Requirement for breaker feeding inverter

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Carultch

Senior Member
Location
Massachusetts
I think this opens the door to violations from the inspector. you have to follow the current code your jurisdiction follows, not the latest NEC. If i do something that the latest NEC permits, but wasn't part of the 2008 NEC, I will get screwed with a violation and a potential change order that the client would have to pay extra for. guess who gets yelled at?! Me.

What happens if products cease to exist, that were once required by previous code cycles?

As an example, in NEC 2017, they introduced the concept of "functionally grounded systems", that most systems would've been since GFCI was required. This applies to both grounded systems on transformered inverters that are grounded through a GFCI device, and ungrounded systems that are "functionally grounded" by being non-isolated from a grounded AC grid. I like the new term, but I don't like that it applies to two fundamentally different kinds of systems. I'd prefer functionally grounded as the first kind I described, and to continue to call the second kind of system ungrounded, as we did prior to 2017. My suggested term for the second kind of system, would be symmetry grounded, since the two polarity voltages are symmetric about ground.

In any case, NEC2017 now specifies that functionally grounded systems of either kind, require disconnecting means on both polarities, and OCPD only on one polarity. This means a 2017-compliant combiner would have disconnect poles on both polarities, a group of fuses on the positive, and a simple busbar on the negative. If pre-2017 combiners cease to be in production, you couldn't build it this way and comply with pre-2017 codes, that required you NOT to disconnect the grounded polarity.
 

Grouch1980

Senior Member
Location
New York, NY
If a product ceases to exist, then yeah you pretty much have no choice at that point. You have to specify what's available and then follow a later code. I agree with that. At that point the inspector can't say much. I haven't come across that happening yet. Probably because most of my work is the usual building systems. something newer such as PV systems that keep modernizing, i can see that being an issue with products being discontinued.
 

Grouch1980

Senior Member
Location
New York, NY
Breakers are always capable of being operated in the forward direction. A backfed rated breaker means that it can operate with power flowing in either direction. Most breakers made today are backfed rated, provided that they don't have extra functionality such as AFCI or GFCI. For breakers with extra functionality, you have to be careful with product selection.
Going back to this... let's say a PV inverter is connected to a panel via a backfeed circuit breaker, located at the bottom of the panel. If the main circuit breaker for the panel, located at the top of the panel, is NOT backfeed rated, the inverter could never feed power back to the utility? Let's assume the panel is for a house. I'm assuming this almost never happens, since from what you said most breakers are both forward and backfeed rated?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Going back to this... let's say a PV inverter is connected to a panel via a backfeed circuit breaker, located at the bottom of the panel. If the main circuit breaker for the panel, located at the top of the panel, is NOT backfeed rated, the inverter could never feed power back to the utility? Let's assume the panel is for a house. I'm assuming this almost never happens, since from what you said most breakers are both forward and backfeed rated?
The main breaker must also be suitable for backfeed. So must all breaker upstream. IRL, this is only an issue on larger feeders and services where the main breaker has ground fault protection because it's over 1000 amps. See 240.13 (2020 NEC reference) or 230.95 (2017 reference), and 705.32 (2017 reference). On a residence it will never be an issue.
 

Grouch1980

Senior Member
Location
New York, NY
The main breaker must also be suitable for backfeed. So must all breaker upstream. IRL, this is only an issue on larger feeders and services where the main breaker has ground fault protection because it's over 1000 amps. See 240.13 (2020 NEC reference) or 230.95 (2017 reference), and 705.32 (2017 reference). On a residence it will never be an issue.
So let's say one screws up, and one of the upstream breakers is not backfeed rated... nothing goes back to the utility?

Also, how does backfeed ratings work for fuses? are those inherently forward and backfeed rated, or must you make sure they are also backfeed rated?
 

Carultch

Senior Member
Location
Massachusetts
So let's say one screws up, and one of the upstream breakers is not backfeed rated... nothing goes back to the utility?

Also, how does backfeed ratings work for fuses? are those inherently forward and backfeed rated, or must you make sure they are also backfeed rated?
Fuses are inherently rated for power to flow in either direction, as the trip element is just a thin section of wire that melts when it overheats as a consequence to overcurrent. A fused disconnect is marked line and load, because of which parts of it are possible to de-energize, as you want the blades, fuses, and load-side terminals to all be de-energized when the unit is shut off. The line side terminals have a plastic shield that guards against accidental contact. A fused disconnect is inherently backfeed-rated, despite the fact that it is marked line and load. Its reasons for being marked line and load are about which side remains energized, rather than which direction power flows.

Given a breaker that isn't backfeed rated, we cannot conclude what will happen when backfeed is attempted from this information alone. The rating doesn't specify the consequences of attempting to backfeed the breaker. The rating only specifies that it hasn't passed the tests required to call it suitable for backfeed. Either because the testing was never done in the first place, or it failed one of the tests such that it doesn't perform as it is intended to perform.

One possibility, is that it fails to trip at its trip rating, when carrying reverse current. Or it nuisance trips at a lower amp value than its trip rating. Or perhaps one of the additional functions such as GFCI or AFCI doesn't work properly. Another possibility is that it works just fine, and the only reason it isn't backfeed rated, is that no one ever bothered to have it formally tested in both directions.

It is very unlikely that a non-backfeed breaker would prevent attempted backfeed entirely. In concept, you could implement zero-export controls, that monitor the current drawn in from the source to this breaker, and curtail your inverter's power to prevent backfeed and only feed the local loads within the panel or switchboard. You'd have to clear this with your AHJ before you commit to a solution like this.

I find it very hard to believe that specific problems would result from backfeeding any AC breaker. Hopefully someone who knows more information about the specific issues, can enlighten us. I can understand why a DC breaker would be sensitive to polarity and current direction, but not for an AC breaker. AC by definition flows in the reverse direction half the time. The only difference between forward feed and reverse feed in AC, is whether voltage is in-phase with current or 180 degrees out of phase with current, and I don't see how this factor has anything to do with the theory of operation of a circuit breaker.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
So let's say one screws up, and one of the upstream breakers is not backfeed rated... nothing goes back to the utility?

Power will flow to the utility as long as the breaker stays closed. There are basically two bad possibilities and one not so bad possibility if this happens by mistake.

- The breaker trips when it is backfed, and the customer loses power.
- The breaker fails to trip when a fault happens while it's being backfed. Potentially disastrous, and the main reason for the rule.
- Nothing bad happens and it's just a code violation.
 
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