Can 10 gauge wire be on a 35 amp breaker?

For air conditioning and refrigeration equipment, yes. NEC 240.4(G) sends those circuits to Article 440, where 440.22 permits a device sized well above the conductor ampacity for starting current. For general purpose circuits, 240.4(D)(7) caps 10 AWG copper at 30 amps.

Why can HVAC circuits exceed the small conductor rule?

Because the compressor overload device provides the overload protection. The branch circuit device is there for short circuit and ground fault protection only, which is why NEC 440.22(A) allows up to 175 percent, or 225 percent where needed to start.

Do electric heat strips follow the same rule?

No. Heat strips are covered by Article 424, and 424.3(B) requires the circuit to be sized at 125 percent of the load. The 240.4(D)(7) 30 amp limit on 10 AWG copper applies to them.

Can 10 AWG MC cable be on a 35 amp breaker for HVAC equipment?

Yes, for air conditioning and refrigeration equipment, and no for a general purpose circuit. NEC 2023 Section 240.4(D)(7) limits the overcurrent protection for 10 AWG copper to 30 amps after any correction or adjustment factors. That is the small conductor rule everyone learns first. Section 240.4(G) is the part that gets overlooked: it lists specific conductor applications that are permitted to be protected in accordance with other articles instead, and air conditioning and refrigeration equipment circuit conductors are on that list, pointed at Article 440 Parts III, IV, V and VI.

How Article 440 sizes the circuit

Section 440.32 requires branch circuit conductors supplying a single motor compressor to have an ampacity of not less than 125 percent of the rated load current or the branch circuit selection current, whichever is greater. Section 440.22(A) then permits the short circuit and ground fault protective device to be rated up to 175 percent of that same current, and up to 225 percent where the smaller device will not carry the starting current. That is why a 10 AWG conductor legitimately sits behind a 35, 40 or even 45 amp device on a condenser or air handler. The conductor is protected against overload by the compressor overload device and the equipment thermal protection, not by the breaker, which is there for short circuit and ground fault only.

The practical version

Read the nameplate. If it says minimum circuit ampacity 30 and maximum overcurrent device 35, install a conductor with an allowable ampacity of at least 30 and a device no larger than 35. Ten AWG copper THHN in MC cable is 35 amps in the 75 degree C column of Table 310.16, which covers a 30 or 32 amp MCA comfortably. Section 110.14(C) holds you to the 75 degree C column for terminations on equipment rated up to 100 amps unless the equipment is marked otherwise, so do not reach for the 90 degree C value.

Where it stops being legal

Take that same 10 AWG MC and feed a receptacle, a lighting circuit or a heater with it, and 240.4(D)(7) applies with no relief: 30 amps maximum. Electric resistance heat inside an air handler is the trap. Heat strips are not motor compressor load, and their circuit is sized under Article 424 with 424.3(B) requiring 125 percent of the load, so a 10 AWG heat strip circuit is capped at 30 amps. A unit with both a compressor and heat strips often needs two circuits sized by two different rules.

Cable and derating details

Article 330 covers MC cable. Section 330.30 requires support within 12 inches of every box or fitting and at intervals not exceeding 6 feet, and 330.24 sets the bending radius. Where the MC runs through a hot attic, apply the ambient correction from Table 310.15(B)(1) before comparing the result to the MCA. A 10 AWG conductor at 90 degrees C is 40 amps, and a 0.87 correction for a 46 degree C attic brings it to 34.8 amps, which still covers a 30 amp MCA.

Field takeaway

Ten AWG on a 35 amp breaker is correct for air conditioning equipment under 240.4(G) and Article 440, provided the conductor meets the nameplate MCA and the device does not exceed the nameplate maximum. For anything that is not motor compressor load, 30 amps is the ceiling.