Wire Amperage Rating Chart: NEC Table 310.16 Ampacity Guide

The standard wire amperage rating chart used by US electricians and inspectors is NEC Table 310.16. For standard residential branch circuits using copper wire, the baseline ampacities are: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, and 6 AWG = 55A. However, simply reading the highest number on the chart is a fast track to a failed inspection or a melted terminal. The correct ampacity depends entirely on your insulation type, terminal temperature ratings, and how many wires are bundled in the conduit.

The Standard Wire Amperage Rating Chart (NEC Table 310.16)

Before sizing a breaker, you need to know how to read the official NFPA 70 (National Electrical Code) Table 310.16. The table is divided by conductor material (Copper vs. Aluminum) and then by temperature rating (60°C, 75°C, and 90°C). The values below assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.

Which Column Applies to Your Installation?

The most common mistake apprentices make is looking at a spool of THHN wire (which has a 90°C rating printed on the jacket), finding the 90°C column, and sizing the breaker based on that higher number. This violates NEC 110.14(C) - Temperature Limitations.

You must match the wire's ampacity column to the temperature rating of the terminals it connects to, not just the wire's insulation. In residential and light commercial panels, standard breakers and receptacles are typically rated for 75°C. Therefore, even if you pull 90°C THHN wire, your final base ampacity is capped at the 75°C column. If you are using NM-B (Romex) cable, NEC 334.80 strictly limits you to the 60°C column, regardless of the fact that the individual conductors inside the sheath might have 90°C insulation.

The 90°C column is not useless, however. It is exclusively used as the starting baseline for derating calculations (explained below) before you apply the final terminal temperature cap.

How Derating Modifies the Base Amperage

Table 310.16 assumes ideal conditions: an ambient temperature of 30°C and a maximum of three current-carrying conductors in a conduit. When you exceed these conditions, the wire cannot dissipate heat as effectively, and you must apply derating factors from NEC Tables 310.15(B)(1) and 310.15(C)(1).

Worked Example: Bundled Conductors
Imagine you are running a multi-wire branch circuit and a dedicated 20A circuit through the same EMT conduit. You have 4 current-carrying 12 AWG THHN copper conductors.

1. Base Ampacity: Start with the 90°C column for 12 AWG copper = 30A.
2. Derating Factor: Table 310.15(C)(1) states that 4 to 6 current-carrying conductors require an 80% adjustment factor.
3. Calculation: 30A × 0.80 = 24A.
4. Terminal Check: The derated ampacity (24A) is now compared to the 75°C terminal limit (25A). Because 24A is less than 25A, the wire is valid.
5. Breaker Sizing: You can still protect this circuit with a standard 20A breaker, as 24A > 20A.

If you had 10 current-carrying conductors in that pipe, the derating factor drops to 50%. Your 12 AWG wire would drop to 15A (30A × 0.50), forcing you to upsize to 10 AWG to maintain a 20A circuit.

What This Wire Amperage Rating Chart Cannot Tell You

While Table 310.16 is the bible for thermal limits, it is blind to several critical jobsite realities:

• Voltage Drop: The chart assumes the wire is short enough that resistance won't cause significant voltage drop. For long feeder runs (e.g., a detached garage 150 feet away), a 6 AWG wire might be thermally rated for 65A, but the voltage drop at that load will starve your equipment. You must calculate voltage drop using Chapter 9, Table 8 resistance values, aiming for a 3% maximum drop on branch circuits.
• Physical Lug Fit: The chart might tell you that 250 kcmil aluminum is rated for 205A, but it won't tell you if that thick, stiff cable will physically bend into the tight radius of a 200A residential panel's main breaker lug without stressing the termination.
• Short-Circuit Withstand: Ampacity is about continuous thermal loading. It does not indicate how long the wire can survive the magnetic and thermal forces of a 10,000A short circuit before the breaker clears the fault.