Wire Gauges Size Chart: NEC Ampacity Table & Sizing Decision Guide

For standard residential copper wiring, the baseline rule is simple: use 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, 6 AWG for 55A/60A, and 4 AWG for 70A. These values are governed by the National Electrical Code (NEC), specifically Table 310.16 (formerly 310.15(B)(16)). However, blindly picking a wire size based on breaker amperage is how beginners melt terminations or fail inspections. Ampacity is not just about the wire; it is about the weakest link in the circuit, which is usually the terminal lug on your breaker or receptacle.

This reference guide provides the complete copper ampacity table, explains the critical temperature columns, and gives you a definitive decision path for sizing your next circuit.

The Master Copper Wire Gauges Size Chart (NEC Table 310.16)

The following table lists the allowable ampacities for insulated copper conductors rated 0 through 2000 volts, based on an ambient temperature of 30°C (86°F). This data is sourced directly from the NFPA National Electrical Code.

How to Read the Chart: Temperature Columns and Termination Limits

The most common mistake DIYers make is looking at the 90°C column because they bought THHN wire, which is stamped "90°C" on the jacket. You cannot use the 90°C column for final ampacity in residential branch circuits.

According to OSHA and NEC 110.14(C), the ampacity of a circuit is limited by the temperature rating of the terminations (the breaker lugs and receptacle screws), not just the wire insulation. Standard residential breakers (up to 100A) and standard 15A/20A receptacles are tested and rated for 60°C terminations. Therefore, the 60°C column governs your final breaker size for almost all interior branch circuits.

The NEC 240.4(D) Small Conductor Rule: For conductors 14, 12, and 10 AWG, the NEC strictly caps the overcurrent protection at 15A, 20A, and 30A respectively. Even though 12 AWG THHN has a 90°C ampacity of 30A, you are legally prohibited from placing it on a 30A breaker. It must be protected at 20A.

When can you use the 75°C column? You may use the 75°C column for circuits rated 100A or greater, or when terminating in equipment explicitly marked for 75°C (like many modern main breaker panels and heavy-duty disconnects). This is why a 4 AWG copper wire can be used for an 85A load in a main panel feeder, but only a 70A load on a subpanel branch circuit.

Derating and Adjustments: When the Base Value Drops

The ampacities in the table above assume you have no more than three current-carrying conductors in a raceway (conduit) and an ambient temperature of 30°C (86°F). If you violate either condition, you must apply derating factors per NEC 310.15(C)(1).

The Professional Secret: Derating calculations always start from the 90°C column, even if your final termination limit is 60°C. You calculate the derated ampacity, and then compare it to the 60°C/75°C termination limit. The lower of the two numbers is your final allowable ampacity.

Note that equipment grounding conductors (bare copper or green) do not count as current-carrying conductors for derating purposes. Furthermore, neutrals in a standard 120V/240V single-phase residential circuit often do not count as current-carrying if they only carry the unbalanced load, though neutrals in multi-wire branch circuits (MWBC) sharing a neutral do count in specific harmonic scenarios.

Decision Tree: Sizing Wire for Common Residential Circuits

Use this decision-tree-table to terminate your sizing process with a concrete pick. Do not guess; match your breaker size and load type to the exact wire specification below.

Default Recommendation: For all branch circuits up to 60A, use solid copper NM-B (for interior walls) or THHN (in conduit). For feeders 60A and above, switch to stranded aluminum XHHW-2. Aluminum feeders (like 2 AWG Al for 90A or 1/0 Al for 100A) cut material costs by up to 40% compared to copper and are perfectly safe when torqued to manufacturer specs on 75°C rated lugs.

What This Chart Cannot Tell You (Voltage Drop & Distance)

Ampacity tables dictate the maximum current a wire can carry before its insulation melts or degrades. They tell you absolutely nothing about voltage drop. A 12 AWG wire is perfectly safe carrying 20A over a 300-foot distance from a thermal perspective, but the voltage at the end of that run will drop to roughly 110V on a 120V nominal system, causing motors to overheat and lights to dim.

While the NEC treats voltage drop as an informational recommendation rather than a strict mandate for most residential dwellings, standard engineering practice (and NEC Article 210.19 Informational Notes) dictates a maximum 3% voltage drop for branch circuits and a combined 5% for feeder plus branch.

Voltage Drop Action Rule: If your one-way wire distance from the panel to the furthest receptacle exceeds 50 feet on a 15A/120V circuit, or 75 feet on a 20A/120V circuit, you must upsize the wire by one gauge to compensate for resistance. For example, a 20A circuit running 120 feet to a detached garage workshop should be wired with 8 AWG copper instead of 12 AWG to maintain tight voltage regulation under heavy tool loads. Always calculate voltage drop using the actual expected load (e.g., 12A for a vacuum), not the breaker rating, for the most accurate and cost-effective sizing.