A twenty-amp circuit is not a twenty-amp circuit for every purpose. For loads that run for extended periods it is a sixteen-amp circuit, and understanding why explains a rule that otherwise looks arbitrary.

A load is treated as continuous when the maximum current is expected to persist for three hours or more. Lighting in a workshop, an electric heater running through a winter evening, an EV charger working overnight and a well pump on a long cycle all qualify. A toaster does not.

Where a load is continuous, the conductor and the overcurrent device have to be sized at a hundred and twenty-five percent of that load. Turned around, this means the continuous load may not exceed eighty percent of the device rating, which is where the familiar shorthand comes from.

The physical reason is heat, and specifically the fact that a breaker sits in an enclosure alongside other breakers and warms up over time. Thermal-magnetic devices respond to temperature, and a breaker running at its full rating in a warm panel for hours behaves differently from the same breaker seeing that current for two minutes. Derating gives it margin.

The same reasoning explains why an EV charger rated at forty amps is installed on a fifty-amp circuit rather than a forty. Charging is the textbook continuous load, running at a steady current for many hours, and it is the appliance that has made this rule familiar to homeowners who never encountered it before.

Mixed circuits use the higher of two calculations, the sum of non-continuous load plus a hundred and twenty-five percent of the continuous portion. In residential work most branch circuits carry intermittent loads and the question rarely arises, which is exactly why it gets overlooked when a heater or a charger is added to an existing circuit.

The mistake it prevents is the quiet one, a circuit that never trips but runs warm every evening for a decade.

This is also the reason existing circuits are a poor place to attach new long-running loads. A garage receptacle that has served occasional tool use for years is not automatically suitable for a charger drawing steady current every night, even though the plug fits and the breaker holds for the first few weeks.

Background reading: originally posted on Wirsuchenjobs