Wiring a 30 amp circuit is a common milestone for DIYers upgrading a home workshop, installing a 240V air compressor, or adding a NEMA 14-30 dryer receptacle. However, the margin for error at 30 amps is slim; undersized wire or loose terminal connections can quickly lead to melted insulation or an electrical fire. This guide provides the exact wire gauges, breaker models, and termination sequences you need to complete the job to NEC standards.

The Direct Answer: Wire Size, Breaker Model, and Terminal Mapping

For a standard 240V 30A load, use 10 AWG copper wire and a double-pole 30A breaker (such as the Square D QO230 or Siemens Q230). The ampacity of 10 AWG copper in the 60°C column of NEC Table 310.16 is exactly 30 amps, making it the mandatory minimum for this breaker size.

Terminal Mapping Summary:

  • Black (Hot 1): Lands on Breaker Terminal 1.
  • Red or Re-identified White (Hot 2): Lands on Breaker Terminal 2.
  • White (Neutral): Lands on the panel's Neutral Bar (only if your appliance requires a 120/240V 4-wire circuit).
  • Bare Copper (Ground): Lands on the panel's Equipment Grounding Bar.

Decision Path: Single-Pole vs. Double-Pole 30A Loads

Before buying materials, you must determine if your load requires 120V or 240V. A 30A breaker can be single-pole (120V) or double-pole (240V). Use the decision matrix below to select the correct configuration.

Application / Load Voltage Required Wire Configuration Breaker Type Needed
RV TT-30 Receptacle 120V 10/2 NM-B (Black, White, Bare) Single-Pole 30A
Heavy 120V Machinery 120V 10/2 NM-B (Black, White, Bare) Single-Pole 30A
Electric Dryer (NEMA 14-30) 120/240V 10/3 NM-B (Black, Red, White, Bare) Double-Pole 30A
240V Air Compressor / EVSE 240V 10/2 NM-B (Black, Red/White, Bare) Double-Pole 30A
DEFAULT RECOMMENDATION: If you are wiring a standard 240V appliance (compressor, EV charger, water heater) and are unsure, your concrete pick is a Double-Pole 30A breaker (Part # Square D QO230 for Square D panels, or Siemens Q230 for Siemens/Murray panels) paired with 10/2 NM-B cable for pure 240V loads, or 10/3 NM-B if the appliance has 120V control boards (like a dryer).

Tools and Materials Checklist

Do not start the installation until you have the following specific items on your workbench. Skipping the torque screwdriver is a direct violation of NEC 110.14(D) and a primary cause of panel fires.

  • Breaker: Square D QO230 (2-Pole, 30A, 120/240V) or equivalent for your specific panel brand. Never mix breaker brands in a panel unless explicitly listed for cross-compatibility.
  • Wire: 10/2 NM-B with ground (Romex) for 240V-only loads, or 10/3 NM-B with ground for 120/240V loads.
  • Torque Screwdriver: Calibrated inch-pound driver (e.g., Klein Tools 70006). Most 10 AWG terminations require 35 to 45 in-lbs of torque.
  • Wire Strippers: Klein 11063W or similar, capable of cleanly stripping 10 AWG without nicking the copper.
  • Voltage Tester: Non-contact voltage tester (NCVT) and a CAT III rated Digital Multimeter (DMM).
  • Screwdrivers: Insulated #2 Phillips and 1/4-inch flathead.

Mains Safety Protocol: De-Energize and Verify

DANGER: MAINS VOLTAGE CAN BE LETHAL.
Working inside an electrical panel exposes you to the utility feed lugs, which remain energized even when the main breaker is turned off. If you are not comfortable identifying the line-side vs. load-side of the main breaker, hire a licensed electrician. Always follow OSHA electrical safety guidelines regarding lockout/tagout procedures.
  1. Turn Off the Main Breaker: Switch the main service disconnect to the OFF position. This de-energizes the branch circuit bus bars.
  2. Verify Dead: Use your NCVT to check the bus bars where the new breaker will seat. Then, use your multimeter set to AC Volts to measure between the main breaker's load-side lugs and the ground bar. It must read 0V.
  3. Flashlight Ready: Ensure you have a hands-free headlamp, as the panel interior will be dark once the main is off.

