Do I size a heat pump circuit from kW or from the nameplate?

From the nameplate MCA and MOCP. NEC 110.3(B) and 440.4(B) make the marked values the governing numbers.

Why does a 16 kW heater need two circuits?

NEC 424.22(B) requires heating equipment over 48 amperes to be subdivided, with each subdivided load at 48 amperes or less protected at 60 amperes or less.

What wire carries 83 amperes?

4 AWG copper at 85 amperes or 2 AWG aluminum at 90 amperes, both in the 75 degrees C column of Table 310.16.

What wire size do I need for a 16 kW single phase heat pump?

The single most important rule on this equipment is that the nameplate governs. Section 110.3(B) makes the listing and labeling instructions enforceable, and 440.4(B) requires the nameplate on multimotor and combination load equipment to be marked with the minimum circuit ampacity and the maximum overcurrent protective device rating. Size the conductors to the MCA and the breaker to the MOCP. Do not calculate around a nameplate that is right in front of you.

Where 16 kW lands if you calculate it

16,000 watts divided by 240 volts is 66.7 amperes for the heat strips alone. Section 424.3(B) treats fixed electric space heating equipment as a continuous load, so branch circuit conductors and the overcurrent device are sized at not less than 125 percent of the total load. 66.7 times 1.25 is 83.3 amperes.

In the 75 degrees C column of Table 310.16, 4 AWG copper is 85 amperes and 3 AWG is 100 amperes. 4 AWG copper clears 83.3 amperes on a 90 ampere breaker. On the aluminum side you need 2 AWG at 90 amperes.

That is the heat strips only. A heat pump also has a compressor and an outdoor fan, and 440.32 and 440.33 govern combination loads. That is exactly why the manufacturer prints an MCA.

The subdivision rule that usually splits the circuit

Section 424.22(B) is the one people miss. Fixed electric space heating equipment rated more than 48 amperes must have the heating elements subdivided, and each subdivided load cannot exceed 48 amperes and must be protected at not more than 60 amperes.

A 16 kW heater at 66.7 amperes is over 48, so the manufacturer subdivides it internally, typically into two sections of about 33 amperes each, each with its own factory installed fuse or breaker. Section 424.22(C) allows those supplementary overcurrent devices to be factory installed.

What this means in the field is that many 16 kW air handlers carry two separate nameplate circuits, each with its own MCA and MOCP, and require two feeders from the panel. Read the nameplate for the circuit count before you pull one big cable. Section 424.22(B) also permits a single supply where the equipment is listed and marked for it.

A realistic single circuit example

If the nameplate reads MCA 83 amperes and MOCP 90 amperes, run 4 AWG copper THHN with an 8 AWG copper equipment grounding conductor from Table 250.122 for a 90 ampere device, on a 90 ampere HACR rated breaker. Section 440.14 requires a disconnecting means within sight from and readily accessible from the equipment.

If the nameplate lists two circuits at MCA 42 amperes and MOCP 45 amperes each, run two circuits of 8 AWG copper at 50 amperes with 10 AWG copper equipment grounding conductors, on two 45 ampere breakers.

Long runs and voltage drop

Heat strips are a resistive load and a sagging voltage costs you output proportional to the square of the voltage. At 83 amperes over 100 feet at 240 volts using 4 AWG copper at 0.308 ohms per 1000 feet from Chapter 9 Table 8, drop is 2 times 100 times 83 times 0.000308, or 5.1 volts, which is 2.1 percent. Acceptable. Double the run to 200 feet and you are at 4.3 percent, and 3 AWG or 2 AWG becomes the practical choice.

Field takeaway

Read the nameplate first and count the circuits. A calculated 16 kW single phase load is 83.3 amperes after the 125 percent continuous multiplier, which is 4 AWG copper on a 90 ampere breaker, but 424.22(B) means the equipment is usually built as two subdivided circuits instead.