Between the disconnect and a boiler, copper conductors must be of which insulation rating?

Between the disconnect and a boiler, copper conductors must be of which insulation rating?

• A. THHN
• B. THWN
• C. THHN/THWN
• D. XHHW

Answer: (C)

The key idea is choosing a conductor insulation that can handle moisture along the circuit path. A run from the disconnect to a boiler in a mechanical space can be damp or wet, so the insulation must be rated for moisture. Using insulation rated only for dry locations would not be acceptable where moisture may be present, while a rating for wet locations alone would be fine, but you want flexibility for any part of the run. THHN/THWN is dual-rated and can be used in either dry or wet locations, making it the correct, most versatile choice for this installation. It covers both conditions you might encounter in the path to the boiler, ensuring code compliance throughout.

The key idea is choosing a conductor insulation that can handle moisture along the circuit path. A run from the disconnect to a boiler in a mechanical space can be damp or wet, so the insulation must be rated for moisture. Using insulation rated only for dry locations would not be acceptable where moisture may be present, while a rating for wet locations alone would be fine, but you want flexibility for any part of the run. THHN/THWN is dual-rated and can be used in either dry or wet locations, making it the correct, most versatile choice for this installation. It covers both conditions you might encounter in the path to the boiler, ensuring code compliance throughout.