Using a sub-panel for PV breaker too big for Main Service Panel

[photonboy]

If a 100A Main Service Panel with a 100A Main Breaker can only receive 20A of PV backfeed would installing a new sub-panel and backfeeding 40A still overload the Main Panel bus? Seems to me that it would since the 40A is still landing on the MSP bus, just going through a sub first.

I've been seeing this a lot but haven't seen it discussed.

Thoughts?

 

[ggunn]

If a 100A Main Service Panel with a 100A Main Breaker can only receive 20A of PV backfeed would installing a new sub-panel and backfeeding 40A still overload the Main Panel bus? Seems to me that it would since the 40A is still landing on the MSP bus, just going through a sub first.

I've been seeing this a lot but haven't seen it discussed.

Thoughts?

705.12 must be complied with no matter what route the PV current takes getting into a panel, so yes, you are correct.

 

[jaggedben]

Back in 2011 the code had some language that said that for 'panelboards in series' you used the 40A breaker (in your example) to calculate the allowance for all panels. When they changed to rule from the breaker rating to '125% of inverter output current' they thought this language was no longer necessary because it should be obvious that any panel that could be fed by the source had to meet the rules. So that's just a tidbit of code history that proves you and ggunn are correct.

 

[Zee]

If the 100A service (MSP) is a main breaker only (MBO) panel (ie no busbar AKA distribution...ie. no other breakers) and this 100A breaker feeds a sub panel, yes you can put over 20A of PV into the sub panel if it complies with the 120% rule vis a vis the sub panel.

Specifically, if you have a 100A Main-Breaker-Only MSP that feeds a 125A rated sub panel, you can tie in up to 50A (!) of PV into the SP.

However, if the MSP has distribution, ie other brkrs, and one of these feeder brkrs feeds the sub.....then the 20% rule applies to the MSP. It applies whether you tie into the sub panel or the MSP.
I.e. 20A PV max.

 

[electrofelon]

If the 100A service (MSP) is a main breaker only (MBO) panel (ie no busbar AKA distribution...ie. no other breakers) and this 100A breaker feeds a sub panel, yes you can put over 20A of PV into the sub panel if it complies with the 120% rule vis a vis the sub panel.

Specifically, if you have a 100A Main-Breaker-Only MSP that feeds a 125A rated sub panel, you can tie in up to 50A (!) of PV into the SP.

However, if the MSP has distribution, ie other brkrs, and one of these feeder brkrs feeds the sub.....then the 20% rule applies to the MSP. It applies whether you tie into the sub panel or the MSP.
I.e. 20A PV max.

My only slight nitpick is I think what you describe would be an "enclosed circuit breaker" not a panelboard. As far as I know, there is not a product standard for ECB's other than UL 489. I did some digging and could not find an ECB with any certifications other than 489 (UL 67 is the standard for panelboards). Is there a way to get a "thing without distribution" to be listed as a panelboard? What if there was a buss assembly that connects the breaker terminals to the connection lugs, but with no allowance for breaker attachment to said buss?

 

[Fred B]

Question. Would it make a difference if majority of load on main panel were moved to a subpanel and then land PV in the main panel, Vs landing PV in subpanel? If I recall there is a requirement that allows for "sum of all breakers" 705.12(B)(3). IF OP is already considering a subpanel, seems you could "strategically" move a lot of the loads over and have them on a feeder breaker much smaller that the sum of the breakers if loads are typical residental non continuous and nonconcurrent. Seen a lot of resi panels that are 200A and have 400A worth of breakers but the actual load doesn't even come close to high enough to trip the main.

 

[ggunn]

If the 100A service (MSP) is a main breaker only (MBO) panel (ie no busbar AKA distribution...ie. no other breakers) and this 100A breaker feeds a sub panel, yes you can put over 20A of PV into the sub panel if it complies with the 120% rule vis a vis the sub panel.

Specifically, if you have a 100A Main-Breaker-Only MSP that feeds a 125A rated sub panel, you can tie in up to 50A (!) of PV into the SP.

However, if the MSP has distribution, ie other brkrs, and one of these feeder brkrs feeds the sub.....then the 20% rule applies to the MSP. It applies whether you tie into the sub panel or the MSP.
I.e. 20A PV max.

The 120% rule in 705.12 has nothing to do with load breakers or MLO service panels. If your main panel is MLO (no longer legal, BTW) your subpanel is connected on the line side of any service OCPD, and the interconnection is governed by 705.11. Your subpanel with a main breaker is governed by 705.12, though, and its busbar must be qualified under one of the provisions of the part of 705.12 that governs panel busbars (it has moved around a bit through the code cycles), and depending on which provision you use, load breakers in the subpanel may or may not be a factor.

