Are Batteries considered loads when sizing feeder wires?

[Solar_ice_caps]

Designing a solar install with battery backup.

I will need to add a "System Controller" that will become the new Service Entrance.

That will effectively make the current Main panel a sub-panel.

310.12 Single-Phase Dwelling Services and Feeders. For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.12(A) through (D).

(B) Feeders. For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

I just got burned on this on the last job because we intercepted the service with a load center outside and so we had to upsize the wires that were going to the old panel because it became a sub-panel that was technically not handling all the loads of the house.

So my question here is, I'll be adding batteries that, while they provide power, also need to charge. Are they considered a load such that the old panel would no longer be "supplying the entire load associated with a one-family dwelling?"

Tangentially, I don't understand how the existence of loads elsewhere in the system makes a feeder of a given size able to handle more amperage.

 

[jaggedben]

...
I just got burned on this on the last job because we intercepted the service with a load center outside and so we had to upsize the wires that were going to the old panel because it became a sub-panel that was technically not handling all the loads of the house.

I'm sorry that happened to you because it's almost certainly wrong. See 310.12(C). Try to argue next time. (Unless the existing feeder was undersized in the first place, which happens sometimes.)

So my question here is, I'll be adding batteries that, while they provide power, also need to charge. Are they considered a load such that the old panel would no longer be "supplying the entire load associated with a one-family dwelling?"

This seems like a totally separate issue and I agree that due diligence requires doing for example a 220.83 load calc with the batteries as additional load. Can't say I've done that every time. But I've definitely done it, especially when it seemed close. Hopefully it doesn't trigger a service upgrade requirement. You might be able to get around this if you have the ESS set to charge only from solar and/or a PCS listed for 'Export Only' mode. But this is a tricky subject, I'm not sure I'd accept that if I were the AHJ.

Tangentially, I don't understand how the existence of loads elsewhere in the system makes a feeder of a given size able to handle more amperage.

It doesn't. See above.

 

[Solar_ice_caps]

I did try to argue it but I’m still really not 100% clear on the interpretation.

In that case we upgraded the service from 125A (#2 Cu) to 200A (4/0 AL) with a 20/40 meter main panel, and put the old panel on a 125A breaker in that. A receptacle and hot tub circuit were added to the meter main outside, along with a PV backfeed. The original #2 Cu that fed the interior panel had to be upsized according to both the inspector and my in-house EC since it no longer served the “entire load” of this single family dwelling.

From what I can tell, in this case 310.12(C) would only mean that the feeder would never have to be larger than the incoming 4/0 service? In this forum even there is conflicting interpretation.

 

[Fred B]

First that reasoning to allow the 83% rule is due to diversity of loads in a dwelling unit and likelyhood of exceeding the rating of the wire low. But, on a subpanel the diversity is lower and can be nonexistant depending what is added so the need to go to full rating for the conductor. The case you site is one of such with the technically loss of diversity. Without the 83% allowance you must use 310.15 for wire size the #2 CU is likely only rated to 115A at 75^o.

From what I can tell, in this case 310.12(C) would only mean that the feeder would never have to be larger than the incoming 4/0 service? In this forum even there is conflicting interpretation.

310.12 is only applicable toward dwelling units and the allowance in 310.12(C) reference application for A or B both of which include limitation of "Entire loads" of the dwelling unit and that could result in the use of 310.15 sizing for the feeder. So your assumption that your feeder never need be larger than the service wire is inaccurate based on the quoted section.

 

[Solar_ice_caps]

First that reasoning to allow the 83% rule is due to diversity of loads in a dwelling unit and likelyhood of exceeding the rating of the wire low. But, on a subpanel the diversity is lower and can be nonexistant depending what is added so the need to go to full rating for the conductor. The case you site is one of such with the technically loss of diversity. Without the 83% allowance you must use 310.15 for wire size the #2 CU is likely only rated to 115A at 75^o.


310.12 is only applicable toward dwelling units and the allowance in 310.12(C) reference application for A or B both of which include limitation of "Entire loads" of the dwelling unit and that could result in the use of 310.15 sizing for the feeder. So your assumption that your feeder never need be larger than the service wire is inaccurate based on the quoted section.

Thanks Fred, so related to my current question, would adding storage batteries upstream of the panel trigger the need to use 310.15 to size these feeders?

By your description, the load diversity of the existing panel wouldn’t change at all, but is that taken into account in interpretation?

 

[jaggedben]

I did try to argue it but I’m still really not 100% clear on the interpretation.

In that case we upgraded the service from 125A (#2 Cu) to 200A (4/0 AL) with a 20/40 meter main panel, and put the old panel on a 125A breaker in that. A receptacle and hot tub circuit were added to the meter main outside, along with a PV backfeed. The original #2 Cu that fed the interior panel had to be upsized according to both the inspector and my in-house EC since it no longer served the “entire load” of this single family dwelling.

Well, the inspector is basically right, although possibly still wrong for a different reason.
It is correct that since you upgraded the service to 200A and the existing feeder no longer serves the entire load that you can no longer use 310.12 on the feeder.
That said, if the #2 copper is rated 75C (that is, it's not NM cable or old TW wire which would be rated 60C) then it's rated for 115A and according to 240.4(B) it can still be protected by a 125A breaker if the load is less than 115A.

The load calc for your feed to the System Controller would include the battery charging load. The load to the subpanel that you backed would not include the battery charging load.

So depending on the wiring type and the load calc for the dwelling the correct answer here might have been:
- what you did was fine
- the System controller was fine on a 125A breaker but the subpanel feeder should have been protected by a 100A breaker
- the whole thing could have been on 100A
- the inspector was correct and the wires needed to be upsized. But I doubt it, that would mean the load calc came out in the small window between 100 and 125A, or the even smaller window between 115 and 125A.

I think the lesson here is that as solar and battery installers we need to know how to do load calcs when necessary. If you're a resi guy most of the time then study in particular 220.82 and 220.83 and look at the examples in Annex D. A quick effort that shows an inspector that a load calc is way under 100A (they often are) should have you not upsizing existing feeder wires.

From what I can tell, in this case 310.12(C) would only mean that the feeder would never have to be larger than the incoming 4/0 service?

That is correct. You can convert aluminum to copper equivalent I think, so for 4/0 AL a copper conductor shouldn't have to be larger than 2/0.