For a standard 30-amp breaker, use 10 AWG copper wire (THHN/THWN-2). This is the minimum safe size per NEC 240.4(D). If your 120V circuit exceeds 40 feet, upsize to 8 AWG copper to prevent voltage drop. Always verify your specific run length, ambient temperature, and terminal ratings before pulling wire.
- Material: Copper (Aluminum requires different sizing)
- Termination Rating: 75°C (Standard for modern breakers and receptacles)
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Raceway (EMT/PVC) or NM-B cable, maximum 3 current-carrying conductors (no derating applied)
NEC Ampacity Rules: Why 10 AWG and Not 12 AWG?
It is a common and dangerous mistake to assume that because a 12 AWG wire can physically fit into a 30-amp breaker lug, it is safe to use. It is not. The National Electrical Code (NEC) strictly governs this via NEC Article 240.4(D), which places hard limits on small conductors to prevent fires.
Under standard conditions, 12 AWG copper wire has an ampacity of 20 amps. If you connect it to a 30-amp breaker and pull 28 amps of current, the wire will overheat, melt its insulation, and potentially ignite surrounding framing long before the breaker's thermal trip mechanism engages. The breaker is sized to protect the wire, not the appliance. By code, 10 AWG copper is the absolute minimum size permitted for a 30-amp overcurrent device.
The Ampacity Table Breakdown
To understand why 10 AWG is the correct baseline, we have to look at NEC Table 310.16 (formerly 310.15(B)(16)). This table lists the allowable ampacities of insulated conductors. Here is the exact row for 10 AWG copper:
| Wire Size (AWG) | 60°C Column (140°F) | 75°C Column (167°F) | 90°C Column (194°F) |
|---|---|---|---|
| 10 AWG Copper | 30 Amps | 35 Amps | 40 Amps |
Notice that in the 90°C column (which applies to THHN/THWN-2 wire insulation), the wire can technically handle 40 amps. However, NEC 110.14(C) requires you to size the conductor based on the temperature rating of the terminations (the breaker lugs and receptacle screws). Modern breakers and 30A receptacles (like NEMA L14-30 or 14-30R) are rated for 75°C. Therefore, you must use the 75°C column, which gives you 35 amps.
Even though the 75°C column allows 35 amps, NEC 240.4(D) overrides this for small conductors, hard-capping 10 AWG copper at a 30-amp breaker. This is your baseline.
Voltage Drop Check: When to Upsize to 8 AWG
Ampacity tells you the wire won't catch fire. Voltage drop tells you your appliance will actually run correctly. The NEC recommends keeping voltage drop under 3% for branch circuits. When you push 30 amps through 10 AWG wire over a long distance, resistance eats your voltage.
Let's run the math for a 50-foot run from the panel to a 30-amp load using the standard DC-resistance approximation formula: VD = (2 × K × I × D) / CM.
- K (Copper constant) = 12.9
- I (Current) = 30 Amps
- D (Distance) = 50 feet
- CM (Circular Mils for 10 AWG) = 10,380
VD = (2 × 12.9 × 30 × 50) / 10,380 = 3.72 Volts.
If this is a 120V circuit (like a 30A RV receptacle), a 3.72V drop is 3.1%. This exceeds the 3% NEC recommendation. You must upsize to 8 AWG for any 120V/30A run over 40 feet.
If this is a 240V circuit (like a dryer or water heater), the 3.72V drop is only 1.55% of 240V. In this case, 10 AWG is perfectly adequate for a 50-foot run, and you can safely push the distance to about 80 feet before needing to upsize.
Decision Tree: Finalizing Your Wire Size
Use this decision path to lock in your exact wire purchase. Do not guess; follow the parameters of your specific installation.
| Circuit Voltage | One-Way Distance | Continuous Load? (3+ Hrs) | Concrete Wire Pick (Copper) |
|---|---|---|---|
| 240V | Under 80 feet | No | 10 AWG THHN/THWN-2 or 10/2 NM-B |
| 240V | 80 to 125 feet | No | 8 AWG THHN/THWN-2 or 8/2 NM-B |
| 120V | Under 40 feet | No | 10 AWG THHN/THWN-2 or 10/2 NM-B |
| 120V | 40 to 65 feet | No | 8 AWG THHN/THWN-2 or 8/2 NM-B |
| 120V / 240V | Any length | Yes (e.g., baseboard heater) | See "Continuous Loads" section below (Requires 8 AWG minimum + 40A breaker) |
What Changes the Answer? (Aluminum, Bundling, and Heat)
The baseline assumes ideal conditions. Real-world jobsites rarely cooperate. Here is what forces you to change your wire size.
1. Switching to Aluminum Wire
Aluminum is lighter and cheaper, but it has higher resistance and expands/contracts more than copper. You cannot use the same AWG for aluminum as you do for copper. According to Table 310.16, 10 AWG aluminum in the 75°C column is only rated for 25 amps. Therefore, if you are pulling aluminum wire (like SER cable for a subpanel feeder), you must use 8 AWG Aluminum for a 30-amp breaker. Never interchange copper and aluminum sizing charts.
2. Conduit Bundling (Derating)
If you are pulling multiple circuits through a single piece of EMT or PVC conduit, the wires heat each other up. NEC 310.15(C)(1) requires ampacity derating when you have more than three current-carrying conductors (CCCs) in a raceway.
- 4 to 6 CCCs: Derate to 80%. (10 AWG THHN 90°C base is 40A × 0.80 = 32A). 10 AWG is still safe.
- 7 to 9 CCCs: Derate to 70%. (40A × 0.70 = 28A). 10 AWG fails. You must upsize to 8 AWG THHN.
3. High Ambient Temperatures
If your conduit runs through an attic in the Southwest US where ambient temperatures hit 110°F (43°C), you must apply a temperature correction factor. At 41-45°C ambient, the 90°C column must be derated by 0.82. (40A × 0.82 = 32.8A). You are still safe with 10 AWG, but you are losing your safety margin. If the attic hits 122°F (50°C), the derating factor drops to 0.75 (40A × 0.75 = 30A), putting you right on the absolute limit. In high-heat environments, pulling 8 AWG is the professional choice.
When an Engineer or the AHJ Must Confirm
There are specific scenarios where standard DIY sizing rules end and professional engineering or Authority Having Jurisdiction (AHJ) approval begins.
The Continuous Load Trap: NEC Article 210.20(A) dictates that if a load is expected to run for 3 hours or more continuously, the overcurrent device must be sized at 125% of the load. If you are wiring a 30-amp continuous load (like a commercial heater or a heavy-duty EV charger pulling max current), you cannot use a 30-amp breaker. You must multiply 30A by 1.25, which equals 37.5A. You must step up to a 40-amp breaker and run 8 AWG copper wire. Conversely, if you are locked into a 30-amp breaker, your maximum continuous load cannot exceed 24 amps (80% of 30A).
Subpanel Feeders: If your 30-amp breaker is feeding a subpanel rather than a single appliance, voltage drop calculations become more complex due to the diversity of downstream loads. Furthermore, if the run exceeds 100 feet, or if you are dealing with high fault-current available at the service entrance, an engineer must verify the let-through current and ensure the wire's short-circuit rating is adequate.
Always treat the NEC as the minimum safety baseline. Your local AHJ inspector has the final authority on your installation, and local amendments may require 8 AWG as a blanket minimum for all 30-amp circuits regardless of distance to future-proof the home. When in doubt, the cost difference between 10 AWG and 8 AWG copper over a short run is negligible compared to the cost of failing an inspection or tearing open drywall to pull new wire.
