The electrical wire ampacity chart dictates the maximum continuous current a conductor can carry before its insulation begins to thermally degrade. For standard residential copper wire evaluated at the 60°C column, the baseline values are: 14 AWG is 15A, 12 AWG is 20A, 10 AWG is 30A, and 8 AWG is 40A. However, picking the right wire requires more than just matching the breaker size to the base chart. You must account for insulation temperature ratings, conduit fill derating, and terminal limitations as defined by the National Electrical Code (NEC).
How to Read the NEC Electrical Wire Ampacity Chart
The definitive source for wire sizing in the US is NEC Article 310.16 (formerly 310.15(B)(16)). Before looking at the numbers, you must understand how to read the columns and the critical exceptions that override them.
| AWG Size | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|
| 14 | 15A * | 20A * | 25A * | N/A | N/A |
| 12 | 20A * | 25A * | 30A * | N/A | N/A |
| 10 | 30A * | 35A | 40A | N/A | N/A |
| 8 | 40A | 50A | 55A | 40A | 45A |
| 6 | 55A | 65A | 75A | 50A | 55A |
| 4 | 70A | 85A | 95A | 65A | 75A |
| 3 | 85A | 100A | 110A | 75A | 85A |
| 2 | 95A | 115A | 130A | 90A | 100A |
| 1 | 110A | 130A | 145A | 100A | 115A |
| 1/0 | 125A | 150A | 170A | 120A | 135A |
| 2/0 | 145A | 175A | 195A | 135A | 150A |
| 3/0 | 165A | 200A | 225A | 155A | 170A |
| 4/0 | 195A | 230A | 260A | 180A | 205A |
* Note on small conductors: While the chart shows higher thermal limits for 14, 12, and 10 AWG at 75°C and 90°C, NEC 240.4(D) strictly limits the overcurrent protection (breaker size) for these small copper wires to 15A, 20A, and 30A respectively, regardless of the insulation's higher thermal capacity.
Applying Derating Factors to Base Ampacity
The numbers in the chart above assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. When your installation deviates from this baseline, you must apply derating factors. The 90°C column is primarily used as the starting point for these derating calculations, even if your final termination limits you to a lower column.
Conduit Fill Adjustment (More than 3 Wires)
When you pull four or more current-carrying conductors through a single conduit, the heat generated by the bundled wires cannot dissipate efficiently. According to NEC Table 310.15(C)(1), you must multiply the base 90°C ampacity by a correction factor.
- 4 to 6 conductors: 80% adjustment factor
- 7 to 9 conductors: 70% adjustment factor
- 10 to 20 conductors: 50% adjustment factor
Ambient Temperature Correction
If your conduit runs through an attic in Texas or a hot boiler room where ambient temperatures exceed 30°C, you must apply the temperature correction factors from NEC Table 310.15(B)(1). For example, if the ambient temperature is 45°C (113°F) and you are using 90°C THHN, you multiply the base ampacity by 0.87. If both high ambient temperature and conduit bundling apply, you multiply the base ampacity by both factors.
What This Electrical Wire Ampacity Chart Cannot Tell You
Relying solely on the ampacity chart is a common trap that leads to failed inspections or melted terminals. The chart only defines the thermal limit of the wire's insulation in free air or standard conduit. It completely ignores two critical real-world constraints:
- Terminal Temperature Limitations (NEC 110.14(C)): This is the 'weakest link' rule. Most residential breakers and receptacles are rated for 60°C (for circuits 100A and under) or 75°C (for circuits over 100A). Even if you pull 90°C THHN wire that charts at 95A for 4 AWG, you cannot terminate it on a 75°C rated breaker and claim 95A. The ampacity is capped at the 75°C column value (85A) at the point of termination.
- Voltage Drop: Ampacity is strictly about heat dissipation; it has nothing to do with voltage drop over distance. A 12 AWG wire might safely carry 20A according to the chart, but if you run it 150 feet to a shed, the resistance will cause a massive voltage drop that can damage motors and electronics. For runs exceeding 50 feet, always calculate voltage drop and upsize the wire accordingly, regardless of what the ampacity chart says.
Electrical Wire Ampacity Chart FAQ
What size wire do I need for a 50-amp breaker?
For a 50-amp breaker, you need 6 AWG copper wire (rated at 55A in the 60°C column or 65A in the 75°C column) or 4 AWG aluminum wire (rated at 65A in the 75°C column). Never use 8 AWG copper for a 50A breaker; while its 90°C rating is 55A, terminal limitations restrict it to the 75°C column (50A), leaving zero safety margin for continuous loads.
Can I use the 90°C column for breaker terminations?
Almost never in residential work. NEC 110.14(C) requires you to use the 60°C column for circuits rated 100A or less, and the 75°C column for circuits over 100A, unless the equipment is explicitly marked and tested for 90°C terminations. You only use the 90°C column to calculate derating adjustments for conduit fill or ambient temperature before checking the final termination limit.
How does bundling wires in a conduit change the ampacity?
When you bundle more than three current-carrying conductors in a single conduit, the trapped heat reduces the wire's ability to dissipate thermal energy. You must apply a derating multiplier (starting at 80% for 4-6 wires) to the 90°C base ampacity. Note that a neutral wire carrying only unbalanced load, or a grounding wire, does not count as a current-carrying conductor for this calculation.
Does voltage drop affect the wire ampacity chart values?
No. Ampacity measures thermal capacity (how much current before the insulation melts), while voltage drop measures electrical resistance over distance. The ampacity chart assumes a theoretical length. If your wire run is long enough to cause a voltage drop exceeding 3% for branch circuits or 5% for the total system, you must upsize the wire. For example, a 12 AWG wire is legally rated for 20A on the chart, but a 20A load at 200 feet requires upsizing to 8 AWG or 6 AWG to maintain acceptable voltage.
