For a standard 30-amp circuit, use 10 AWG copper wire paired with a 30-amp breaker. This applies to typical residential branch circuits like RV receptacles, dryers, or water heaters. Sizing relies on NEC Table 310.16 and the small conductor rules in NEC 240.4(D), assuming copper conductors and standard 75°C terminal ratings.
• Conductor: Copper
• Insulation: THHN/THWN-2 or NM-B
• Terminal Temperature Rating: 75°C
• Ambient Temperature: 30°C (86°F)
• Installation: Single circuit in a standard raceway or wall cavity
Note: NEC-style guidance provided here is for educational purposes; your local Authority Having Jurisdiction (AHJ) has final legal authority.
The Baseline Sizing Rules and Ampacity Table
When sizing wire, you cannot simply look at the highest temperature column in the ampacity table. Under NEC 110.14(C), the ampacity of a circuit is limited by the lowest temperature rating of any connected component—usually the breaker terminals or the receptacle. Most modern 30-amp breakers and heavy-duty receptacles are rated for 75°C. Therefore, we must size our wire using the 75°C column of NEC Table 310.16.
| AWG Size | 60°C Column (NM-B) | 75°C Column (THHN in conduit) | 90°C Column (THHN derating) |
|---|---|---|---|
| 14 AWG | 15A | 20A | 25A |
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Looking at the table, 10 AWG copper in the 60°C column is rated for exactly 30 amps. In the 75°C column, it is rated for 35 amps. Because our breaker limits us to 30 amps, 10 AWG is the mathematically correct and code-compliant baseline.
Why 10 AWG and Not 12 AWG? (The 240.4(D) Rule)
A common bench mistake is assuming that because 12 AWG wire has a 90°C ampacity of 30 amps, it can be used on a 30-amp breaker. This is a severe fire hazard and a direct violation of NEC 240.4(D), which governs small conductors.
NEC 240.4(D) places hard caps on overcurrent protection for small wires to prevent the wire from acting as a fuse. Under this rule:
- 14 AWG maximum breaker size: 15 Amps
- 12 AWG maximum breaker size: 20 Amps
- 10 AWG maximum breaker size: 30 Amps
If you wire a 12 AWG cable to a 30-amp breaker and the circuit develops a 28-amp fault, the breaker will not trip. However, the 12 AWG wire will overheat, melt its insulation, and potentially ignite surrounding framing long before the 30-amp breaker detects an overload. The breaker protects the wire, not the load; the wire gauge must safely handle the maximum current the breaker will allow through.
Voltage Drop: When 10 AWG Fails the Distance Test
Ampacity tells you the wire won't catch fire. Voltage drop tells you your equipment will actually run. The NEC recommends a maximum 3% voltage drop on branch circuits. When current travels down a wire, resistance causes the voltage to sag. If the sag exceeds 3%, motors will overheat, electronics will brown out, and heaters will underperform.
We calculate voltage drop using the formula: VD = (2 × K × I × D) / CM
- K = 12.9 (resistivity of copper)
- I = 30 Amps
- D = One-way distance in feet
- CM = 10,380 (circular mils for 10 AWG)
• On a 120V circuit: Max VD is 3.6V. 10 AWG is only good for 48 feet.
• On a 240V circuit: Max VD is 7.2V. 10 AWG is good for 96 feet.
If you are wiring a 120V, 30-amp RV receptacle at the end of a 70-foot driveway, 10 AWG will yield a 5.2% voltage drop. Your RV's air conditioner compressor will struggle to start and may burn out. For that 70-foot 120V run, you must upsize to 8 AWG copper to keep the drop under 3%. Always verify long runs using a reliable tool like the Southwire Voltage Drop Calculator.
Decision Tree: What Changes Your Wire Gauge?
The baseline 10 AWG / 30A breaker rule holds true for 80% of residential jobs. But real-world conditions force derating. Use this decision path to lock in your final material pick.
| Condition / Constraint | Code / Physics Reason | Concrete Action & Pick |
|---|---|---|
| Standard run < 48ft (120V) or < 96ft (240V) | Baseline ampacity and V-drop acceptable. | Pick: 10 AWG Copper, 30A Breaker |
| Long run > 48ft (120V) or > 96ft (240V) | Voltage drop exceeds 3% limit. | Pick: 8 AWG Copper, 30A Breaker |
| 4 to 6 current-carrying conductors in one conduit | NEC 310.15(C)(1) requires 80% derating. | Pick: 8 AWG Copper (40A × 0.8 = 32A) |
| Continuous Load (runs 3+ hours, e.g., EV charger) | NEC 210.20(A) requires 125% breaker sizing. | Pick: 8 AWG Copper, 40A Breaker |
| Using Aluminum wire instead of Copper | Aluminum has higher resistance; requires upsizing. | Pick: 8 AWG Aluminum (if terminals are rated AL/CU) |
| Ambient temp exceeds 30°C (e.g., hot attic) | NEC Table 310.15(B)(1) correction factors apply. | Pick: 8 AWG Copper (or consult AHJ) |
When an Engineer or the AHJ Must Confirm
While the decision tree above covers standard branch circuits, certain scenarios require a licensed professional or explicit AHJ sign-off. Do not guess on the following:
- Aluminum Terminals: Many modern 30-amp breakers and receptacles are strictly rated for copper. If the terminal does not explicitly state 'AL' or 'AL/CU', you cannot use aluminum wire, regardless of gauge. Furthermore, aluminum requires specific anti-oxidant compound (like Noalox) and exact torque settings to prevent high-resistance connections that cause fires.
- Service Entrance or Feeder Upgrades: If your 30-amp load is part of a larger subpanel feeder calculation, the sizing shifts from branch circuit rules (Article 210) to feeder rules (Article 215) and standard residential calculations (Article 220).
- High Ambient Environments: If your conduit runs across a roof or through an attic where ambient temperatures regularly exceed 104°F (40°C), the 90°C column must be used for derating, but the final adjusted ampacity must still meet the 75°C terminal limits. This often forces a jump to 8 AWG or even 6 AWG copper.
For standard, ambient-temperature, short-run branch circuits, buy your 10 AWG copper THHN or 10/2 NM-B, torque your 30-amp breaker terminals to the manufacturer's spec (usually 20-25 in-lbs), and you are good to close up the panel.
