Base table conditions matter
Public Table 310.16 material identifies ampacities for not more than three current-carrying conductors in a raceway, cable or earth and points users to correction and adjustment provisions. Using the table outside those assumptions requires additional work.
Derating is not an extra percentage to subtract from the voltage-drop result. It belongs to the ampacity calculation; voltage drop remains a separate check.
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.
Conditions that trigger a review
| Condition | Why it matters | Information needed |
|---|---|---|
| Ambient above table basis | Conductor sheds heat less effectively | Temperature and insulation rating |
| More current-carrying conductors | Combined heating increases | Circuit arrangement and conductor count |
| Rooftop or hot location | Local temperature can be higher | Actual installation environment |
| Mixed terminal ratings | Lowest applicable limit can control | Both endpoint markings |
Professional calculation workflow
- 01
Choose the applicable base ampacity
Use the correct material, size, insulation and conditions.
- 02
Apply required correction
Evaluate ambient temperature using the applicable factor.
- 03
Apply required adjustment
Evaluate current-carrying conductor count and arrangement.
- 04
Respect terminal limits
Confirm the final value does not exceed applicable equipment limitations.
- 05
Check voltage drop separately
Run the resulting candidate with actual circuit inputs.
Information a calculator cannot infer
- Which conductors count as current carrying
- Actual ambient temperature along the route
- Whether special wiring-method provisions apply
- Terminal ratings at both ends
- Which jurisdiction and code edition govern