Wire gauge for a 240V circuit is determined strictly by the breaker amperage and conductor material, not the voltage itself. For a standard 240V 30A circuit (like an electric dryer), use 10 AWG copper. For a 50A circuit (like a range or EV charger), use 6 AWG copper when using NM-B cable, or 8 AWG copper if pulling THHN through conduit with 75°C rated terminations. Always size the wire to protect the weakest link in the circuit based on the ampacity columns of NEC Table 310.16.
How to Read This 240V Wire Gauge Chart
Before pulling wire, you must understand which temperature column applies to your specific installation. The National Electrical Code (NEC) Table 310.16 provides ampacities based on insulation temperature ratings (60°C, 75°C, and 90°C), but your terminations dictate the legal limit.
Assumptions for this chart:
- Conductor material: Copper and Aluminum (ACM/AAC).
- Maximum of 3 current-carrying conductors in a raceway or cable.
- Ambient temperature of 30°C (86°F).
- Standard residential single-phase 240V split-phase systems.
Complete 240V Ampacity & Wire Size Reference Table
The following data is derived from NEC Table 310.16. Use the 60°C column for NM-B cable and standard residential receptacles. Use the 75°C column for THHN/THWN-2 wires in conduit connected to 75°C-rated breakers and hardwired lugs.
| Breaker Size (Amps) | Copper AWG (60°C Column) | Copper AWG (75°C Column) | Aluminum AWG (75°C Column) | Common 240V Application |
|---|---|---|---|---|
| 15A | 14 AWG | 14 AWG | 12 AWG | Baseboard heaters, small window ACs |
| 20A | 12 AWG | 12 AWG | 10 AWG | 240V window ACs, small water heaters |
| 30A | 10 AWG | 10 AWG | 8 AWG | Electric dryers, RV outlets (TT-30) |
| 40A | 8 AWG | 8 AWG | 6 AWG | Older electric ranges, large AC condensers |
| 50A | 6 AWG | 8 AWG | 6 AWG | EV chargers (Level 2), modern ranges, hot tubs |
| 60A | 4 AWG | 6 AWG | 4 AWG | Subpanel feeders, large HVAC heat strips |
| 70A | 4 AWG | 4 AWG | 3 AWG | Heavy duty subpanels |
| 80A | 3 AWG | 3 AWG | 2 AWG | Large subpanels, commercial HVAC |
| 90A | 2 AWG | 2 AWG | 1 AWG | Tankless electric water heaters |
| 100A | 1 AWG | 3 AWG | 1 AWG | Main subpanel feeders, workshop service |
Note: Aluminum wire terminations are almost universally rated 75°C in residential panels. Never use the 60°C column for aluminum unless specifically mandated by legacy equipment markings.
Derating, Voltage Drop, and What the Table Cannot Tell You
Ampacity tables assume ideal conditions. On the jobsite, you must adjust for bundling, ambient heat, and distance. Here is how derating modifies the base values and where the chart falls short.
Applying NEC 310.15(C)(1) Bundling Derating
If you pull more than three current-carrying conductors through a single conduit, the wires heat each other up, and you must de-rate their ampacity. Crucially, derating is applied to the 90°C column of THHN wire, but the final result cannot exceed the termination temperature limit.
Worked Example: You are pulling two 240V circuits (4 current-carrying conductors total) through one EMT conduit for two 40A EV chargers.
1. Base 90°C ampacity for 8 AWG THHN is 55A.
2. NEC Table 310.15(C)(1) requires an 80% multiplier for 4-6 conductors.
3. 55A × 0.80 = 44A derated ampacity.
4. Check termination limit: 8 AWG at 75°C is 50A. Since 44A is less than 50A, the wire is legally rated for 44A.
5. Because 44A exceeds the 40A breaker requirement, 8 AWG THHN is legal and safe for this bundled run. If you had used NM-B cable, you could not use the 90°C column for derating, forcing an upsizing to 6 AWG.
What the Table Cannot Tell You
- Voltage Drop: NEC 310.16 does not account for distance. For 240V runs exceeding 100 feet, calculate voltage drop. A 3% maximum drop is standard practice (7.2V on a 240V circuit). A 50A EV charger 150 feet away requires upsizing from 6 AWG to 4 AWG copper to prevent charging faults.
- Conduit Fill: Chapter 9 of the NEC dictates how many wires physically fit inside a conduit. You might need 6 AWG for ampacity, but if you are pulling 4 conductors, you may need to jump from 3/4-inch to 1-inch EMT to meet the 40% fill capacity rule.
- Local AHJ Overrides: Some municipalities mandate minimum wire sizes (e.g., no 14 AWG allowed anywhere in the house) or require aluminum feeders to be upsized one additional step. Always verify with your local inspector.
Frequently Asked Questions
What size wire do I need for a 240V 50 amp breaker?
If you are using NM-B (Romex) cable, you must use 6 AWG copper because NM-B is legally restricted to the 60°C ampacity column (which rates 6 AWG at 55A, safely covering a 50A breaker). If you are pulling individual THHN/THWN-2 conductors through conduit and your breaker terminals are rated 75°C, you can legally use 8 AWG copper, as 8 AWG is rated 50A in the 75°C column. For aluminum, use 6 AWG.
Can I use 10 AWG wire for a 240V 30 amp circuit?
Yes. 10 AWG copper wire is rated for exactly 30 amps in both the 60°C and 75°C columns. This is the standard, code-compliant wire size for a 240V electric dryer on a 30A double-pole breaker. Ensure you are using a 4-wire setup (two hots, one neutral, one ground) for modern dryers to comply with NEC 250.140.
Does a 240V circuit require a different wire gauge than 120V for the same amperage?
No. Wire gauge (ampacity) is determined by the current (amps) generating heat in the conductor, not the voltage. A 20A breaker requires 12 AWG copper wire whether it is protecting a 120V kitchen receptacle or a 240V baseboard heater. The insulation thickness (e.g., 300V vs 600V rating) handles the voltage, while the copper cross-section handles the amperage.
Do I need a neutral wire for a pure 240V load?
No. Pure 240V appliances—such as electric baseboard heaters, well pumps, and dedicated AC condensers—only require two hot wires and an equipment grounding conductor (EGC). However, appliances that utilize both 120V and 240V (like electric dryers, ranges, and ovens that have 120V control boards or interior lights) require a 4-wire cable: two hots, one neutral, and one ground. Never use the ground wire as a neutral substitute.
