Feeder tap.

[msi99]

I am attaching a pictorial representation of a scenario where the solar point of interconnection is proposed to be on the load side feeders of one of the panels.
According to 705.12(A)(2)(b), we are permitted to perform a feeder tap on the existing 4/0Al feeders as long as we install an OCPD on the load side to ensure that the current does not exceed 200A.
However, the AHJ is requiring both the service and feeder wires to be upsized, citing nonconforming service conductors from the meter to the main that do not meet the requirements of 310.12(A) or (B) for feeding the entire load. As we are performing a feeder tap, the AHJ is requiring us to upgrade the service conductors and feeder to meet 310.16 and 705.12(A)(2)(B).
I don't think this is the correct interpretation of the code and should not require the upsizing of the conductors. The key issue is that the city is requiring the service and feeder conductors to be rated for the entire house load ( let's say it's at 320A) rather than just handling 200A to loads specific to the panel. However, 310.12(A) does not explicitly state that feeders must be rated for the entire load; instead, it permits feeders to be sized ( 83%) based on their association with the entire load, which implies that they can be part of, but not necessarily equal to, the entire load.

Please let me know your thoughts.

 

[wwhitney]

310.12 applies to conductors that carry the entire load of a dwelling unit (via a service under (A) or via a feeder under (B)), or for any downstream feeder of the same rating (under (C)). If only one of the 200A panels in your diagram is for the dwelling unit with solar, and the other two serve other, then you could use 4/0 Al for a full 200A.

I infer that more than one of the 200A panels serve the dwelling unit. As such you have no conductors that qualify for (A) or (B). And even if you did, they would need to be protected at more than 200A, so 310.12 wouldn't help you with a 200A feeder.

However, 4/0 Al has a 75C ampacity of 180A, and so under 240.4(B) may be protected at 200A when the calculated load is 180A. That load calculation would have to be via the standard calculation, as each feeder is only carrying part of the dwelling unit load. Assuming that load calculation works out, there is nothing wrong with the existing 4/0 Al protected at 200A.

705.12(A)(2)(b), in contrast, just refers to the ampacity of the conductor, and there's no allowance for upsizing the OCPD to the next higher standard size. So to use 705.12(A)(2)(b) with the existing 4/0 Al, you'd need to use a 175A main breaker in the bottom 200A panel in your drawing. And confirm that the panel load is at most 175A.

Or you could keep the 200A main breaker in the bottom 200A panel if you upsize just the conductor between the feeder tap and that panel to a full 200A ampacity, i.e. 3/0 Cu or 250 kcmil Al.

Cheers, Wayne

 

[msi99]

310.12 applies to conductors that carry the entire load of a dwelling unit (via a service under (A) or via a feeder under (B)), or for any downstream feeder of the same rating (under (C)). If only one of the 200A panels in your diagram is for the dwelling unit with solar, and the other two serve other, then you could use 4/0 Al for a full 200A.

I infer that more than one of the 200A panels serve the dwelling unit. As such you have no conductors that qualify for (A) or (B). And even if you did, they would need to be protected at more than 200A, so 310.12 wouldn't help you with a 200A feeder.

However, 4/0 Al has a 75C ampacity of 180A, and so under 240.4(B) may be protected at 200A when the calculated load is 180A. That load calculation would have to be via the standard calculation, as each feeder is only carrying part of the dwelling unit load. Assuming that load calculation works out, there is nothing wrong with the existing 4/0 Al protected at 200A.

705.12(A)(2)(b), in contrast, just refers to the ampacity of the conductor, and there's no allowance for upsizing the OCPD to the next higher standard size. So to use 705.12(A)(2)(b) with the existing 4/0 Al, you'd need to use a 175A main breaker in the bottom 200A panel in your drawing. And confirm that the panel load is at most 175A.

Or you could keep the 200A main breaker in the bottom 200A panel if you upsize just the conductor between the feeder tap and that panel to a full 200A ampacity, i.e. 3/0 Cu or 250 kcmil Al.

Cheers, Wayne

I appreciate the detailed explanation. As I was reading through 310.12 (A) and (B), I couldn't find where it specifically states that the code applies only to the conductors feeding the entire load of a dwelling unit via a single service or feeder. When it mentions "conductors associated with the entire load," it seems to imply that there could be more than one feeder or service and 310.12 can still be applied.

 

[wwhitney]

When it mentions "conductors associated with the entire load,"

No, that's not the phrase used. It says "conductors supplying the entire load associated with a one-family dwelling". That's unambiguous to me.

Cheers, Wayne

 

[jaggedben]

Maybe you can do a load calc and prove that all those 200A overcurrent devices can be 175A. After all, the total load can't be more than 400A on a 320 meter, right? Replacing breakers or fuses should be a lot easier than replacing the conductors.

...
...The key issue is that the city is requiring the service and feeder conductors to be rated for the entire house load ( let's say it's at 320A) rather than just handling 200A to loads specific to the panel. ...

Please let me know your thoughts.

For the service conductors on the line side of the meter, which you haven't shown, they need to be sized for 83% of the entire calculated load. For the conductors from the meter (or tap point) to each individual panel they only need to be sized for the load connected to that panel. It may be tedious to diligently divide the lighting load square footage between the three panels but I'm sure you can come up with something reasonable and I'd still rather do that than replace those conductors.