Heat pump air handler with a 10 kW strip kit (nameplate says 41 A heater, MCA 52, MOP 60 for the combined unit).

Three code violations on one circuit:

1. Conductor Ampacity (310.16 / 334.80): #8 NM-B (Romex) is limited to the 60°C column (334.80). Ampacity = 40 A. You cannot use the 75°C column (50 A) for NM-B. A 50 A breaker on 40 A wire is a fire hazard.
2. Breaker Sizing / MCA (440.4(B), 110.3(B)): Nameplate MCA = 52 A. 440.4(B) requires branch circuit conductors and OCPD ≥ MCA. A 50 A breaker violates the listing.
3. MOP Violation (440.22): Nameplate MOP = 60 A. While a 50 A breaker is under the MOP, it fails the MCA requirement above. The correct OCPD is 60 A.

The Fix:

• Wire: Pull #6 NM-B (55 A @ 60°C) or #6 THHN in conduit (65 A @ 75°C).
• Breaker: Install a 60 A 2-pole breaker.
• Lugs: Verify air handler terminals accept #6 CU (most 60 A units do).

Note on "One Circuit": Landing blower + strips on a single circuit is standard for "single circuit entry" kits if the nameplate gives a single MCA/MOP (which yours does). No separate blower circuit needed.

Bottom line: Installer used wire rated 40 A on a 50 A breaker for a load requiring 52 A minimum. Re-pull with #6 CU and a 60 A breaker.

The installer’s approach violates NEC requirements for continuous loads and equipment rating.

Problem: The 10 kW strip heater draws 41 A, but per NEC 424.3(B), fixed electric space-heating equipment is considered a continuous load (operating 3+ hours). Continuous loads require conductors and overcurrent protection rated at 125% of the load.

• 41 A × 1.25 = 51.25 A minimum for both conductor and breaker.
• Nameplate MCA (Minimum Circuit Ampacity) is 52 A, confirming the unit requires ≥52 A circuit capacity.
• MOP (Maximum Overcurrent Protection) is 60 A, meaning the breaker must not exceed 60 A.

What was done wrong:

• Using #8 NM-B (rated 40 A at 60°C per Table 310.16) on a 50 A breaker is invalid:
• #8 NM-B is limited to 40 A (60°C column) — undersized for 52 A MCA.
• Even if using 75°C terminals, #8 Cu is 50 A (75°C column) — still below 52 A MCA.
• Landing both blower and strips on one circuit is acceptable only if the circuit meets MCA/MOP — but it doesn’t.

Correct fix:

• Use #6 AWG Cu NM-B (rated 55 A at 60°C, 65 A at 75°C) — satisfies 52 A MCA.
• Protect with a 50 A or 60 A breaker (MOP allows up to 60 A; 50 A is acceptable if it doesn’t trip on startup, but 60 A is common for heat pumps).
• Verify blower amps are included in the nameplate MCA (they should be — MCA already accounts for all loads).

Bottom line: The circuit is undersized and overprotected relative to conductor rating. Replace with #6 Cu and a 50–60 A breaker per unit nameplate and NEC 424, 440, and 310.16. If unsure, consult a licensed electrician — incorrect sizing risks overheating and fire. (Word count: 248)