Under the NEC, a continuous load is any load where the maximum current is expected to continue for three hours or more, and circuits serving one must be sized at 125 percent of that current for both the conductor and the overcurrent device. The rule exists because wiring and breakers heat up as current flows, and after about three hours of steady draw they reach a thermal plateau that pushes materials toward their design limits. Get the classification right and the math falls into place. Get it wrong and you end up with nuisance trips, failed inspections, or damaged insulation hidden behind drywall.
The Three-Hour Rule in Article 100
NEC Article 100 defines a continuous load as one whose maximum current is expected to continue for three hours or more. The three-hour figure is not arbitrary. Laboratory testing shows that most wiring assemblies reach a stable maximum temperature after roughly three hours of steady current, and once a conductor hits that plateau it will not cool until the load drops.
The operative word is “expected.” You do not need to prove a circuit actually ran for three straight hours on a given day. If the load is the kind that routinely operates that long during normal use, it is continuous by definition. A parking lot light that runs from dusk to dawn is continuous even on a night you shut it off early.
Which Loads Count and Which Don’t
Commercial office lighting is the textbook continuous load: on when the building opens, off when it closes, eight or more hours later. Electric water heaters with large storage tanks, fixed space heaters, and EV charging stations all qualify for the same reason. Industrial examples add conveyor systems, process machinery running full shifts, commercial kitchen equipment, and welding stations. Data center server racks and their cooling systems run around the clock, so every branch circuit feeding that equipment is continuous.
Non-continuous loads cycle or run briefly. A refrigerator compressor rarely runs for three consecutive hours. Toasters, microwaves, and hair dryers operate for minutes at a time. Garbage disposals and power tools see even shorter duty cycles. These devices can pull significant amperage, but the brief run time lets the wiring cool between uses, so the 125 percent bump does not apply. If every circuit in a house were sized to continuous-load standards, material costs would climb for no safety benefit.
The 125 Percent Rule
NEC Section 210.19(A)(1) requires branch-circuit conductors to have an ampacity no less than 125 percent of the continuous load they serve. Section 210.20(A) imposes the same 125 percent requirement on the overcurrent protection device.1UpCodes. 210.19 Conductors – Minimum Ampacity and Size Feeder conductors follow the same logic under NEC 215.2.2UpCodes. 215.2 Minimum Rating and Size
The reason is thermal. A standard breaker tested to UL 489 is designed to carry 80 percent of its rated current indefinitely without exceeding safe temperature limits. Push it to 100 percent of its rating for hours and the internal bimetallic strip heats beyond its design envelope, which produces nuisance trips or, worse, gradual damage that lets an overcurrent condition pass undetected. Bumping the conductor and breaker up to 125 percent of the load is the same as keeping the load at or below 80 percent of the device’s rating. Two ways of saying the same thing.
Running the Numbers
The practical side of the 125 percent rule is often called the “80 percent rule” because it tells you how much continuous load a given breaker can safely carry. Multiply the breaker’s rating by 0.80:
- 15-amp breaker: 12 amps continuous
- 20-amp breaker: 16 amps continuous
- 30-amp breaker: 24 amps continuous
To convert to watts, multiply by system voltage. A 15-amp, 120-volt circuit has a total capacity of 1,800 watts but a continuous capacity of only 1,440 watts. A 20-amp circuit at the same voltage tops out at 2,400 watts total and 1,920 watts continuous.
Here is where that math saves you from a common mistake. A 1,500-watt space heater on a 15-amp, 120-volt circuit draws 12.5 amps. That is within the breaker’s rating, so it looks fine on paper. But if the heater runs for more than three hours, it is a continuous load pulling 12.5 amps on a circuit that can only safely carry 12 amps continuously. The breaker will eventually trip. Or worse, it will not. Moving the heater to a 20-amp circuit puts the load well inside the 16-amp continuous limit.
Check the manufacturer label on any appliance you plan to run for extended periods. The wattage or amperage printed there is what you compare against these limits. If no label exists, a clamp meter on the circuit gives you the actual draw under operating conditions.
Circuits With Both Continuous and Non-Continuous Loads
Most real circuits do not serve purely one type. A commercial kitchen might have lighting that runs all day along with a mixer that runs twenty minutes at a time on the same panel. The NEC accounts for this with a combined formula: the overcurrent device must be rated for at least the full non-continuous load plus 125 percent of the continuous load.3UpCodes. Continuous and Noncontinuous Loads
Suppose a branch circuit feeds 10 amps of continuous lighting and 6 amps of non-continuous receptacle loads. The minimum breaker size is:
(10 × 1.25) + 6 = 18.5 amps
Breakers come in standard sizes, so you round up to a 20-amp breaker and size the conductors to match. Skipping the 125 percent multiplier and just adding 10 + 6 = 16 would technically fit on a 20-amp breaker, but you would be at 80 percent of the breaker’s rating with the non-continuous load alone before accounting for the thermal penalty of lighting that runs all day.
