Selected copper table values
How do you calculate wire size?
The size of an electrical conductor refers to its cross-sectional area (measured in circular mils (CM) or AWG in the US). The larger the cross-sectional area, the more current the wire can transmit with lower resistance—very similar to water flowing through a wider pipe with less pressure loss. We derive the required conductor area from Ohm's law and conductor resistivity:
where:
- V_drop – Voltage drop calculated between the supply source and the farthest end of the wire, measured in volts (V_drop = V_source × (% drop ÷ 100));
- I – Maximum current running through the conductor, measured in amps (convert power using the watts to amps calculator);
- K – Specific resistance constant of the conductor material at 75°C (12.9 Ω·CM/ft for copper and 21.2 Ω·CM/ft for aluminum);
- L – One-way length of the wire run from panel to load, measured in feet; and
- CM – Cross-sectional area of the conductor, measured in circular mils (where 1,000 CM = 1 kcmil).
Calculating wire sizes for three-phase AC
For a balanced three-phase AC circuit, three current-carrying phase conductors are used instead of a two-wire loop. The calculator accepts line-to-line voltage and line current, modifying the required area of a single conductor to:
The factor of √3 ≈ 1.732 converts between phase and line quantities in a balanced three-phase system, replacing the factor of 2. The calculator automatically selects the next AWG size up from its listed 14 AWG – 4/0 table so the voltage-drop limit is never exceeded. To check apparent power feeding that circuit, use the kVA to amps calculator.
| Copper AWG | 60°C column | 75°C column | 90°C column |
|---|---|---|---|
| 14 | 15 A | 20 A | 25 A |
| 12 | 20 A | 25 A | 30 A |
| 10 | 30 A | 35 A | 40 A |
| 8 | 40 A | 50 A | 55 A |
| 6 | 55 A | 65 A | 75 A |
| 4 | 70 A | 85 A | 95 A |
Why three columns exist
The columns represent conductor temperature ratings under the table's stated conditions. The complete installation may be limited by equipment terminations or another applicable rule.
The 90°C column is often important for correction or adjustment calculations, but the final allowable ampacity can still be capped by the terminal rating. That workflow requires professional code application.
Four markings to record
- 01
Conductor insulation
Record the exact type and temperature marking.
- 02
Equipment terminals
Check the temperature and conductor-material markings at both ends.
- 03
Ambient condition
Document temperature around the wiring method rather than assuming 30°C.
- 04
Conductor grouping
Count current-carrying conductors under the applicable rules.
Do not use this table alone
- Small-conductor overcurrent limitations may apply
- Continuous and special loads can change required ampacity
- Ambient correction may reduce the value
- More than three current-carrying conductors can require adjustment
- Equipment and local rules can impose additional limits