For standard residential copper branch circuits, the direct answers are: use 14 AWG for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 55A (or 65A with 75°C terminations). This amp size wire chart is based on the copper ampacity limits defined in NFPA 70 (National Electrical Code) Table 310.16. However, pulling a wire size straight from a chart without accounting for termination temperatures, conduit fill, and continuous loads is how terminals melt and breakers trip. Below is the definitive reference to size your copper conductors correctly the first time.
How to Read the Amp Size Wire Chart (Which Column Applies?)
The most common mistake DIYers and junior apprentices make is looking at the 90°C column because it offers the highest ampacity. In practice, you are almost never allowed to use the 90°C column for your final breaker sizing. Here is how to determine which column applies to your installation:
- The 60°C Column: Under NEC 110.14(C), circuits rated 100A or less using 14, 12, or 10 AWG wire must be sized using the 60°C column, regardless of the wire's actual insulation rating. This applies to standard NM-B (Romex) cable and most standard receptacles.
- The 75°C Column: For circuits over 100A, or for wires 8 AWG and larger on circuits 100A or less, you can use the 75°C column—provided the equipment terminations (breakers, lugs, receptacles) are explicitly rated for 75°C. Most modern THHN/THWN-2 wire and standard breakers meet this requirement.
- The 90°C Column: This column is only used as a mathematical starting point for derating calculations (e.g., when bundling multiple wires in a conduit). The final ampacity after derating must still be compared against the 60°C or 75°C base limits.
The Master Amp Size Wire Chart (NEC Table 310.16)
The following spec-sheet-table provides the allowable ampacities for insulated copper conductors rated up to 2000 volts, in an ambient temperature of 30°C (86°F). Source standard: NFPA 70 (NEC) Table 310.16. Bookmark the quick-jump rows for the most queried residential sizes.
| AWG / kcmil | 60°C (TW, UF) | 75°C (THW, THWN, XHHW) | 90°C (THHN, THWN-2) | Common Application |
|---|---|---|---|---|
| 14 AWG | 15A | 20A | 25A | 15A Lighting/Receptacle Branch (NM-B) |
| 12 AWG | 20A | 25A | 30A | 20A Kitchen/Bathroom/Laundry Branch |
| 10 AWG | 30A | 35A | 40A | 30A Dryer / Water Heater / RV Outlet |
| 8 AWG | 40A | 50A | 55A | 40A-50A Range / EV Charger (Short Run) |
| 6 AWG | 55A | 65A | 75A | 50A-60A EV Charger / Subpanel Feeder |
| 4 AWG | 70A | 85A | 95A | 70A-85A Subpanel Feeder |
| 3 AWG | 85A | 100A | 110A | 100A Subpanel Feeder |
| 2 AWG | 95A | 115A | 130A | 100A-125A Heavy Feeder |
| 1 AWG | 110A | 130A | 145A | 125A Subpanel / Service Entrance |
| 1/0 AWG | 125A | 150A | 170A | 150A Service Entrance |
| 2/0 AWG | 145A | 175A | 195A | 150A-200A Service Entrance |
| 3/0 AWG | 165A | 200A | 225A | 200A Residential Service Entrance |
| 4/0 AWG | 195A | 230A | 260A | 200A-225A Heavy Service / Long Runs |
Derating: How Bundling Modifies Your Base Value
The ampacities in the chart above assume you have no more than three current-carrying conductors (hot, neutral, and one more hot or neutral) in a single raceway or cable. If you bundle more wires together, they cannot dissipate heat effectively. Under NEC 310.15(C)(1), you must apply a derating factor to the 90°C column to find your new allowable ampacity.
How derating rows modify the base value:
- 4 to 6 conductors: Multiply 90°C ampacity by 80%
- 7 to 9 conductors: Multiply 90°C ampacity by 70%
- 10 to 20 conductors: Multiply 90°C ampacity by 50%
Decision Path: Pick Your Wire Size in 4 Steps
Use this decision-tree-table to terminate your sizing process with one concrete pick. Never guess; follow the math.
| Step | Action & Rule | Concrete Example (24A Continuous Load) |
|---|---|---|
| 1. Calculate True Load | Is the load continuous (on for 3+ hours)? If yes, multiply by 1.25 (125%). If no, use base load. | 24A × 1.25 = 30A minimum circuit ampacity. |
| 2. Find Base Wire Size | Look at the 60°C column (for 14-10 AWG) or 75°C column (8 AWG+). Pick the first wire that meets/exceeds Step 1. | 30A requires 10 AWG (60°C column = 30A) or 8 AWG (75°C column = 40A). Let's select 10 AWG THHN. |
| 3. Apply Derating | Count current-carrying wires in the conduit. If >3, multiply the 90°C column value by the derating factor. | Assume 4 current-carrying wires. 10 AWG 90°C is 40A. 40A × 0.80 = 32A derated ampacity. |
| 4. Final Selection | Compare Step 2 and Step 3. The wire must satisfy both. Select the standard breaker size (NEC 240.4(B)) that does not exceed the lowest ampacity found. | 32A (derated) > 30A (required). 10 AWG THHN passes. Terminate on a 30A breaker. |
What This Chart Cannot Tell You (Voltage Drop & Local Code)
An amp size wire chart dictates the thermal limits of the insulation and the copper, but it completely ignores voltage drop. According to the Copper Development Association and NEC informational note 310.15(B), you should limit voltage drop to 3% on branch circuits and 5% overall to ensure equipment operates efficiently and motors don't overheat.
If you are running a 20A circuit using 12 AWG copper to a detached garage 150 feet away, the chart says 12 AWG is thermally safe for 20A. However, the voltage drop at 150 feet on a 120V circuit will be roughly 6.4V (over 5%). In this scenario, the chart fails you. You must bump up to 10 AWG (dropping the loss to ~4%) or 8 AWG (dropping it to ~2.5%) purely for voltage stability, even though a 20A breaker protects it.
Final Rule of Thumb: Always use the amp size wire chart to establish your absolute minimum AWG based on thermal limits and breaker sizing. Then, run a voltage drop calculation based on your exact one-way wire length. If the drop exceeds 3%, increase your wire size by one or two AWG steps regardless of what the ampacity chart permits. Always verify final conduit fill ratios and grounding requirements with your local electrical inspector, as municipal amendments frequently override baseline NEC tables.
