Can I use Romex for an air handler?

Yes in a dry indoor location where NM-B is permitted, sized to the nameplate MCA and supported per NEC 334.30.

What breaker size goes on an air handler?

At or below the MOCP number on the equipment nameplate. NEC 440.4(B) requires that marking and 110.3(B) makes it enforceable.

Do I need a separate ground in the flex whip?

Yes for liquidtight nonmetallic conduit, and for metallic flex outside the length and size limits of NEC 348.60 or 350.60. Pulling one every time is the safe practice.

What type of wire can you run to an AC air handler?

The wiring method is whatever Chapter 3 permits for the location, but the conductor size and the overcurrent device come off the equipment nameplate, not off a standard branch circuit table. Look for two numbers on the air handler label: minimum circuit ampacity, or MCA, and maximum overcurrent protective device, or MOCP. Section 440.4(B) requires the manufacturer to mark both on multimotor and combination load equipment, and 110.3(B) requires the equipment to be installed according to that listing.

Sizing the conductor

Size the conductor for at least the MCA using the 60 or 75 degree C column of Table 310.16, whichever matches the terminals per 110.14(C). An air handler with an MCA of 26.9 amperes needs a conductor rated at least 26.9 amperes, so 10 AWG copper at 30 amperes in the 60 degree C column works. Do not round the MCA down. Section 440.6 and 440.32 set the conductor rules for hermetic motor compressors, and for an air handler with electric strip heat, 424.4(B) requires the branch circuit conductors and overcurrent device to be sized at 125 percent of the total heater load.

The MOCP is a ceiling, not a target. If the label says MCA 26.9 and MOCP 45, you may use a 30, 35, 40, or 45 ampere device, but never a 50. The larger device is permitted because the motor inrush would nuisance trip a device sized to the conductor. That is why the conductor can look undersized relative to the breaker.

Wiring methods that work

Type NM-B is common on residential air handlers in an attic or closet, as long as the location is dry and the cable is protected per 334.15 and supported per 334.30. Type MC cable is the usual commercial choice and handles the run from a junction box to the unit cleanly. THHN/THWN-2 in EMT or PVC is used where the run is exposed or in a mechanical room.

The final connection almost always uses flexible conduit to absorb vibration. Section 348.60 and 350.60 allow flexible metal conduit and liquidtight flexible metal conduit as an equipment grounding path only within listed length and size limits, so pull a separate equipment grounding conductor sized from Table 250.122 in any flex whip. Section 348.20(A) limits FMC smaller than trade size 1/2 to specific uses. Liquidtight flexible nonmetallic conduit under Article 356 always requires a separate EGC.

Disconnect and GFCI

Section 440.14 requires the disconnecting means to be located within sight from and readily accessible from the air conditioning or refrigerating equipment, and it may not be installed on panels designed to allow access to the equipment. Section 210.8(F) requires GFCI protection for outdoor outlets supplying HVAC equipment rated 150 volts to ground or less and 60 amperes or less, single phase. An indoor air handler is not an outdoor outlet, so 210.8(F) generally does not reach it, but a rooftop or exterior unit does fall under that rule.

Field takeaway

Read the nameplate. Size the conductor to the MCA at the terminal temperature rating, pick a breaker at or below the MOCP, use NM-B, MC, or THHN in raceway depending on the location, and land the final connection with a flex whip carrying its own equipment grounding conductor. If the unit has strip heat, run the 125 percent heater calculation from 424.4(B) as well.