The ampacity of wire—the maximum continuous current a conductor can carry without exceeding its insulation temperature rating—depends strictly on gauge, material, and insulation type. For standard residential copper branch circuits, the baseline ampacities are: 14 AWG is 15A, 12 AWG is 20A, and 10 AWG is 30A. These values are derived from the 60°C column of NEC Table 310.16, which governs terminations for circuits rated 100 amps or less.

However, pulling a single number from a chart without understanding the temperature columns and derating factors is how DIYers melt terminal lugs and trip breakers under load. Below is the complete reference data, the rules for selecting the correct column, and a concrete decision path to size your next run.

The Master Ampacity of Wire Chart (NEC Table 310.16)

The following table reproduces the most queried rows for copper and aluminum conductors based on the 2023/2024 National Electrical Code.

How to read this table: The columns are split by insulation temperature rating (60°C, 75°C, 90°C) and conductor material (Copper vs. Aluminum). The numbers represent the maximum allowable ampacity before any ambient temperature or bundling derating is applied. Always assume copper unless your wire is explicitly marked AL or ALUM.
Source: NEC Table 310.16 (formerly 310.15(B)(16)). Allowable Ampacities of Insulated Conductors.
Wire Size (AWG/kcmil) 60°C Copper (TW, UF) 75°C Copper (THHW, THWN) 90°C Copper (THHN, XHHW) 60°C Aluminum 75°C Aluminum 90°C Aluminum
14 AWG15A20A25A———
12 AWG20A25A30A15A20A25A
10 AWG30A35A40A25A30A35A
8 AWG40A50A55A30A40A45A
6 AWG55A65A75A40A50A60A
4 AWG70A85A95A55A65A75A
3 AWG85A100A115A65A75A85A
2 AWG95A115A130A75A90A100A
1 AWG110A130A145A85A100A115A
1/0 AWG125A150A170A100A120A135A
2/0 AWG145A175A195A115A135A150A
3/0 AWG165A200A225A130A155A175A
4/0 AWG195A230A260A150A180A205A

Which Temperature Column Actually Applies to Your Installation?

The most common mistake in wire sizing is looking at the 90°C column because it offers the highest ampacity, then sizing the breaker to that number. This violates NEC 110.14(C) termination rules and creates a fire hazard at the lugs.

  • The 60°C Column: You must use this column for the final breaker sizing if the circuit is rated 100 amps or less, or if the wire is 14 AWG through 1 AWG. This applies to 95% of residential branch circuits (outlets, lighting, standard appliances) because standard breakers and receptacles are only tested and rated for 60°C terminations.
  • The 75°C Column: You may use this column for final breaker sizing only if the circuit is rated over 100 amps (like a main service feeder or a large subpanel feed) AND the equipment terminals are explicitly marked for 75°C.
  • The 90°C Column: You never use this column to size your breaker. The 90°C column is used exclusively as the starting baseline for calculating derating factors (bundling and ambient temperature). Once derated, the final ampacity must still be capped at the 60°C or 75°C termination limit.

Derating: When the Base Ampacity Drops

The chart above assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) in a raceway. When you exceed these parameters, NEC Article 310 mandates derating.

Safety Rule: Neutral conductors that carry unbalanced current from a multi-wire branch circuit (MWBC) count as CCCs. Grounding wires and equipment grounding conductors (EGCs) never count as CCCs.

How derating modifies the base value:
Imagine you are pulling four 12 AWG THHN copper wires (two hots, two neutrals for two MWBCs) through a single conduit.

  1. Identify the 90°C baseline: 12 AWG THHN in the 90°C column is 30A.
  2. Apply the bundling factor: NEC Table 310.15(C)(1) states that 4 to 6 CCCs require an 80% adjustment factor.
  3. Calculate derated ampacity: 30A × 0.80 = 24A.
  4. Apply the termination cap: Because this is a <100A circuit, you must cap the final ampacity at the 60°C column value for 12 AWG, which is 20A.

Since 24A is greater than the 20A termination limit, you are legally permitted to use a standard 20A breaker. If you had 10 CCCs in the conduit (50% derating), the math would be 30A × 0.50 = 15A. Because 15A is less than the 20A termination limit, you would be forced to upsize to 10 AWG wire to maintain a 20A circuit.

Decision Tree: Sizing Your Wire and Breaker in 4 Steps

Use this decision path to terminate your sizing process with a concrete material list. Do not guess; follow the load calculation down the tree.

Step 1: Calculate Continuous Load Step 2: Apply 125% Safety Margin Step 3: Select Wire (60°C Col) Step 4: Final Concrete Pick (Wire + Breaker)
Lighting / Standard Receptacles (Max 12A expected) 12A × 1.25 = 15A 14 AWG (Rated 15A) Buy: 14/2 NM-B + 15A Standard Breaker
Kitchen Small Appliance / Bathroom (Max 16A expected) 16A × 1.25 = 20A 12 AWG (Rated 20A) Buy: 12/2 NM-B + 20A Standard Breaker
Electric Dryer / EV Charger Level 1 (Max 24A expected) 24A × 1.25 = 30A 10 AWG (Rated 30A) Buy: 10/3 NM-B (or THHN in conduit) + 30A Breaker
Electric Range / Oven (Max 40A expected) 40A × 1.25 = 50A 8 AWG Cu (Rated 40A at 60°C, but 50A at 75°C if terminals rated) Buy: 6/3 NM-B (Safest default) + 50A Breaker
100A Subpanel Feeder (Max 80A continuous) 80A × 1.25 = 100A 3 AWG Cu or 1/0 AWG Al (75°C column applies >100A) Buy: 1/0-1/0-1/0-2 SER Aluminum Cable + 100A Breaker

What the Ampacity Table Cannot Tell You

NEC Table 310.16 dictates thermal limits, but it ignores two critical physical realities that will ruin your installation if overlooked.

1. Voltage Drop

Ampacity tables assume the wire can handle the heat, but they do not guarantee the voltage will reach the load. NEC 210.19(A) Informational Note recommends a maximum 3% voltage drop on branch circuits and 5% total from service to outlet. If you are running a 12 AWG wire 150 feet to a 15A window AC unit at the end of a driveway, the wire won't melt (it's within ampacity limits), but the voltage will drop below 114V, causing the compressor to stall and burn out. Rule of thumb: For runs over 100 feet, upsize your wire by one gauge strictly for voltage drop, even if the ampacity chart says the smaller wire is thermally safe.

2. Conduit Fill and Physical Fit

You might calculate that three 4 AWG THHN wires are perfectly rated for a 60A circuit. However, if you try to pull them through a half-inch EMT conduit, you will violate NEC Chapter 9, Table 1, which limits conduit fill to 40% for three or more wires. Furthermore, bending three 4 AWG wires into a standard single-gang junction box will violate box fill calculations (NEC 314.16). Always cross-reference your ampacity pick with conduit fill tables and physical bend radius limitations before purchasing.