Using romex for Electric Vehicle Chargers

#6 Romex (Type NM-B) is not rated for a 60-amp circuit for a continuous load like an electric vehicle (EV) charger. The National Electrical Code (NEC) specifies two key reasons why this is not permitted.

The 60°C temperature rating

• Insulation limit: Even though the individual copper wires inside a #6 Romex cable have a higher temperature rating, the outer plastic sheath (the "jacket") is only rated for 60°C. The NEC requires that you size the conductor based on this lower temperature rating.
• Ampacity restriction: According to the NEC ampacity table for the 60°C column, #6 copper wire is rated for a maximum of 55 amps. A breaker's rating cannot exceed the wire's ampacity.

Continuous load calculation

EV chargers are considered a "continuous load" because they draw their maximum current for three or more consecutive hours.

• The NEC requires that continuous loads not exceed 80% of the circuit's rated ampacity.
• For a 60-amp circuit, the maximum continuous load is

60 amps×80%=48 amps60 amps cross 80 % equals 48 amps

60 amps×80%=48 amps

.

• To support a 48-amp continuous load, the conductor must have an ampacity of at least 125% of the continuous load, or

48 amps×1.25=60 amps48 amps cross 1.25 equals 60 amps

48 amps×1.25=60 amps

.

• Since #6 Romex is only rated for 55 amps, it does not meet this 60-amp requirement.

Correct wiring for a 60-amp EV charger circuit

For a hardwired EV charger on a 60-amp circuit, you must use a conductor that meets the continuous load requirements. The correct options are:

• #6 AWG THHN/THWN conductors in a conduit: This type of wire has a higher temperature rating (typically 90°C), which allows it to be rated for 65 amps and comply with code for a 60-amp breaker.
• #4 AWG Romex (NM-B): This larger gauge wire is needed to meet the ampacity requirements when constrained by the 60°C rating of the Romex jacket.