The correct wire size for your circuit depends on the load amperage, insulation temperature rating, and installation method, as governed by NEC Table 310.16. For a standard residential 20A circuit, you need 12 AWG copper; for 30A, 10 AWG; for 50A, 6 AWG. However, picking the right wire goes beyond matching a single number to a breaker size. You must account for termination temperature limits, conduit fill derating, and specific overcurrent protection rules for small conductors.

This reference guide provides the exact ampacity values you need, explains how to read the temperature columns, and highlights the real-world jobsite constraints that the base chart ignores.

How to Read the NEC Wire Chart for Amperage

Before pulling wire, you need to understand how to read the ampacity table. The chart below is based on NEC Table 310.16 for copper conductors. It is divided into three temperature columns: 60°C, 75°C, and 90°C. These columns represent the maximum allowable ampacity based on the thermal limits of the wire's insulation and the environment it is installed in.

Bookmark this section: The most queried residential sizes (12, 10, 8, and 6 AWG) are highlighted and anchored below for quick jobsite lookups. Note that these values assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway.

NEC Table 310.16: Copper Conductor Ampacity (Standard Residential Sizes)
AWG / kcmil 60°C (140°F)
NM-B, UF-B		 75°C (167°F)
SER, THWN (Wet)		 90°C (194°F)
THHN (Dry Conduit)
14 AWG 15A* 20A* 25A*
12 AWG 20A* 25A* 30A*
10 AWG 30A* 35A* 40A*
8 AWG 40A 50A 55A
6 AWG 55A 65A 75A
4 AWG 70A 85A 95A
3 AWG 85A 100A 110A
2 AWG 95A 115A 130A
1 AWG 110A 130A 145A
1/0 AWG 125A 150A 170A

*Crucial Safety Note: NEC 240.4(D) strictly limits overcurrent protection for small conductors. Regardless of the 75°C or 90°C column values, 14 AWG is capped at a 15A breaker, 12 AWG at 20A, and 10 AWG at 30A. You cannot put 12 AWG wire on a 25A or 30A breaker just because the 90°C column says it can handle the heat.

Which Temperature Column Applies to Your Installation

The most common mistake DIYers and junior apprentices make is using the 90°C column simply because they bought 90°C rated THHN wire. To determine your actual allowable ampacity, you must apply the "weakest link" rule defined in NEC 110.14(C). The ampacity of your circuit is limited by the lowest temperature rating of any connected component, including the breaker terminals, lugs, and the wire insulation itself.

The 60°C Column: Use this for Non-Metallic Sheathed Cable (NM-B, commonly known as Romex) and Underground Feeder (UF-B). Even if your breaker terminals are rated 75°C, NEC 334.80 mandates that NM-B ampacity must be determined using the 60°C column. A 6 AWG NM-B cable is strictly limited to 55A, not 65A.
The 75°C Column: Use this for Service Entrance Rated cable (SER), USE-2, and individual THWN conductors in wet locations. Most modern breakers and panelboard lugs (rated 100A and below) are tested and listed for 75°C terminations. Therefore, your final allowable ampacity for individual wires in conduit usually maxes out here.
The 90°C Column: Use this only as the starting baseline for derating calculations (ambient temperature or conduit fill). You cannot use the 90°C ampacity for your final breaker sizing unless every single termination in the circuit is explicitly listed and marked for 90°C, which is exceptionally rare in residential gear.

Derating Factors: When the Base Chart Value Changes

The base wire chart for amperage assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you exceed these conditions, the wires trap heat, and you must apply derating factors to the 90°C column to find your new adjusted ampacity.

Conduit Fill Derating (NEC 310.15(C)(1))

If you pull four or more current-carrying conductors through a single raceway (like PVC or EMT conduit), you must multiply the 90°C base ampacity by the adjustment factor below. Note that equipment grounding conductors (bare or green) do not count toward this total.

Number of Current-Carrying Conductors Adjustment Factor
1 through 3 100% (No derating)
4 through 6 80%
7 through 9 70%
10 through 20 50%

Worked Example: You are running two 20A multi-wire branch circuits (MWBC) in a single 3/4" EMT conduit. This means you have 4 current-carrying conductors (two hots, two neutrals). You are using 12 AWG THHN.
1. Base 90°C ampacity for 12 AWG = 30A.
2. Derating factor for 4-6 conductors = 80%.
3. Adjusted ampacity = 30A × 0.80 = 24A.
4. Check termination limits: The 75°C termination limit for 12 AWG is 25A. Since 24A is less than 25A, the wire is thermally safe. However, per NEC 240.4(D), the breaker cannot exceed 20A. Since 24A > 20A, the installation is fully code-compliant.

What the Ampacity Table Cannot Tell You

Relying solely on NEC Table 310.16 will get you past a rough-in inspection, but it won't guarantee a high-performance or physically installable system. Here is what the chart leaves out:

1. Voltage Drop Over Distance

The ampacity table assumes a short run where resistance is negligible. If you are feeding a 50A detached garage subpanel 150 feet away using 6 AWG copper, the wire is legally sized for the ampacity (65A at 75°C). However, at 50A, that 150-foot run will experience a voltage drop of roughly 4.2 volts (about 3.5%). While not a strict NEC violation for branch circuits, it can cause motors to overheat and lights to dim. For runs over 100 feet, use the Southwire Voltage Drop Calculator and plan to upsized to 4 AWG or 3 AWG to keep the drop under 3%.

2. Physical Termination Constraints

The chart tells you that 4 AWG copper is rated for 85A, making it perfect for a 70A or 80A subpanel feeder. What it doesn't tell you is that the lugs on many standard 70A/80A residential breakers and disconnect switches are physically too small to accept 4 AWG stranded or solid wire. In 2026, with copper prices hovering around $1.40/ft for 4 AWG THHN, discovering you need to buy a $45 lug upgrade kit or pigtail down to 6 AWG inside the panel is a frustrating jobsite delay. Always check the manufacturer's lug sizing datasheet before purchasing heavy-gauge wire.

3. Short-Circuit Withstand Ratings

Ampacity measures continuous thermal loading, not fault conditions. If a dead short occurs, thousands of amps can flow for the milliseconds before the breaker's magnetic trip engages. Smaller wires can literally vaporize or weld themselves to busbars before a standard thermal-magnetic breaker clears the fault. For service entrance conductors or high-AIC (Ampere Interrupting Capacity) environments, engineers must calculate let-through current to ensure the wire's thermal mass can survive the fault. The standard residential ampacity table offers zero guidance on this dynamic.