What the calculator is selecting
The calculator converts the selected percentage limit into allowed volts, calculates the circular-mil area required by its stated formula and then chooses the next listed conductor with at least that area. Because AWG sizes are discrete, the selected conductor can be larger than the exact calculated area.
This answers one planning question: which listed size holds estimated voltage drop within the selected limit under the simplified inputs. It does not calculate every condition used to approve a conductor installation.
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.
Inputs to verify before trusting the comparison
- 01
Actual load current
Do not substitute breaker rating when the design load is different unless the applicable design method requires it.
- 02
One-way distance
Enter source to load. The selected circuit formula applies its own multiplier.
- 03
Copper or aluminum
Material changes the resistance constant and the calculated circular-mil requirement.
- 04
DC, single phase or balanced three phase
Choose the circuit type explicitly because the multiplier changes.
- 05
Maximum voltage drop
Record the chosen planning limit; the calculator does not decide which limit governs a specific project.
Worked example: why 10 AWG moves to 8 AWG
For 240 V, 30 A, a 100 ft one-way copper run and a 3% planning limit, allowed drop is 7.2 V. The stated DC/single-phase formula produces a requirement of approximately 10,750 circular mils.
10 AWG is approximately 10,380 circular mils, which falls below the calculated requirement. The calculator therefore moves to 8 AWG at approximately 16,510 circular mils. Rechecking the selected size gives an estimated drop near 1.95% under the same model.
| Conductor / target | Circular mils | Check vs. 10,750 CM requirement |
|---|---|---|
| Calculated minimum | 10,750 CM | Target for 7.2 V allowed drop |
| 10 AWG copper | 10,380 CM | 370 CM short, fails 3% limit |
| 8 AWG copper | 16,510 CM | Selected candidate (~1.95% drop) |
Why the final installed size may differ
- Ampacity and continuous-load rules may require a larger conductor.
- Temperature, bundling and the number of current-carrying conductors can require adjustment.
- Terminals and equipment listings can limit conductor material and size.
- Motors, starting current, power factor and nonlinear loads need fuller analysis.
- Parallel or larger-than-4/0 conductors are outside this tool's range.
- Local rules, permits and inspection requirements still apply.
Pair sizing with a voltage-drop check
After the calculator selects a candidate AWG, use the voltage-drop calculator with that conductor and the same inputs. This makes the two steps visible: first determine required area, then estimate the performance of the selected standard size.
Save both results with the input assumptions. If a qualified professional specifies a different conductor, compare the load, distance, phase, material, temperature and installation conditions rather than assuming the arithmetic is the only difference.