The correct size of wire for a 30 amp breaker is 10 AWG copper with THHN/THWN-2 insulation in conduit, or 8 AWG copper if you are using NM-B (Romex) cable. This assumes a standard 30°C ambient temperature and no more than three current-carrying conductors bundled together.
- Material: Copper (Aluminum requires different sizing, covered below).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit/Cable Type: Raceway (EMT/PVC) for THHN, or standard residential framing for NM-B.
- Bundling: Maximum of 3 current-carrying conductors in a single raceway.
Note: NEC-style guidance provided here is for educational planning; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Core Ampacity Table (NEC 310.16)
To understand wire sizing, you have to look at NEC Table 310.16, which dictates the allowable ampacity of insulated conductors. The most common mistake DIYers make is looking exclusively at the 90°C column because THHN wire is rated for 90°C. However, the National Electrical Code (NEC) requires you to base your final ampacity on the lowest temperature rating of any connected component, which is usually the breaker terminal (rated 75°C) or the cable jacket itself.
| AWG Size | 60°C Column (NM-B / Romex) |
75°C Column (THHN in Conduit) |
90°C Column (Derating Baseline) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
| 6 AWG | 55A | 65A | 75A |
As highlighted in the table, 10 AWG copper in the 75°C column is rated for 35 amps. Because 35A is greater than your 30A breaker, 10 AWG THHN is the minimum legal size for conduit runs. However, if you are pulling NM-B cable, you are legally restricted to the 60°C column, where 10 AWG is only rated for 30A. This brings us to the continuous load rule.
Why 10 AWG (and When You Must Step Up to 8 AWG)
Why use 10 AWG and not 12 AWG? A 12 AWG wire in the 75°C column is only rated for 25A. If you pull 25A through it and protect it with a 30A breaker, the wire will overheat and melt before the breaker ever trips. The breaker must be sized to protect the wire's weakest link.
The Continuous Load Trap (NEC 210.20)
The NEC defines a "continuous load" as any load where the maximum current is expected to continue for 3 hours or more. Common 30A continuous loads include EV chargers, large window AC units, and commercial water heaters.
If your 30A load is continuous, NEC 210.20(A) requires the branch circuit rating to be 125% of the continuous load.
- Math: 30A × 1.25 = 37.5A.
- Result: You cannot use a 30A breaker for a continuous 30A load. You must step up to a 40A breaker and use 8 AWG wire (rated 40A at 60°C / 50A at 75°C).
If your load is non-continuous (like a standard residential dryer that cycles on and off, or an RV outlet used intermittently), a 30A breaker and 10 AWG THHN wire is perfectly code-compliant.
The NM-B (Romex) Restriction
Even for non-continuous loads, most professional electricians will pull 8 AWG NM-B for a 30A circuit rather than 10 AWG. Why? Because NM-B is restricted to the 60°C column (NEC 334.80). At 60°C, 10 AWG is rated for exactly 30A. Running a wire at 100% of its absolute maximum ampacity inside insulated walls causes terminal heating and degrades the insulation over time. Stepping up to 8 AWG NM-B (40A at 60°C) provides a thermal buffer and makes the physical termination into the breaker lugs much easier, as 8 AWG holds a torque screwdown more securely than the stiffer, smaller 10 AWG.
Variables That Force a Larger Wire Size
The baseline assumptions at the top of this article rarely survive contact with the actual jobsite. Here is what changes the answer and forces you to buy thicker wire.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for branch circuits. Let's look at a 240V, 30A circuit using 10 AWG copper (resistance ≈ 1.24 Ω per 1,000 ft).
- At 50 feet: Voltage drop is ~1.1V (0.4%). 10 AWG is fine.
- At 100 feet: Voltage drop is ~2.2V (0.9%). 10 AWG is fine.
- At 200 feet: Voltage drop is ~4.4V (1.8%). 10 AWG is fine.
Wait, what about 120V? If this is a 120V, 30A circuit (like a heavy-duty RV TT-30 outlet), the current travels out on the hot and back on the neutral, doubling the effective wire length. At 100 feet of physical distance, you have 200 feet of wire. The drop becomes ~7.4V, which is a 6.1% drop on a 120V circuit. This exceeds the 3% recommendation. For a 120V/30A run over 60 feet, you must step up to 8 AWG, and over 100 feet, you need 6 AWG.
2. Conductor Bundling (Derating)
If you pull multiple circuits through the same piece of EMT conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires derating. If you have 4 to 6 current-carrying conductors in a raceway, you must multiply the wire's 90°C ampacity by 80%.
- 10 AWG THHN (90°C column = 40A): 40A × 0.80 = 32A. (Still passes the 30A breaker requirement).
- 7 to 9 conductors (70% derating): 40A × 0.70 = 28A. Fails. You must step up to 8 AWG THHN.
3. Aluminum vs. Copper
Aluminum is cheaper but has higher resistance and expands/contracts more under heat. You cannot use the copper chart for aluminum wire. For a 30A breaker, 10 AWG aluminum is illegal (it's only rated 25A at 75°C). The minimum size of aluminum wire for a 30 amp breaker is 8 AWG (rated 40A at 75°C). Always use anti-oxidant paste (like Noalox) on aluminum terminations and torque to the manufacturer's exact spec, as aluminum is highly prone to arcing if the lug loosens over time.
When to Call an Engineer or the AHJ
While the rules above cover 95% of residential and light commercial 30A circuits, there are edge cases where you must defer to a licensed professional or your local inspector. According to NFPA 70 (National Electrical Code) guidelines, involve an engineer or the AHJ when:
- High Ambient Temperatures: If the conduit runs through an attic or rooftop where temperatures regularly exceed 30°C (86°F), you must apply ambient temperature correction factors. A 50°C attic requires multiplying your 90°C ampacity by 0.82, which can quickly invalidate 10 AWG wire.
- Harmonic Loads: If the 30A load is fed by a large variable frequency drive (VFD) or UPS system that generates heavy triplen harmonics, the neutral conductor can carry more current than the phase conductors, requiring oversized neutrals and specialized breaker types.
- Service Entrance Conductors: If this 30A breaker is part of a main service panel upgrade or involves the utility meter base, this is strictly licensed electrician territory. Utilities have their own "green books" that supersede standard NEC branch circuit rules.
Ultimately, sizing wire isn't just about preventing a fire today; it's about ensuring the termination points don't degrade over a decade of thermal cycling. When in doubt between 10 AWG and 8 AWG for a 30A breaker, the cost difference in copper is usually less than $15 for a standard run. Step up to 8 AWG, torque the lugs to the breaker manufacturer's specification (usually printed on the breaker label, often around 35 in-lbs), and you'll have a circuit that runs cool and passes any inspection.
