Roof mounted microinverter pv system - combiner box not needed

[RJM60]

System specs: 24 JAM60S02-310/PR panels with IQ7-Plus microinverters (max output 290 va/1.29 a) split into 3 arrays. Array 1 has 16 panels split into two 8 panel strings. Arrays 2 and 3 each have 4 panels. Each array is on a different roof section.

Whenever I see an Enphase microinverter system, it always runs each string to a combiner (roof mounted or at ground level) with each string going to a 20 amp breaker, or fuse.

On DIY systems, I've noticed that if the DIY'r doesn't use a combiner box, they still run each string to a 20 amp breaker before combining the strings and connecting to a back fed breaker in their service panel. The Enphase design guide promotes this as well.

My question is this: Since the only limiting factor for the number of microinverters in a string is the 12 AWG Q cable ampacity rating (20 a), why can't the strings just be wire nutted together in a J-Box on the roof with a 10 AWG home run taking all the power to the back fed breaker I the service panel.

I don't understand the purpose of a combiner box with a 20 amp breaker for each string. The breaker will never flip, especially with only 8 microinverters (total current = 10.32 a).

Could this be an unneeded leftover from early microinverter systems?

[Ampster]

The breaker is there to protect the wire. In the case of the Q cable it is 12 AWG which is rated at 20 Amps. I don't believe it is a problem to combine strings up to 16 Amps. The NEC rules state that you can only run a continuous load at 80 percent of the breaker size. In this case if each microinverter is rated at 1.29 Amps then the most you could have on one string is 12.This may present challenges with regard to the location of the various groups of panels but the logistics will probably be determined by the cost of wire and conduit and the cost of labor to run to the various locations. Don't confuse the physical location (also called strings) with the electrical string.

[RJM60]

The breaker is there to protect the wire. In the case of the Q cable it is 12 AWG which is rated at 20 Amps. I don't believe it is a problem to combine strings up to 16 Amps because the NEC rules state that you can only run a continous load at 80 percent of the breaker size. In this case if each microinverter is rated at 1.29 Amps then to most you could have on one string is 12.

Protecting it from what? Somehow one or more inverters break and put out a lot more current - or is it just an NEC thing - must protect the wire even if it's impossible to overload it?

Are you saying it's okay to combine the strings without an intermediate 20 amp breaker or fuse?

[Ampster]

Protecting it from what? Somehow one or more inverters break and put out a lot more current - or is it just an NEC thing - must protect the wire even if it's impossible to overload it?

Yes it is a NEC thing and short circuits do happen. A short circuit could be in the inverter or a cable getting pinched. Remember, even if the inverters are not producing power, the cable is connected to a live electrical panel.

Are you saying it's okay to combine the strings without an intermediate 20 amp breaker or fuse?

I literally said you could combine up to 12 inverters. Since the inverters are actually in parallel you can combine several cables as long as you have no more than 12 inverters on each pair of 12 AWG wire going to a 20 Amp breaker at your electrical panel or a breaker on the roof.

If you don;t have a basic level of understanding of electrical codes, I would suggest increasing your life insurance of find a friend who does understand. The shock of 240 volts might not kill you but falling off the roof after getting shocked might kill you. You don't want to risk your life based on the word of some anonymous poster like me. Trust but verify.

[RJM60]

If you don;t have a basic level of understanding of electrical codes, I would suggest increasing your life insurance of find a friend who does understand. The shock of 240 volts might not kill you but falling off the roof after getting shocked might kill you. You don't want to risk your life based on the word of some anonymous poster like me. Trust but verify.

This was unnecessary but thanks for the rest.

[SunEagle]

Yes it is a NEC thing and short circuits do happen. A short circuit could be in the inverter or a cable getting pinched. Remember, even if the inverters are not producing power, the cable is connected to a live electrical panel.

I literally said you could combine up to 12 inverters. Since the inverters are actually in parallel you can combine several cables as long as you have no more than 12 inverters on each pair of 12 AWG wire going to a 20 Amp breaker at your electrical panel or a breaker on the roof.

If you don;t have a basic level of understanding of electrical codes, I would suggest increasing your life insurance of find a friend who does understand. The shock of 240 volts might not kill you but falling off the roof after getting shocked might kill you. You don't want to risk your life based on the word of some anonymous poster like me. Trust but verify.

