the sum of the ratings of the PV breakers be no more than 120% of the bus rating (and of the feeder wire rating if it is an MLO panel).

 

[erickench]

If I had to find some wording to justify my interpretation, I would say this:

The text of 705.12(D)(2) talks about the sum of the ratings of all of the breakers supplying the panel.
In the first case, the presence of loads tells us that the main breaker can also supply (deliver) power to the panel if the load exceeds the PV output power.
In the second case, not counting the conveniently ignored trivial amount of monitoring power going to the GTIs when the sun is down, there are no loads, so the main breaker or feeder wires of the panel will never be supplying power. So that leaves (D)(2) only requiring that the sum of the ratings of the PV breakers be no more than 120% of the bus rating (and of the feeder wire rating if it is an MLO panel).

Then what is the purpose of this panel if it's not used to supply loads? I could only assume that there is no utility power so you would feed the current from the four 50A CBs through the 225A main breaker. But if this is so then how could it be an interconnected power source if there is no connection from the utility?

 

[GoldDigger]

Then what is the purpose of this panel if it's not used to supply loads? I could only assume that there is no utility power so you would feed the current from the four 50A CBs through the 225A main breaker. But if this is so then how could it be an interconnected power source if there is no connection from the utility?

Given that we have a bunch of utility interactive inverters (at least two, maybe a lot more than that, in this case 4), then it is usually a good design to combine the inverter AC outputs somewhere close to the inverters and run one larger feeder back to the panel where we actually make the backfeed to the utility connection.
The panel that does that will be a combiner, just as we would use a combiner box on the DC leads from several panel strings going to the DC input of one string inverter.
All of the loads on the site will then be fed from the existing panel where the single combined circuit lands or other panels connected upstream of it or parallel to it.

 

[Smart $]

Then what is the purpose of this panel if it's not used to supply loads? I could only assume that there is no utility power so you would feed the current from the four 50A CBs through the 225A main breaker. But if this is so then how could it be an interconnected power source if there is no connection from the utility?

I agree with GD...

Note 705.12(D) says the PV system can be connected on the load side of the service disconnecting means at any distribution equipment. The key is the term distribution equipment. The supply side side of the "point of connection" ?which is the title of 705.12? is still regarded as the PV system output and not yet part of the distribution system. As such, the "120% rule" applies only on the load side of the point of connection.