A continuous load, one that runs for three hours or more at a stretch, heats a breaker differently than an intermittent one, and the breaker must be sized for that reality. Lighting circuits in commercial spaces, electric heaters, and EV chargers are classic continuous loads: they draw their full current for hours, the breaker and the wiring warm up and stay warm, and a breaker sized merely to the load current will eventually trip on thermal overload. The rule is simple and non-negotiable: size the breaker and the conductors at 125 percent of the continuous load.
Why continuous loads get special treatment
Circuit breakers trip on heat: a bimetallic element bends as it warms, and sustained current near the breaker's rating keeps that element warm enough to eventually trip even though nothing is wrong. Intermittent loads give the breaker time to cool between cycles; continuous loads do not. The 125 percent rule provides the thermal headroom the breaker needs to carry the load indefinitely without nuisance tripping.
The same logic applies to the conductors. Wire carrying current heats up, and a continuous load holds the wire at its elevated temperature for hours. Sizing conductors at 125 percent of the continuous load keeps the wire within its temperature rating during those long runs. Breaker and wire are sized together because they protect each other: the breaker protects the wire, and the wire must be able to carry whatever the breaker allows.
The calculation
Multiply the continuous load current by 1.25 to get the minimum breaker rating, then select the next standard breaker size at or above that value. A 16-amp continuous load, for example, requires a minimum 20-amp breaker (16 x 1.25 = 20). A 32-amp continuous load needs a 40-amp breaker. Then size the conductors for that breaker rating per the applicable ampacity tables, accounting for ambient temperature and conduit fill derating where they apply.
Where a circuit mixes continuous and noncontinuous loads, apply the 125 percent factor to the continuous portion and add the noncontinuous portion at 100 percent, then size the breaker to the total. This is the standard calculation method, and it prevents both the under-sizing of the continuous share and the over-sizing that a blanket 125 percent on everything would produce.
Breaker types and the 100 percent exception
Most standard branch breakers are rated for 80 percent continuous loading, which is where the 125 percent rule comes from (1 divided by 0.8 equals 1.25). Some breakers are listed for 100 percent continuous operation, typically larger-frame commercial breakers in properly rated enclosures, and these may be loaded to their full rating continuously. Unless the breaker is specifically marked for 100 percent continuous duty and installed in the enclosure conditions its listing requires, assume the 80 percent limit applies.
Do not try to dodge the rule by splitting a continuous load across multiple breakers unless each resulting circuit is genuinely independent and properly wired. The rule follows the load's behavior, not the wiring convenience.
Common continuous loads in the shop and home
Electric vehicle chargers are the continuous load most homeowners meet first: an EVSE draws its rated current for the entire charging session, often many hours, and undersized breaker sizing is the leading cause of nuisance tripping on new EV circuits. Electric heat, including baseboards and shop heaters, runs for hours in cold weather. Grow lights, aquarium heaters, and any lighting expected to run all evening all count.
Workshop dust collectors and large air compressors deserve a careful look: if they genuinely run for three hours or more continuously, they are continuous loads for sizing purposes. Intermittent use, even heavy intermittent use, does not trigger the rule, but be honest about the duty cycle; a compressor that runs all afternoon during a big project is continuous that afternoon.
Safety and code compliance
This sizing rule comes from the electrical code, and it exists because undersized breakers on continuous loads cause both nuisance tripping and, worse, the temptation to replace the breaker with a larger one without upgrading the wire. Never upsize a breaker without verifying the conductors are rated for it; the breaker is the wire's protection, and defeating that protection to stop nuisance trips trades an annoyance for a fire risk.
When in doubt, or when the load is large, the circuit is long, or the installation is anything but straightforward, have the work reviewed by a licensed electrician. Breaker sizing is simple arithmetic wrapped around serious consequences.
Continuous load times 1.25, next standard breaker size up, conductors to match, and never defeat the protection to silence a trip. The breaker is telling the truth about the heat; listen to it.
Related Guides
- Sizing Neutral Wires for Non-Linear Loads: In a balanced three-phase system, the neutral carries only the imbalance between phases, which is why neutrals were traditionally sized smaller than
- Three-Wire Cable Sizing for Multiway Switched Circuits: Multiway switching, controlling one light from two or more locations, runs on three-wire cable between the switches: two travelers plus a common
- Conduit Fill Capacity Ratios for Commercial Romex: Running Romex (nonmetallic sheathed cable) through conduit seems straightforward until the conduit fill rules enter: the electrical code limits how
- Derating Conductor Ampacity in Multi-Cable Bundles: A conductor's ampacity, the current it can carry safely, is really a temperature rating in disguise. The copper can handle far more current than the