A standard 30 amp breaker requires 10 AWG copper wire for branch circuits up to 50 feet. This assumes 75°C equipment terminations, 90°C THHN/THWN-2 insulation in conduit, and a 30°C ambient environment.
- Conductor Material: Copper (unless aluminum is explicitly stated).
- Insulation Type: THHN/THWN-2 (90°C rated) or XHHW-2.
- Termination Rating: 75°C (standard for modern breakers and NEMA 14-30/L14-30 receptacles).
- Ambient Temperature: 30°C (86°F) or lower.
- Installation Method: Single circuit in standard EMT conduit or NM-B cable in an open wall cavity (no more than 3 current-carrying conductors).
Why 10 AWG? Decoding NEC Table 310.16
To understand why 10 AWG is the correct pick, we have to look at how the National Electrical Code (NEC) rates wire ampacity versus how it rates equipment terminations. This is where many DIYers get tripped up by reading the wrong column on the ampacity chart.
NEC Section 110.14(C) dictates that you must size your wire based on the lowest temperature rating of any connected component. While modern THHN wire is insulated for 90°C, standard residential breakers and 30-amp receptacles are typically rated for 75°C. Therefore, you must use the 75°C column to determine your baseline ampacity.
| Wire Size (AWG) | 60°C Column (Older Equipment) | 75°C Column (Standard Terminations) | 90°C Column (Wire Insulation Only) |
|---|---|---|---|
| 12 AWG | 20A | 25A | 30A |
| 10 AWG | 30A | 35A | 40A |
| 8 AWG | 40A | 50A | 55A |
Why not use 12 AWG? If you look at the 90°C column, 12 AWG shows 30A. However, because your breaker lugs are rated for 75°C, the wire is legally and thermally limited to 25A. If you pull 30 amps through 12 AWG wire terminated at 75°C lugs, the heat will degrade the termination point, potentially melting the breaker lug or causing a fire before the 30-amp breaker ever trips. 10 AWG copper at 75°C is rated for 35A, providing a safe 5-amp thermal buffer above your 30-amp breaker threshold.
The Voltage Drop Trap: When to Upsize to 8 AWG
Ampacity tables only tell you what size wire will prevent a fire. They do not guarantee your equipment will actually run. NEC 210.19(A) Informational Note recommends keeping voltage drop under 3% for branch circuits to ensure efficient equipment operation.
10 AWG copper has a resistance of roughly 1.21 ohms per 1,000 feet. Using the standard voltage drop formula (VD = 2 × L × R × I / 1000):
- At 50 feet: VD = 3.63V (3.02% drop) — Acceptable.
- At 60 feet: VD = 4.35V (3.62% drop) — Exceeds 3% recommendation.
- At 100 feet: VD = 7.26V (6.05% drop) — Unacceptable; motors will overheat.
The Fix: If your 120V, 30-amp run exceeds 50 feet, you must upsize to 8 AWG copper to maintain a 3% or lower voltage drop. (Note: For a 240V circuit, like a standard dryer or EV charger, 10 AWG is fine up to 100 feet because the percentage drop is halved).
Variables That Force an Upsize: Bundling and Aluminum
Your environment and material choices can invalidate the standard 10 AWG recommendation. Here is what changes the math on the jobsite.
Conduit Bundling (Derating)
When you pull multiple circuits through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to derate the ampacity of the wire's 90°C insulation rating based on the number of current-carrying conductors.
- 4 to 6 conductors: Derate to 80%. (10 AWG at 90°C is 40A × 0.80 = 32A). 10 AWG is still safe for a 30A breaker.
- 7 to 9 conductors: Derate to 70%. (10 AWG at 90°C is 40A × 0.70 = 28A). 10 AWG is now undersized. You must upsize to 8 AWG.
Switching to Aluminum Wire
Aluminum is cheaper and lighter, but it has higher resistance and expands more under heat. Standard building wire does not typically come in 10 AWG aluminum; it usually starts at 8 AWG. If you are using aluminum (like 8-8-8-6 SER cable for a subpanel feed), 8 AWG aluminum is your absolute minimum for a 30-amp breaker. At 75°C, 8 AWG aluminum is rated for 40A, safely clearing the 30A requirement. Never attempt to terminate aluminum wire in lugs not explicitly marked "AL/CU" or rated for 75°C.
Decision Tree: Finalizing Your Wire Pick
Use this matrix to lock in your exact wire purchase. Follow the conditions down to your specific installation scenario.
| Installation Condition | Circuit Voltage | Run Length | Final Wire Pick (Buy This) |
|---|---|---|---|
| Standard single circuit, open wall or single conduit | 120V or 240V | Under 50 ft | 10 AWG Copper (THHN or NM-B) |
| Standard single circuit, open wall or single conduit | 120V | 51 ft to 85 ft | 8 AWG Copper (THHN) |
| Standard single circuit, open wall or single conduit | 240V | 51 ft to 100 ft | 10 AWG Copper (THHN or NM-B) |
| Conduit contains 7-9 current-carrying conductors | Any | Any | 8 AWG Copper (THHN) |
| Using Aluminum wire (e.g., SER cable feed) | 120V or 240V | Under 50 ft | 8 AWG Aluminum |
When the AHJ or an Engineer Must Confirm
The recommendations above apply to standard, non-continuous resistive or intermittent motor loads (like a table saw, RV outlet, or occasional-use welder). However, specific load profiles trigger NEC rules that require a larger breaker and larger wire. If your project falls into these categories, consult your local Authority Having Jurisdiction (AHJ) or an Electrical Training Alliance certified professional.
Continuous Loads (The 125% Rule)
NEC Article 100 defines a continuous load as one where the maximum current is expected to continue for 3 hours or more. Examples include hardwired kilns, continuous-duty air compressors, and Level 2 EV chargers. NEC 210.20(A) requires the branch circuit to be rated at 125% of the continuous load.
- The Math: A 30A continuous load × 1.25 = 37.5A.
- The Result: You cannot use a 30-amp breaker. You must install a 40-amp breaker and pull 8 AWG copper wire (rated 50A at 75°C).
Motor Full-Load Current (FLC)
If your 30-amp breaker is protecting a large motor, you do not size the wire based on the breaker. You size the wire based on the motor's nameplate Full-Load Current (FLC) multiplied by 125% (NEC 430.22), and then size the breaker based on NEC Table 430.52 (which often allows 250% of FLC for inverse-time breakers to handle startup inrush). In motor circuits, the wire and the breaker sizing decouple entirely. Always defer to the motor manufacturer's wiring diagram and a qualified electrician for these setups.
