Yes, a #6/3 NMB cable feeding a subpanel can be on a 60 amp breaker, provided certain conditions are met. Here's a breakdown

1. Ampacity of #6 NMB (Romex):
#6 AWG copper NMB cable has an ampacity of 55 amps when operating at 60°C (140°F). This is per NEC Table 310.16 for conductors in raceways or cables.2. Overcurrent Protection

NEC 240.4(B) states that if the ampacity of a conductor does not correspond to a standard breaker size, you can go up to the next higher standard overcurrent device rating if three criteria are met: The calculated load is less than the rating of the overcurrent device. The standard rating of the overcurrent device is no higher than 800 amperes. The next standard overcurrent device rating above the ampacity of the conductors does not exceed the values in the Table 240.4(B).

3. Applying the NEC to Your Situation

You have #6 NMB with an ampacity of 55 amps. You want to use a 60 amp breaker. Your calculated load is 40 amps (less than the 60 amp breaker).Following 240.4(B), since the conductor is rated at 55A, and the next standard size is 60A, you can use a 60A breaker as long as your load calculation is 40 amps.

Important Considerations and Cautions



Temperature Rating of Terminations

Ensure that the terminals of the breakers and the subpanel are rated for the ampacity of the wire. Often, equipment is only rated for 60°C. Using the 60°C column in Table 310.16 is often safest.

Subpanel Load Calculations

Your 40 amp calculated load must be accurate and complete. Factor in everything that will be connected to the subpanel, including future additions. It is generally a good idea to overestimate slightly. Inadequate load calculations can lead to overloads and dangerous situations.

Local Codes and AHJ (Authority Having Jurisdiction)

Local codes may be stricter than the NEC. Always check with your local building department or electrical inspector before installing or modifying electrical systems. They have the final say on what is permitted.

Voltage Drop

Consider voltage drop, especially over longer distances. While a 60 amp breaker might be permitted, voltage drop could become excessive with a long run of #6 wire, potentially affecting the performance of equipment connected to the subpanel.

GFCI Protection

Be sure to meet all GFCI (Ground Fault Circuit Interrupter) requirements as needed.

In Summary:

Based solely on the information provided and the NEC, a 60 amp breaker can be used for a #6/3 NMB cable feeding a subpanel if your calculated load is 40 amps and the above conditions are met. However*, it is highly recommended to consult with a qualified electrician to verify your calculations, ensure compliance with all applicable codes, and assess voltage drop and other factors. They can provide a professional assessment of your specific situation. Flag for review