For a standard 30 amp circuit breaker, you need 10 AWG copper wire or 8 AWG aluminum wire. This assumes a 75°C temperature rating, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway. Always verify voltage drop if the run exceeds 50 feet.
- Material: Copper (unless aluminum is explicitly specified)
- Insulation: THHN/THWN-2 in conduit, or NM-B (Romex) in walls
- Temperature Column: 75°C for terminations (standard for modern 30A breakers), 60°C for NM-B cable
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors in a single raceway
Note: Sizing guidance aligns with NEC-style practice; your local AHJ (Authority Having Jurisdiction) has final authority on all installations.
The Baseline Sizing: Why 10 AWG Copper Wins
When determining what size wire for 30 amp circuit breaker applications, 10 AWG copper is the undisputed baseline. To understand why, we have to look at the ampacity tables and the physics of overcurrent protection. You cannot use 12 AWG wire because its maximum allowable ampacity is 20A (at 60°C) or 25A (at 75°C). If you pull 30A through 12 AWG wire, the insulation will degrade, and the wire will overheat before the 30A breaker ever trips, creating a severe fire hazard.
Conversely, 10 AWG copper wire is rated for 30A in the 60°C column (which applies to NM-B cable per NEC 334.80) and 35A in the 75°C column (which applies to THHN in conduit with 75°C rated terminations). A 30A breaker perfectly protects both configurations.
| Wire Size (AWG) | Material | 60°C Column (NM-B / TW) | 75°C Column (THWN / THHN Terminals) | 90°C Column (THHN Derating Base) |
|---|---|---|---|---|
| 14 | Copper | 15A | 20A | 25A |
| 12 | Copper | 20A | 25A | 30A |
| 10 | Copper | 30A | 35A | 40A |
| 8 | Copper | 40A | 50A | 55A |
| 8 | Aluminum | 30A | 40A | 45A |
| 6 | Aluminum | 40A | 50A | 55A |
As shown in the table above, if you are pulling individual THHN/THWN-2 conductors in EMT conduit, you use the 75°C column for your final ampacity check because most modern 30A breakers and receptacles (like the NEMA 14-30R or L5-30R) have 75°C rated lugs. 10 AWG yields 35A, which safely exceeds the 30A breaker rating. If you are running NM-B (Romex) through wall cavities, NEC 334.80 forces you to use the 60°C column regardless of the wire's actual insulation rating. In the 60°C column, 10 AWG is rated for exactly 30A, making it the absolute minimum size permitted.
When 10 AWG Fails: Voltage Drop and Derating Factors
The baseline answer assumes a short, straightforward run. In the real world, physics and the National Electrical Code introduce variables that can force you to upsize to 8 AWG copper. The two primary culprits are voltage drop over distance and ampacity derating due to conductor bundling.
Voltage Drop: The 50-Foot Threshold
While the NEC recommends keeping voltage drop under 3% for branch circuits, it is a design recommendation rather than a strict enforcement rule in most jurisdictions. However, exceeding it causes motors to overheat, heaters to underperform, and electronics to brown out.
Let's run a voltage drop check for a 30A, 240V circuit (like a dryer or RV pedestal) using 10 AWG copper over a 100-foot one-way distance. Using the standard formula VD = (2 x K x I x D) / CM (where K=12.9 for copper, I=30A, D=100ft, and CM=10,380 for 10 AWG):
- VD = (2 x 12.9 x 30 x 100) / 10,380
- VD = 77,400 / 10,380 = 7.45 Volts
- Percentage = 7.45V / 240V = 3.1%
At 240V, a 3.1% drop is marginally acceptable. But if this is a 120V, 30A circuit (like a TT-30R RV receptacle), that same 7.45V drop represents a 6.2% loss, which is entirely unacceptable. For any 30A run exceeding 50 feet at 120V, or 100 feet at 240V, you must upsize to 8 AWG copper to maintain power quality. You can verify these calculations dynamically using the Cerro Wire Voltage Drop Calculator.