The 100 Percent Rated Breaker Exception
The 125 percent rule has one notable exception. If the entire assembly, including the breaker and its enclosure, is specifically listed and labeled for operation at 100 percent of its rating, the breaker only needs to be sized to carry the full load without the extra margin.3UpCodes. Continuous and Noncontinuous Loads These breakers are tested under stricter thermal conditions with mandated enclosure dimensions, cable sizes, and termination methods.
In practice, 100 percent rated breakers appear almost exclusively in large commercial and industrial panels. They are available only in frame sizes of 250 amps and above for circuits at 250 volts or below, or in any frame size for multi-pole breakers above 250 volts. You will not find them in a residential loadcenter. You also cannot swap a standard breaker for a 100 percent rated one in the same panel. The rating depends on the specific enclosure the manufacturer tested it in, and that enclosure catalog number is printed on the breaker’s label. Install it in the wrong box and the 100 percent rating does not apply.
HVAC Follows Its Own Rules
Air conditioners and refrigeration compressors are governed by NEC Article 440, which overrides the standard 125 percent calculation. Start with the equipment nameplate. If the manufacturer specifies a maximum breaker or fuse size, that number controls.
When no nameplate maximum is given, NEC 440.22(A) caps the branch-circuit overcurrent device at 175 percent of the motor-compressor’s rated-load current. If the motor cannot start without tripping a breaker sized at 175 percent, the next standard size up is permitted, with an absolute ceiling of 225 percent. The disconnecting means for a hermetic compressor must be rated at no less than 115 percent of the compressor’s rated-load current under NEC 440.12(A)(1).
These percentages run higher than 125 percent because motor-compressors draw a large inrush current at startup that would trip a tighter breaker. The trade-off is that HVAC circuits rely more heavily on the equipment’s internal overload protection for running-condition faults. If you are sizing a circuit for a central air conditioner, ignore the 125 percent rule and follow the nameplate or Article 440. Mixing the two methods is the fastest way to fail an inspection.
Picking the Wire
Once you have the minimum ampacity, you match that number against the NEC’s ampacity table (Table 310.16, renumbered 310.15(B)(16) in recent editions). Common residential copper sizes at the 60°C column:
- 14 AWG: 15 amps
- 12 AWG: 20 amps
- 10 AWG: 30 amps
- 8 AWG: 40 amps
- 6 AWG: 55 amps
For standard 60°C-rated terminations, 14 AWG wire tops out at exactly 15 amps. That is the full breaker rating, not the continuous rating. A continuous 12-amp load on 14 AWG is fine by the numbers, but it leaves zero margin for future load additions or temperature derating. In commercial work, where most loads are continuous, 12 AWG on 20-amp circuits is the practical minimum because it gives more headroom.
Conductors with higher-rated insulation (75°C or 90°C) can carry more current per gauge, but NEC 110.14(C) generally requires you to use the 60°C column for terminations on circuits rated 100 amps or less unless the equipment is specifically listed for higher-temperature connections. The higher insulation rating still helps when you apply ambient temperature correction factors; you just cannot use the 75°C ampacity as your starting point on a standard residential breaker.
When your continuous-load calculation lands between two sizes, round up to the larger gauge.
Ambient Temperature Derating
The ampacity numbers in NEC tables assume an ambient temperature of 30°C (86°F). When wiring runs through hotter spaces like attics, boiler rooms, or southern rooftops, the conductor’s effective ampacity drops. The NEC provides correction factors for copper conductors with a 75°C insulation rating:
- 35°C (95°F): multiply by 0.94
- 40°C (104°F): multiply by 0.88
- 45°C (113°F): multiply by 0.82
- 50°C (122°F): multiply by 0.75
For a circuit already sized at 125 percent for continuous loading, hot ambient temperatures can push you up another gauge. A 12 AWG copper conductor rated for 25 amps at 75°C in a 30°C environment drops to roughly 22 amps in a 40°C attic. If your continuous load after the 125 percent calculation requires 24 amps, that 12 AWG wire no longer qualifies and you need 10 AWG.
Voltage Drop on Long Runs
The NEC recommends, though does not mandate, keeping branch-circuit voltage drop at or below 3 percent, with no more than 5 percent total across the combined feeder and branch circuit. Long runs serving continuous loads are especially prone to voltage drop because steady current produces constant voltage loss along the full conductor length. Undersized wiring on a 200-foot run to an outbuilding can cause motors to overheat and lighting to dim. Upsizing the conductor beyond the minimum required by the 125 percent rule is the standard fix.