The 125% rule is a requirement for sizing electrical conductors and overcurrent protective devices for continuous loads.
A continuous load is an electrical load where the maximum current is expected to continue for 3 hours or more. When current flows for that long, conductors and breakers can run hotter than they would under a short-duration load.
This extra heat can damage wire insulation, nuisance-trip a breaker, or shorten equipment life. To reduce that risk, the National Electrical Code requires the conductor and its overcurrent protection to be sized larger than the actual continuous load.
Here is how it works:
Determine the continuous load current: Find the amperes the continuous load will draw.
Multiply by 125% (or 1.25): Take the continuous load current and multiply it by 1.25. This gives the new minimum sizing current.
Size components based on the new value: Conductors must be rated to carry the higher current, and the overcurrent protective device must be selected to protect those conductors.
Example:
If a continuous load draws 10 amps, then 10 amps × 1.25 = 12.5 amps. Select conductors that can safely carry at least 12.5 amps and select an overcurrent protective device that protects the conductor ampacity.
Where this shows up in the NEC:
NEC 210.20(A) addresses branch-circuit overcurrent protection for continuous loads. NEC 215.2(A)(1) addresses feeder conductors. NEC 230.42(A)(1) addresses service conductors. [Source: NEC 100 Continuous Load; Source: NEC 210.20(A); Source: NEC 215.2(A)(1); Source: NEC 230.42(A)(1)]
The answers we give you are for educational purposes only. Please verify with your code book, your journeyman or master electrician, and your local Authority Having Jurisdiction (AHJ).
Answer summary: The 125% rule means continuous load amps × 1.25. This protects conductors and overcurrent devices from long-duration heat by preventing the circuit from being sized exactly at the running current.