Backfeed Breaker in Subpanel

[ohmti787]

Hello,

In these scenarios, do I still need to install a main breaker in the subpanel to protect the feed through conductors?



Thanks!

 

[ggunn]

No.

 

[PaulMmn]

Hello,

In these scenarios, do I still need to install a main breaker in the subpanel to protect the feed through conductors?

View attachment 2566756

Thanks!

Looking at your diagram, I'm assuming (!) the sub-panel is to handle circuits to remain live in the event of a power failure.
Won't you need an interlocked main breaker in the sub panel to prevent back-feeding the mains?

 

[ggunn]

Looking at your diagram, I'm assuming (!) the sub-panel is to handle circuits to remain live in the event of a power failure.
Won't you need an interlocked main breaker in the sub panel to prevent back-feeding the mains?

I didn't see anything in his electrical drawing that indicates that there is a battery inverter in the system. Am I missing something?

 

[ohmti787]

Looking at your diagram, I'm assuming (!) the sub-panel is to handle circuits to remain live in the event of a power failure.
Won't you need an interlocked main breaker in the sub panel to prevent back-feeding the mains?

no sir, that's the existing subpanel inside the home, being fed from the meter main outside. instead of installing our backfeed breaker in the meter main outside, and having to install a main breaker to protect the subpanel downstream, we decided to put the PV breaker in the subpanel instead so we don't need to buy that main breaker. but i'm having second thoughts about the potential of overloading the feed through conductors.

 

[ohmti787]

I didn't see anything in his electrical drawing that indicates that there is a battery inverter in the system. Am I missing something?

it's just solar. one string of panels with micros.

 

[PaulMmn]

I didn't notice the bi-directional utility meter.

 

[ggunn]

no sir, that's the existing subpanel inside the home, being fed from the meter main outside. instead of installing our backfeed breaker in the meter main outside, and having to install a main breaker to protect the subpanel downstream, we decided to put the PV breaker in the subpanel instead so we don't need to buy that main breaker. but i'm having second thoughts about the potential of overloading the feed through conductors.

How would that happen? Unless all the loads in the sub are all turned off, when it is running the PV will reduce the current in the feedthrough conductors, and if they are turned off, the PV can only push at most 20A back through the 150A conductors to the main panel. 705.12(B) is satisfied in both panels; I don't see the problem.

 

[winnie]

I have a slightly different take.

With the connection to the subpanel having no OCPD, the cables are essentially bus bars.

In which case having a 150A OCPD and 150A cables limits your maximum allowed solar circuit.

I think that the 20A solar circuit is ok (150 + 1.25*20 < 150*1.2) but it is close to the maximum.

Add the 150A breaker or increase the ampacity of those conductors on the 200A panels and you can go up to a 70A PV breaker.

Jon

 

[wwhitney]

With the connection to the subpanel having no OCPD, the cables are essentially bus bars.

Perhaps abstractly, but not in the sense that matters here. Treating the assembly as a single composite panel is not supported by the code language, or by the reasons for it.

Busbars not otherwise tested for multiple sources of supply are limited by the 705.12(B) rules due to the additional heat that can be generated by the additional current that can flow through the connections to those busbars, occurring within the limited space of the busbar enclosure. That consideration does not apply to the feed thru conductors, which have only two connections, one at each end, spatially dispersed; and since the curren

So the 120% rule can be applied to each busbar separately. If the busbars were both 200A, with the feed-thru conductors say 150A and protected by a 150A breaker, it would still be fine to interconnect with a 40A breaker in the subpanel.

Cheers, Wayne

 

[PWDickerson]

If you only have a single circuit of grid-interactive micro inverters, with no battery in the system, then no you don't need a main breaker in the sub-panel as long as it is in the same building as the meter-main. As ggunn indicated, the solar current reduces the current in the feeders rather than adds to it (unless there are no loads, then the current flows "backwards" and is limited to the output of the PV system).

 

[ohmti787]

Thank you guys. I appreciate the input