Wire Fill for Mini-Split Heat Pump
Mini-split wire fill: 9k-18k BTU uses 14-12 AWG on 15-20A breaker, 24k-36k BTU needs 10-8 AWG on 30-40A. NEC 440 with LFNC whip to outdoor unit.
Wire fill for mini-split heat pump circuits is governed by NEC Article 440 (HVAC equipment) and NEC 430 for the motor compressor. A typical 12,000 BTU (1-ton) ductless mini-split uses a 15A 2-pole breaker, 14 AWG copper THHN to an outdoor disconnect, and a short LFNC whip to the condenser. Larger 24,000-36,000 BTU multi-zone systems need 30-40A breakers with 10 or 8 AWG copper. The interconnect cable between outdoor and indoor units is usually a 14/4 stranded shielded cable provided by the installer.
Load characteristics
Mini-splits are highly efficient inverter-driven systems with low FLA but inrush spikes:
| Mini-split size | Typical MCA | MOCP | Breaker |
|---|---|---|---|
| 9,000 BTU (3/4-ton) | 9-12A | 15A | 15A |
| 12,000 BTU (1-ton) | 10-15A | 15-20A | 15A or 20A |
| 18,000 BTU (1.5-ton) | 14-18A | 20-25A | 20A or 25A |
| 24,000 BTU (2-ton) | 18-22A | 25-30A | 25A or 30A |
| 36,000 BTU (3-ton multi-zone) | 22-28A | 30-40A | 30A or 40A |
| 48,000 BTU (4-ton multi-zone) | 25-32A | 35-50A | 40A or 50A |
Always confirm with the unit nameplate — variable-speed inverter systems often have much lower MCA than equivalent-tonnage conventional split systems.
NEC code references
- NEC 440.4 — nameplate markings (MCA, MOCP)
- NEC 440.14 — disconnect within sight of outdoor unit
- NEC 440.22 — branch-circuit overcurrent protection
- NEC 440.32 — conductor sizing (125% of largest motor RLA)
- NEC 350 — LFMC (liquidtight flexible metal conduit)
- NEC 356 — LFNC (liquidtight flexible nonmetallic conduit)
Conductor sizing
Use 75°C column of NEC 310.16, size to MCA:
| MCA | Copper THHN | Aluminum |
|---|---|---|
| up to 15A | 14 AWG | 12 AWG |
| up to 20A | 12 AWG | 10 AWG |
| up to 25A | 10 AWG | 8 AWG |
| up to 30A | 10 AWG | 8 AWG |
| up to 50A | 6 AWG | 4 AWG |
EGC per NEC 250.122 based on MOCP:
| MOCP | Copper EGC |
|---|---|
| 15A | 14 AWG |
| 20A | 12 AWG |
| 30A | 10 AWG |
| 40A | 10 AWG |
| 50A | 10 AWG |
Conduit sizing
Mini-split outdoor circuits are 2-wire 240V (no neutral) + EGC. Most installs use:
- Home-run from panel to outdoor disconnect: EMT, PVC, or NM-B through framing
- Whip from disconnect to condenser: 4-6 ft of LFNC (most common) or LFMC
| Circuit | Conductors | Area | 1/2" EMT | 3/4" EMT | 1/2" LFNC |
|---|---|---|---|---|---|
| 15A (14 AWG) | 2 × 14 + 1 × 14 EGC | 0.0291 in² | 14.8% | 6.2% | 16.7% |
| 20A (12 AWG) | 2 × 12 + 1 × 12 EGC | 0.0399 in² | 20.3% | 8.4% | 22.9% |
| 30A (10 AWG) | 2 × 10 + 1 × 10 EGC | 0.0780 in² | 39.6% (pass tight) | 16.5% | 44.7% — needs 3/4" |
| 40A (8 AWG) | 2 × 8 + 1 × 10 EGC | 0.0855 in² | over 40% — fails | 18.1% | over 40% — fails |
| 50A (6 AWG) | 2 × 6 + 1 × 10 EGC | 0.1225 in² | over 40% — fails | 25.9% | 70%+ — needs 3/4" |
1/2-inch EMT handles 15A and 20A circuits with margin. 3/4-inch EMT for anything 25A and larger.
For the LFNC whip, use 1/2-inch for 14-12 AWG and 3/4-inch for 10-6 AWG.
Special requirements
- Disconnect within sight: Non-fused pullout (60A typical) NEMA 3R, on exterior wall.
- Whip to outdoor unit: 4-6 ft of LFNC is the universal standard. Bonding bushings if using LFMC over 6 ft.
- Indoor unit power: Most single-zone mini-splits power the outdoor unit and use a 14/4 interconnect to the indoor head. Multi-zone systems often power the outdoor unit only.
- Communication cable: Run separately from line voltage in commercial installs. For residential, the 14/4 shielded cable typically runs alongside the refrigerant line set in the wall sleeve.
- Drain heater: Cold-climate outdoor units may have a base pan heater drawing 0.5-1A continuous — already included in MCA.
- Outdoor receptacle: NEC 210.8(F) requires GFCI for outdoor receptacles within sight of HVAC equipment.
Worked example — 18,000 BTU single-zone mini-split, 25 ft from panel
Nameplate: MCA = 14.5A, MOCP = 20A
- Breaker: 20A 2-pole in main panel
- Home-run conductors: 12 AWG copper THHN × 2 (L1, L2) + 12 AWG copper EGC
- Home-run raceway: 1/2" EMT, 25 ft from panel to outdoor disconnect
- Fill check (home-run): 3 × 0.0133 = 0.0399 in² in 1/2" EMT (0.122 in² 40% limit) = 32.7%, pass.
- Disconnect: 30A non-fused pullout, NEMA 3R, mounted within 3 ft of condenser.
- Whip: 4 ft of 1/2" LFNC with 12 AWG conductors + 12 AWG EGC. Fill: 3 × 0.0133 = 0.0399 in² in 1/2" LFNC (~22.9%), pass.
- Indoor head: Powered from outdoor unit via 14/4 stranded shielded cable per manufacturer.
For a 36,000 BTU multi-zone:
- Breaker: 40A 2-pole
- Conductors: 8 AWG copper THHN × 2 + 10 AWG copper EGC in 3/4" EMT
- Whip: 3/4" LFNC, same 8 AWG conductors + 10 AWG EGC
Cross-check with the Conduit Fill Calculator.
Quick reference
| BTU | MCA | Breaker | Copper wire | Home-run conduit | LFNC whip |
|---|---|---|---|---|---|
| 9,000 | 10A | 15A | 14 AWG × 2 | 1/2" EMT | 1/2" |
| 12,000 | 13A | 15A | 14 AWG × 2 | 1/2" EMT | 1/2" |
| 18,000 | 16A | 20A | 12 AWG × 2 | 1/2" EMT | 1/2" |
| 24,000 | 20A | 25A | 10 AWG × 2 | 1/2" EMT | 3/4" LFNC |
| 36,000 | 26A | 35-40A | 8 AWG × 2 | 3/4" EMT | 3/4" LFNC |
| 48,000 | 30A | 50A | 6 AWG × 2 | 3/4" EMT | 3/4" LFNC |
Related
FAQ
This page's figures are transcribed from NEC 2023 (NFPA 70) Chapter 9, Tables 1, 4 and 5, and checked by a version-controlled automated test suite — they are not signed off by a licensed professional. WireFillChart is an independent reference, not affiliated with or endorsed by NFPA. Always verify against the code edition adopted by your local authority having jurisdiction (AHJ).