A 60-gallon boiler rated 8.2 kW at 240 V, 1-phase requires a minimum size THHN/THWN copper wire between the required disconnect and the boiler of which gauge?

• A. 12
• B. 10
• C. 8
• D. 6

Answer: (C)

Sizing the conductors is based on the actual current the boiler will draw and picking the smallest conductor that can safely carry that current. The boiler uses 8.2 kW at 240 V, so the current is about 8,200 W / 240 V ≈ 34 A. For THHN/THWN copper in a conduit, the ampacity table shows that 8 AWG copper is rated around 40 A, while 10 AWG is about 30 A and 12 AWG about 20 A. Since 34 A exceeds 30 A but is less than 40 A, the next standard conductor size that can handle the load is 8 AWG. Therefore, the minimum wire gauge between the disconnect and the boiler is 8 AWG copper. (If this were treated as a continuous load, you’d size at 125% of the load, which could require a larger conductor, but the problem’s answer uses the non-continuous sizing assumption.)