The correct wire size for 30 amps is 10 AWG copper or 8 AWG aluminum, which provides sufficient cross-sectional area to carry the current safely without exceeding the insulation's temperature rating under standard National Electrical Code (NEC) ampacity tables. When sizing conductors, you are fundamentally balancing the electrical resistance of the metal against the heat generated by the current flow to prevent insulation meltdown and fire hazards.
The Core Data: Wire Sizes and Ampacity for 30A Circuits
Before pulling any wire through a stud bay, you need to know exactly what the code allows. The NEC does not just look at the wire gauge; it looks at the insulation type, the metal (copper vs. aluminum), and the temperature rating of the terminals you are connecting to. Below is the definitive spec sheet for 30-amp circuits based on manufacturer ampacity charts and NEC Table 310.16.
| Wire Material & Gauge | Insulation / Cable Type | Temp Column Used | Wire Ampacity | Max Breaker Size |
|---|---|---|---|---|
| 10 AWG Copper | NM-B (Romex) / UF-B | 60°C | 30A | 30A |
| 10 AWG Copper | THHN / THWN-2 (in conduit) | 60°C (termination limit) | 40A (at 90°C wire rating) | 30A (per NEC 240.4(D)) |
| 8 AWG Aluminum | NM-B / USE | 60°C | 30A | 30A |
| 8 AWG Aluminum | THHN / THWN-2 (in conduit) | 75°C | 40A | 30A or 40A |
What Changes in a Real Installation (and Common Confusions)
Selecting the wire size for 30 amps changes three physical realities in your installation: the heat dissipation inside the wall cavity, the voltage delivered to the load at the end of the run, and the physical size of the conduit or cable clamps required. A thicker wire runs cooler and delivers voltage more efficiently, but it is stiffer to bend and harder to terminate in tight junction boxes.
When talking to DIYers and junior apprentices, I see two major confusions regarding 30A circuits:
Confusion 1: The "90°C Trap"
People look at a spool of 10 AWG THHN, see it is rated for 40 amps at 90°C, and assume they can protect it with a 40A breaker. This is a code violation. NEC 110.14(C) dictates that for equipment rated 100A or less, you must use the 60°C column for termination limits unless the equipment is specifically listed for 75°C. Furthermore, NEC 240.4(D) places a hard cap on small conductors: 10 AWG copper cannot be protected by a breaker larger than 30A, regardless of the insulation's 90°C rating. The 90°C rating is only useful for applying derating factors (like bundling multiple wires in a conduit), not for sizing the breaker.
Confusion 2: Breaker Rating vs. Continuous Load
A 30-amp breaker does not mean you can pull 30 amps continuously. NEC 210.20(A) requires that for continuous loads (anything expected to run for 3 hours or more, like an EV charger, a space heater, or a shop dust collector), the circuit must be derated to 80%. Therefore, a 30A breaker can only safely handle 24 amps of continuous load. If your load draws 28A continuously, you must step up to a 40A breaker and 8 AWG copper wire.
Worked Numeric Example: Sizing a 30A RV Receptacle Feeder
Ampacity tables assume standard conditions, but distance introduces voltage drop. Let's look at a real-world scenario: running a 120V, 30A RV receptacle (NEMA TT-30R) to a driveway pedestal 80 feet away from the main panel.
Voltage: 120V (1-phase)
Distance (D): 80 feet (one-way)
Continuous Load (I): 24A (RV air conditioner + battery charger)
Proposed Wire: 10 AWG Copper (Circular Mils [CM] = 10,380)
Conductor Material Constant (K): 12.9 for copper
We use the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- Calculate the numerator: 2 × 12.9 × 24 × 80 = 49,536
- Divide by Circular Mils: 49,536 / 10,380 = 4.77 volts dropped
- Calculate percentage: (4.77V / 120V) × 100 = 3.97% voltage drop
The Verdict: The NEC recommends (via Informational Note in 210.19) keeping branch circuit voltage drop under 3%, and the combined feeder/branch drop under 5%. At 3.97%, 10 AWG copper is legally compliant and safe from a fire perspective, but it is slightly outside the optimal 3% efficiency target. If this RV will be running sensitive inverter-based electronics, I would step up to 8 AWG copper to drop the loss to roughly 2.5%, ensuring the RV's internal power supply doesn't brown out on hot days when the grid voltage sags.
Where You Meet This in Practice
You will encounter the 30-amp / 10 AWG requirement frequently in residential and light commercial wiring. Here is where this specific sizing applies on the jobsite:
- Electric Clothes Dryers: Standard NEMA 14-30R receptacles require a 30A double-pole breaker. Modern code requires 10/3 NM-B (Black, Red, White, Bare) to provide an isolated equipment grounding conductor.
- Electric Water Heaters: Most standard 4500W, 240V tank water heaters draw 18.75A. Because this is a continuous load (18.75 × 1.25 = 23.4A), a 30A breaker and 10/2 NM-B (Black, White re-taped Red, Bare) is the standard, code-compliant configuration.
- RV Pedestals (TT-30R): As calculated above, these are 120V single-pole 30A circuits. Do not confuse a TT-30R (120V) with a NEMA 14-30R (240V); wiring a TT-30 with 240V will instantly destroy the RV's electronics.
- Heavy Shop Tools: A 5HP, 240V air compressor or a large MIG welder often requires a 30A double-pole breaker. Always check the manufacturer's nameplate for the Minimum Circuit Ampacity (MCA) rather than guessing based on horsepower.
FAQ: Edge Cases and Code Caveats
Can I use 8 AWG wire on a 30-amp breaker?
Yes. The NEC allows you to upsize your conductors to mitigate voltage drop or for future-proofing. The only physical constraint is whether the breaker's terminal lug is rated to accept 8 AWG wire. Most modern 30A breakers from Square D, Eaton, and Siemens accept a range of 14 to 8 AWG, but always verify the label on the breaker itself.
What color wires do I use for a 30A 240V circuit?
For a standard 240V circuit (like a water heater) using NM-B cable, you use the Black and White wires as your two hot legs. NEC 200.7(C) requires you to permanently re-identify the White wire with black or red electrical tape (or paint) at both ends to indicate it is a hot conductor, not a neutral. If you are pulling individual THHN wires in conduit, use Black and Red for the hots, and Green or Bare for the ground.
Does ambient temperature change my wire size for 30 amps?
Yes. If you are routing 10 AWG THHN through an attic in a climate where ambient temperatures exceed 86°F (30°C), you must apply the temperature correction factors from NEC Table 310.15(B)(1). For example, in a 110°F attic, you multiply the 90°C ampacity (40A) by 0.87, yielding 34.8A. Because 34.8A is still above your 30A breaker, 10 AWG remains safe, but if you were bundling multiple cables together, the combined derating could force you to upsize to 8 AWG.
Is aluminum wire safe for a 30-amp circuit?
Yes, provided you use the correct gauge and terminations. For 30A, you must use 8 AWG aluminum. You must also ensure the breaker and receptacle lugs are rated for aluminum (marked AL or CU/AL) and apply an antioxidant compound like Noalox to the stripped wire ends to prevent galvanic corrosion and high-resistance heating over time. For more on termination standards, refer to the NFPA 70 National Electrical Code guidelines.
