Compare equal power, not equal amperage
Voltage alone never determines wire gauge. Start with real power in watts. In a resistive single-phase circuit, current equals power divided by voltage: a 3,600 W heater draws 30 A at 120 V, but only 15 A at 240 V. Cutting current in half while doubling system voltage reduces percentage voltage drop by a factor of four on the same wire.
Only connect equipment to the exact supply voltage stamped on its nameplate. This comparison explains how voltage affects conductor math, not how to rewire a 120 V appliance.
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.
Same 3,600 W load at two voltages
Look at the 120 V row: it carries twice the current (30 A vs. 15 A) and allows only half the voltage loss (3.6 V vs. 7.2 V) to stay within 3%. That fourfold difference pushes the 100 ft 120 V run to 6 AWG copper, whereas the 240 V run meets the drop target with 10 AWG.
Still, a voltage-drop candidate is only one piece of the circuit design. Overcurrent protection, continuous-load multipliers, and terminal temperature limits must all pass inspection.
| System | Calculated current | 3% allowed volts | Voltage-drop candidate | Estimated drop |
|---|---|---|---|---|
| 120 V | 30 A | 3.6 V | 6 AWG | About 2.46% |
| 240 V | 15 A | 7.2 V | 10 AWG | About 1.55% |
The five inputs that must stay visible
- 01
Equipment voltage
Use only the voltage shown by the equipment and approved design.
- 02
Actual design current
Use the applicable load calculation or nameplate information, not a guessed conversion.
- 03
One-way distance
Measure source to load. The calculator applies the circuit multiplier.
- 04
Material and phase
Copper versus aluminum and single-phase versus balanced three-phase change the formula.
- 05
Selected drop target
Record the percentage as a planning assumption; the tool does not decide what governs the project.
When 240 V can help, and when it does not
- Higher voltage reduces current for an equal-power dual-voltage load.
- Lower current cuts calculated conductor voltage drop over long runs.
- A 240 V circuit requires a two-pole breaker, proper grounding, disconnects, and 240 V listed equipment.
- Dual-voltage 120/240 V appliances need a neutral conductor and a complete wiring-diagram review.
- Motors, welders, and HVAC compressors follow dedicated NEC sizing articles rather than simple watts-divided-by-volts math.
- Existing branch-circuit cable must be checked for conductor count, insulation condition, and breaker compatibility before reuse.
Use the two calculators as a cross-check
First use the wire-size calculator to obtain a candidate based on the selected voltage-drop limit. Then enter that AWG in the voltage-drop calculator with the same material, phase, current and distance. Saving both results makes the assumptions auditable.
If a professional result differs, compare the load method, voltage, distance, temperature, conductor count, terminals and equipment rules. A difference does not mean one calculation is automatically wrong; it often means the simplified example omitted a governing condition.