What is the 80 percent rule for a breaker?

A standard-rated breaker or fuse may carry a continuous load of no more than 80 percent of its ampere rating. So a 20 A breaker is limited to 16 A of continuous load, a 30 A breaker to 24 A, and a 100 A breaker to 80 A. The 80 percent limit is the mirror image of the 125 percent sizing rule in NEC 210.20(A): 80 percent of the rating equals the rating divided by 1.25. A purely noncontinuous load may use the full rating.

What is a continuous load in the NEC?

Article 100 defines a continuous load as a load where the maximum current is expected to continue for 3 hours or more. Store lighting, an EV charger, an electric water heater, and heat tracing are common continuous loads. A receptacle circuit that only sees intermittent tool use is noncontinuous. The classification is yours to make from how the equipment runs; this calculator applies the code math once you separate the continuous amps from the noncontinuous amps.

How do I size a breaker for a continuous load?

NEC 210.20(A) and 215.3 require the overcurrent device to be rated at not less than the noncontinuous load plus 125 percent of the continuous load. For a 100 A continuous load that is 100 x 1.25 = 125 A. For a 30 A continuous load it is 37.5 A, which rounds up to the next standard 240.6(A) size of 40 A. The conductor gets the same minimum ampacity under 210.19(A)(1) before any temperature or conductor-count adjustment.

How much continuous load can a 20 amp breaker handle?

A 20 A breaker handles 16 A of continuous load (20 divided by 1.25, or 80 percent of 20). If the load is a mix, subtract 125 percent of the continuous part and 100 percent of the noncontinuous part from the 20 A rating to find the remaining headroom. A load that runs less than 3 hours is noncontinuous and may use the full 20 A.

What is a 100 percent rated breaker?

Where the assembly, including its overcurrent device, is listed for operation at 100 percent of its rating, the 210.20(A) and 215.3 exceptions let you drop the 125 percent factor and use the full rating for a continuous load. These listed 100 percent rated devices are typically only available at 400 A and larger and require specific terminations and enclosures. Standard breakers are 80 percent rated, so this calculator uses the 125 percent factor unless you check the 100 percent rated option.

Continuous Load Calculator

Apply the NEC continuous-load rule in both directions. Enter a continuous and noncontinuous load to get the minimum overcurrent device and conductor ampacity (210.20(A), the 125% rule), or enter an existing breaker rating to find the maximum load it can carry (the 80% rule). No wire-size guesswork buried inside a bigger tool, just the rule itself.

How to use this calculator

Choose a direction. "Size a circuit from a load" finds the minimum breaker and conductor; "Find an existing circuit's capacity" applies the 80% rule to a breaker you already have.
Separate your load into continuous (running 3 hours or more) and noncontinuous amps. If your load is in volt-amps, switch the unit to VA and enter the voltage and phase to convert at unity power factor.
Check the 100%-rated assembly box only if the equipment is listed for 100% continuous operation (usually 400 A and up). Leave it clear for ordinary breakers, which are 80% rated.
Read the result. In size mode you get the minimum standard 240.6(A) device and the conductor ampacity to carry to the wire-size step; in capacity mode you get the maximum continuous and noncontinuous load, and the room left over when loads are already present.
Take the conductor ampacity to the Wire Size or Wire Ampacity calculator to pick the actual AWG with your temperature and conductor-count conditions.

NEC reference

Sizing follows NEC 2020. Section 210.20(A) (branch circuits) and 215.3 (feeders) require the overcurrent device to be rated at not less than the noncontinuous load plus 125% of the continuous load. Sections 210.19(A)(1) and 215.2(A)(1) apply the same minimum to the conductor ampacity before any ambient-temperature or conductor-count adjustment. Article 100 defines a continuous load as one whose maximum current is expected to continue for 3 hours or more. The 125% factor exists because standard overcurrent devices are listed for continuous operation at only 80% of their rating, so the two numbers are the same rule seen from opposite ends. The 210.20(A) and 215.3 exceptions allow a factor of 100% where the assembly is listed for operation at 100% of its rating. Standard ratings come from 240.6(A). These rules are unchanged across the 2017, 2020, and 2023 editions.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

The 80% rule at a glance: continuous load per breaker

Breaker / fuse	Max continuous load (80%)	Max noncontinuous load (100%)
15 A	12 A	15 A
20 A	16 A	20 A
30 A	24 A	30 A
50 A	40 A	50 A
100 A	80 A	100 A
200 A	160 A	200 A

Max continuous load is the rating divided by 1.25, which is the same as 80% of the rating. A load that runs less than 3 hours is noncontinuous and may use the full rating. Mixed loads fall between the two columns; enter them above for the exact headroom.

125% and 80% are one rule seen from two ends

NEC 210.20(A) says the overcurrent device is rated at not less than the noncontinuous load plus 125% of the continuous load. Turn that around for an all-continuous load and it says the load may be at most the rating divided by 1.25, which is 80% of the rating. Standard breakers and fuses are tested and listed to carry only 80% of their rating continuously, which is why the code builds the 25% margin into the sizing. Sizing a circuit and checking a circuit are therefore the same equation; this tool just solves it for whichever quantity you are missing.

