Wire Sizing Chart for Amps: NEC Ampacity Table & Quick Reference

If you need a fast answer for standard residential branch circuits, here are the baseline copper wire sizes for common breakers: 15A requires 14 AWG, 20A requires 12 AWG, 30A requires 10 AWG, 40A requires 8 AWG, and 50A requires 6 AWG. These values assume copper conductors, a maximum of three current-carrying wires in a raceway, and an ambient temperature of 30°C (86°F).

However, electrical work is rarely that simple. The definitive authority for conductor sizing in the United States is the National Electrical Code (NEC). Below is the complete reference data you need to size wire safely, legally, and efficiently for both branch circuits and heavy feeder loads.

The Master Wire Sizing Chart for Amps (NEC Table 310.16)

This table is derived directly from NEC Table 310.16 (formerly 310.15(B)(16)). It lists the allowable ampacities for insulated conductors rated up to 2000 volts. Before using this chart, understand how to read the temperature columns: the 60°C, 75°C, and 90°C ratings refer to the thermal limit of the wire's insulation (e.g., TW, THHW, THHN), not the temperature of the copper itself. Always match the column to the lowest temperature rating of any connected component, terminal, or breaker in your circuit.

Which Column Applies and How Derating Modifies the Base Value

The most common mistake DIYers and junior apprentices make is reading the 90°C column and assuming they can use that higher ampacity for breaker sizing. You almost never use the 90°C column for final breaker sizing. Here is the exact framework for choosing your column:

• The 60°C Column: Applies to 14, 12, and 10 AWG wires due to NEC 240.4(D) small conductor rules, and anytime you are using NM-B cable or connecting to older terminals explicitly marked 60°C.
• The 75°C Column: The standard for modern residential and commercial work. Most breakers, lugs, and terminal bars manufactured today are rated for 75°C. Use this column for 8 AWG and larger THHN/THWN wires in conduit.
• The 90°C Column: Used exclusively as the starting point for derating calculations. You cannot terminate a wire on a 75°C lug using its 90°C ampacity.

How Derating Rows Modify the Base Value

When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to 'derate' the wire's capacity. You apply the derating percentage to the 90°C column, then compare the result to your terminal temperature limit.

Worked Example: You are pulling four 12 AWG THHN wires (two hots, one neutral, one ground) through a conduit for a multi-wire branch circuit. Only the two hots and the neutral are 'current-carrying' (the ground does not count). Wait, if the neutral carries only the unbalanced load, it might not count, but let's assume four current-carrying conductors for a worst-case scenario.

1. Look at the 90°C column for 12 AWG Copper: 30A.
2. Find the derating factor for 4-6 conductors in NEC Table 310.15(C)(1): 80%.
3. Multiply: 30A × 0.80 = 24A.
4. Check the terminal limit (75°C column for 12 AWG): 25A.
5. The derated value (24A) is lower than the terminal limit (25A), so your final allowable ampacity is 24A. You can still safely use this wire on a standard 20A breaker.

What This Table Cannot Tell You (Voltage Drop & Environment)

The ampacity table assumes ideal conditions: a short wire run and an ambient temperature of 30°C (86°F). If your installation deviates from these baselines, the table alone will lead to an undersized, dangerous, or inefficient circuit.

Voltage Drop Over Long Distances

NEC Table 310.16 does not account for voltage drop. If you are running a 50A circuit to a detached garage 200 feet away, 6 AWG copper will safely handle the 50A heat load, but the resistance of the wire will cause the voltage to drop below the acceptable 3% threshold (dropping below 114V on a 120V leg). For long runs, you must upsize the wire. Using a dedicated voltage drop calculator, a 50A load at 200 feet requires upsizing to 4 AWG copper or 2 AWG aluminum to maintain proper voltage at the load.

Ambient Temperature Corrections

If you are routing wires through a hot attic in the middle of summer, the ambient temperature easily exceeds 30°C. NEC Table 310.15(B)(1) provides temperature correction factors. If your attic reaches 113°F to 122°F (45°C to 50°C), you must multiply the 90°C ampacity by 0.82. A 10 AWG THHN wire (40A at 90°C) derates to 32.8A in a hot attic, which is still fine for a 30A breaker, but a 6 AWG wire running a 60A subpanel might suddenly fall below the required threshold.

Wire Sizing Chart for Amps FAQ

What size wire do I need for a 50 amp breaker?

For a standard 50A breaker (like an electric range or EV charger), you need 6 AWG copper or 4 AWG aluminum. This assumes the wire is in a raceway or cable with an ambient temperature of 30°C and no more than three current-carrying conductors. If you are using NM-B (Romex) cable, you are legally bound to the 60°C column, which also dictates 6 AWG copper for 55A (the next standard breaker size down is 50A, making 6 AWG compliant).

Can I use 14 AWG wire on a 20 amp breaker?

No. NEC 240.4(D) explicitly mandates that 14 AWG copper wire must be protected at a maximum of 15 amps. Even if the 75°C or 90°C columns show higher theoretical thermal limits, the small-conductor rule overrides them to prevent fire hazards from high-fault currents. If you have a 20A breaker, you must use a minimum of 12 AWG copper.

Does the ground wire count when derating for conduit fill?

No. When calculating the derating factors for multiple wires in a single conduit (NEC 310.15(C)(1)), equipment grounding conductors (bare copper or green insulated) are not counted as 'current-carrying conductors.' They only carry current during a fault condition, which should trip the breaker instantly, so they do not contribute to steady-state heat buildup in the conduit.

How do I size wire for a 100 amp subpanel?

For a 100A subpanel feeder, you need 3 AWG copper or 1/0 AWG aluminum (using the 75°C column, which is standard for panel lugs). Many electricians prefer 2 AWG aluminum (rated 90A at 75°C) for 100A panels if the calculated continuous load does not exceed 90A, but to safely land on a 100A main breaker without load calculations, 1/0 AWG aluminum is the safest, most code-compliant choice. Always ensure your feeder includes four wires: two hots, one neutral, and one separate equipment ground.