Panel backfeed issues

[Eros]

I have a 200a service in a home that I need to connect/backfeed 2 - 50a PV breakers to. I can either increase the service or re-configure the existing service. Any thoughts

 

[ron]

Depending on the calculated load for the home, you might be able to reduce the main breaker low enough OR, just do a line side tap.

 

[ggunn]

I have a 200a service in a home that I need to connect/backfeed 2 - 50a PV breakers to. I can either increase the service or re-configure the existing service. Any thoughts

If you downsize the main breaker it must be less than 140A.

50A + 50A + 140A = 240A = 1.2 X 200A

 

[jaggedben]

There are four options to consider in this situation (not all of them will necessary be an actual option on any given job). Three have already been mentioned:

1) Upgrade the service (if the customer is willing to pay)
2) Do a line side tap (not possible on many meter/main combos)
3) Downsize the main breaker (if the existing load allows)

The final option...

4) Revise the contract to be for a smaller system.

 

[Carultch]

1) Upgrade the service (if the customer is willing to pay)

You don't need to upgrade the service, you need to upgrade the busbar rating of the main panelboard.

Granted, it is major surgery to do that still. But at least you wouldn't need to get the utility involved to change any equipment upstream this device. Provided that you can fit a 200A breaker in a 400A panelboard.

 

[GoldDigger]

You don't need to upgrade the service, you need to upgrade the busbar rating of the main panelboard.

Granted, it is major surgery to do that still. But at least you wouldn't need to get the utility involved to change any equipment upstream this device. Provided that you can fit a 200A breaker in a 400A panelboard.

Not that it is likely to happen in other jurisdictions, but Los Angeles DWP is particularly anal about this point: They will not allow the panel rating (regardless of the actual main breaker installed) to be higher than the nominal service size. This causes all kinds of grief for solar installers there.

 

[jaggedben]

You don't need to upgrade the service, you need to upgrade the busbar rating of the main panelboard.

Granted, it is major surgery to do that still. But at least you wouldn't need to get the utility involved to change any equipment upstream this device. Provided that you can fit a 200A breaker in a 400A panelboard.

Correct, I should have said "service panel upgrade."

 

[ActionDave]

Not that it is likely to happen in other jurisdictions, but Los Angeles DWP is particularly anal about this point: They will not allow the panel rating (regardless of the actual main breaker installed) to be higher than the nominal service size. This causes all kinds of grief for solar installers there.

Seems like it would cause grief for others too. Around here a 200A feed through panel can be bought for about $75 less than a 150A.

 

[Eros]

What if the 200a MB panel is converted to a sub-panel with a 200a main disconnect. Does the 120% rule still apply to the sub-panel - it no longer has a main breaker to include in the calculation

 

[Eros]

Also, does the 120% rule apply to the service, the main breaker, or the bussbar rating the solar breakers are connected to ?

 

[jaggedben]

What if the 200a MB panel is converted to a sub-panel with a 200a main disconnect. Does the 120% rule still apply to the sub-panel - it no longer has a main breaker to include in the calculation

The 120% rule still applies (it applies to all panels) and the rating of the fuses in the disconnect still count towards it.

 

[jaggedben]

Also, does the 120% rule apply to the service, the main breaker, or the bussbar rating the solar breakers are connected to ?

The busbar.

 

[GoldDigger]

The busbar.

And, in the case of a subpanel, the feeder conductors to that sub. (At least according to some AHJs.)

 

[electrofelon]

Not that it is likely to happen in other jurisdictions, but Los Angeles DWP is particularly anal about this point: They will not allow the panel rating (regardless of the actual main breaker installed) to be higher than the nominal service size. This causes all kinds of grief for solar installers there.

That is the silliest thing I have ever heard. I guess in the interest of avoiding a complete crisis and paradigm shift in their lives, we will keep it a secret that manufacturers use the same bus bars for many different panel ratings....What do they think of a SUSE MLO panel with 6 service disconnects?

 

[shortcircuit2]

What if the 200a MB panel is converted to a sub-panel with a 200a main disconnect.

You may be able to reconfigure the service by adding a new 200amp main panel with 225amp bus, then back feed the existing main panel with a 125amp breaker (if load calculations allow) and feed the two 50amp solar breakers into the new main panel. Then you would be OK under the 100% rule with the a 125amp sub-feed and 2-50amp solar breakers=225amps

Here is a Square D piece of equipment that may have 225amp bussing for this conversion...

http://www.homedepot.com/p/Square-D...tdoor-Main-Breaker-CSED-RC816F200CH/100152735

Otherwise do a line side connection.

 

[ggunn]

What if the 200a MB panel is converted to a sub-panel with a 200a main disconnect. Does the 120% rule still apply to the sub-panel - it no longer has a main breaker to include in the calculation

The 120% rule applies to all the panels through which the power from the inverters travels on the way back to the service. If the power is fed through a subpanel, then the inverter output breakers count toward the 120% rule in both the sub and the main. If there are no loads in the sub then in some jursdictions only the backfed breakers are counted, but they still count in the main as well.

 

[GoldDigger]

If the sub is MLO then you use the size of the feeder breaker in the main.
It is only in a supply side connection that you do not have a supply breaker to count.

 

[ggunn]

If the sub is MLO then you use the size of the feeder breaker in the main.
It is only in a supply side connection that you do not have a supply breaker to count.

It depends on the AHJ. I have designed load side connected systems that used AC combiners sized only to accommodate the inverter breakers which passed muster with the AHJ.

 

[GoldDigger]

It depends on the AHJ. I have designed load side connected systems that used AC combiners sized only to accommodate the inverter breakers which passed muster with the AHJ.

Correct. Combiner-only panels are a special case.
But for a general sub with loads the feed ampacity is counted.

 

[jaggedben]

And, in the case of a subpanel, the feeder conductors to that sub. (At least according to some AHJs.)

To be even more precise, it depends on the code cycle.
Before 2014, the wording suggests that it applies to both the busbars and the feeder conductors. (705.12(D)(2))
The 2014 NEC, while still a little vague, has a specific section of rules for Feeders (705.12(D)(2)(1)) and another set of rules for Busbars(705.12(D)(2)(3)).

The Feeders section applies where an "inverter output connection is made to a feeder at a location other than at the opposite end of the feeder from the primary source overcurrent device". The implication is that the rules in this section do not apply if the location is the opposite end. The section also contains no 120% rule, and the old language that applied that rule to both busbars and conductors is gone. The 120% rule is now only in the Busbars section.

One option that is gone in the 2014 is to make a tap on a feeder and argue that the combined primary and interactive sources are allowed to supply the feeder 'up to 120%.' You are now explicitly required to put overcurrent protection on the load end of the feeder that protects the feeder at its ampacity.