For a standard 30-amp circuit, you need 10 AWG copper wire or 8 AWG aluminum wire. This baseline assumes a standard residential installation using the 60°C ampacity column of NEC Table 310.16, with no more than three current-carrying conductors in a raceway, and an ambient temperature not exceeding 30°C (86°F). If your installation deviates from these baseline conditions—such as running wire through a hot attic or bundling multiple circuits together—you will need to upsize your wire.
The 30 Amp Wire Size Chart (NEC Table 310.16)
The following data is extracted directly from NFPA 70 (National Electrical Code) Table 310.16. To read this table correctly, you must understand the temperature columns. Wire insulation (like THHN or XHHW) is rated for high temperatures (90°C), but the terminations at your breaker and receptacles are often rated lower. NEC Article 110.14(C) requires you to use the lowest temperature rating of any connected component, which for circuits 100A or less is typically the 60°C column.
| AWG Size | Copper 60°C (Amps) | Copper 75°C (Amps) | Copper 90°C (Amps) | Aluminum 75°C (Amps) |
|---|---|---|---|---|
| 12 AWG | 20 | 25 | 30 | 25 |
| 10 AWG | 30 | 35 | 40 | 30 (Requires 75°C terminals) |
| 8 AWG | 40 | 50 | 55 | 40 |
| 6 AWG | 55 | 65 | 75 | 50 |
| 4 AWG | 70 | 85 | 95 | 65 |
Notice that 10 AWG copper in the 90°C column is rated for 40 amps. A common DIY mistake is buying THHN wire, reading the 90°C column, and assuming 10 AWG can safely carry 40 amps on a 40A breaker. This is a code violation. Because standard residential breakers and receptacles are rated for 60°C or 75°C, the allowable ampacity of 10 AWG copper is capped at 30A or 35A, respectively. For a 30A breaker, the 60°C column dictates 10 AWG is the exact minimum.
Which Column Applies and How Derating Modifies the Base Value
The chart above represents ideal, baseline conditions. Real-world jobsites rarely match the baseline. You must apply derating factors from NEC Tables 310.15(B)(1) (Ambient Temperature) and 310.15(C)(1) (Adjustment Factors for More Than Three Current-Carrying Conductors). Derating is always calculated using the 90°C column as your starting baseline, even if your final termination limit is 60°C.
Which Column Applies to Your Installation?
- 60°C Column: Applies to circuits rated 100A or less using wire sizes 14 AWG through 1 AWG, unless the equipment is explicitly marked for 75°C. This is your default for standard 30A residential branch circuits.
- 75°C Column: Applies if your breaker and receptacle are explicitly stamped '75°C' (common on modern commercial gear and some high-end residential panels) and you are using 75°C-rated wire like THWN or XHHW.
- 90°C Column: Used only for derating calculations and for specific high-temp industrial terminations. It is almost never used for final ampacity sizing in residential branch circuits.
How Derating Rows Modify the Base Value
Let's run a real-world scenario. You are wiring a 30-amp compressor in a garage. You run 10 AWG THHN (90°C rating = 40A base) through a conduit in an uninsulated attic where the ambient temperature reaches 45°C (113°F). You are also pulling two other circuits in the same conduit, giving you 6 current-carrying conductors total.
- Base Ampacity (90°C column): 40A
- Temperature Correction (45°C ambient): Multiply by 0.79
- Conductor Adjustment (4-6 wires): Multiply by 0.80
- Final Derated Ampacity: 40A × 0.79 × 0.80 = 25.28 Amps
Even though you are using 10 AWG wire, your derated ampacity has dropped to 25.28A. Putting this on a 30-amp breaker is a fire hazard and a direct NEC violation. You must upsize to 8 AWG THHN (Base 55A × 0.79 × 0.80 = 34.76A) to safely protect a 30A load under these conditions.
What the Chart Cannot Tell You (Edge Cases & Limits)
Ampacity charts assume a perfect, short run with zero voltage drop and non-continuous duty cycles. Here is what the chart leaves out, and how to handle it on the bench.
1. Voltage Drop Over Distance
NEC Table 310.16 does not account for resistance over long distances. If you are wiring a 240V, 30-amp RV pedestal or a detached garage subpanel 100 feet away, 10 AWG copper will result in unacceptable voltage drop. Using the standard voltage drop formula Vd = (2 × K × I × L) / CM (where K=12.9 for copper, I=30A, L=100ft, and CM=10,380 for 10 AWG), the drop is 7.45 volts. On a 240V circuit, that is a 3.1% drop, which exceeds the NEC's recommended 3% maximum for branch circuits (Informational Note to NEC 210.19). For a 100-foot run at 30 amps, upsize to 8 AWG copper to keep the drop under 2%.
2. Continuous Loads (The 125% Rule)
If your 30-amp load is considered 'continuous' by the NEC (defined as operating for 3 hours or more), you cannot size the wire and breaker at exactly 100% of the load. Examples include heavy-duty space heaters, continuous-duty EV chargers, or commercial dehumidifiers. Per NEC 210.20(A), continuous loads must be multiplied by 125%.
30 Amps × 1.25 = 37.5 Amps.
A 10 AWG wire (rated 30A at 60°C) will overheat, and a 30A breaker will eventually nuisance-trip. For a 30A continuous load, you must upsize to 8 AWG copper (rated 40A at 60°C) and install a 40-amp breaker.
3. Motor Startup Currents (LRA vs. FLA)
If your 30-amp circuit feeds a large motor (like a 5HP air compressor), the wire is sized based on the motor's Full Load Amps (FLA) at 125%, but the breaker is sized to handle the Locked Rotor Amps (LRA) during startup. A motor drawing 24A FLA requires wire sized for 30A (10 AWG), but the NEC allows the breaker to be sized up to 250% of the FLA to prevent tripping during startup. In this specific edge case, you might see a 60A breaker protecting 10 AWG wire. This is legal only under the specific motor overload protection rules of NEC Article 430, and the motor's internal thermal overloads must be properly calibrated. Do not apply this logic to standard resistive loads like dryers or heaters.
