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❄️What size wire does a mini split need?

✓ Tested & verified Updated: How we calculate this

Mini split circuit

A
ft
°F

14 AWG copper

on a 15 A device · 14 AWG ground · 1/2" conduit at 9.6% fill

1.4% drop at 35 ft

  • Ampacity wants14 AWG
  • Voltage drop wants14 AWG
Current the conductor has to carry
15 A
Usable ampacity after derating
20 A (table 20)
Longest run before the drop passes 3%
76 ft
In metric sizes
2.1 mm²

MCA already includes the 125% multiplier, so do not multiply it again. MOCP on the nameplate is a ceiling, not a target.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

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What size wire does a mini split need?

The AmpSizer mini split result for a nameplate MCA of 15 amps is 14 AWG copper with a 14 AWG ground, in half-inch conduit. Voltage drop over a 35 foot run at 240 volts is 1.4%.

MCA is not the running current. The manufacturer has already taken 125% of the largest motor and added the other loads, per 440.33, so the conductor is sized directly from MCA with no further multiplier — and multiplying it again is the most common mini split error.

The breaker comes from the nameplate too, but differently. MOCP is a maximum, not a required size: a unit with MCA 15 and MOCP 20 accepts a 15 or a 20 amp device, and never a 25.

Typical nameplate figures by system size, with the circuit each one wants. Always read your own nameplate — these vary by manufacturer and by efficiency rating far more than they vary by capacity.

SystemTypical MCATypical MOCPCopperGroundConduit
9,000 BTU, 240 V10–12 A15–20 A14 AWG14 AWG1/2 in
12,000 BTU, 240 V13–16 A20 A14 AWG14 AWG1/2 in
18,000 BTU, 240 V17–20 A25–30 A12 AWG12 AWG1/2 in
24,000 BTU, 240 V21–25 A30–35 A10 AWG10 AWG1/2 in
36,000 BTU, 240 V28–32 A40–50 A8 AWG10 AWG3/4 in
Multi-zone, 48,000 BTU35–45 A50–60 A8 AWG10 AWG3/4 in

Note how far apart MCA and MOCP drift as the system gets larger. At 36,000 BTU the conductor is sized for about 30 amps and the device may be 50 — which looks wrong on paper and is exactly what 440.22 intends, because the breaker’s job there is to survive the compressor start rather than to protect against overload.

Choosing the capacity first

This page answers the electrical half. The other half — how many BTU the room actually needs — is a different calculation with different inputs, and getting it wrong costs more in comfort than any wire size does: an oversized system short-cycles, never dehumidifies properly, and wears its compressor out doing it. Our sibling site BTU Sizer works out room-by-room capacity if you have not settled that yet.

Where mini split circuits fail inspection

  • No disconnect within sight of the outdoor unit
  • A breaker above the nameplate MOCP, which voids the listing
  • MCA multiplied by 1.25 a second time, producing a conductor two sizes larger than needed and, more importantly, a breaker above the MOCP to go with it
  • The interconnecting cable substituted for something not specified by the manufacturer
  • No weatherproof means of support where the line set and the circuit enter the wall

Frequently asked questions

Why does my mini split nameplate say MCA and not amps?

Because air conditioning equipment contains a hermetic compressor motor, and 440.33 makes the manufacturer do the multiplier arithmetic for you: minimum circuit ampacity is 125% of the largest motor plus the remaining loads. You size the conductor straight from it. It saves you from having to know which internal motor is largest.

What is the difference between MCA and MOCP?

MCA sizes the conductor and is a minimum. MOCP sizes the overcurrent device and is a maximum. They are usually different numbers — MCA 15 with MOCP 20 is typical — because the device has to let the compressor start without tripping while the conductor only has to carry the running current.

Can I use a breaker smaller than the MOCP?

Yes, as long as it is at least large enough for the MCA and does not nuisance-trip on startup. MOCP is the ceiling. Using a device larger than the MOCP is what is not allowed, and it is the one that voids the listing of the equipment.

Does a mini split need a disconnect outside?

The outdoor unit needs a disconnecting means within sight of it, per 440.14 — usually a small pull-out or a non-fused disconnect on the wall beside it. It is not the same thing as the breaker in the panel, and it is what lets a service technician work on the unit safely.

Do I need a neutral for a 240 volt mini split?

Usually not. Most single-zone 240 V mini splits are two hots and a ground; the indoor head is powered from the outdoor unit over the interconnecting cable. Some multi-zone systems and 120 V units do need a neutral, and the nameplate wiring diagram is the only reliable answer for a specific model.

How we calculate this

Air conditioning and heat pump circuits are sized from the nameplate minimum circuit ampacity, which the manufacturer has already calculated under NEC 440.33 as 125% of the largest motor plus the other loads — so no further multiplier is applied. Conductors then satisfy allowable ampacity from Table 310.16 corrected by Tables 310.15(B)(1) and 310.15(C)(1), and the voltage drop target, whichever asks for more. The overcurrent device must not exceed the nameplate maximum overcurrent protective device rating (440.22 and 110.3(B)), and the figure this tool shows is the smallest standard rating from 240.6(A) that covers the MCA — compare it with the nameplate MOCP and use whichever the nameplate permits. The equipment grounding conductor is Table 250.122, the raceway is Chapter 9, Table 4 at 40% fill, and the disconnecting means within sight of the outdoor unit is 440.14.

Worked example

With Minimum circuit ampacity (MCA) 15, One-way run 35, System voltage 240 V, Metal Copper, Terminal rating 75 °C — the usual breaker, Ambient 86, Current-carrying conductors in the raceway 3, this page works out 14 AWG copper. on a 15 A device · 14 AWG ground · 1/2" conduit at 9.6% fill

  • Current the conductor has to carry15 A
  • Usable ampacity after derating20 A (table 20 )
  • Longest run before the drop passes 3%76 ft
  • In metric sizes2.1 mm²

Sources:

The wiring a mini split actually needs

The line-voltage circuit. Panel to outdoor unit, sized from MCA. This is what the calculator answers.

The interconnecting cable. Outdoor unit to indoor head, carrying power and communications. It is specified by the manufacturer in size and conductor count, and it is not a circuit you size yourself.

The disconnect. Within sight of the outdoor unit, per 440.14. Cheap, small, and the item most often missing at inspection on an owner-installed system.