Select the conductor size from the table and enter a combined derating factor as a percent. The base ampacity comes from the National Electrical Code table for the insulation temperature rating and conductor material. Derating factors reduce ampacity for high ambient temperature and for more than three current-carrying conductors in a raceway or cable, and factors multiply together.
Use the rating of the lowest-rated part of the circuit, including terminals.
This is an adjusted reference value, not a final conductor or breaker selection. Apply the actual installation method, permitted correction starting column, terminal limits and equipment rules. The general continuous-load sizing rule has conditions, including permitted applications of listed 100%-rated assemblies.
The National Electrical Code edition and local requirements govern the actual circuit.
Base table ampacity (amps) from conductor
| Conductor | 12 AWG copper (25A @ 75°C) |
|---|---|
| Combined derating factor | 100.0% |
| Base table ampacity (amps) | 25 |
| Derated ampacity (amps) | 25 |
Common error: Using the base table value for a bundled cable in a hot space. The derated ampacity can be well below it, and the breaker must be sized to the lower number.
Wire Ampacity Sizing: formula
NFPA 70: National Electrical Code · Schneider Electric: standard and 100%-rated circuit breakers
Wire Ampacity Sizing: changing Conductor
| Conductor | Base table ampacity (amps) | Derated ampacity (amps) |
|---|---|---|
| 20 | 20 | 20 |
| 25 (example) | 25 | 25 |
| 35 | 35 | 35 |
| 50 | 50 | 50 |
| 65 | 65 | 65 |
| 85 | 85 | 85 |
| 100 | 100 | 100 |
| 130 | 130 | 130 |
Wire Ampacity Sizing: compare input ranges
| Input | Tested from | To | Base table ampacity (amps): low input | Base table ampacity (amps): high input | Swing |
|---|---|---|---|---|---|
| Conductor | 14 AWG copper (20A @ 75°C) | 1 AWG copper (130A @ 75°C) | 20 | 130 | 110 (260%) |
| Combined derating factor | 98.0% | 102.0% | 25 | 25 | 0 |
Wire Ampacity Sizing: individual input comparisons
| Input | Value | Base table ampacity (amps) | Derated ampacity (amps) |
|---|---|---|---|
| Conductor | 14 AWG copper (20A @ 75°C) | 20 | 20 |
| 12 AWG copper (25A @ 75°C) | 25 | 25 | |
| 10 AWG copper (35A @ 75°C) | 35 | 35 | |
| 8 AWG copper (50A @ 75°C) | 50 | 50 | |
| 6 AWG copper (65A @ 75°C) | 65 | 65 | |
| 4 AWG copper (85A @ 75°C) | 85 | 85 | |
| 3 AWG copper (100A @ 75°C) | 100 | 100 | |
| 1 AWG copper (130A @ 75°C) | 130 | 130 | |
| Combined derating factor | 96.0% | 25 | 24 |
| 98.0% | 25 | 24.5 | |
| 100.0% | 25 | 25 | |
| 102.0% | 25 | 25.5 | |
| 104.0% | 25 | 26 |
Wire Ampacity Sizing: values and definitions
| Quantity and definition | Value used |
|---|---|
| Conductor — Copper conductor size and its NEC Table 310.16 base ampacity at 75°C. | 12 AWG copper (25A @ 75°C) |
| Combined derating factor — Combined temperature-correction and conductor-bundling adjustment factor from NEC 310.15(B), as a percent of the base ampacity. | 100.0% |
| Base table ampacity (amps) | 25 |
| Derated ampacity (amps) | 25 |
A wire's rated ampacity is the current it can carry without overheating under stated conditions, and real conditions usually call for a reduction. A conductor with a base ampacity of 25 amps keeps that value only with a derating factor of 100 percent. Bundling more than three current-carrying conductors in a raceway, high ambient temperatures and insulation limits all reduce it, and four to six conductors commonly bring an 80 percent factor.
The result is what the wire may carry.
Apply every derating that applies and then check the result against the breaker. The wire, not the breaker, sets the safe current, and the terminals set a limit too.
See conduit capacity on the conduit fill calculator, voltage loss on the voltage drop page and current from watts on the watts to amps calculator. The three-phase power and transformer sizing pages cover larger systems, and the Ohm's law calculator gives basic relationships.
Does derated ampacity alone determine the breaker?
No. Small-conductor limits, terminal ratings, continuous-load requirements and equipment-specific code provisions also apply.
Are these aluminum-conductor values?
No. The selector is explicitly the copper-conductor 75°C reference column.