Wire Size Calculator: Find the Right Wire Gauge

How do I determine the required ampacity?

The required ampacity is typically 125% of the continuous load current. For motors, use 125% of the full-load current rating. Always round up to the next standard wire size.

What's the difference between copper and aluminum wire?

Copper has better conductivity and requires smaller wire sizes for the same current. Aluminum is lighter and less expensive but requires larger sizes. Aluminum connections need special consideration due to thermal expansion.

How does temperature affect wire sizing?

Higher temperature ratings allow higher current carrying capacity. However, terminal ratings often limit the usable ampacity. Most installations use 75°C ratings for safety.

Should I account for future expansion?

It's good practice to size wires with some capacity for future load growth. Consider using the next larger size when the calculation is close to a standard size.

What is voltage drop and why is it important?

Voltage drop is the reduction in voltage that occurs as current flows through a wire. Excessive voltage drop can cause equipment malfunction, reduced efficiency, and increased power consumption. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% for combined feeder and branch circuits.

How do I calculate conduit fill?

Conduit fill is based on the total cross-sectional area of all wires compared to the conduit's internal area. The NEC limits conduit fill to 40% for three or more conductors. Use conduit fill calculators or tables to ensure compliance.

What are THHN, THWN, and other wire insulation types?

These designations indicate the wire's insulation properties: - THHN: Thermoplastic High Heat-resistant Nylon-coated - THWN: Thermoplastic Heat and Water-resistant Nylon-coated - XHHW: Cross-linked High Heat-resistant Water-resistant Choose the appropriate type based on installation environment and local codes.

When should I use stranded vs. solid wire?

Use stranded wire for flexible applications and sizes larger than 10 AWG. Solid wire is typically used for smaller sizes (14-10 AWG) in residential wiring where flexibility is less important. Stranded wire is better for applications with vibration or movement.

How do I handle parallel conductors?

Parallel conductors must be the same length, material, and size. They're typically used for high current applications where single conductors would be too large to handle. Each conductor must be sized to carry its share of the total current.

What's the difference between wire size and circuit breaker size?

Circuit breaker size must match or be smaller than the wire's ampacity rating. For example, 14 AWG copper wire is rated for 15A, so it must use a 15A or smaller breaker. Never use a larger breaker than the wire's rating.

How do I account for ambient temperature?

Wire ampacity must be derated for high ambient temperatures. Use temperature correction factors from the NEC when ambient temperature exceeds 30°C (86°F). For example, at 40°C (104°F), multiply the base ampacity by 0.82.

What about wire sizing for motors?

Motor circuits require special consideration due to starting current. Size the wire for 125% of the motor's full-load current rating. The circuit breaker or fuse may be sized up to 250% of full-load current for motor starting.

How do I size ground wires?

Equipment grounding conductors are sized based on the circuit's overcurrent protection. For example, a 15A circuit requires a 14 AWG ground, while a 20A circuit needs a 12 AWG ground. Refer to NEC Table 250.122 for specific requirements.

What about voltage drop in DC circuits?

DC circuits use a similar voltage drop calculation but only consider one-way distance since current flows in a single direction. The formula is modified to remove the factor of 2 used in AC calculations.

How do environmental factors affect wire sizing?

Consider these environmental factors when sizing wires: - Ambient temperature - Exposure to sunlight - Wet locations - Underground installation - Proximity to other current-carrying conductors Each factor may require derating or special wire types.