 

[electrofelon]

The 120% rule in 705.12 has nothing to do with load breakers or MLO service panels. If your main panel is MLO (no longer legal, BTW) your subpanel is connected on the line side of any service OCPD, and the interconnection is governed by 705.11. Your subpanel with a main breaker is governed by 705.12, though, and its busbar must be qualified under one of the provisions of the part of 705.12 that governs panel busbars (it has moved around a bit through the code cycles), and depending on which provision you use, load breakers in the subpanel may or may not be a factor.

I don't think he was talking about a MLO. He was talking about something that has no distribution capabilities, such as a ECB or safety switch.

 

[electrofelon]

Question. Would it make a difference if majority of load on main panel were moved to a subpanel and then land PV in the main panel, Vs landing PV in subpanel? If I recall there is a requirement that allows for "sum of all breakers" 705.12(B)(3). IF OP is already considering a subpanel, seems you could "strategically" move a lot of the loads over and have them on a feeder breaker much smaller that the sum of the breakers if loads are typical residental non continuous and nonconcurrent. Seen a lot of resi panels that are 200A and have 400A worth of breakers but the actual load doesn't even come close to high enough to trip the main.

You could certainly do that, bit I don't think it would get you what you need for the "sum of all breakers" rule in most cases. You would have to combine quite a bit of breakers under a relatively small feeder breaker. There are limits to what most people would feel comfortable with.

 

[jaggedben]

Question. Would it make a difference if majority of load on main panel were moved to a subpanel and then land PV in the main panel, Vs landing PV in subpanel? If I recall there is a requirement that allows for "sum of all breakers" 705.12(B)(3). IF OP is already considering a subpanel, seems you could "strategically" move a lot of the loads over and have them on a feeder breaker much smaller that the sum of the breakers if loads are typical residental non continuous and nonconcurrent. Seen a lot of resi panels that are 200A and have 400A worth of breakers but the actual load doesn't even come close to high enough to trip the main.

Yes, this idea can work, sometimes. It tends to work better when you're doing battery backup and both the loads and sources are getting relocated and/or aggregated to one breaker in the main that feeds the microgrid. It also tends to be more practical when the main service panel isn't chock full of the 15-20A single pole branch circuits.

 

[ggunn]

I don't think he was talking about a MLO. He was talking about something that has no distribution capabilities, such as a ECB or safety switch.

Oh, right; sorry, I misread it. Something I have seen in the field, mostly in rural locations, is a 320A meter feeding a pair of 200A feedthough panels with no loads in them out at the pole and conductors running from the feedthrough lugs to 200A subpanels on the buildings. I guess the FT panels are cheaper than 200A fused discos; we usually treated them as discos and installed PV in the subpanels with an ADD NO LOADS placard on any FT panel that had PV connected in its subpanel.

 

[electrofelon]

Oh, right; sorry, I misread it. Something I have seen in the field, mostly in rural locations, is a 320A meter feeding a pair of 200A feedthough panels with no loads in them out at the pole and conductors running from the feedthrough lugs to 200A subpanels on the buildings. I guess the FT panels are cheaper than 200A fused discos; we usually treated them as discos and installed PV in the subpanels with an ADD NO LOADS placard on any FT panel that had PV connected in its subpanel.

So does having something with no capabilities for distribution gain you anything? Take the case zee was talking about:. If that first device was a feed thru panelboard with no branch breakers, you could just use the sum of all breakers rule for it. Wasn't there a time, like pre 2017 code, where it made a difference whether something had the capability for other loads? Maybe the sum of all breakers rule didn't exist?

 

[ggunn]

So does having something with no capabilities for distribution gain you anything? Take the case zee was talking about:. If that first device was a feed thru panelboard with no branch breakers, you could just use the sum of all breakers rule for it. Wasn't there a time, like pre 2017 code, where it made a difference whether something had the capability for other loads? Maybe the sum of all breakers rule didn't exist?

Yes; the sum of all breakers rule came into the NEC in 2011 or 2014. Before that, most of the AHJs I dealt with let it slide when an AC combiner panel was technically in violation of 705.12.

 

[jaggedben]

So does having something with no capabilities for distribution gain you anything? Take the case zee was talking about:. If that first device was a feed thru panelboard with no branch breakers, you could just use the sum of all breakers rule for it. Wasn't there a time, like pre 2017 code, where it made a difference whether something had the capability for other loads? Maybe the sum of all breakers rule didn't exist?

Until 2023 the first paragraph of the "Load Side Source Connections" or equivalent section of 705.12 referred to 'distribution equipment ... capable of supplying multiple branch circuits or feeders, or both'. In 2023 that is gone and the rules now generally apply to 'distribution equipment or feeders'. Make of that what you will.

Some people used to believe that a breaker or fused disconnect was subject to the 120% rule but I haven't dealt with anyone who thought that for almost ten years now.

The 'sum of all breakers' rule was added in the 2014 NEC, and that was also when the 120% rule was changed to apply to panelboards instead of 'busbars or conductors'. (In the 2023 NEC it applies to 'busbars'.) So arguably before the 2014 NEC having distribution didn't make a difference, and after that it did; opposite of what you said.