Worked example: a 100 A continuous load needs 100 x 1.25 = 125 A of overcurrent protection and conductor ampacity, so a 125 A device. Run it the other way and a 125 A device carries 125 / 1.25 = 100 A of continuous load. A 30 A continuous load needs 37.5 A, which is not a standard size, so it rounds up to a 40 A device under 240.6(A).

Mixed continuous and noncontinuous loads

Most real circuits carry both kinds of load. The rule adds 125% of the continuous part to 100% of the noncontinuous part. A circuit with 24 A of continuous load and 10 A of noncontinuous load needs 10 + (24 x 1.25) = 40 A of protection. Going the other way, a 50 A feeder that already carries 20 A continuous and 10 A noncontinuous has used 10 + (20 x 1.25) = 35 A of its 50 A, leaving room for 12 A more continuous load or 15 A more noncontinuous load before it needs to grow.

This tool stops at the ampere rule, on purpose

The 210.20(A) continuous-load rule is one step inside every conductor-sizing job. This calculator isolates that step so you can see it clearly and run it backward. It reports the minimum device rating and the conductor ampacity, but it deliberately does not pick a wire gauge: choosing the AWG needs Table 310.16, the 110.14(C) termination limit, ambient and conductor-count derating, and the 240.4(B) and 240.4(D) conductor-protection rules. Carry the conductor ampacity this tool returns into the Wire Size or Wire Ampacity calculator to finish that step. Motor loads are also out of scope: a motor branch circuit is sized from the Table 430.248 or 430.250 full-load current and Article 430 percentages, not the 125% continuous rule.

Frequently asked questions

Why 125% and not exactly 100%?

A standard molded-case breaker is calibrated and listed to carry 80% of its rating continuously without its terminals overheating. Adding 25% to a continuous load (1 / 0.80 = 1.25) restores that margin, so the device runs within its listing for the full 3 hours or more that a continuous load stays on.

Is a receptacle circuit a continuous load?

Usually not. General-purpose receptacles serving cord-and-plug tools and appliances that come and go are noncontinuous. But a receptacle dedicated to a load that runs 3 hours or more, such as an EV charger or a sign, is continuous. Classify by how the load actually runs, not by the outlet type.

Does this apply to feeders and services?

Yes. NEC 215.3 applies the identical noncontinuous-plus-125%-continuous rule to feeder overcurrent devices, and 215.2(A)(1) to feeder conductors. Service and feeder load calculations under Article 220 are a separate step that produces the load figure you then feed into this rule.

Does the 80% number change the wire size?

The 125% figure sets the minimum ampacity the conductor must have before derating, which can push the wire larger. It is not the same as the temperature and conductor-count adjustments, which come after. This tool gives you the required ampacity; the Wire Size calculator turns it into an AWG.

Related calculators

Breaker Size Calculator: size the breaker and the wire together from a load, with the 240.4 conductor limits applied.
Wire Size Calculator: turn the conductor ampacity from this tool into an actual AWG with your conditions.
Wire Ampacity Calculator: Table 310.16 ampacity with ambient and conductor-count derating and the 110.14(C) cap.
EV Charger Circuit Calculator: the 125% continuous rule applied to an EVSE branch circuit.

Seasonal safety guide

Loading a holiday display onto a household circuit rather than sizing a branch circuit? This homeowner guide uses the same 80 percent rule.

Will a Halloween Fog Machine Set Off a Smoke Alarm?: indoors a dense fog often does, and the 1,000-watt heater behind it is 8.3 amps against the 12-amp continuous ceiling, so total it here first.
Can You Leave a Halloween Fog Machine On All Night?: no; the makers say never run one unattended, and all evening it is a continuous load, so budget the full 8.3 amps of a 1000-watt fogger against the 12-amp ceiling.
Are Outdoor Outlets 15 or 20 Amps?: either one; the breaker feeding it decides, not the slot shape, and that sets the 12 or 16 amp display budget you have to fit inside.
How Many Watts of Halloween Decorations Can You Put on One Circuit?: about 1,440 watts on a 15-amp circuit and 1,920 on a 20-amp, with the worked add-up-the-watts example for a yard display.
How Many Watts Can a Power Strip Handle for Halloween Decorations?: the same 80 percent rule applied to a power strip, which carries about 1,440 watts and never adds power on top of the wall circuit.
Can You Plug a Space Heater Into the Same Circuit as Halloween Decorations?: why a single 1,500-watt space heater, at 12.5 amps, already overruns a 15-amp circuit's safe ceiling.
Can You Plug Halloween Decorations Into the Same Circuit as a Freezer?: the same 80 percent budget, plus the compressor start surge that tips a loaded circuit over and thaws the freezer silently.
Can You Plug an Electric Turkey Fryer Into a Regular Outlet?: a 1,650-watt indoor fryer is about 13.75 amps, nearly the whole 15-amp circuit, so total it here before adding anything else to the breaker.
Can You Run a Roaster Oven and Microwave on the Same Circuit?: two Thanksgiving appliances near 12 amps each overrun a 20-amp counter circuit; add the input watts here and split them across the two kitchen circuits.

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For deeper NEC training on continuous-load and branch-circuit sizing, Mike Holt's NEC courses are the industry standard.

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