Main lug only

[SAP]

When I'm I required to use a main breaker in a sub panel, we have a 400 amp 3 phase main service, we are back feeding 70 amps, plans are asking me to put a 70amp in my 400 amp service, and another in my solar sub panel, FYI they are next to each other main and solar Sub-panel thanks

 

[Smart $]

No such requirement jumping out and grabbing me at the moment. Sounds a bit off the cuff if you ask me.

 

[ggunn]

When I'm I required to use a main breaker in a sub panel, we have a 400 amp 3 phase main service, we are back feeding 70 amps, plans are asking me to put a 70amp in my 400 amp service, and another in my solar sub panel, FYI they are next to each other main and solar Sub-panel thanks

I'm having a little trouble understanding your question. Which panel(s) are MLO? 70 amp what? Main breaker? Backfed breaker? What is your question, exactly? A little more/clearer information would help.

 

[ron]

Is the 70A sub panel acting as a PV source? You need to protect the feeder, so if it is a source, a MLO panel will not protect the 70A feeder to the main.

 

[SAP]

I will try to be more clear, plans are showing a 70 amp MCB in my solar load center, plans are also showing a 70 amp backfeed breaker in my 400 amp main switch board, why can't I use a main lug only in my solar load center

 

[SAP]

I will try to be more clear, plans are showing a 70 amp MCB in my solar load center, plans are also showing a 70 amp backfeed breaker in my 400 amp main switch board, why can't I use a main lug only in my solar load center

 

[Carultch]

Is the 70A sub panel acting as a PV source? You need to protect the feeder, so if it is a source, a MLO panel will not protect the 70A feeder to the main.

You bring up an important issue.

I see it is quite common to have a system with string inverters, each grouped with subpanels as AC combiners. Then all the subpanels feed a common master AC Combiner (possibly a switchboard). There would be breakers in the common master AC combiner for each subpanel feed, thus protecting the feeders. But do they protect the feeders as the NEC requires?

Would in fact, a main breaker be required on each subpanel AC combiner? I often put one in, out of ignorance. Also consider that it could be distant, out of sight, and require a walking path possibly through ladders to get between the panelboards. But when reviewing the design of others, how do I know if it is required or not?

 

[ggunn]

I will try to be more clear, plans are showing a 70 amp MCB in my solar load center, plans are also showing a 70 amp backfeed breaker in my 400 amp main switch board, why can't I use a main lug only in my solar load center

You can; we do it all the time. The backfed breaker in the main protects the sub and the feeder to it.

You do not need OCP on the inverter end of the feeder because if the feeder conductors are sized correctly the inverter is incapable of delivering enough current to endanger them.

 

[jaggedben]

Is the 70A sub panel acting as a PV source? You need to protect the feeder, so if it is a source, a MLO panel will not protect the 70A feeder to the main.

The individual branch breakers for each inverter provide the same protection.

 

[Smart $]

The individual branch breakers for each inverter provide the same protection.

Not if the sum of the breakers adds up to more than the ampacity of the feeder... which is possible in 2011 Code, right?

 

[Smart $]

You can; we do it all the time. The backfed breaker in the main protects the sub and the feeder to it.

You do not need OCP on the inverter end of the feeder because if the feeder conductors are sized correctly the inverter is incapable of delivering enough current to endanger them.

I consider the feeders as bus extensions.

I think OP needs to get clarification from plan review, don't you?

No one has even mentioned a section for debate.

 

[ggunn]

I consider the feeders as bus extensions.

I think OP needs to get clarification from plan review, don't you?

No one has even mentioned a section for debate.

In the event of a fault, the fault current comes from the service. There is no need to protect the conductors from the inverters.

 

[jaggedben]

Not if the sum of the breakers adds up to more than the ampacity of the feeder... which is possible in 2011 Code, right?

Perhaps I should have just pointed out that the inverters are current limited sources. It amounts to the same thing.

 

[Smart $]

Perhaps I should have just pointed out that the inverters are current limited sources. It amounts to the same thing.

Sorta kinda. Regardless, I agree with the gist.

 

[Smart $]

In the event of a fault, the fault current comes from the service. There is no need to protect the conductors from the inverters.

I realize this and am in agreement. I was asking if you agree the OP needs to get clarification from the plan reviewer for why they want an MCB panelboard.

 

[ggunn]

I realize this and am in agreement. I was asking if you agree the OP needs to get clarification from the plan reviewer for why they want an MCB panelboard.

Almost. I think the plan reviewer needs to be educated on how solar works.