In residential and commercial wiring, 30 amp wire refers to a conductor—typically 10 AWG copper—sized to safely carry a continuous or non-continuous electrical load of up to 30 amperes without exceeding the thermal limits of its insulation. This sizing dictates the physical thickness of the copper, the minimum insulation rating required, and the maximum overcurrent protective device (breaker) you can install at the panel. The most common confusion I see on the bench and in the field is DIYers assuming any 10-gauge wire can handle 30A regardless of insulation type, or mixing up the breaker’s trip rating with the wire’s baseline ampacity under the National Electrical Code (NEC).
Where You Meet 30-Amp Circuits in Practice
You will not find 30-amp circuits powering standard lighting or general-purpose receptacles. This wire size is reserved for dedicated, medium-to-high draw appliances and specialized receptacles. When you pull 10 AWG wire, you are almost always wiring a specific, high-wattage load.
| Application | Nominal Voltage | Standard Wire Type | Breaker Configuration |
|---|---|---|---|
| RV Receptacle (TT-30R) | 120V AC | 10/2 UF-B or THHN in PVC | 1-Pole 30A |
| Electric Water Heater (Small/Medium) | 240V AC | 10/2 NM-B or THHN | 2-Pole 30A |
| Electric Dryer (Older/Specific Models) | 240V AC | 10/3 NM-B (with ground) | 2-Pole 30A |
| Shop Air Compressor (3-5 HP) | 240V AC | 10/2 THHN in EMT conduit | 2-Pole 30A |
The Physics of the 60°C vs 75°C Ampacity Columns
To understand what 30 amp wire actually is, you have to look at NEC Table 310.16. The code does not just care about the copper diameter; it cares about how hot the insulation gets before it melts or degrades. This is where installations fail inspection.
If you look at a spool of 10 AWG NM-B (Romex), the jacket often prints "90°C". However, per NEC Article 334.80, the ampacity of NM-B cable must be determined using the 60°C column of Table 310.16, regardless of the 90°C rating on the jacket. In the 60°C column, 10 AWG copper is rated for exactly 30 amps. You cannot use the 90°C column (which allows 40A) to upsize your breaker.
Conversely, if you pull individual 10 AWG THHN/THWN-2 conductors through a conduit to a subpanel or a hardwired appliance, you are allowed to use the 75°C column for termination limits (per NEC 110.14(C)), provided your breaker and device lugs are rated for 75°C. In the 75°C column, 10 AWG copper is rated for 35 amps. However, because standard breakers jump from 30A to 35A, and standard appliance cords are often limited to 30A, we still universally protect 10 AWG circuits with a 30A breaker.
Worked Numeric Example: Calculating Voltage Drop
Ampacity tells you if the wire will catch fire. Voltage drop tells you if the appliance will actually work. Let us run the math on a standard 240V electric water heater located 100 feet from the main panel, using 10 AWG THHN copper.
The resistance of 10 AWG solid copper at 75°C is approximately 1.24 ohms per 1,000 feet. The formula for single-phase voltage drop is:
Voltage Drop (Vd) = (2 × Length × Resistance per ft × Current) / 1000
- Length (one-way): 100 feet
- Resistance: 1.24 ohms/kft
- Current: 30 Amps (maximum continuous draw for this example)
- Calculation: (2 × 100 × 1.24 × 30) / 1000 = 7.44 Volts
To find the percentage drop: (7.44V / 240V) × 100 = 3.1%. The NEC recommends a maximum of 3% voltage drop on a branch circuit for reasonable efficiency. At 3.1%, this is borderline but generally acceptable for a resistive load like a water heater, which will simply take a few seconds longer to heat the tank. For precise calculations on your specific wire brand, always reference a manufacturer tool like the Southwire Voltage Drop Calculator.
Scenario Walkthrough: The Stalled RV Air Conditioner
Theory is clean; the jobsite is not. Here is a real-world scenario that demonstrates why treating 30 amp wire as a "one-size-fits-all" solution leads to catastrophic equipment behavior.
The Numbers: The RV plugs in. The microwave works. The lights turn on. Then, the 15,000 BTU roof air conditioner kicks on. The AC compressor requires a massive Locked Rotor Amperage (LRA) spike to start, and draws a steady 18 amps while running. Let us look at the voltage drop at a full 30A continuous load (or a heavy 20A motor starting surge) over 150 feet of 10 AWG UF-B:
Vd = (2 × 150 × 1.24 × 30) / 1000 = 11.16 Volts
The Outcome: The voltage at the RV pedestal drops from 120V down to 108.8V (a 9.3% drop). When the AC compressor tries to start at this depressed voltage, it cannot generate enough magnetic torque to spin the rotor. The motor hums violently, stalls, and draws maximum locked-rotor current. Because the voltage is low, the motor draws even more current to try and meet its power requirement (Power = Voltage × Current). The internal thermal overload in the compressor trips, shutting the AC down. Ten minutes later, it tries to restart, fails again, and eventually, the 30A breaker at the main panel trips from the sustained thermal buildup.
What Went Wrong: The wire was sized correctly for fire safety (ampacity), but incorrectly for performance (voltage drop). Because 120V circuits have half the voltage headroom of 240V circuits, a 9.3% drop is devastating to inductive motor loads. The Fix: For a 150-foot 120V run at 30A, you must upsize the conductors to 8 AWG copper or even 6 AWG to keep the voltage drop under 3% and ensure the motor starts cleanly.
FAQ: Sizing and Terminating 30A Conductors
Can I use aluminum wire for a 30-amp circuit?
Yes, but you must upsize. Aluminum has higher resistance than copper. According to the 75°C column of NEC Table 310.16, you need a minimum of 8 AWG aluminum to safely carry 30 amps (it is rated for 40A at 75°C, but 8 AWG is the smallest standard aluminum building wire size available). Never use aluminum for the short pigtails inside a standard residential breaker panel unless the lugs are explicitly rated for it (most modern ones are marked AL/CU), and always apply anti-oxidant paste to the terminations.
Why does my 10 AWG wire have a 90°C rating if I can only use it for 30 amps?
The 90°C rating on THHN or NM-B jackets is used for derating calculations, not for final ampacity. If you have more than three current-carrying conductors in a single conduit, or if the conduit runs across a hot attic space, the NEC requires you to multiply the wire's capacity by a derating factor. You start your derating math using the 90°C column (40A for 10 AWG), apply the penalty, and if the final number drops below 30A, you must upsize to 8 AWG. The final termination, however, is still capped at the 60°C or 75°C column limits.
What torque should I use when terminating 10 AWG copper on a 30A breaker?
Always check the manufacturer's datasheet printed on the breaker label. For standard Square D Homeline or QO 30A breakers, the typical torque specification for 10 AWG copper is 35 in-lbs. For Eaton BR/Cutler-Hammer, it is often 40 in-lbs. Using a calibrated inch-pound torque screwdriver is no longer just a best practice; NEC 110.14(D) mandates that connections must be torqued to the manufacturer's specified values to prevent loose lugs, which cause arcing and fires.
