How this instrument works
A continuous load is one the NEC defines as running at its maximum current for three hours or more without a break — a store's overnight lighting circuit, a water heater element, an EV charger topping off a fleet vehicle. The 125% multiplier in I_min = I_load × 1.25 is not an arbitrary safety margin; it comes straight from how ordinary breakers are rated. A breaker not specifically listed for 100% continuous duty is only certified to carry 80% of its printed rating when the current lasts three hours or longer. Turn that around and a breaker sized at load ÷ 0.8 — mathematically the same as load × 1.25 — keeps the continuous current at or under that 80% ceiling.
Electricians reach for this multiplier whenever a circuit schedule flags a load as continuous — a countertop oven bank running through dinner service, a rooftop compressor cycling on for hours in a heat wave, permanently wired signage lit dusk to dawn. The common mistake is skipping the factor because the equipment's nameplate current already looks safely under the breaker rating: 16 A on a 20 A breaker looks fine at a glance, but 16 A is already 80% of 20 A, exactly the continuous-duty ceiling, with zero margin left for a hot afternoon.
The raw multiplication rarely lands on a size you can actually buy. Breakers come only in the standard ratings listed in NEC 240.6(A) — 15, 20, 25, 30, 35, 40 A and up — so a computed minimum of 18.75 A, from a 15 A continuous load, forces a jump to the next size, 20 A, never a round down to 15 A. The one exception is equipment specifically listed and marked for 100% continuous duty, common in larger industrial gear, which drops the multiplier entirely because it is already certified to run at full rating indefinitely.
- Enter the amperes in Continuous load current — the current the circuit draws when running at full output for three hours or longer.
- Leave NEC continuous-load multiplier at 1.25 unless the equipment is specifically listed for 100% continuous duty, in which case set it to 1.
- Read Minimum breaker rating (round up to standard size) — already stepped up to the next size a breaker is actually sold in.
- Check the branch-circuit conductor separately: NEC 210.19(A)(1)(a) applies the same 1.25 factor to wire ampacity, not just the breaker.
Worked example — a 16 A continuous kitchen load
A dedicated circuit feeds a countertop convection oven bank that runs at a steady 16 A through dinner service, well past the NEC's three-hour continuous-load threshold. Set Continuous load current to 16 and leave the multiplier at 1.25: I_min = 16 × 1.25 = 20.0 A exactly, matching I_min = I_load × 1.25 with no remainder to round away.
Twenty amperes lands exactly on a standard breaker size from NEC 240.6(A), so the Minimum breaker rating reads 20 A with no further step-up needed. A 15 A breaker would be undersized — 16 A already exceeds 80% of a 15 A rating — and 16 A itself is not a standard breaker size available to buy in any case.
Questions
Why multiply by 1.25 instead of matching the breaker to the load current?
Because standard breakers not listed for 100% continuous duty are only certified to carry 80% of their rating for three hours or more. Sizing the breaker at load ÷ 0.8 — the same arithmetic as load × 1.25 — keeps the continuous current at or below that 80% ceiling, following NEC 210.19(A)(1)(a) and 210.20(A).
What counts as a continuous load under the NEC?
Any load expected to run at its maximum current for three hours or more, per NEC Article 100 — overnight store lighting, EV chargers, water heaters, or kitchen equipment on a long service run. Loads that cycle on and off, like most motors, usually fall short of this definition and skip the multiplier.
Why does the breaker rating round up to a number like 20 A?
Because breakers only come in the standard sizes listed in NEC 240.6(A) — 15, 20, 25, 30, 35, 40 A and so on — not arbitrary values. A calculated minimum of 18.75 A can't be bought as-is, so the next size up, 20 A, becomes the actual breaker; the multiplication only sets the floor a real breaker has to clear.
Does the 125% factor also apply to the wiring, not just the breaker?
Yes. NEC 210.19(A)(1)(a) applies the same factor to branch-circuit conductor ampacity, so the wire feeding a continuous load must also be rated for at least load × 1.25 before any temperature-correction adjustments. Sizing the breaker correctly while leaving the conductor undersized still leaves the wire as the weak point.
What if my load is not continuous?
Set NEC continuous-load multiplier to 1. A non-continuous load only needs a breaker rated to carry its actual current, without the 25% headroom that the Code reserves specifically for loads sustained past the three-hour mark.
Can a breaker be used at 100% of its rating for a continuous load?
Only if it is specifically listed and marked for 100% continuous duty — common in larger commercial and industrial panels. Those breakers are certified to carry their full nameplate rating indefinitely, so the minimum size equals the load current itself, with the multiplier set to 1 instead of 1.25.