For a 30-amp load at 100 feet, use 8 AWG copper wire protected by a 30-amp breaker. While 10 AWG copper safely carries 30 amps thermally, 8 AWG is necessary to keep voltage drop below the recommended 3% threshold over a 100-foot run.
Baseline Assumptions for This Sizing
Wire sizing is not one-size-fits-all. The 8 AWG recommendation above is based on the following strict parameters. If your installation deviates from these, you must recalculate:
- Conductor Material: Copper
- Insulation Type: THHN/THWN-2
- Temperature Column: 75°C (Standard for most residential/commercial breakers and receptacles, per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Installation Method: Raceway/conduit (up to 3 current-carrying conductors)
- System: Single-phase AC
Wire Size vs. Voltage Drop at 100 Feet
The National Electrical Code (NEC) Table 310.16 dictates the thermal limit of a wire—how much current it can carry before the insulation melts. However, the NEC also includes Informational Notes regarding voltage drop (VD), recommending a maximum 3% drop for branch circuits and a 5% maximum overall. Over a 100-foot distance, voltage drop becomes the governing constraint, not heat.
Using the standard single-phase voltage drop formula VD = (2 × K × I × L) / CM (where K=12.9 for copper, I=30A, L=100ft, and CM is circular mils), we can map out exactly why 10 AWG fails and 8 AWG wins.
| Wire Size (AWG) | Ampacity (75°C) | Circular Mils (CM) | VD at 240V (Volts) | VD % (240V) | VD at 120V (Volts) | VD % (120V) |
|---|---|---|---|---|---|---|
| 10 AWG | 35A | 10,380 | 7.45V | 3.1% (Borderline) | 7.45V | 6.2% (Fails >5%) |
| 8 AWG | 50A | 16,510 | 4.68V | 1.95% (Passes) | 4.68V | 3.9% (Passes <5%) |
| 6 AWG | 65A | 26,240 | 2.94V | 1.2% (Excellent) | 2.94V | 2.45% (Passes <3%) |
| 4 AWG | 85A | 41,740 | 1.85V | 0.77% (Overkill) | 1.85V | 1.54% (Overkill) |
The Verdict: If your 30-amp load is 240V (like a dryer, RV receptacle, or baseboard heater), 8 AWG keeps you well under the 3% recommendation at 1.95%. If your load is 120V, 8 AWG yields a 3.9% drop, which is under the 5% absolute maximum but slightly above the 3% ideal. For a 120V circuit where sensitive electronics are involved, bumping to 6 AWG is the professional choice to hit that 2.45% sweet spot.
Why Not Use 10 AWG? (The Thermal vs. Distance Conflict)
A common mistake among DIYers is looking solely at the breaker size. A 30-amp breaker requires a wire rated for at least 30 amps. According to the 75°C column of NEC Table 310.16, 10 AWG copper THHN is rated for 35 amps. Thermally, 10 AWG will not overheat or trip the breaker under a 30A load.
However, electricity behaves like water in a long, narrow hose. Over 100 feet, the resistance of 10 AWG wire chokes the voltage. If you run a 30A compressor on a 100-foot 10 AWG extension or branch circuit at 240V, the equipment only sees ~232V. At 120V, it sees ~112V. Motors draw more current when voltage drops to maintain their power output (P = V × I), which leads to overheating, shortened equipment life, and nuisance tripping of the motor's internal thermal overload. Sizing up to 8 AWG eliminates this hidden stress.
