The 80% Rule for Circuit Loading
The 80% rule is shorthand for the continuous-load requirement: a load expected to run for three hours or more must be limited to 80% of the circuit rating, which is the same as sizing the circuit at 125% of the load.
One rule, two ways of saying it
Sizing the circuit at 125% of a continuous load, and limiting a continuous load to 80% of the circuit rating, are the same statement from opposite ends.
A 20 A circuit carrying a continuous load is limited to 16 A. The breaker is still 20 A; what changes is how much load you may put on it.
What counts as continuous
The test is duration: three hours or more of maximum current. Lighting in a commercial space, EV charging, water heating and similar long-running loads are the usual examples.
If a load is not continuous, the 125% factor does not apply.
Frequently asked
- Does the 80% rule apply to a 15 A lighting circuit?
- It applies to the continuous portion of the load. If the load runs three hours or more, it is limited to 80% of the circuit rating.
- Does 125% change the wire or just the breaker?
- Both, effectively: the conductor must carry the larger calculated load, and the overcurrent device is sized from the same number.
- Is the 80% rule the same as the 40% conduit rule?
- No. The 40% is about physical space for conductors in a raceway; the 80% is about electrical loading over time.
Related calculators
Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.