Equal-power comparison
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.
| System | Current | Allowed volts lost | Formula candidate |
|---|---|---|---|
| 12 V | 40 A | 0.36 V | 1/0 AWG |
| 24 V | 20 A | 0.72 V | 6 AWG |
| 48 V | 10 A | 1.44 V | 12 AWG |
Why low voltage is demanding
At 12 V, a 0.36 V loss is already 3%. The equal-power 12 V circuit also carries four times the current of the 48 V example. Both effects increase the area required by the simplified formula.
This comparison is not permission to change system voltage. The source, load, controls, charging equipment, insulation, connectors and protective devices must all be designed for the selected DC system.
Inputs to confirm
- 01
Maximum operating current
Use the applicable equipment or design value, including expected operating conditions.
- 02
Actual source voltage
Nominal battery labels may not equal minimum operating voltage.
- 03
Round-trip path
Enter one-way length only when the tool applies the return multiplier.
- 04
DC-rated protection
Verify overcurrent and disconnect devices are suitable for DC voltage and available fault current.
Do not reuse automotive charts blindly
- Chassis-return assumptions may not match a two-conductor run
- Insulation temperature and bundling conditions differ
- Voltage-drop targets vary by equipment sensitivity
- Marine, solar, vehicle and building installations can follow different standards
- Connector losses are not represented by a simple conductor formula