Correct Wire Size for a 30 Amp Circuit
A 30-amp circuit breaker requires a minimum of 10 AWG (American Wire Gauge) copper wire. If using aluminum conductors, the minimum size is 8 AWG. These ratings are based on NEC Table 310.16 for conductors rated at 60 degrees Celsius in standard residential installations. Using wire smaller than these minimums on a 30-amp breaker is a code violation and a fire hazard.
Common 30-Amp Circuit Applications
30-amp circuits are used for specific dedicated appliance circuits in residential wiring. The most common applications include electric clothes dryers (30A/240V, typically using a 10/3 NM-B cable with ground), electric water heaters (30A/240V, typically using 10/2 NM-B cable with ground), window and through-wall air conditioning units, small sub-panel feeds, certain workshop equipment circuits, and RV outlet receptacles (NEMA TT-30).
Each of these applications requires a dedicated circuit, meaning the 30-amp breaker serves only one appliance or outlet. Sharing a 30-amp circuit across multiple outlets is not typical and may not meet code depending on the installation.
Wire Type Selection
The correct wire gauge is only half the equation. You also need the right wire type for the installation environment.
Indoor Installations
For standard indoor runs through walls, ceilings, and floors, use 10/2 or 10/3 NM-B (Romex) cable with ground. The “10” indicates the wire gauge, and the “2” or “3” indicates the number of insulated conductors (plus a bare ground). Use 10/2 for 240V-only loads (like water heaters) and 10/3 for 120/240V loads (like dryers) that require both 240V power and a neutral conductor.
Outdoor and Underground
For outdoor circuits or direct burial, use 10/2 or 10/3 UF-B (Underground Feeder) cable. UF-B has a solid, moisture-resistant jacket rated for wet locations and direct burial at a minimum depth of 24 inches (or 18 inches if protected by PVC conduit).
In Conduit
For runs through conduit (EMT, PVC, or flexible metal), use individual 10 AWG THHN/THWN-2 conductors. Pull the required number of conductors (two hots plus ground for 240V, two hots plus neutral plus ground for 120/240V) through the conduit. THHN is rated for both wet and dry locations, making it versatile for indoor and outdoor conduit installations.
Voltage Drop and Long Runs
For 30-amp circuits with wire runs under 100 feet, 10 AWG copper meets both ampacity and voltage drop requirements. For runs between 100 and 150 feet, the voltage drop at full load approaches or exceeds the NEC-recommended 3% maximum. In these cases, upgrading to 8 AWG copper reduces voltage drop to acceptable levels and ensures the connected appliance receives adequate voltage for proper operation. For runs beyond 150 feet, a voltage drop calculation specific to your circuit length and load is essential.
240V vs. 120V 30-Amp Circuits
Most 30-amp residential circuits are 240V (using a double-pole 30-amp breaker). These serve high-draw appliances that require 240V service. A 30-amp 120V circuit (single-pole breaker) is less common in residential settings but may be used for certain equipment. The wire gauge requirement (10 AWG copper minimum) is the same regardless of voltage.
Quick Reference
For a 30-amp dryer circuit, use 10/3 NM-B with ground (four conductors total: two hots, one neutral, one ground). For a 30-amp water heater circuit, use 10/2 NM-B with ground (three conductors total: two hots, one ground, no neutral needed). For a 30-amp outdoor or buried circuit, use 10/2 or 10/3 UF-B with ground. For a 30-amp circuit in conduit, use individual 10 AWG THHN conductors.
IB Lighting stocks 10 AWG wire in NM-B, UF-B, and THHN configurations in multiple spool sizes. Call (800) 674-9019 for pricing and availability, or browse our electrical wire selection.