To properly size wire for a 30 amp breaker, you must use 10 AWG copper wire (or 8 AWG aluminum). This assumes standard THHN/THWN-2 insulation in a 30°C ambient environment. The 30-amp breaker protects the 10 AWG wire, which has an allowable ampacity of 35A in the 75°C column, safely derated to 30A for terminal limits.
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Column: 75°C for termination limits (NEC 110.14(C)), 90°C for derating
- Ambient Temperature: 30°C (86°F)
- Installation Method: EMT conduit, PVC, or NM-B cable in standard framing
The Core Sizing Rule: Why 10 AWG Copper?
When sizing conductors, we look at the National Electrical Code (NEC) Table 310.16. The goal is to match the wire's ampacity to the breaker's trip rating while respecting the physical limits of the breaker's screw terminals.
| AWG Size | 60°C Column | 75°C Column (Termination) | 90°C Column (Derating) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG (Target) | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Why not use 12 AWG? Looking at the 90°C column, 12 AWG shows 30A. However, NEC 240.4(D) strictly caps overcurrent protection for 12 AWG copper at 20A, regardless of insulation type. Using a 30A breaker on 12 AWG wire is a severe fire hazard; the wire will melt before the breaker trips. 10 AWG is the absolute minimum permitted size for a 30-amp overcurrent device.
Variables That Change the Answer: Distance, Bundling, and Material
The 10 AWG baseline assumes a short run in a cool environment with no more than three current-carrying conductors in a raceway. Real-world jobsites rarely stay this simple. Here is a decision matrix for when you must upsize to 8 AWG.
| Condition | Impact on 10 AWG | Required Action |
|---|---|---|
| Voltage Drop (120V, 100 ft) | 24A load yields ~4.9% drop (exceeds 3% NEC recommendation) | Upsize to 8 AWG |
| Voltage Drop (240V, 100 ft) | 24A load yields ~2.4% drop (acceptable) | Keep 10 AWG |
| Conduit Bundling (7-9 conductors) | 90°C derating (70%) drops 40A to 28A (below 30A breaker) | Upsize to 8 AWG |
| Aluminum Conductors | 10 AWG Aluminum is only rated for 25A at 75°C | Upsize to 8 AWG Aluminum |
| High Ambient Temp (50°C attic) | Correction factor (0.82) drops 90°C ampacity to 32.8A | Upsize to 8 AWG |
The Voltage Drop Math in Practice
According to data from the Copper Development Association, 10 AWG copper has a resistance of roughly 1.24 ohms per 1,000 feet at 75°C. If you are running a 240V circuit for a 30A RV receptacle 100 feet away, your total wire length is 200 feet.
At a continuous 24A draw (80% of 30A), the voltage drop is 24A × (1.24 × 0.2) = 5.95V. On a 240V system, that is a 2.48% drop, which is perfectly fine. However, if you are wiring a 120V 30A travel trailer outlet at that same distance, 5.95V represents a 4.9% drop. The NEC recommends keeping branch circuit drop under 3%. For a 120V run over 75 feet, you must pull 8 AWG copper to maintain voltage stability.
Aluminum vs. Copper: Never Interchangeable
If you are using aluminum wire (like SER cable for a subpanel feed), 10 AWG is physically too small and electrically inadequate. Aluminum has higher resistance and different thermal expansion properties. You must use 8 AWG aluminum for a 30A circuit. Furthermore, verify the breaker lugs are stamped 'CU/AL'. Many modern 30A breakers are rated for copper only; forcing aluminum into a copper-only lug will cause high resistance, arcing, and eventual terminal failure.
When to Call an Engineer or the AHJ
While 10 AWG copper covers 90% of residential 30A circuits (like standard dryers, water heaters, and RV plugs), specific scenarios require professional sign-off or an upgrade to 8 AWG and a 40A breaker.
- Continuous Loads (NEC 210.20(A)): If the 30A load will run for 3 hours or more continuously (e.g., commercial EV chargers, heavy-duty shop heaters), you must size the breaker at 125% of the load. A 30A continuous load requires a 40A breaker, which mandates 8 AWG copper wire.
- Local AHJ Amendments: Some municipalities have blanket rules requiring 8 AWG minimum for all 30A circuits to provide a buffer for future upgrades or to account for poor local grid voltage. Always check with your local Authority Having Jurisdiction (AHJ) before rough-in.
- Voltage Sags: If your utility supply routinely sags below 114V (for 120V nominal) or 228V (for 240V nominal), an electrical engineer should calculate the exact voltage drop profile. Undersized wire on a sagging grid will cause motors to draw locked-rotor amperage and burn out.
Frequently Asked Questions: Sizing Wire for 30 Amp Breakers
Can I use 8 AWG wire on a 30 amp breaker?
Yes. The NEC allows you to use a larger wire than the minimum required. 8 AWG copper on a 30A breaker is perfectly safe and actually reduces voltage drop. The only physical challenge is that 8 AWG wire is thicker and stiffer; ensure your breaker's terminal lugs are rated to accept up to 8 AWG, and use a torque screwdriver to tighten the set screw to the manufacturer's spec (usually around 35-45 in-lbs) to prevent cold flow loosening.
What size wire for a 30 amp breaker at 100 feet?
For a 240V circuit at 100 feet, 10 AWG copper is sufficient (yielding a ~2.5% voltage drop at 24A). However, if you are wiring a 120V 30A circuit at 100 feet, 10 AWG will result in a nearly 5% voltage drop. For 120V applications at this distance, you must upsize to 8 AWG copper to keep the drop under the recommended 3% threshold.
Is it safe to use 12 AWG wire with a 30 amp breaker?
Absolutely not. This is a direct violation of NEC 240.4(D) and a severe fire hazard. 12 AWG wire is legally capped at 20 amps of overcurrent protection. If a fault occurs drawing 28 amps, the 30A breaker will not trip, but the 12 AWG wire will overheat, melt its insulation, and potentially ignite surrounding framing. Always match or exceed the wire ampacity to the breaker size.
What size wire for a 30 amp 240V dryer circuit?
A standard residential electric dryer requires a 30-amp, 240V circuit. The correct wire size is 10 AWG copper. Most electricians use 10/3 NM-B cable (which includes two 10 AWG hot conductors, one 10 AWG neutral, and a ground) for runs inside walls, or three strands of 10 AWG THHN pulled through 1/2-inch EMT conduit if the run is exposed or goes through masonry.