Bundling and Derating: The 90°C Column Saves the Day
If you are pulling multiple circuits through a single conduit, the heat generated by adjacent wires reduces the ampacity of each wire. NEC Table 310.15(C)(1) mandates derating when you have more than three current-carrying conductors.
Here is where the 90°C column becomes useful. Even though your terminations are limited to 75°C, you are allowed to use the 90°C column (40A for 10 AWG THHN) as your starting point for derating calculations.
| Scenario | Current-Carrying Conductors | Derating Factor | 10 AWG THHN Adjusted Ampacity | Action Required |
|---|---|---|---|---|
| Standard single circuit | 1 to 3 | 100% | 35A (75°C term limit) | Use 10 AWG |
| Two circuits in one conduit | 4 to 6 | 80% | 32A (40A x 0.80) | Use 10 AWG (32A > 30A) |
| Three circuits in one conduit | 7 to 9 | 70% | 28A (40A x 0.70) | Upsize to 8 AWG |
| Ambient temp 41-45°C (Attic) | 1 to 3 | 82% (90°C col) | 32.8A (40A x 0.82) | Use 10 AWG |
If you bundle 7 to 9 current-carrying conductors, 10 AWG derates down to 28A. Because 28A is less than your 30A breaker, the breaker will no longer adequately protect the wire under full load. You must step up to 8 AWG copper.
Aluminum, Continuous Loads, and AHJ Inspections
Copper is the standard for residential branch circuits, but aluminum is frequently used for feeder lines, subpanels, and heavy appliance circuits due to its lower cost. If you are using aluminum wire for a 30A breaker, 8 AWG is the absolute minimum. Aluminum has higher resistance and lower thermal conductivity than copper, meaning it generates more heat at the same current level. Never interchange copper and aluminum sizing tables; doing so is a direct path to a melted terminal lug or a structural fire.
The Continuous Load Trap (NEC 210.20)
One of the most common mistakes DIYers make is ignoring the definition of a continuous load. The NEC defines a continuous load as any load where the maximum current is expected to continue for 3 hours or more. Examples include EV chargers, baseboard heaters, and commercial lighting.
If your 30A load is continuous, you must size the overcurrent protection and the conductors at 125% of the load.
- 30A x 1.25 = 37.5A.
- You must use a 40A breaker (the next standard size up).
- You must use wire rated for at least 37.5A, which requires 8 AWG copper (rated 40A at 60°C / 50A at 75°C).
If you wire a continuous 30A EV charger with 10 AWG wire and a 30A breaker, the breaker will eventually nuisance-trip as the bimetallic strip heats up from hours of sustained load, and the wire will operate at its absolute thermal limit. For a deep dive on continuous load calculations, refer to the NFPA National Electrical Code (NEC) Article 210.
When to Call an Engineer or the AHJ
While the rules above cover 95% of residential and light-commercial 30A circuits, there are scenarios where you must defer to a licensed professional or your local inspector:
- Equipment Nameplate Overrides: NEC 110.3(B) requires you to follow the manufacturer's installation instructions. If a 30A HVAC condenser nameplate explicitly states 'Minimum Circuit Ampacity 35A' or 'Maximum Fuse Size 40A', you must size the wire and breaker to the nameplate, not general rules.
- Extreme Ambient Temperatures: If your conduit runs across a commercial roof or through an unventilated attic where ambient temperatures routinely exceed 45°C (113°F), standard derating tables may not suffice. An engineer must calculate the exact thermal dissipation profile.
- Mixed Material Terminations: If you are transitioning from aluminum feeder wire to copper pigtails at a 30A disconnect, you must use AL/CU rated connectors and specific anti-oxidant compound. Improper mixed-metal terminations cause galvanic corrosion, leading to high-resistance faults and arcing.
Always verify your specific wire gauge, insulation type, and local amendments with your local AHJ before pulling wire. For further reading on conductor sizing and overcurrent protection coordination, EC&M's National Electrical Code resources provide excellent field-tested case studies on complex derating scenarios.
