Feeder For ADU (Accessory Dwelling Unit).

[Will Wire]

Is it Code compliant to use NEC Table 310.12 to determine the feeder size conductors for an ADU (Accessory Dwelling Unit) for an existing house?

2020 NEC

Thank you,
Trent

 

[wwhitney]

Is it Code compliant to use NEC Table 310.12 to determine the feeder size conductors for an ADU (Accessory Dwelling Unit) for an existing house?

Yes, as long as the ADU meets the NEC definition of dwelling unit, and the feeder meets the predicates in 310.12(B).

Note, however, that if this feeder for the ADU originates in a panel also supplying the existing house, the supply to that panel will no longer qualify to use 310.12, and this could mean upsizing existing conductors or downsizing existing breakers. Best case is to split the supply just after the meter to two separate services/feeders, one for each dwelling unit. Then each such service or feeder gets to use 310.12. The conductors that are part of a common supply to both do not.

Cheers, Wayne

 

[Will Wire]

Thank you, Wayne,

The Feeder for the ADU originates in the Main Service supplied by PG&E. My load calculation determined that the existing Service could handle the additional load, and the ADU meets the definition of a Dwelling. If I install the feeder wires based on NEC Table 310.16, I will have to use # 3 Copper THHN/THWN-2. If I can use NEC Table 312, I can use # 4 Copper THHN/THWN-2. I’ve always thought that NEC Table 312 was for single Services, Duplexes, and apartments.

Given this new information, do you see any problems with this installation?

 

[jaggedben]

I don't see any issue with using 4awg t9 the ADU in this situation. As Wayne explained, whether the existing service remains compliant is potentially an issue, but it is a separate issue.

310.12 is for an individual dwelling unit, whether it is a single family home, one unit of a duplex, or one unit in a 30 unit apartment building. It cannot be applied to any part of a circuit that supplies more than one unit. By adding an ADU to the existing panel that was previously only for a single family home, you make the service conductors feeding that panel no longer eligible to use 310.12. The likely worst case, depending on your load calc and whether the AHJ actually notices or cares, is that you may have to downsize the main breaker in the service panel to 175A.