You need 10 AWG copper wire for a standard 30-amp breaker. If you are using aluminum wire, you must step up to 8 AWG. This assumes a standard 75°C termination rating, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway.
The Direct Answer & Baseline Assumptions
Before pulling any wire through a conduit or stapling NM-B to a stud, you must establish the baseline conditions for your circuit. Wire ampacity is not a fixed number; it is a variable dependent on insulation type, termination temperature limits, and environmental factors. The 10 AWG copper answer applies strictly to the following baseline parameters:
- Material: Copper (Aluminum requires 8 AWG)
- Temperature Column: 75°C (per NEC 110.14(C) for terminations)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors in a single raceway
- Load Type: Non-continuous (operates for less than 3 hours at a time)
If any of these assumptions change—such as running wire through a hot attic or bundling multiple circuits in a single PVC conduit—the required wire gauge will increase. Always size the wire to protect the weakest point in the circuit, which is almost always the breaker lug or the device terminal.
30-Amp Wire Sizing Matrix (NEC 310.16)
The National Electrical Code (NEC) Table 310.16 is the master reference for allowable ampacities of insulated conductors. Below is the specific data matrix for 30-amp circuits, comparing common insulation types and materials. Notice how the 90°C column is largely irrelevant for final breaker sizing, a concept we will break down in the next section.
| Material | Wire Gauge (AWG) | Insulation Type | Temp Rating | Base Ampacity (75°C Col) | Max Breaker Size |
|---|---|---|---|---|---|
| Copper | 10 AWG | THHN / THWN-2 | 90°C (use 75°C col) | 30A (at 75°C) | 30A |
| Copper | 10 AWG | NM-B (Romex) | 60°C | 30A | 30A |
| Aluminum | 8 AWG | THHN / THWN-2 | 90°C (use 75°C col) | 40A (at 75°C) | 30A |
| Aluminum | 8 AWG | USE-2 / RHW-2 | 90°C (use 75°C col) | 40A (at 75°C) | 30A |
For a comprehensive look at how these tables are applied in the field, refer to the National Fire Protection Association's NEC guidelines, which govern these ampacity limits to prevent insulation meltdown and structural fires.
Why 10 AWG? The 75°C Terminal Rule Explained
A common point of confusion for DIYers is looking at a spool of THHN wire, seeing it rated for 90°C, and checking the 90°C column in NEC Table 310.16. In that column, 10 AWG copper is rated for 35 amps, and 12 AWG is rated for 25 amps. So why can't you put 12 AWG THHN on a 30-amp breaker?
The answer lies in NEC 110.14(C): Temperature Limitations at Terminations. The wire is only as good as its connection. Standard residential breakers (like Square D QO or Homeline) and receptacles are tested and rated for a maximum termination temperature of 75°C (or 60°C for older/smaller devices). If you push 30 amps through a 12 AWG wire, the wire itself might survive at 90°C, but the heat will conduct down the copper into the breaker lug. The breaker lug will overheat, degrade, and potentially cause a fire at the panel before the 30-amp thermal-magnetic trip mechanism activates.
Therefore, you must size the wire based on the 75°C column (or 60°C for NM-B cable). In the 75°C column, 10 AWG copper is exactly 30 amps. Using 12 AWG on a 30-amp breaker is a direct violation of NEC 240.4(D) and is a severe fire hazard.
Voltage Drop: When 10 AWG Copper Isn't Enough
Ampacity tells you the wire won't melt. Voltage drop tells you the equipment at the end of the wire will actually function. The NEC recommends a maximum voltage drop of 3% for branch circuits. While not strictly enforceable as a hard rule in all jurisdictions, ignoring it leads to tripped breakers, dim lights, and burned-out motors.
Let's run a voltage drop calculation for a 30-amp load on 10 AWG copper wire over a 100-foot run. According to the Southwire voltage drop parameters, 10 AWG copper has an approximate resistance of 1.21 ohms per 1,000 feet at 75°C.
Formula: VD = 2 × K × I × L / CM (or simplified using resistance per 1000ft)
VD = 2 × 0.00121 Ω/ft × 30A × 100 ft = 7.26 Volts dropped
- On a 120V circuit: 7.26V / 120V = 6.05% drop. This exceeds the 3% recommendation. You must step up to 8 AWG copper for a 100-foot 120V run.
- On a 240V circuit: 7.26V / 240V = 3.02% drop. This is right on the borderline. For 240V circuits (like a dryer or RV outlet), 10 AWG is acceptable up to about 80 feet. Beyond 80 feet, bump to 8 AWG to maintain optimal performance and prevent motor startup sags.
Derating Factors: What Changes the Wire Size?
The 10 AWG baseline assumes ideal conditions. In the real world, jobsite conditions often force you to increase the wire gauge. Here is the decision tree for when 10 AWG is no longer sufficient.
1. Conductor Bundling (NEC 310.15(C)(1))
When current flows through a wire, it generates heat. If you bundle multiple circuits in a single conduit, the heat cannot dissipate. If you have 4 to 6 current-carrying conductors in a raceway, you must apply an 80% derating factor.
If your load requires 30 amps, you divide by 0.80: 30A / 0.80 = 37.5A. You must now find a wire with a base ampacity of at least 37.5A. Because 10 AWG THHN is only rated for 35A in the 90°C column, it fails. You must pull 8 AWG THHN copper (rated 50A at 90°C) to safely carry the bundled load.
2. High Ambient Temperatures
If you are routing conduit through an unventilated attic in a hot climate where ambient temperatures regularly exceed 30°C (86°F), you must apply ambient temperature correction factors. At 41-45°C (105-113°F), the correction factor for 90°C insulation is 0.82. A 30A load divided by 0.82 requires 36.5A of base capacity, again forcing an upgrade to 8 AWG THHN.
3. Aluminum vs. Copper
Aluminum has a higher resistance than copper and expands/contracts more under thermal load. You can never use 10 AWG aluminum for a 30-amp circuit. The absolute minimum is 8 AWG aluminum, and you must use an anti-oxidant compound (like Noalox) and a torque screwdriver to terminate it, as aluminum is highly susceptible to loose-connection arcing over time. The Copper Development Association provides extensive comparative data on why copper remains the standard for branch circuits under 100 amps.
When to Consult the AHJ or an Engineer
You must involve your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer if your 30-amp load is classified as a continuous load. A continuous load is defined by the NEC as any load where the maximum current is expected to continue for 3 hours or more (e.g., a kiln, a commercial heater, or an EV charger).
For continuous loads, NEC 210.20(A) requires the circuit to be sized at 125% of the load. A 30-amp continuous load requires 30A × 1.25 = 37.5A. This means you cannot use a 30-amp breaker at all; you must install a 40-amp breaker and pull 8 AWG copper wire. Never attempt to bypass this rule by simply running the load in intervals; if the equipment's duty cycle allows for 3+ hours of runtime, it is legally a continuous load.
