Continuous Load + 125% Rule
Also known as: 80% rule125% rulecontinuous dutyNEC 210.19
The NEC defines a "continuous load" as any load expected to run for 3 hours or longer. Examples: EV chargers, electric heat strips, motors running continuously, HVAC blower fans. The corresponding sizing rule (NEC 210.19, 215.2, 230.42): circuits and equipment serving continuous loads must be sized at 125% of the continuous load.
Practical effect: a 48A continuous EV charger needs a 48 × 1.25 = 60A breaker and 60A-rated wire. You cannot put a 48A charger on a 50A breaker even though the math says 50A > 48A — the 125% rule says no. Same logic: the main service breaker on your panel can only safely carry 80% of its rating continuously (100A breaker, 80A continuous max).
This is the math behind "your 100A panel can only really carry 80A continuous." Combined with the code's demand factors, it's what determines whether your panel handles a new EV charger or heat pump — not the nameplate rating alone.
When we run a load calculation for a panel, we show both numbers: the calculated peak demand and the 80% threshold of the main breaker. If your calculated demand is below 80% of the main, you have headroom for more load. If it's close to or above, we recommend the upgrade.
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