Four values people call cable size
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.
| Term | What it describes | Used for |
|---|---|---|
| AWG | Nominal individual conductor size | Area and electrical calculations |
| Circular mils | Conductor cross-sectional area | Voltage-drop formula |
| Conductor count | Number and function of conductors in an assembly | Circuit and installation selection |
| Outside diameter | Physical finished cable or conductor dimension | Raceway fill, fittings and handling |
Why an AWG answer is not a cable selection
The Calc for Trades result identifies an AWG candidate by voltage drop. It does not choose insulation, jacket, conductor count, grounding conductor, wet-location rating, sunlight resistance, burial suitability or raceway method.
Two products with the same conductor AWG can have different outside dimensions and listings. Use the exact manufacturer data after the electrical design is verified.
Translate the project request
- 01
Identify the circuit
Record voltage, phase, load and protection.
- 02
Size each required conductor
Separate ungrounded, grounded and equipment-grounding functions as applicable.
- 03
Select a listed wiring method
Match location, environment and equipment.
- 04
Check physical installation
Use product dimensions for raceway, fittings, bending and supports.
Questions to ask before ordering
- Is the requested value conductor AWG or cable outside diameter?
- How many conductors and which functions are required?
- Is copper, aluminum or copper-clad aluminum permitted?
- What environment and wiring method apply?
- Are every connector and terminal listed for the product?