To size wire for 30 amp circuits, use 10 AWG copper wire paired with a 30-amp breaker. This baseline applies to standard residential and commercial branch circuits like dryers, RV receptacles, and heavy-duty power tools.
The Baseline Sizing Rule (and Why 10 AWG Wins)
Before pulling any wire, you must lock in your baseline assumptions. Wire ampacity is not a single fixed number; it changes based on insulation type, ambient temperature, and termination ratings. The 10 AWG recommendation relies on the following standard jobsite conditions:
- Material: Copper (never assume aluminum without explicit derating)
- Insulation: THHN/THWN-2 (standard modern building wire)
- Termination Temperature: 75°C (standard for modern breakers and receptacles per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: No more than 3 current-carrying conductors in a single raceway
Under these conditions, we look at the standard ampacity charts derived from NEC Table 310.16. While 10 AWG THHN copper has an absolute thermal limit of 40A in the 90°C column, we are legally bound to use the 75°C column (35A) for final sizing because standard breaker lugs are rated for 75°C. Furthermore, NEC 240.4(D) explicitly caps the overcurrent protection for 10 AWG copper at 30 amps.
| Wire Size (AWG) | 75°C Ampacity | Max Standard Breaker | Common Use Case |
|---|---|---|---|
| 14 AWG | 20A | 15A | Lighting, basic receptacles |
| 12 AWG | 25A | 20A | Kitchen/bathroom small appliance |
| 10 AWG | 35A | 30A | Dryers, RV plugs, 30A tools |
| 8 AWG | 50A | 40A | EV chargers, large HVAC, subpanels |
Why Not One Size Smaller (12 AWG)?
A common beginner mistake is assuming that because 12 AWG wire can physically fit into a 30-amp breaker lug, it is safe to use. It is not. If you push 30 amps through 12 AWG wire, the conductor will overheat and melt its insulation long before the 30-amp breaker's thermal trip mechanism engages. The breaker is there to protect the wire, not the appliance. NEC 240.4(D) strictly forbids placing 12 AWG copper on anything larger than a 20-amp breaker.
When You Must Upsize to 8 AWG (Distance, Heat, and Bundling)
The 10 AWG rule holds true for short, standard runs. However, real-world jobsite conditions frequently force you to upsize to 8 AWG copper. Use this decision tree to determine if your specific installation requires a larger conductor.
| Condition | Threshold | Action Required |
|---|---|---|
| Voltage Drop (120V Circuit) | Run exceeds 50 feet | Upsize to 8 AWG copper |
| Voltage Drop (240V Circuit) | Run exceeds 100 feet | Upsize to 8 AWG copper |
| Conduit Bundling | 4 to 6 current-carrying conductors | Upsize to 8 AWG copper (80% derating) |
| High Ambient Heat | Attic/conduit temp exceeds 104°F (40°C) | Upsize to 8 AWG copper (82% derating) |
| Continuous Load | Appliance runs for 3 hours or more | Upsize to 8 AWG copper (125% rule) |
The Voltage Drop Calculation
The NEC recommends a maximum 3% voltage drop for branch circuits to ensure motors and compressors don't overheat from undervoltage. For a 30-amp load on 10 AWG copper, the voltage drop is roughly 0.075 volts per foot.
- On a 120V circuit: 3% drop is 3.6V. (3.6V / 0.075V = 48 feet). If your RV outlet is 60 feet from the panel, 10 AWG will result in a 4.5V drop (3.75%), which can damage sensitive RV electronics. Upsize to 8 AWG.
- On a 240V circuit: 3% drop is 7.2V. (7.2V / 0.075V = 96 feet). A 240V dryer receptacle can safely use 10 AWG up to about 95 feet before requiring 8 AWG.
Derating for Bundling and Heat
When you pull multiple circuits through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to apply an adjustment factor. If you have 4 to 6 current-carrying conductors in a pipe, you must multiply the wire's 90°C ampacity (40A for 10 AWG) by 80%. That yields 32A. While 32A is technically above your 30A load, you are still capped by the 75°C termination limit (35A) and the 240.4(D) breaker cap (30A). In high-heat environments like an uninsulated attic in summer, the ambient temperature correction factor drops your capacity even further, making 8 AWG the only safe choice.
Aluminum vs. Copper and Continuous Load Exceptions
Material choice and load duration are two massive variables that alter your wire size. Never treat aluminum and copper interchangeably; aluminum has higher resistance and expands/contracts more under heat, requiring larger gauges and specific termination compounds.
Sizing for Aluminum Conductors
If you are using aluminum wire (such as SER cable for a subpanel feed or an RV pedestal), 10 AWG aluminum is not sufficient. According to the 75°C column for aluminum, 10 AWG is only rated for 30A thermally, but standard practice and safety margins dictate upsizing. For a 30-amp aluminum circuit, you must use 8 AWG aluminum (rated 40A at 75°C). Furthermore, you must apply an antioxidant compound (like Noalox) to aluminum terminations to prevent galvanic corrosion and high-resistance heating over time.
The Continuous Load 125% Rule
NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. Examples include commercial lighting, EV chargers, and certain workshop dust collection systems.
If your 30-amp load is continuous, you must size the wire and breaker for 125% of the load:
- 30A × 1.25 = 37.5 amps.
- A 30-amp breaker is no longer legal; you must use a 40-amp breaker.
- 10 AWG copper (35A at 75°C) is now undersized. You must pull 8 AWG copper (50A at 75°C) to safely feed the 40-amp breaker.
Note: Standard residential dryers and RV plugs are generally not considered continuous loads, so the 10 AWG / 30A breaker rule holds.
When to Call the AHJ or an Engineer
While the National Electrical Code (NFPA 70) provides the framework, local inspectors (the AHJ) have the final say on compliance. You must pull a permit and consult a licensed electrical engineer or master electrician in the following scenarios:
- Mixed Termination Ratings: If you are connecting a new 30-amp circuit to an older piece of equipment or a legacy panel where the lug temperature rating is unknown or marked 60°C, the ampacity of 10 AWG drops to 30A. While this technically matches the breaker, an inspector may require 8 AWG for thermal headroom.
- Complex Derating Stacking: If your conduit run passes through a high-temperature area and contains more than 3 current-carrying conductors, the derating factors multiply. Calculating the exact required ampacity under stacked NEC 310.15 conditions requires professional load calculation software.
- Feeder vs. Branch Circuit: If this 30-amp wire is acting as a feeder to a subpanel rather than a direct branch circuit to a single receptacle, the load diversity and future expansion calculations change the sizing requirements entirely.
By starting with 10 AWG copper for standard runs and systematically checking for voltage drop, bundling, and continuous load exceptions, you ensure your 30-amp circuits remain safe, code-compliant, and capable of handling heavy inductive startup currents without nuisance tripping.
