Oversizing back feed OCPD breaker, and wire sizing

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Anode

Member
Location
Washington, USA
Simple question. Does the feeder from the inverter output circuit, either single or combined, always have to match the rating of the back feed ocpd installed in the main panel or switchboard?

I know that we can oversize the back feed ocpd breaker for the inverter output circuit, beyond the 125%. For example:

if you have a residential sized pv array where the ocpd would equal say 35 Amps, it is commonly upsized to a 40Amp common trade size breaker.

In this example, my question again rephrased would be; in any case could the wire still be sized to 35 Amps, which again is adequately oversized for the max continuous output rating of the inverter, when the back feed ocpd is 40Amps.



Have a commercial project where EC installed larger breaker in the switchboard 250A, our plans for the pv system show 200A. We are most likely going to either ask them to provide the smaller breaker, or upsize our conduit and conductors between panelboard collecting inverter output circuits and the main switchboard to match the 250A breaker that is now existing.

So, my question is hypothetical in nature, as we are going to work with the circumstances. Regardless, your input is appreciated. I should note that I searched this forum, and couldn't find a thread dedicated specifically to this.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Simple question. Does the feeder from the inverter output circuit, either single or combined, always have to match the rating of the back feed ocpd installed in the main panel or switchboard?

I know that we can oversize the back feed ocpd breaker for the inverter output circuit, beyond the 125%. For example:
if you have a residential sized pv array where the ocpd would equal say 35 Amps, it is commonly upsized to a 40Amp common trade size breaker.

In this example, my question again rephrased would be; in any case could the wire still be sized to 35 Amps, which again is adequately oversized for the max continuous output rating of the inverter, when the back feed ocpd is 40Amps.



Have a commercial project where EC installed larger breaker in the switchboard 250A, our plans for the pv system show 200A. We are most likely going to either ask them to provide the smaller breaker, or upsize our conduit and conductors between panelboard collecting inverter output circuits and the main switchboard to match the 250A breaker that is now existing.

So, my question is hypothetical in nature, as we are going to work with the circumstances. Regardless, your input is appreciated. I should note that I searched this forum, and couldn't find a thread dedicated specifically to this.
Your wire must always be protected by the OCPD. That is to say that your conductor ampacity derated for conditions of use must be greater than the rating of the next size down OCPD from the one you are using. That's for current less than 800A; for 800A or more it must be greater than the rating of the OCPD itself.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What about 690.8(b)2?
What about it? 690.8(B) just says that the wire needs to be the larger of the two sizes calculated by (1) and (2). 240.4 says conductors must be protected according to their ampacities. You can oversize the breakers all you want, but when you do you must make sure that your wiring is still protected by the OCPD from fault current from the service, and to do that you may have to upsize the conductors.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Consider that the wiring is subject to both the inverter output, which is strictly limited and to fault current coming back from the grid side through the OCPD.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Consider that the wiring is subject to both the inverter output, which is strictly limited and to fault current coming back from the grid side through the OCPD.
Yes, but if the conductors are properly sized the inverter is incapable of endangering them. Realistically, fault current from the service is the only consideration for OCP.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Simple question. Does the feeder from the inverter output circuit, either single or combined, always have to match the rating of the back feed ocpd installed in the main panel or switchboard?

I don't think you're using terms precisely. If it's a 'feeder' then it connects to more than the inverter. The inverter output branch circuit wiring can be oversize but must be protected by the OCPD.

[/QUOTE]I know that we can oversize the back feed ocpd breaker for the inverter output circuit, beyond the 125%. For example: if you have a residential sized pv array where the ocpd would equal say 35 Amps, it is commonly upsized to a 40Amp common trade size breaker. [/QUOTE]

I see no justification for that in the code. I buy 35A breakers all the time. Not saying it isn't common but I've got too many AHJs to worry about to remember which ones have no objection to that. Some do.

In this example, my question again rephrased would be; in any case could the wire still be sized to 35 Amps, which again is adequately oversized for the max continuous output rating of the inverter, when the back feed ocpd is 40Amps.

Not code compliant in my opinion. It's probably safe if the inverter is not rated to output more than 28 amps, but such an inverter may come with instructions that the max ocpd is 35A. It's not so safe if the inverter is rated to output 32 amps. Or at least not to the Code's theory of derating for continuous output.

Have a commercial project where EC installed larger breaker in the switchboard 250A, our plans for the pv system show 200A. We are most likely going to either ask them to provide the smaller breaker, or upsize our conduit and conductors between panelboard collecting inverter output circuits and the main switchboard to match the 250A breaker that is now existing.

Maybe a third option is to install a fused disconnect somewhere in between with 200A fuses. I know such discos are expensive, but it might be less than upsizing the conductors if they are long. Especially if you already have a non-fused disco planned somewhere, that isn't bought yet.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Yes, but if the conductors are properly sized the inverter is incapable of endangering them. Realistically, fault current from the service is the only consideration for OCP.
+1.
My point was that sizing the conductors to the inverter output alone is not sufficient.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Realistically, fault current from the service is the only consideration for OCP.

There can also be issues when mistakes are made. Say microinverters don't get divided up properly between circuits and you have too many connected to one trunk cable.
 
Do any inverters have instructions as to a max size OCPD? It makes sense - and sort of analogous to an HVAC unit that always has this info - to not have an excessively sized OCPD in case of an internal fault or failure of the inverter.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Do any inverters have instructions as to a max size OCPD? It makes sense - and sort of analogous to an HVAC unit that always has this info - to not have an excessively sized OCPD in case of an internal fault or failure of the inverter.

ABB (formerly Power-One) datasheets have them. Micros inverters always have them as far as I've seen.
 

Carultch

Senior Member
Location
Massachusetts
Do any inverters have instructions as to a max size OCPD? It makes sense - and sort of analogous to an HVAC unit that always has this info - to not have an excessively sized OCPD in case of an internal fault or failure of the inverter.

If that ends up being the case, it is simply a multiple OCPD solution that you would have to have, if you were in a situation similar to that of our OP. Some datasheets/manuals do specify this, others do not. It is uncommon that slightly higher than default ratings of an inverter OCPD will matter, as its main purpose is to protect against short circuit/ground fault, rather than simple overload.

The interconnection breaker can be any size greater than 1.25*system current, and if the inverter manufacturer specifically requires a smaller OCPD, you simply put that smaller OCPD between the inverter and the interconnection OCPD. It might already be there, if you have an AC combining panelboard to combine the outputs of smaller string inverters.

In any event, you size the wire per 690.8 for PV-specific methods, and in the general sense to "be protected" by the OCPD. 240.4(B), the "next size up rule", applies as well. This means if you want to use 250A of interconnection breaker, you need at least 225.0000001 Amps of wire in the final connection to it. Once you are behind a smaller OCPD (perhaps 200A), then you would only need 175.000001A or 1.25*system-current worth of wire and terminations.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
There can also be issues when mistakes are made. Say microinverters don't get divided up properly between circuits and you have too many connected to one trunk cable.
There is no OCPD made that can protect against incompetency. My comments are based on the assumption that the system is correctly designed. If it's not, there's no telling what could go wrong.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
There is no OCPD made that can protect against incompetency. My comments are based on the assumption that the system is correctly designed. If it's not, there's no telling what could go wrong.

I don't mean incorrectly designed, I mean incorrectly installed. Anyway, point taken.
 
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