Wire Chart Ampacity: NEC 310.16 Copper Reference & Derating Guide

The base wire chart ampacity for common residential copper wire (THHN/THWN-2) in the 90°C column is 30A for 10 AWG, 40A for 8 AWG, and 55A for 6 AWG. However, your usable ampacity is almost always capped by the 60°C or 75°C terminal ratings of your breakers and devices. Sizing wire strictly by the highest number on the chart is a fast track to melted terminals and failed inspections.

How to Read the NEC Wire Chart Ampacity Table

Before pulling wire, you need to know how to read the source standard: NFPA 70 (National Electrical Code) Table 310.16. This table assumes copper conductors in an ambient temperature of 30°C (86°F). The columns are divided by the temperature rating of the wire's insulation. Most modern THHN/THWN-2 wire is rated for 90°C, but as we will cover below, you rarely get to use that full 90°C capacity for final overcurrent protection sizing.

Below is the data-dense reference for the most queried residential and light-commercial sizes. Bookmark this section for quick job-site lookups.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at a spool of 90°C THHN wire, finding the 90°C column, and sizing the breaker accordingly. The NFPA NEC has strict rules preventing this, primarily governed by NEC 110.14(C) regarding termination provisions.

Here is the practical decision framework for selecting your column:

• The 60°C Column: Use this for NM-B (Romex) cable. Even though modern NM-B insulation might withstand higher heat, NEC 334.80 explicitly limits its ampacity to the 60°C column. Also use this column if your equipment (like older receptacles or specific light fixtures) is only marked for 60°C.
• The 75°C Column: Use this for most standard breakers, lugs, and devices rated 100A or less, provided you are using wire with a 75°C or 90°C rating (like THHN in conduit). Most modern commercial and residential panels feature 75°C rated terminals.
• The 90°C Column: You can only use this column as your starting point for derating calculations (ambient temperature and bundling). Your final adjusted ampacity cannot exceed the 75°C or 60°C limit of the terminals it lands on.

Derating Factors: When the Base Chart Isn't Enough

The wire chart ampacity table assumes two ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled together in a raceway. When you deviate from these baselines, you must apply derating factors found in NEC 310.15(B) and Table 310.15(C)(1).

Derating math always starts at the 90°C column for THHN/THWN-2, regardless of your terminal temperature limits. This is the one major advantage of using 90°C wire: it gives you a thermal buffer to absorb derating penalties.

Worked Example: Bundling in Conduit

Imagine you are pulling four separate 20A non-continuous circuits through a single 1-inch EMT conduit. That gives you 8 current-carrying conductors (4 hot, 4 neutral; grounds do not count as CCCs).

1. Base Ampacity: You choose 12 AWG THHN. The 90°C base ampacity is 30A.
2. Derating Factor: According to NEC Table 310.15(C)(1), 7 to 9 current-carrying conductors require a 70% adjustment factor.
3. Calculation: 30A × 0.70 = 21A.
4. Terminal Check: The adjusted ampacity (21A) is greater than the 20A breaker. However, you must also verify that 21A does not exceed the 60°C or 75°C terminal limits. The 60°C limit for 12 AWG is 20A. Because 21A > 20A, this installation is compliant for a 20A non-continuous load.

If you were running 10 CCCs in that same conduit, 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. For deeper dives into complex bundling scenarios, industry resources like EC&M's National Electrical Code hub provide excellent visual breakdowns of conduit fill and derating matrices.

What the Ampacity Table Cannot Tell You

While Table 310.16 is the bible for thermal limits, it is blind to several critical real-world physics constraints. Relying solely on ampacity will result in functional failures, even if you pass the rough-in inspection.

1. Voltage Drop

Ampacity measures how much heat the wire can safely dissipate before the insulation melts. It does not measure how much voltage is lost over distance due to conductor resistance. NEC 310.15(B) includes an Informational Note recommending a maximum 3% voltage drop on branch circuits and 5% on the combined feeder and branch.

Example: A 12 AWG copper wire on a 20A breaker is perfectly compliant for ampacity. But if that circuit runs 150 feet to a window AC unit drawing 15A, you will experience roughly a 4.7% voltage drop. The compressor will struggle, draw higher amperage to compensate, and potentially burn out. In this scenario, you must upsize to 8 AWG or 6 AWG strictly for voltage drop mitigation, even though the ampacity table says 12 AWG is 'legal'.

2. Short-Circuit Withstand Ratings

The ampacity table assumes normal operating conditions. It does not tell you if the wire can survive the massive thermal and magnetic forces of a short circuit before the breaker trips. For high-available-fault-current services (like a 200A panel close to the utility transformer), smaller gauge wires might vaporize before a standard thermal-magnetic breaker clears the fault in the required milliseconds.

3. Local AHJ Overrides

The NEC is a model code. Your local Authority Having Jurisdiction (AHJ) or municipal inspector has the final legal say. Some jurisdictions mandate minimum wire sizes regardless of calculated loads (e.g., banning 14 AWG entirely and requiring 12 AWG minimum for all 15A and 20A receptacle circuits). Always check local amendments before buying wire in bulk.