You're on the right track, but let's refine the process and ensure accuracy. Here's a breakdown

1. Understanding the Nameplate Data
HKTS10X1

This is your heater kit, and its specifications are crucial. You noted the input voltage is 240V, so that's good.

MCA (Minimum Circuit Ampacity)

This is the minimum allowable ampacity for the conductors feeding the air handler and the heater. You're correct to use the 55.6A MCA as this corresponds with the 240V input.

MOCP (Maximum Overcurrent Protective Device)

This is the maximum size of the breaker or fuse you can use. According to the nameplate, the MOCP is 60A.

2. Conductor Sizing (NEC 310.15) You're using NMC cable, which is correct for residential wiring. Termination Temperature

This is where you need to be cautious. The air handler and breaker terminations may only be rated for 60°C, or even 75°C. You must verify the temperature rating of the terminations at both the air handler and the breaker. If either is limited to 60°C, you must use the 60°C column in NEC 310.16. If both terminations are rated for 75°C, you can use the 75°C column.

3. Conductor Ampacity (NEC Table 310.16) If using the 60°C column

You are correct that #6 AWG NMC is only rated for 55A, which is below the 55.6A MCA. Therefore, you must use #4 AWG NMC (rated at 70A in the 60°C column).

If using the 75°C column

#6 AWG NMC is rated for 65A. This exceeds the 55.6 MCA, so you can use #6 AWG with the 75°C ampacity rating. If the conductors are rated for 90°C, then the correction factors from NEC 310.15(B)(2) apply.

4. Overcurrent Protection (Breaker Size - NEC 240.4) The nameplate states the MOCP is 60A. NEC 240.4(B) allows you to "round up" to the next standard breaker size only if the conductor ampacity, after all adjustments and corrections, does not correspond to a standard breaker size. You cannot exceed the MOCP of 60A. The 60A MOCP listed on the nameplate is the maximum breaker size, so a 60 amp breaker would be the right choice if you use a #4 AWG conductor, or a #6 AWG conductor and the terminations are rated for 75°C.

Recap and Summary



Verify Termination Temperatures

This is the most critical step.

#4 AWG

This is the safest option if you are unsure of the termination temperatures, or if the terminations are rated for 60°C. Use a 60A breaker.

#6 AWG

This is acceptable only if both the air handler and breaker terminations are rated for 75°C. Use a 60A breaker.

Final Recommendation



Prioritize Safety and Code Compliance

If there's any doubt about the termination temperatures, or if the terminations are rated for 60°C, use #4 AWG with a 60A breaker. It's better to be slightly over-sized than to risk overheating.

Double-Check Termination Ratings

Look closely at the manufacturer's documentation for the air handler and the breaker to determine their termination temperature ratings. This information is critical.

Important Considerations



Voltage Drop

Don't forget to calculate voltage drop, especially if the run is long. NEC Article 210.19(A)(1) Note 4 recommends no more than a 3% voltage drop for the branch circuit. A voltage drop calculator will help you with this. This may make you increase the conductor size.*

Local Codes: Always check with your local electrical inspector for any specific requirements or interpretations of the NEC that may apply in your area.Good luck with your wiring project! Flag for review