Continuous Load & the 125% Rule

The definition: three hours or more

Article 100 defines a continuous load as a load where the maximum current is expected to continue for three hours or more. That is the entire definition — three hours. There is no list of load types, no occupancy-based exception. A lighting circuit in a warehouse that runs an overnight shift is continuous. A kitchen exhaust hood that runs for four hours during a dinner service is continuous. A receptacle circuit serving a data center that is always on is continuous.

On the exam, the three-hour threshold is either given in the question or implied by the load type. Commercial and industrial lighting, continuous process equipment, and always-on circuits are the scenarios where you should immediately flag "continuous" without being told explicitly. Residential circuits are less often tested as continuous, but a lighting circuit in a common area of an apartment building can qualify.

The 125% sizing rule and where it appears

The NEC addresses continuous loads at three key locations, and the journeyman exam draws from all three:

• 210.19(A)(1) — Branch-circuit conductors must have an ampacity of not less than the noncontinuous load plus 125 percent of the continuous load.
• 210.20(A) — Branch-circuit overcurrent devices must be rated not less than the noncontinuous load plus 125 percent of the continuous load.
• 215.2(A)(1) — Feeder conductors carry the identical requirement: noncontinuous load at 100 percent plus continuous load at 125 percent.

In each case the formula is the same: add the full noncontinuous load to 125 percent of the continuous load. The result is the minimum ampacity or rating required before you go to a conductor table or breaker selection.

Worked example

A branch circuit serves a 32-ampere noncontinuous load and a 40-ampere continuous load. What is the minimum conductor ampacity and overcurrent device rating?

• Continuous load at 125%: 40 A × 1.25 = 50 A
• Noncontinuous load at 100%: 32 A
• Minimum required: 50 + 32 = 82 amperes

You would select a conductor with at least 82 amperes of ampacity (at the correct termination temperature column) and a breaker rated at least 82 amperes — in practice, the next standard size up, which is 90 amperes under 240.6(A). If the circuit had only the 40-ampere continuous load with no noncontinuous component, the minimum would be 40 × 1.25 = 50 amperes, meaning a 50-ampere breaker and a conductor rated at least 50 amperes. That straightforward case — a purely continuous load — is the most common exam scenario.

The 80% rule: the same requirement, stated differently

The 125% sizing rule is sometimes expressed as "do not load a breaker above 80 percent of its rating for continuous use." Both statements describe the same mathematical relationship:

• A 50-ampere breaker loaded to 80% carries 40 amperes of continuous load.
• A 40-ampere continuous load requires a breaker rated at least 40 × 1.25 = 50 amperes.

Dividing by 1.25 is the same as multiplying by 0.80. On the exam, knowing both formulations matters because questions are written both ways: "size the breaker for a 40-amp continuous load" and "what is the maximum continuous load on a 50-amp breaker?" have the same underlying answer — 40 amperes and 50 amperes respectively — but require opposite operations.

One important caveat: some overcurrent devices are listed for continuous operation at 100 percent of their rating. When a question or equipment specification calls out a 100-percent-rated breaker, the 125% derating does not apply to that device. Those devices are less common and the exam will tell you when one is involved; the default assumption is always 125 percent unless stated otherwise.

Interaction with ampacity adjustment factors

The 125% figure is applied to the load to set a minimum ampacity requirement. Ampacity adjustment factors — temperature correction and conduit fill — are applied to the conductor's rated ampacity to determine what the conductor can actually carry in that installation. The two calculations work in opposite directions and must both be satisfied.

A conductor running through a hot attic, or bundled with many other conductors, has a derated ampacity that may be lower than its table value. The derated ampacity still has to meet or exceed the minimum calculated from the continuous load rule. If the 125% calculation gives you 82 amperes but the conductor's derated ampacity is only 78 amperes, the conductor is undersized regardless of the breaker rating. Load calculations that involve continuous loads must account for both effects.