Conduit Fill for Central A/C Disconnect
Central A/C disconnect wire fill: 3-ton uses 10 AWG copper on 30A breaker, 5-ton uses 8 AWG on 40A. NEC Article 440 with MCA/MOCP from nameplate.
Wire fill for central A/C disconnect is governed by NEC Article 440 (air conditioning and refrigerating equipment). A typical 3-ton residential central A/C uses a 30A 2-pole breaker, 10 AWG copper THHN to the outdoor disconnect, and a 6-foot LFMC or LFNC whip to the condensing unit. A 5-ton unit usually gets 40-60A breaker and 8 AWG copper. The Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOCP) on the equipment nameplate dictate sizing, not standard motor circuit tables.
Load characteristics
Central A/C condensing units have a compressor (largest motor) and condenser fan (smaller motor):
| A/C tonnage | Typical MCA | Typical MOCP | Breaker |
|---|---|---|---|
| 2 ton (~24,000 BTU) | 13-18A | 25-30A | 25A or 30A |
| 2.5 ton (~30,000 BTU) | 16-22A | 30-35A | 30A or 35A |
| 3 ton (~36,000 BTU) | 18-25A | 30-40A | 30A or 40A |
| 3.5 ton (~42,000 BTU) | 22-28A | 35-45A | 40A or 45A |
| 4 ton (~48,000 BTU) | 25-32A | 40-50A | 40A or 50A |
| 5 ton (~60,000 BTU) | 30-40A | 50-60A | 50A or 60A |
Always confirm with the equipment nameplate — high-efficiency variable-speed units may have lower MCA than older single-stage units of the same tonnage.
NEC code references
- NEC 440.4 — markings on nameplate (MCA, MOCP)
- NEC 440.14 — disconnect within sight of motor compressor
- NEC 440.22 — application of MOCP rules
- NEC 440.32 — branch-circuit conductors at 125% of largest motor compressor RLA
- NEC 440.33 — multiple-motor and combination loads
- NEC 250.122 — equipment grounding conductor sizing
Conductor sizing
Size to MCA on the nameplate. Use 75°C column of NEC 310.16:
| 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 35A | 8 AWG | 6 AWG |
| up to 50A | 6 AWG | 4 AWG |
EGC per NEC 250.122 sized to MOCP (not MCA):
| MOCP | Copper EGC |
|---|---|
| 30A | 10 AWG |
| 40-60A | 10 AWG |
| 100A | 8 AWG |
Conduit sizing
A/C circuits are typically 3-wire (2 hots + EGC, no neutral). Single-phase 240V hermetic compressors don't need a neutral.
| Circuit | Conductors | Area | 1/2" EMT | 3/4" EMT |
|---|---|---|---|---|
| 3-ton (30A, 10 AWG) | 2 × 10 + 1 × 10 EGC | 0.0780 in² | 39.6% pass tight | 16.5% |
| 4-ton (40A, 8 AWG) | 2 × 8 + 1 × 10 EGC | 0.0855 in² | over 40% — fails | 18.1% |
| 5-ton (50A, 6 AWG) | 2 × 6 + 1 × 10 EGC | 0.1225 in² | over 40% — fails | 25.9% |
| 5-ton (60A, 6 AWG) | 2 × 6 + 1 × 10 EGC | 0.1225 in² | over 40% — fails | 25.9% |
3/4-inch EMT or PVC handles essentially all residential central A/C circuits.
The whip from disconnect to condenser is usually 6 ft of liquidtight flexible conduit (LFMC or LFNC). Common sizes:
| Whip size | Typical for |
|---|---|
| 1/2" LFNC | 10 AWG conductors |
| 3/4" LFNC | 8 AWG conductors |
| 3/4" LFMC | 6 AWG with grounding bushing |
LFMC is preferred when used as the EGC; LFNC requires a separate copper EGC inside.
Special requirements
- Disconnect within sight — NEC 440.14. Mounted on exterior wall within sight of condenser, usually pullout-style with fuse holders.
- Fused vs non-fused disconnect — NEC doesn't require fuses if the upstream breaker provides protection per nameplate MOCP. Many AHJs prefer non-fused pullout for residential.
- Outdoor location — Use weather-resistant enclosure, NEMA 3R minimum.
- Bonding — LFMC requires bonding bushings if used as the EGC over a certain length; separate EGC inside the whip is the safer practice.
- GFCI for service receptacle — NEC 210.8(F) requires GFCI on outdoor receptacles within sight of HVAC equipment.
Worked example — 3-ton central A/C, 35 ft from main panel
Nameplate: MCA = 22.6A, MOCP = 40A, RLA = 16.5A
- Breaker: 40A 2-pole in main panel (or 35A if available)
- Conductors: 10 AWG copper THHN × 2 + 10 AWG copper EGC (MCA of 22.6A requires 25A wire — 10 AWG at 75°C is 35A, fine)
- Raceway home-run: 3/4" EMT from main panel to outdoor disconnect, 35 ft.
- Disconnect: 60A non-fused pullout, NEMA 3R, mounted within 3 ft of condenser.
- Whip: 6 ft of 1/2" LFNC with 10 AWG conductors + separate EGC.
- Fill check (home-run): 3 × 0.0260 = 0.0780 in² in 3/4" EMT = 16.5%, pass.
For a 5-ton: bump to 8 AWG (some prefer 6 AWG for margin), 50A or 60A breaker, 3/4" EMT.
Quick reference
| A/C tonnage | Typical MCA | Breaker | Copper wire | Conduit |
|---|---|---|---|---|
| 2 ton | 16A | 25A | 10 AWG | 3/4" EMT |
| 3 ton | 23A | 30A or 40A | 10 AWG | 3/4" EMT |
| 4 ton | 28A | 40A | 8 AWG | 3/4" EMT |
| 5 ton | 36A | 50-60A | 8 AWG or 6 AWG | 3/4" EMT |
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).