Step-by-Step Installation: Wiring the 30 Amp Breaker

Follow these steps sequentially. Never route wires behind the panel's interior deadfront where they can be pinched, and ensure all wire colors are correctly mapped to their respective bars and terminals.

  1. Prep the Cable: Strip back 3/4-inch of the outer NM-B jacket to expose the conductors. Strip exactly 1/2-inch of insulation from the tips of the Black, Red (or White), and White wires. Do not nick the copper; a nicked 10 AWG wire creates a localized hot spot under load.
  2. Route the Cable: Feed the 10 AWG NM-B cable through the appropriate panel knockout, securing it with a proper NM cable clamp within 8 inches of the panel box.
  3. Land the Ground (Bare Copper): Route the bare copper wire to the Equipment Grounding Bar. Insert it into an open lug, ensuring no insulation is under the screw and no bare wire is exposed past the lug. Tighten securely.
  4. Land the Neutral (White - If Applicable): If you are running a 4-wire circuit (10/3 NM-B for a dryer), route the White neutral wire to the Neutral Bar. Note: If you are running a pure 240V circuit (10/2 NM-B), you will not have a neutral wire, and the White wire in a 10/2 cable must be wrapped in black or red electrical tape at both ends to re-identify it as a hot conductor.
  5. Seat the Breaker: Align the double-pole 30A breaker with the two adjacent bus stabs. Press down firmly and evenly until the breaker clips snap into place. Ensure it sits flush against the panel base.
  6. Land Hot 1 (Black Wire): Insert the stripped Black wire into Terminal 1 on the breaker. Ensure the wire is fully seated and the insulation jacket is not pushed under the pressure plate. Torque the screw to the manufacturer's specification (typically 35 in-lbs for Square D QO 10 AWG terminations).
  7. Land Hot 2 (Red or Re-identified White Wire): Insert the second hot wire into Terminal 2 on the breaker. Torque to the exact same specification as Terminal 1.
  8. Dress the Wires: Neatly route the conductors along the sides of the panel, avoiding sharp bends. Keep the hot wires separated from the neutral and ground wires where possible to reduce inductive heating and make future troubleshooting easier.

Verify and Test: Expected Meter Readings

Before replacing the panel deadfront cover and energizing the circuit, double-check that no stray wire strands are touching adjacent terminals. Once the cover is secured, restore power and test.

  1. Energize: Turn the Main Breaker ON, then flip the new 30A double-pole breaker ON.
  2. Set Multimeter: Set your DMM to AC Voltage (V~), expecting a range up to 600V.
  3. Test Hot-to-Hot: Place one probe on the Black wire termination (or the receptacle's X slot) and the other on the Red wire termination (or Y slot). Expected Reading: 240V (Acceptable range: 228V - 252V per NEC standards and ANSI C84.1).
  4. Test Hot-to-Ground: Place one probe on the Black wire and the other on the Bare ground wire. Expected Reading: 120V.
  5. Test Neutral-to-Ground (4-wire only): Place one probe on the White neutral wire and the other on the Bare ground wire. Expected Reading: Less than 2V (ideally 0.0V to 0.5V). A reading higher than 2V indicates a loose neutral connection at the panel or utility transformer.

The Most Common Botch: Loose Lugs and the "Melted Bus" Symptom

The single most frequent failure mode on DIY 30A installations is the undertorqued terminal lug. Because 10 AWG copper is thick and stiff, it exerts outward pressure against the breaker's pressure plate. If you only tighten the screw "hand-tight" with a standard screwdriver, the connection will have high electrical resistance.

The Symptom: Under a continuous 24A+ load (like an EV charger or air compressor running for 30 minutes), the high-resistance connection generates intense heat. You will hear a faint buzzing or crackling sound from the panel. Eventually, the breaker will trip randomly due to thermal overload, or worse, the heat will melt the wire insulation and scorch the panel's metal bus stab, permanently ruining that breaker slot.

The Fix: NEC 110.14(D) mandates that terminals must be tightened to the manufacturer's specified torque using a calibrated torque tool. For Schneider Electric Square D QO breakers, this is typically 35 in-lbs for 10 AWG wire. Buy a dedicated electrical torque screwdriver, set the dial, and tighten until the driver clicks. Do not guess, and do not use a standard driver and "feel" for tightness.

By following this exact wire sizing, termination sequence, and torque protocol, your 30 amp circuit will operate safely and reliably for decades, passing any professional inspection without a second look.