The electrician who looked at our garage said something I didn't understand at the time: the circuit was fine, but I should treat it as a twelve-amp circuit rather than a fifteen. I asked whether the breaker was weak or the wire undersized, because those were the only two explanations I had. Neither, he said. The circuit is exactly what it says it is. It's the way I was using it — a heater running all afternoon while I worked out there — that made the last three amps the wrong ones to plan on. That sent me looking for where the number twelve came from, and it turns out to be one of the most quoted and most misquoted figures in residential wiring.
The figure is 125 percent, seen from the other side
Nobody actually wrote a rule that says eighty percent. What the National Electrical Code, NFPA 70, says in Section 210.20(A) is that where a branch circuit supplies a continuous load, or a mix of continuous and noncontinuous loads, the overcurrent device must be rated at not less than the noncontinuous load plus 125 percent of the continuous load. Section 210.19(A)(1) applies the same 125 percent to the conductors, so the wire and the breaker are enlarged together rather than one outgrowing the other.
Divide instead of multiply and the familiar number appears: one divided by 1.25 is 0.8. Sizing a device at 125 percent of a load is arithmetically identical to loading it to 80 percent of its rating. Both sentences describe one requirement, which is why an electrician sizing a new circuit says 125 percent while a homeowner reading about an existing one hears eighty. Same rule, opposite ends of the telescope.
The reasoning is heat. Current through a breaker, and through the terminations at each end of the run, produces heat continuously, and the parts have to shed it as fast as it arrives. Short bursts are absorbed by the thermal mass of the metal. Hours of steady current are not, and the temperature climbs until it settles — at a level the margin is there to keep well below where insulation and terminations start to suffer. It's the same mechanism behind a wall plate that has gone noticeably warm.
Three hours is the line, and it's in the definitions
The word doing the work in both sections is continuous, and the Code defines it in Article 100 rather than leaving it to judgment: a continuous load is one where the maximum current is expected to continue for three hours or more. That threshold is the whole game, and it's where most kitchen-table versions of the rule go wrong.
A toaster pulls a large current for ninety seconds. A microwave runs for two minutes. Neither is a continuous load, and no part of the Code asks you to derate a circuit for them. Meanwhile a baseboard heater on a January afternoon, a car charging overnight, an aquarium heater, a window air conditioner during a heat wave, and grow lights on a twelve-hour timer all clear three hours without effort. The question isn't how much a device draws. It's how long it keeps drawing it.
Where the breaker's own listing comes in
There's a second half to this, outside the Code, in the standard the breaker is built and tested to. Unless a molded-case circuit breaker is specifically marked as suitable for continuous operation at 100 percent of its rating, it is intended for use at no more than 80 percent of its rated current where the load continues for three hours or more. Those 100-percent-rated devices exist, but they're large-frame equipment in specified enclosures, not the single-pole breakers in a house panel.
So the residential answer is settled from both directions at once: the Code tells the designer to size at 125 percent, and the breaker's listing says it wasn't tested to sit at its full number for hours in a closed panel. This is a separate question from the one on the breaker's face, which is matched to the wire it protects.
What the number works out to at 120 volts
The arithmetic is short enough to do in your head, using the relationship between volts, amps, and watts. Eighty percent of 15 amps is 12 amps; at 120 volts that's 1,440 watts. Eighty percent of 20 amps is 16 amps, or 1,920 watts. Those two figures are the continuous budgets for the circuit sizes behind nearly every general-purpose circuit in a house.
They're worth holding next to the wattage printed on things you already own, because one very common appliance lands on the wrong side of the line.
The rule that reaches the things you plug in
Most of the Code speaks to whoever installs the circuit. One part of it speaks to what gets plugged into an existing one: Section 210.23(A)(1) says that on a 15- or 20-amp circuit serving two or more outlets, the rating of any single cord-and-plug-connected item not fastened in place must not exceed 80 percent of the branch circuit's rating. On an ordinary 15-amp bedroom circuit, that ceiling is 1,440 watts for one portable device.
Portable electric heaters are commonly sold at 1,500 watts. That is 12.5 amps, which is above the 1,440-watt ceiling on a 15-amp multi-outlet circuit and comfortably within it on a 20-amp one — and it is a load that runs for hours by design. It explains why the standing advice about space heaters is so specific: plug it straight into a wall receptacle, never a cord or power strip, and don't ask that receptacle to run anything else. The Electrical Safety Foundation International makes the same point more broadly in its guidance on not overloading your home, which also lists the symptoms of a circuit being asked for too much: dimming lights, breakers that trip repeatedly, warm or discolored wall plates, and buzzing or a burning odor from a receptacle.
Appliances the Code already treats as continuous
For a few loads, nobody has to decide whether three hours will be reached, because the Code decides in advance. A fixed storage-type water heater of 120 gallons or less has its conductors and overcurrent device sized at not less than 125 percent of the appliance's rating. Fixed electric space-heating equipment is likewise considered a continuous load. Vehicle charging gets the same treatment, which is why an EV charger's circuit comes out a size larger than the charger's own amperage suggests — a 40-amp charger on a 50-amp circuit is the rule working, not an installer being generous.
That pattern is useful when reading a quote: if a proposal for a heat-producing or long-running appliance shows a circuit sized right at the nameplate amps, it's worth asking about before the work starts.
Using this on the circuits you already have
- Sort your loads by duration first, not by wattage. Three hours or more is what decides whether the margin applies at all.
- Keep the two numbers handy: 1,440 watts continuous on a 15-amp circuit, 1,920 on a 20-amp one, at 120 volts.
- Read nameplates on anything that runs long — heaters, dehumidifiers, window air conditioners, pumps, aquariums, chargers — and add up only the ones that overlap in time.
- Give a portable heater a receptacle to itself, and don't run it through a cord or strip.
- Treat a breaker that trips after an hour or two, rather than at switch-on, as information about duration — that pattern points at an overload rather than a fault.
- Work out which receptacles share a circuit before assuming two devices in different rooms are independent.
- Warm cover plates, a warm plug body, or a cord that's hot near the plug mean stop and unplug, regardless of what the arithmetic said.
Our garage ended with a second circuit rather than a cleverer schedule, which is usually how this goes. The useful part wasn't the twelve-amp figure — it was learning that a circuit's rating describes what it can carry, not what you should plan to draw from it all afternoon. The roughly 24,200 residential electrical fires the U.S. Fire Administration counted in 2021 sit downstream of that distinction somewhere.