What Size Wire for a 30 Amp Breaker? The Exact 30amp Wire Size Guide

The Baseline: Sizing Wire for a 30-Amp Breaker

When matching a conductor to an overcurrent protection device, the National Electrical Code (NEC) relies on the ampacity tables found in NEC 310.16. The goal is to ensure the wire can handle the maximum current the breaker will allow before tripping, without the insulation melting or degrading.

Because 10 AWG copper is rated for 30 amps even in the more conservative 60°C column, it is the universal, bulletproof choice for a 30-amp breaker. This is critical because while most modern breakers and terminals are rated for 75°C, NEC 334.80 mandates that NM-B (Romex) cable must be sized using the 60°C column regardless of the termination temperature rating. By using 10 AWG, you satisfy both columns simultaneously.

The Most Common Confusion: Breaker Size vs. Continuous Load

The most frequent mistake DIYers make with 30-amp circuits is confusing the breaker rating with the continuous load capacity. According to NEC 210.20(A) for continuous loads, a breaker can only be loaded to 80% of its rating if the load will run for three hours or more.

A 30-amp breaker can only safely supply 24 amps continuously. If your equipment actually draws 30 amps continuously (like a heavy-duty kiln or a large EV charger), you cannot use a 30-amp breaker and 10 AWG wire. You must step up to a 40-amp breaker and 8 AWG copper wire. Sizing for the actual load profile is what separates a safe installation from a nuisance-tripping fire hazard.

What Changes in the Circuit When You Size Correctly

Wire size dictates two fundamental physical realities in your circuit: heat dissipation and voltage drop.

Heat Dissipation (The Fire Limit): Every wire has resistance. When current flows, it generates heat proportional to the square of the current (I²R). If you incorrectly install 12 AWG wire (rated for 20A) on a 30-amp breaker, the breaker will not trip at 25 amps. The 12 AWG wire will overheat, the THHN or NM-B insulation will soften and melt, and a short circuit or fire will occur before the breaker's bimetallic strip ever bends to open the circuit. Correct sizing ensures the wire acts as a safe conduit, not a heating element.

Voltage Drop (The Performance Limit): Even if a wire doesn't melt, undersizing causes voltage to sag over distance. Think of current as cars on a highway; 10 AWG is a three-lane highway that keeps 30 amps of traffic moving smoothly without friction, while 12 AWG is a two-lane road where 30 amps causes a friction-heavy traffic jam that drops the voltage at the destination. Motors running on low voltage will draw higher amperage to compensate for the missing wattage, leading to premature burnout.

Where You Meet 30-Amp Circuits in Practice

You will typically encounter the need for 30amp wire size in specific high-draw residential and workshop applications:

• RV Receptacles: The NEMA TT-30R (120V, 30A) for travel trailers, and the NEMA 14-30R (240V, 30A) for larger fifth-wheel RVs.
• Electric Dryers: Older homes often feature NEMA 10-30 (ungrounded) or 14-30 (grounded) receptacles. While modern code often pushes dryers to 50-amp circuits, 30-amp is still standard for compact or 120V/240V combo units.
• Workshop Machinery: 240V single-phase equipment like 3HP air compressors, midi-lathes, and MIG welders frequently require 30-amp double-pole breakers.
• HVAC Equipment: Large window air conditioners, heavy-duty dehumidifiers, and some mini-split condenser disconnects are wired to 30-amp circuits.

Worked Example: 240V RV Receptacle at 50 Feet

Let's run the math on a real-world installation to prove why 10 AWG is the correct pick, and when you might need to change it.

The Voltage Drop Formula:
Voltage Drop (VD) = (2 × K × I × L) / CM

• K (Resistance constant for copper) = 12.9
• I (Current) = 30A (We calculate using the breaker's max capacity to ensure the wire handles worst-case scenarios)
• L (One-way length) = 50 feet
• CM (Circular Mils for 10 AWG) = 10,380

The Calculation:
VD = (2 × 12.9 × 30 × 50) / 10,380
VD = 38,700 / 10,380 = 3.72 Volts

The Result:
To find the percentage, divide the drop by the system voltage: 3.72V / 240V = 1.55%.
The NEC recommends a maximum voltage drop of 3% for branch circuits. At 1.55%, 10 AWG copper is well within safe limits. However, if this same run was 120 feet long, the drop would be 8.92V (3.71%), which exceeds the 3% recommendation. In that specific edge case, you would step up to 8 AWG copper (CM = 16,510) to bring the drop back down to 2.34%.

For exact calculations on your specific run length, use the Southwire Voltage Drop Calculator to verify your math before pulling wire.

The 30-Amp Wire Size Decision Tree

Use this decision path to finalize your materials list. Follow the conditions from top to bottom until you hit your specific scenario.

Circuit Condition	If YES	If NO
Is the continuous load (running 3+ hours) greater than 24 Amps?	STOP. You need a 40A breaker and 8 AWG Copper.	Proceed to next row.
Is the one-way wire run longer than 100 feet?	Step up to 8 AWG Copper to mitigate voltage drop.	Proceed to next row.
Are you required to use Aluminum wire (e.g., feeding a subpanel)?	Use 8 AWG Aluminum (minimum) or 6 AWG for termination ease.	Proceed to next row.
Is this a standard run under 100ft, under 24A continuous, using Copper?	DEFAULT PICK: Buy 10 AWG THHN-2 Copper (e.g., Southwire or Cerro) or 10/3 NM-B.

Frequently Asked Questions