Battery back up for half of a multi-wire branch circuit

[SunFish]

Just finished installing a battery backup system for a customer. We have a dedicated "backed-up loads" panel to supply critical loads during grid outages. One of the 120 V circuits the customer wants backed up is on a multi-wire branch circuit and the other half has loads that we do not want to back up because they will draw more power than the inverter can handle during backup. Line 1 circuit = kitchen outlets (want to back up) ; Line 2 = Dishwasher + Garbage Disposal (do not want to back up)

We subbed out the AC work for moving the circuits from the main panel to the backed up loads panel. The existing wiring for the MW branch circuit is 12/3 NM from the main panel to the loads. The AC electrician used 12/2 to extend the Line 1 conductor of the MW branch circuit from the main panel over to the new breaker in the backed-up loads panel.

For the neutral of the 12/2 extension circuit between panels they left the 12/3 MW branch circuit neutral landed on the neutral bus of the main panel and ran the neutral of the 12/2 extension circuit from the neutral bus in the main panel to the neutral bus in the backed up loads panel.

Question 1: Anyone see any NEC code violations here with the MW branch circuit L1 breaker in the backed up loads panel, the L2 breaker in the main panel, and the neutral for the circuit extended between each panel's neutral bus?

Question 2: Any way that during back up mode the neutral return current might find it's way back onto the utility. At first glance I don't see how this could happen, the current will want to flow back to the source (inverter) but the electrician is now second guessing this wiring and concerned that we could potentially backfeed power from the neutral onto the utility transformer. Anyone see any concerns, electrically speaking (not code), with how this would operate?

 

[LarryFine]

Just finished installing a battery backup system for a customer. We have a dedicated "backed-up loads" panel to supply critical loads during grid outages. One of the 120 V circuits the customer wants backed up is on a multi-wire branch circuit and the other half has loads that we do not want to back up because they will draw more power than the inverter can handle during backup. Line 1 circuit = kitchen outlets (want to back up) ; Line 2 = Dishwasher + Garbage Disposal (do not want to back up)

Just don't use the D/W or G/D during outages.

We subbed out the AC work for moving the circuits from the main panel to the backed up loads panel. The existing wiring for the MW branch circuit is 12/3 NM from the main panel to the loads. The AC electrician used 12/2 to extend the Line 1 conductor of the MW branch circuit from the main panel over to the new breaker in the backed-up loads panel.

A waste, in my opinion, considering what I said above.

For the neutral of the 12/2 extension circuit between panels they left the 12/3 MW branch circuit neutral landed on the neutral bus of the main panel and ran the neutral of the 12/2 extension circuit from the neutral bus in the main panel to the neutral bus in the backed up loads panel.

So, this conductor is now carrying approximately half of the sub-panel's neutral current. Not good.

Question 1: Anyone see any NEC code violations here with the MW branch circuit L1 breaker in the backed up loads panel, the L2 breaker in the main panel, and the neutral for the circuit extended between each panel's neutral bus?

Yes. All of the MWBC's conductors should originate in one panel. Plus the above.

Question 2: Any way that during back up mode the neutral return current might find it's way back onto the utility. At first glance I don't see how this could happen, the current will want to flow back to the source (inverter) but the electrician is now second guessing this wiring and concerned that we could potentially backfeed power from the neutral onto the utility transformer. Anyone see any concerns, electrically speaking (not code), with how this would operate?

No. However, is the inverter okay with the input and output neutrals tied together.

 

[jaggedben]

Larry basically nailed it. You cannot back up only one leg of a MWBC. It's either both, neither, or take it apart and rewire as non-MWBCs.

Just finished installing a battery backup system for a customer. ...

We subbed out the AC work for moving the circuits from the main panel to the backed up loads panel. The existing wiring for the MW branch circuit is 12/3 NM from the main panel to the loads. The AC electrician used 12/2 to extend the Line 1 conductor of the MW branch circuit from the main panel over to the new breaker in the backed-up loads panel.

For the neutral of the 12/2 extension circuit between panels they left the 12/3 MW branch circuit neutral landed on the neutral bus of the main panel and ran the neutral of the 12/2 extension circuit from the neutral bus in the main panel to the neutral bus in the backed up loads panel.

So in addition to what Larry said, you will have the hot current for that circuit L1 going out on the feeder L1 to the subpanel, then going to the load on the 12/2 cable, neither of which will be balanced by neutral or L2 current as is required. Which is a code violation and a possible source of inductive heating.

Question 2: Any way that during back up mode the neutral return current might find it's way back onto the utility. At first glance I don't see how this could happen, the current will want to flow back to the source (inverter) but the electrician is now second guessing this wiring and concerned that we could potentially backfeed power from the neutral onto the utility transformer. Anyone see any concerns, electrically speaking (not code), with how this would operate?

No. The L1 and L2 would be completely disconnected when the system is in backup mode. (You will definitely have backfeed problems if they are not. The inverter should object and shut down.) But the neutral is just one wire, and thus without another wire it basically cannot complete a circuit to the utility transformer and carry any current. That said, your parallel neutral paths between the sub and main will still be carrying objectionable current.

 

[winnie]

You _can_ back up half of an MWBC, but not as described in the original post.

Simplest would be to supple the complete MWBC from the backup panel, and then have some sort of load shed device to remove power from the half that shouldn't be on backup.

-Jon

 

[gadfly56]

You _can_ back up half of an MWBC, but not as described in the original post.

Simplest would be to supple the complete MWBC from the backup panel, and then have some sort of load shed device to remove power from the half that shouldn't be on backup.

-Jon

The only way to do that is at each device to be shed. The signal is likely to be loss of voltage from the non-backed up panel, which means running cable of some sort or finding a local non-backed up device to monitor voltage. You might as well split the MWBC.

 

[jaggedben]

The only way to do that is at each device to be shed. The signal is likely to be loss of voltage from the non-backed up panel, which means running cable of some sort or finding a local non-backed up device to monitor voltage. You might as well split the MWBC.

There's no reason you can't shed one leg of the branch circuit. I think Jon's idea is a good one, although most often not worth the effort.

 

[SunFish]

You _can_ back up half of an MWBC, but not as described in the original post.

Simplest would be to supple the complete MWBC from the backup panel, and then have some sort of load shed device to remove power from the half that shouldn't be on backup.

-Jon

Yeah, this was the alternative option we came up with since it's going to be hard to rewire as two separate circuits. Pretty easy fix with the system we're using so we'll go this route.