- Material: Copper (unless Aluminum is explicitly stated)
- Insulation: THHN/THWN-2
- Temperature Column: 75°C (per NEC 110.14(C) termination limits)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Raceway (conduit) or cable assembly, not more than 3 current-carrying conductors
NEC-style guidance; your local AHJ has final authority.
The Baseline: NEC Ampacity and the 10 AWG Rule
When sizing conductors, you must look at NEC Table 310.16 and NEC Article 240.4(D). While the 90°C column for 10 AWG THHN copper shows an ampacity of 40A, you are almost never allowed to use that number for final sizing. Breaker lugs and receptacle terminals are typically rated for 75°C. Per NEC 110.14(C), you must size the wire based on the 75°C column, which lists 10 AWG copper at 35A.
Furthermore, NEC 240.4(D) places a hard cap on small conductors to prevent overheating at termination points. It explicitly states that the overcurrent device for 10 AWG copper shall not exceed 30 amps. Therefore, a 30A breaker and 10 AWG copper wire are a perfectly matched, code-compliant pair for standard branch circuits like dryer outlets, RV receptacles, or heavy-duty workshop equipment.
| AWG Size | 60°C Column | 75°C Column (Termination Limit) | 90°C Column (Derating Start) |
|---|---|---|---|
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Variables That Force an Upsize: Length, Heat, and Bundling
The 10 AWG baseline assumes ideal conditions. On a real jobsite, three main variables will force you to pull 8 AWG wire instead.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for branch circuits. Let us run the math for a 240V, 30A load (like a window AC unit or welder) using 10 AWG copper at a distance of 100 feet.
- Formula: VD = (2 x K x I x D) / CM
- K (Copper): 12.9
- I (Current): 30A
- D (Distance): 100 ft
- CM (Circular Mils for 10 AWG): 10,380
Calculation: (2 x 12.9 x 30 x 100) / 10,380 = 7.45 Volts.
Percentage: 7.45V / 240V = 3.1%.
Because 3.1% exceeds the 3% recommendation, any 240V/30A run longer than 80 feet requires an upsize to 8 AWG copper to maintain equipment efficiency and prevent motor burnout.
2. Conduit Bundling (Derating)
If you are pulling multiple circuits through a single conduit, the wires heat each other up. Per NEC 310.15(C)(1), you must apply a derating factor to the 90°C column of your wire. If you have 7 to 9 current-carrying conductors in a raceway, the derating factor is 70%.
For 10 AWG THHN (90°C rating = 40A): 40A x 0.70 = 28A. Because 28A is less than your 30A breaker, 10 AWG is now a code violation. You must upsize to 8 AWG THHN (90°C rating = 55A; 55 x 0.70 = 38.5A, which safely clears the 30A threshold).
3. High Ambient Temperatures
If your conduit runs through an unconditioned attic in a hot climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors. At 50°C (122°F), the correction factor for 90°C insulation is 0.82. Again, 40A x 0.82 = 32.8A, which leaves almost no headroom for a continuous 30A load.
Copper vs. Aluminum: Sizing for Feeders and RV Outlets
Aluminum wire is lighter and cheaper, making it popular for long feeder runs to subpanels. However, aluminum has higher resistance and expands/contracts more under thermal load, which can loosen terminations if not torqued correctly. You cannot swap copper and aluminum interchangeably.
| Criteria | 10 AWG Copper | 8 AWG Aluminum |
|---|---|---|
| Cost (approx. per 100ft) | $45 - $65 | $25 - $40 |
| Termination Torque | Standard | Requires strict inch-pound compliance & anti-oxidant paste |
| Physical Flexibility | High (easy to pull in tight boxes) | Low (stiff, requires larger bend radius) |
| Best Application | Indoor branch circuits, RV outlets, appliance whips | Long outdoor feeder runs to subpanels |
Note: While 10 AWG aluminum technically exists in some catalogs, standard building practice and most AHJs require a minimum of 8 AWG aluminum for a 30A breaker to account for voltage drop and termination reliability.
Frequently Asked Questions
Why can't I use a smaller wire like 12 AWG for a 30 amp breaker?
Using 12 AWG wire (rated for 20A) on a 30A breaker is a severe fire hazard. The breaker's job is to protect the wire, not the appliance. If a fault or overload draws 25 amps, a 12 AWG wire will overheat, melt its insulation, and potentially ignite surrounding framing long before the 30A breaker trips. NEC 240.4(D) strictly forbids this mismatch to prevent exactly this failure mode.
What specific conditions change the 10 AWG wire size requirement?
You must abandon 10 AWG and move to 8 AWG copper if:
1. Your one-way wire length exceeds 80-100 feet (voltage drop).
2. You are pulling more than 6 current-carrying conductors in a single conduit (bundling derating).
3. The conduit is exposed to ambient temperatures above 110°F (temperature derating).
4. You are using aluminum instead of copper.
When do I need a licensed engineer or AHJ to confirm my wire sizing?
While the NEC provides the baseline, you must consult your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer if your installation involves continuous loads running at exactly 30A for 3 hours or more (which requires sizing the wire at 125% of the load, pushing you to 8 AWG), if you are wiring specialized industrial machinery with high inrush currents, or if your local municipality has adopted amendments that restrict aluminum wire or mandate larger grounding conductors. Always pull a permit and schedule a rough-in inspection before closing up walls.
