You need 10 AWG copper wire and a 30-amp breaker for a standard 30 amp circuit. This assumes THHN/THWN-2 insulation in a conduit, a 75°C temperature rating, and an ambient temperature of 30°C (86°F). If using aluminum, you must step up to 8 AWG.
The Baseline Sizing Rules & Assumptions
Wire sizing is never a one-size-fits-all answer; it depends entirely on the installation environment. Before pulling wire or terminating a breaker, you must confirm your installation matches the baseline assumptions below. If your jobsite conditions differ, the required wire size will change.
Baseline Assumptions for 10 AWG Copper
- Conductor Material: Solid or stranded Copper
- Insulation Type: THHN / THWN-2 (rated for 90°C in dry/damp locations)
- Termination Temperature Rating: 75°C (Standard for most modern breakers and receptacles per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Installation Method: Raceway (conduit) or cable assembly, with no more than 3 current-carrying conductors bundled together
- Load Type: Non-continuous (operates for less than 3 hours at a time)
To understand why 10 AWG is the correct choice, we look at the Cerrowire Ampacity Charts, which reflect the National Electrical Code (NEC) Table 310.16. Notice how the allowable ampacity shifts depending on the temperature column you are legally permitted to use.
| Wire Size (AWG) | Material | 60°C Column (NM-B / Older Terminals) | 75°C Column (THHN in Conduit / Modern Terminals) | 90°C Column (Derating Calculations Only) |
|---|---|---|---|---|
| 12 AWG | Copper | 20A | 25A | 30A |
| 10 AWG | Copper | 30A | 35A | 40A |
| 8 AWG | Copper | 40A | 50A | 55A |
| 10 AWG | Aluminum | 25A | 30A | 35A |
| 8 AWG | Aluminum | 30A | 40A | 45A |
Why 10 AWG? Understanding Termination Limits & Derating
A common mistake on the bench or jobsite is looking at the 90°C column (where 10 AWG copper is rated for 40A) and assuming you can put it on a 40-amp breaker. You cannot. The National Fire Protection Association (NFPA) enforces NEC 110.14(C), which states that your circuit ampacity is limited by the lowest temperature rating of any connected component. Most 30-amp receptacles (like a NEMA L14-30R or 10-30R) and standard breakers have 75°C terminations. Therefore, you must use the 75°C column, limiting 10 AWG copper to 35A. A 30-amp breaker perfectly protects this wire.
Why not one size smaller? 12 AWG copper is rated for 20A at 60°C and 25A at 75°C. If you pull 30 amps through 12 AWG wire, the insulation will overheat, degrade, and potentially cause a fire before the 30-amp breaker ever trips. The breaker is there to protect the wire, not the appliance.
Why not aluminum? While 10 AWG aluminum is technically rated for 30A at 75°C, running a conductor at exactly 100% of its ampacity limit leaves zero margin for termination heating. Furthermore, many modern breakers are only rated for copper, and using aluminum requires specific CO/ALR lugs and anti-oxidant paste. Stepping up to 8 AWG aluminum (rated 40A at 75°C) is the standard, safe practice for aluminum branch circuits.
What Changes the Answer: Bundling and Derating
If you are pulling multiple circuits through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to derate the ampacity based on the 90°C column when you have 4 or more current-carrying conductors.
| Current-Carrying Conductors in Raceway | Derating Factor | Adjusted Ampacity (40A x Factor) | Can it carry 30A? | Action Required |
|---|---|---|---|---|
| 1 to 3 | 100% | 40A | Yes | Use 10 AWG |
| 4 to 6 | 80% | 32A | Yes | Use 10 AWG |
| 7 to 9 | 70% | 28A | No | Upsize to 8 AWG |
Voltage Drop: When Distance Forces a Larger Wire
Ampacity tables only tell you what the wire can handle thermally. They do not account for voltage drop over distance. The NEC recommends a maximum 3% voltage drop for branch circuits to ensure equipment operates efficiently. For a 240V circuit (like a dryer, welder, or RV outlet), 3% equals 7.2 volts.
Let us run the math for a 30-amp load on 10 AWG copper at 240V using the standard voltage drop formula: VD = (2 × K × I × L) / CM.
- K (Copper resistivity) = 12.9
- I (Current) = 30A
- CM (Circular mils for 10 AWG) = 10,380
Scenario A: 50-foot run
VD = (2 × 12.9 × 30 × 50) / 10,380 = 3.72V.
3.72V / 240V = 1.55% drop. 10 AWG is perfectly fine.
Scenario B: 150-foot run
VD = (2 × 12.9 × 30 × 150) / 10,380 = 11.18V.
11.18V / 240V = 4.65% drop. This exceeds the 3% recommendation. You must upsize to 8 AWG copper to maintain proper voltage at the receptacle. Always use a voltage drop calculator for runs exceeding 75 feet.
When an Engineer or AHJ Must Confirm
While the rules above cover 95% of residential and light commercial 30-amp circuits, you must consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer under the following conditions:
- Continuous Loads: If the 30-amp load will run for 3 hours or more continuously (like a commercial kiln or heavy duty compressor), NEC 210.20 requires the breaker to be sized at 125% of the load (30A × 1.25 = 37.5A). You would need a 40-amp breaker and 8 AWG copper wire.
- High Ambient Temperatures: If the conduit runs through an attic or boiler room where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors from NEC Table 310.15(B)(1). At 41-45°C, the 90°C column must be derated to 87%, which may force an upsizing to 8 AWG.
- Service Entrance or Feeder Upgrades: If this 30-amp circuit is actually a feeder to a subpanel rather than a branch circuit to a single receptacle, different sizing rules and grounding conductor requirements apply.
Frequently Asked Questions
Can I use 12 AWG wire on a 30 amp breaker?
No. 12 AWG copper wire has a maximum allowable ampacity of 20A (for standard NM-B cable) or 25A (for THHN in conduit at 75°C). A 30-amp breaker will not trip until the current exceeds 30 amps, meaning the 12 AWG wire will overheat, melt its insulation, and create a severe fire hazard long before the breaker protects it. NEC 240.4(D) explicitly limits 12 AWG copper to a 20-amp overcurrent device.
What size wire for a 30 amp 240V circuit?
The voltage of the circuit (120V vs 240V) does not change the thermal ampacity requirements of the wire. You still need 10 AWG copper wire for a 30-amp 240V circuit (like a NEMA 10-30 or 14-30 dryer receptacle). However, because it is 240V, you will need two 10 AWG hot conductors (typically Black and Red), plus a neutral and a ground wire if required by the specific receptacle configuration (like a 14-30R).
How does voltage drop change the wire size for a 30 amp circuit at 100 feet?
At 100 feet on a 240V circuit, 10 AWG copper will experience roughly a 3.1% voltage drop (7.45V). Because this slightly exceeds the NEC's recommended 3% maximum for branch circuits, it is highly advisable to upsize to 8 AWG copper for a 100-foot run. If the circuit is 120V, the drop is doubled to 6.2%, making 8 AWG an absolute necessity to prevent equipment damage and motor burnout.
Can I use aluminum wire for a 30 amp circuit?
Yes, but you must use 8 AWG aluminum, not 10 AWG. While 10 AWG aluminum is technically rated for 30A at 75°C, best practice dictates stepping up one size to 8 AWG (rated 40A at 75°C) to compensate for aluminum's higher thermal expansion rate and oxidation risks at terminations. Ensure your breaker and receptacle lugs are explicitly rated for aluminum (marked AL or CU/AL) and apply Noalox anti-oxidant paste to the stripped wire ends before torquing.
