For a standard 30 amp circuit, use 10 AWG copper wire paired with a 30A breaker. If using aluminum, step up to 8 AWG. This ensures the overcurrent protective device trips before the conductor insulation fails under fault conditions.
- Material: Copper (unless aluminum is explicitly stated)
- Temperature Rating: 75°C termination limits (standard for modern breakers and receptacles)
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Standard raceway/conduit or NM-B cable, with no more than 3 current-carrying conductors bundled together.
The Core Sizing Rule: Why 10 AWG Copper and Not 12 AWG?
A common point of confusion for DIYers reading the National Electrical Code (NEC) is looking at NEC Table 310.16 and seeing that 10 AWG THHN copper is rated for 40A in the 90°C column. If the wire can handle 40A, why can't you put it on a 35A or 40A breaker? More importantly, why can't you use 12 AWG (rated 25A at 90°C) on a 30A breaker for a very short run?
The answer lies in NEC 240.4(D), which places strict overcurrent protection limits on small conductors to prevent fire hazards from high-resistance faults that might not trip a larger breaker instantly. Under this article, the maximum overcurrent protection for 12 AWG copper is hard-capped at 20A, and for 10 AWG copper, it is capped at 30A.
| Wire Size (AWG) | 75°C Column Ampacity | Max Breaker Size (NEC 240.4(D)) |
|---|---|---|
| 14 AWG | 20A | 15A |
| 12 AWG | 25A | 20A |
| 10 AWG | 35A | 30A |
| 8 AWG | 50A | 40A / 50A* |
*8 AWG is not restricted by 240.4(D), so it can be used on standard 40A or 50A breakers depending on the specific equipment rating.
Variables That Change Your Wire Size (Length, Bundling, Aluminum)
The 10 AWG baseline assumes ideal conditions. In the real world, jobsite variables force you to upsize your wire. Here is what changes the math:
1. Conductor Bundling (Derating)
If you are pulling wire through a conduit that contains more than three current-carrying conductors, the heat generated by the wires cannot dissipate properly. You must apply a derating factor from NEC Table 310.15(C)(1).
Let's say you are pulling two 240V circuits (4 current-carrying conductors) in a single EMT conduit. The derating factor for 4-6 conductors is 80%. You must use the 90°C column of Table 310.16 for derating math. 10 AWG THHN in the 90°C column is rated 40A.
- 40A × 0.80 = 32A. Since 32A is greater than your 30A load, 10 AWG is still legally acceptable.
- However, if you pull three circuits (6 conductors) and add a neutral, you might hit 7 current-carrying conductors. The derating factor drops to 70%. 40A × 0.70 = 28A. Your wire is now underrated for a 30A breaker. You must step up to 8 AWG copper.
2. Aluminum Conductors
Aluminum has higher resistance than copper and requires a larger cross-section to carry the same current safely. Furthermore, aluminum is more susceptible to thermal expansion and creep at termination points. For a 30A circuit using aluminum (like SER cable for a feeder), you must use 8 AWG aluminum, which is rated 40A in the 75°C column. Never use 10 AWG aluminum for a 30A circuit; it is only rated for 30A at 75°C, leaving zero margin for termination heating, and many local AHJs prohibit aluminum branch circuits under 8 AWG entirely.
Voltage Drop Check: The 100-Foot Threshold
Ampacity tells you if the wire will melt; voltage drop tells you if your equipment will actually run. The NEC recommends (via Informational Notes, though often enforced as local code) a maximum 3% voltage drop for branch circuits.
Let's run the math for a 30A load on a 240V circuit using 10 AWG copper at exactly 100 feet. Using the standard single-phase voltage drop formula ($VD = \frac{2 \times K \times I \times D}{CM}$):
- K (Copper resistance constant) = 12.9
- I (Current) = 30A
- D (Distance) = 100 ft
- CM (Circular mils for 10 AWG) = 10,380
$VD = \frac{2 \times 12.9 \times 30 \times 100}{10380} = 7.45V$
On a 240V circuit, 7.45V is a 3.1% drop. This is marginally over the recommended 3% limit. If this were a 120V circuit (like a TT-30 RV outlet), that same 7.45V drop represents a massive 6.2% drop, which will cause motors to overheat and electronics to brown out.
| One-Way Distance | Voltage Drop (10 AWG Cu) | Required Wire Size |
|---|---|---|
| Under 50 ft | 1.5% | 10 AWG Copper |
| 50 ft - 95 ft | 1.5% - 2.9% | 10 AWG Copper |
| 96 ft - 150 ft | 3.0% - 4.6% | 8 AWG Copper |
| 151 ft - 200 ft | 4.7% - 6.2% | 6 AWG Copper |
Pro Tip: You can verify these calculations on the fly using the Southwire Voltage Drop Calculator, which accounts for specific insulation types and AC impedance.
When to Call an Engineer or the AHJ
The 10 AWG / 30A breaker rule applies to standard, non-continuous, resistive or general-use loads. You must consult a licensed electrical engineer or your local Authority Having Jurisdiction (AHJ) when dealing with the following edge cases:
- Continuous Loads: The NEC defines a continuous load as one where the maximum current is expected to continue for 3 hours or more (e.g., baseboard heaters, EV chargers, server racks). NEC Article 210.20 requires the branch circuit rating to be 125% of the continuous load. A 30A continuous load requires a circuit rated for 37.5A. This means you must install a 40A breaker and 8 AWG copper wire.
- Motor Circuits: Motors have massive inrush currents (Locked Rotor Amps). NEC Article 430 allows you to size the breaker much higher than the wire ampacity (often up to 250% of the Full Load Amps) to prevent nuisance tripping during startup, while the wire is sized to 125% of the FLA. An HVAC disconnect or air compressor will not follow standard 240.4(D) rules.
- High Ambient Temperatures: If your conduit runs through an attic that reaches 120°F (49°C) in the summer, you must apply ambient temperature correction factors from Table 310.15(B)(1), which will severely reduce the ampacity of 10 AWG wire, forcing an upsize.
Frequently Asked Questions
Can I use 12 AWG wire for a very short 30 amp run?
No. The length of the wire does not change its thermal limits or the NEC 240.4(D) overcurrent protection cap. Even if the run is only 5 feet long, a 30A breaker will not trip in time to save 12 AWG wire from melting during a sustained 25A-29A overload. You must use 10 AWG copper minimum for any circuit protected by a 30A breaker.
What size wire and breaker for a 30 amp RV outlet?
For a standard 120V 30A RV receptacle (NEMA TT-30), you need 10 AWG copper wire, a single-pole 30A breaker, and a dedicated equipment ground. Because it is a 120V circuit, voltage drop is highly aggressive. If the outlet is more than 40 feet from the panel, you should upsize to 8 AWG copper to keep the voltage drop under 3% when the RV's air conditioner kicks on.
Does the equipment grounding conductor need to be 10 AWG too?
Yes. According to NEC Table 250.122, the minimum size for an equipment grounding conductor on a 30A circuit is 10 AWG copper. While some older DIY guides suggest you can downsize the ground wire, modern code requires the ground to be sized proportionally to the breaker to ensure it can safely carry fault current long enough to trip the breaker without vaporizing. If you upsize your current-carrying conductors to 8 AWG for voltage drop, you are not strictly required to upsize the ground, but it is considered best practice on the bench to keep all conductors in the cable assembly the same gauge.
