Backfed Main Lug Only
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jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
To give a firm answer, we need to know the rating of the breaker feeding the panel, and the rating of the feeder.
If those ratings are 800A then 160A is correct. To install a 400A solar breaker, the rating of the breaker would have to be 560A or less, and the rating of the feeder would have be 800A or greater. This is "the 120% rule."
The relevant code is in 705.12(D)(1-7).
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
Then it is a supply side connection and can be backfed up to its rating. 705.12(A). 400A is fine.
jaggedben
Senior Member
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- Northern California
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- Solar and Energy Storage Installer
In the 2011 code, new language was added limiting backfeed to the "rating of the service." So that is whatever is the limiting factor, be it the busbars, the service conductors, the CT equipment, etc.
EDIT: I notice that at 500mcm the service conductors may or may not be rated for 400A of backfeed, depending on the insulation type. So when I said "400A is fine", that might have been premature.
No. The solar is not a load, and cannot add any more current to the busbar than the load breakers already allow. If the current configuration is safe, it will also be safe with the solar.
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- Placerville, CA, USA
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- Retired PV System Designer
Right.
For an MB panel here is no code requirement that the sum of the load breakers be less than the capacity of the bus or the service. That protection is expected to come from the main breaker or whatever is feeding it.
But if there is no OCPD anywhere on the feed side of the panel and the (six or fewer) breakers in the panel form the service disconnect, then there is a potential problem unless the sum of those breakers is less than or equal to 800 amps.
The concern behind the 120% rule is that the total current drawn by the loads on the bus could be greater than the bus rating and with the potential source being the sum of the service capacity (unlimited???) and the backed amps also being greater than the bus rating the bus could actually end up carrying that current.
As long as the sum of the load breakers is less than or equal to the bus rating, there cannot be a problem resulting from parallel sources.
The "common" line side tap attaches to the line side conductors and not to an unprotected bus, so this is an unusual situation which requires some interpretation. An actual line side tap, connecting a new panel for back feed to the same service wire,s would be unconditionally OK as long as the backfeed is less than the service rating as jaggedben said.
Another consideration that I didn't see mentioned (only skimmed through post) was allowable breaker size by manuafacturer listing. Has to do with the breaker stab capacity. 400A breaker in an 800A panel probably isn't prohibited, but you need to check panel label or documentation to be certain.
If this was directed at me, the PV system breaker will be installed in a load position and backfed. Manufacturers restrict the sizes that can be used in the load positions. You need to verify a 400A breaker is permitted to be used in whatever position it will be located.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I think it might have been directed at me, in response to my speculation on where, if anywhere, the OCPD was feeding the MLO panel.
If the sum of the load breakers is strictly limited to 800A, then as you said there is no problem with bus loading.
McT
Member
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- Peoria, IL
What if I have a 250A MLO AC combiner panel fed from a 125A breaker in the main switchgear? (3-phase, 3-wire) The feeders from the 125A breaker are #1 AWG THWN-2 CU. It seems like the limiting factor would be the conductors and not the bus of the panel. Therefore, the feeders can carry 150A which limits the total disconnect amperage to 150A x 120% = 180A. Section 705.12D of the 2011 NEC does not limit the 120% rule to just bus but also conductors.
Another question raised to me by one of our more senior engineers is what is protecting the feeders when a MLO AC combiner panel is used? For example, I have 5 inverters feeding an AC combiner panel. The AC combiner panel is 250A MLO. Each inverter feed has a 45A breaker. The circuit breaker in the switchgear feeding the AC combiner panel is 225A and the wiring from the switchgear to the AC combiner panel is 4/0. Say there was a short in the AC wiring between the Switchgear and the AC combiner. What is protecting that feeder? Is each individual 45A breaker enough protection? I get the feeling that all AC combiner panels need to have a main at the panel although I know it's commonly done otherwise.
Another question raised to me by one of our more senior engineers is what is protecting the feeders when a MLO AC combiner panel is used? For example, I have 5 inverters feeding an AC combiner panel. The AC combiner panel is 250A MLO. Each inverter feed has a 45A breaker. The circuit breaker in the switchgear feeding the AC combiner panel is 225A and the wiring from the switchgear to the AC combiner panel is 4/0. Say there was a short in the AC wiring between the Switchgear and the AC combiner. What is protecting that feeder? Is each individual 45A breaker enough protection? I get the feeling that all AC combiner panels need to have a main at the panel although I know it's commonly done otherwise.
I've had this argument before... 705.12(D)(2) does not kick in until you get to "distribution equipment" [see 705.12(D) general statement]. The AC combiner panel and its feeders are not distribution equipment... they are supply equipment as long as the serve no loads via the other source(s) of power.
This is one of the conundrums of multi-source supply systems. In short the reality of it is, if you do have a short in the feeder, there is no longer any reason to protect the conductors. It then becomes a personel and equipment protection issue.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
It's been established on this forum that different AHJs will interpret the rules differently for AC combiners and the feeders that feed them. I think the code has been sufficiently vague that this is to be expected. The 2014 code will completely revise 705.12 and make a lot of things more specific.
In reality what should happen is that the short will create a voltage spike which will cause the inverters to go offline. Once that happens it's back to the 225A breaker on the utility side. It might take a few extra cycles to trip that breaker, but unless it's a very transitory or very resistive fault then it will still trip like normal. If it's not a bolted fault then unpredictable things could happen anyway.
In reality what should happen is that the short will create a voltage spike which will cause the inverters to go offline. Once that happens it's back to the 225A breaker on the utility side. It might take a few extra cycles to trip that breaker, but unless it's a very transitory or very resistive fault then it will still trip like normal. If it's not a bolted fault then unpredictable things could happen anyway.
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