Minimum copper THHN feeder size for 12 EVSEs rated at 24 amperes on a 3-phase, 4-wire, 120/208-volt system, assuming four EVSE per phase? (Use 1.732 for the square root of 3.)

Minimum copper THHN feeder size for 12 EVSEs rated at 24 amperes on a 3-phase, 4-wire, 120/208-volt system, assuming four EVSE per phase? (Use 1.732 for the square root of 3.)

• A. 250 kcmil
• B. 350 kcmil
• C. 4/0 AWG
• D. 2/0 AWG

Answer: (C)

The situation tests how to size a three‑phase feeder when the loads are connected line‑to‑line, not line‑to‑neutral. In a 120/208 V system, if each EVSE is tied between two hot legs and draws 24 A, each unit delivers about 208 V × 24 A ≈ 4,992 W. With twelve EVSEs, the total power is about 12 × 4,992 ≈ 59,904 W. For a balanced three‑phase system, the line current is I = P total / (√3 × V line‑to‑line). Using √3 ≈ 1.732 and V_LL = 208 V gives I ≈ 59,904 / (1.732 × 208) ≈ 166 A. Copper THHN ampacities at 75°C show that 2/0 AWG has about 165 A, which would be just short of 166 A, while 4/0 AWG provides around 230 A, comfortably exceeding the 166 A requirement. Therefore, the minimum copper THHN feeder size is 4/0 AWG. The other options either exceed what’s needed (larger conductors) or fall short of the calculated requirement (2/0 AWG isn’t enough for 166 A).

The situation tests how to size a three‑phase feeder when the loads are connected line‑to‑line, not line‑to‑neutral. In a 120/208 V system, if each EVSE is tied between two hot legs and draws 24 A, each unit delivers about 208 V × 24 A ≈ 4,992 W. With twelve EVSEs, the total power is about 12 × 4,992 ≈ 59,904 W.

For a balanced three‑phase system, the line current is I = P total / (√3 × V line‑to‑line). Using √3 ≈ 1.732 and V_LL = 208 V gives I ≈ 59,904 / (1.732 × 208) ≈ 166 A.

Copper THHN ampacities at 75°C show that 2/0 AWG has about 165 A, which would be just short of 166 A, while 4/0 AWG provides around 230 A, comfortably exceeding the 166 A requirement. Therefore, the minimum copper THHN feeder size is 4/0 AWG.

The other options either exceed what’s needed (larger conductors) or fall short of the calculated requirement (2/0 AWG isn’t enough for 166 A).