What size feeder wire does an ADU subpanel need?

Large enough to carry the ADU's calculated load and to be protected by the breaker feeding it, using the ampacity table in Article 310 of the California Electrical Code, and often larger than that to keep voltage drop in check on a long run. Panel size alone doesn't settle it: conductor material, the temperature rating of the terminals, the conduit and the run length all change the answer.

Step 1: Start from the load calculation

The Article 220 load calculation for the ADU sets the minimum feeder rating. The breaker in the main panel that protects the feeder is chosen from that, and the conductors must be sized so that breaker protects them. If the calculation lands close to a breaker size, you may choose to step up so the ADU has room for later additions.

Step 2: Choose conductors from the ampacity table

• Table 310.16 lists how much current each conductor size can carry, in columns by insulation temperature rating.
• Terminal ratings limit the column you use. Section 110.14(C) ties usable ampacity to the temperature rating of the breaker and panel lugs, so a 90°C wire landing on 75°C lugs is sized from the 75°C column.
• Copper or aluminum. Aluminum feeder conductors are permitted, but they must be a larger size than copper for the same ampacity and must land on lugs rated for aluminum.
• The dwelling allowance. Section 310.12 allows reduced conductor sizes for 100- to 400-amp services and feeders that carry a dwelling unit's entire load. Whether that applies to an ADU feeder is an interpretation question, so a conservative design sizes straight from Table 310.16 unless your plan reviewer agrees.

Step 3: Check voltage drop

Voltage drop isn't an enforceable limit for a typical ADU feeder, but informational notes in the NEC suggest keeping it to about 3 percent on a feeder and about 5 percent for feeder and branch circuit combined. On a long run to a backyard ADU, meeting that recommendation can push the conductor up one or more sizes. Heat pumps, induction cooktops and EV chargers all work best with steady voltage.

Step 4: Finish the rest of the feeder

• A detached ADU feeder has four conductors: two hots, a neutral and an equipment grounding conductor. The grounding conductor is sized from Table 250.122 based on the breaker rating, and it must be increased proportionally if the hot conductors are upsized for voltage drop.
• The conduit must be large enough for the conductors under the fill limits in Chapter 9. A larger conduit makes the pull easier and leaves room for a future upgrade.

What this means for your electrical plan

The plan should list the feeder breaker size, conductor size and material, grounding conductor size, conduit type and size, run length and voltage-drop result, all tied back to the load calculation. Read why the load calculation comes first, how a detached ADU subpanel works and what panel size an ADU needs.

Sources: dgs.ca.gov, ecmag.com, forums.mikeholt.com.