Hmmm. I wonder if the Tesla panels that are burning up on the Walmart roofs have had over current protection for the wiring. If not the fires do happen.

[sdold]

Strings is probably the wrong term, the Enphase inverters are wired in parallel on a "branch circuit" to the electrical service panel that will have a limit to the number of inverters on that circuit. I think the limit for each circuit with the IQ 7+ is 13, for my M215s it was 17. I had two rows of panels, 16 in total, both rows were paralleled in a box but it was still considered one 16-panel branch circuit.

[RJM60]

Hmmm. I wonder if the Tesla panels that are burning up on the Walmart roofs have had over current protection for the wiring. If not the fires do happen.

Another unnecessary snide remark .

From what I understand, the panels sparked, and the sparks caused the fires. I'd also guess that since it's a commercial installation, it doesn't use microinverters and is therefore a DC system running at voltages closer to 400 or 600 VDC rather than 240 VAC and that here were fuses installed.

[Ampster]

Hmmm. I wonder if the Tesla panels that are burning up on the Walmart roofs have had over current protection for the wiring. If not the fires do happen.

Apparently in one case a fuse was replaced with a bolt. From what I read it wasn't the panels per se but in some cases the H4 connectors and in others wire chaffed on metal and caused sparks.

[RJM60]

Strings is probably the wrong term, the Enphase inverters are wired in parallel on a "branch circuit" to the electrical service panel that will have a limit to the number of inverters on that circuit. I think the limit for each circuit with the IQ 7+ is 13, for my M215s it was 17. I had two rows of panels, 16 in total, both rows were paralleled in a box but it was still considered one 16-panel branch circuit.

yeah - once you add the breaker, I guess branch circuit is the term but it's still a string even if string isn't a defined term in the NEC, or CEC.

In any case, I'll have to add some kind of combiner with OCP for each branch. Makes sense when you think about - a short could happen,

[sdold]

I don't understand the purpose of a combiner box with a 20 amp breaker for each string.

Probably to allow a single backfeed breaker to the panel that exceeds 20A, while still having a 20A breaker on each branch. I've never seen this done with a combiner box on the roof, only with a sub-panel near the main panel.

A "string" usually refers to several panels wired in series using a single inverter instead of microinverters.

[Ampster]

yeah - once you add the breaker, I guess branch circuit is the term but it's still a string even if string isn't a defined term in the NEC, or CEC

Since microinverters are AC devices branch circuit is an easier term for me to relate to with regard to AC wiring concepts. You can use any term that works for you.

In any case, I'll have to add some kind of combiner with OCP for each branch. Makes sense when you think about - a short could happen,

I have only done two micro inverter installations and they were less than 20 Amps. if I were to do a larger microinverter system I would spend the extra money on wiring to bring each 20 Amp branch circuit to the electrical panel instead of OCP protection on the roof. If the runs were long I could always use 10 AWG to reduce voltage loss. EDIT: As simultaneously suggested by sdold

[RJM60]

Since microinverters are AC devices branch circuit is an easier term for me to relate to with regard to AC wiring concepts. You can use any term that works for you.

I have only done two micro inverter installations and they were less than 20 Amps. if I were to do a larger microinverter system I would spend the extra money on wiring to bring each 20 Amp branch circuit to the electrical panel instead of OCP protection on the roof. If the runs were long I could always use 10 AWG to reduce voltage loss.

I guess it's called an aggregator. I just read an Enphase guide that said it could go on the roof but I think it also said it contains fuses that are not serviceable, so I don't think I'd use that.

I really don't know what I was thinking. You can't just change wire current carrying gauge without adding protection for the smaller wire.

[RJM60]

I have only done two micro inverter installations and they were less than 20 Amps. if I were to do a larger microinverter system I would spend the extra money on wiring to bring each 20 Amp branch circuit to the electrical panel instead of OCP protection on the roof. If the runs were long I could always use 10 AWG to reduce voltage loss. EDIT: As simultaneously suggested by sdold

I'm not sure AHJ would let me do that but not gonna anyway.

[Ampster]

I'm not sure AHJ would let me do that but not gonna anyway.

Do what? Run larger wire?
I think you can use larger wire but you may need to label it as a lower amperage circuit. That is to prevent a later repair person upgrading the breaker because he only sees the larger wire and not the smaller wire on the roof.