Variables That Change Your Wire Size
The 8 AWG answer assumes a perfect-world installation. Jobsite realities often force you to size up again. Here is what changes the math and how to adapt.
| Variable | Impact on Sizing | Required Action |
|---|---|---|
| Increased Length (150+ ft) | Voltage drop scales linearly with distance. At 150 ft, 8 AWG drops to 2.9% at 240V and 5.8% at 120V. | Upgrade to 6 AWG copper for 240V, or 4 AWG for 120V. |
| Conduit Bundling (4-6 wires) | NEC 310.15(C)(1) requires an 80% derating factor for 4-6 current-carrying conductors in a single raceway. | 8 AWG (50A × 0.8 = 40A) still passes. 10 AWG (35A × 0.8 = 28A) fails thermal limits. Stick to 8 AWG minimum. |
| High Ambient Heat (Attics) | Attics can reach 50°C+ in summer. The 90°C column must be used for derating, reducing ampacity significantly. | Apply temperature correction factors. Usually requires bumping to 6 AWG to maintain 30A capacity after derating. |
| Using Aluminum Wire | Aluminum has higher resistance and lower thermal capacity. 10 AWG and 8 AWG aluminum are not sufficient. | Use 4 AWG aluminum (rated 65A at 75°C) to satisfy both thermal and VD requirements at 100 ft. |
The Aluminum Trap
Never interchange copper and aluminum sizing charts. Aluminum is lighter and cheaper, making it attractive for long feeder runs, but it requires roughly two AWG sizes larger than copper for the same current. For a 30-amp load at 100 feet, 2 AWG aluminum is the safest bet to keep voltage drop manageable, though 4 AWG aluminum is the absolute thermal minimum. Always use anti-oxidant paste (like Noalox) on aluminum terminations and torque to the manufacturer's exact specification to prevent cold creep and arcing.
Termination, Torque, and Installation Realities
Sizing the wire correctly is only half the battle; terminating it properly prevents fires. When pulling 8 AWG THHN through conduit for a 100-foot run, friction is your enemy. Use a high-quality wire pulling lubricant (like Polywater J) to prevent scoring the insulation, which compromises the dielectric strength of the wire.
Once the wire is at the panel and the receptacle, you must address torque. The 2017 NEC (and all subsequent editions, including the 2026 standards) strictly mandate that terminations be torqued to the values marked on the equipment. Most 30-amp breakers and receptacles require between 25 and 30 in-lbs of torque. Use a calibrated inch-pound torque screwdriver. Under-torquing 8 AWG wire causes high-resistance connections that generate massive heat; over-torquing can shear the screw or damage the conductor strands.
For the receptacle end, if you are wiring a NEMA 14-30 (common for dryers and EV chargers), ensure you are using a 4-wire setup (Hot, Hot, Neutral, Ground) and that the ground and neutral are strictly separated at the subpanel or receptacle. Never bootleg a ground or tie neutral to ground on the load side of the main disconnect.
When an Engineer or the AHJ Must Confirm
While the guidelines above cover 95% of residential and light-commercial 30-amp scenarios, specific conditions require a stamped engineering calculation or a direct call to your local Authority Having Jurisdiction (AHJ).
- Continuous Loads: If your 30-amp load will run for 3 hours or more continuously (e.g., commercial heaters, continuous-duty compressors, or certain EV charging setups), NEC Article 210.20 requires the branch circuit to be sized at 125% of the load. A 30A continuous load requires wire and breaker sizing for 37.5 amps. You will need a 40-amp breaker and 6 AWG copper wire minimum, regardless of distance.
- Complex Conduit Fill: If you are pulling this circuit alongside multiple other heavily loaded circuits in a single conduit, the mutual heating effect requires a professional derating calculation per NEC Chapter 9, Note 4 to the tables.
- Utility Interconnection: If this 30-amp circuit feeds an inverter or a back-fed breaker for solar/battery systems, the AHJ will require a line-side tap analysis or a busbar rating calculation (the 120% rule) to ensure the panel can handle the bidirectional current.
For quick field verification of your math on complex runs, cross-reference your calculations with the Southwire Voltage Drop Calculator, which accounts for specific insulation types, conduit materials (PVC vs. Steel), and power factor adjustments that manual formulas often simplify.
By starting with 8 AWG copper for a standard 30-amp, 100-foot run, you satisfy both the thermal limits of the NEC and the practical physics of voltage drop, ensuring your equipment runs cool, efficient, and safe for decades.
