Wire Size Calculator
Find the exact wire gauge you need for your specific electrical load.
1Load Specifications
Continuous load current
Usually 75°C for modern breakers
Sizing Logic
Load Requirement
Your load requires 40 Amps continuous
NEC 310.16 Lookup
Find smallest Copper wire @ 75C column
Ampacity Calculation
NEC 310.16 Allowable Ampacities
NEC Table 310.16 is the primary reference for conductor ampacity. It lists allowable currents for copper and aluminum conductors at different temperature ratings (60°C, 75°C, 90°C).
Frequently Asked Questions
What is the difference between 60°C, 75°C, and 90°C columns?
These columns represent the temperature rating of the wire insulation. However, you must typically use the 75°C column for terminations on breakers and devices, even if the wire is rated 90°C (like THHN).
Does THHN count as 90°C?
Yes, THHN is rated for 90°C in dry locations. This higher rating allows for derating adjustments, but the final ampacity is often limited to 75°C at the termination points.
When is a load considered continuous?
A load is continuous if it operates for maximum current for 3 hours or more. Continuous loads require the overcurrent device and conductors to be sized at 125% of the load.