What does the National Electrical Code specify for sizing breakers on continuous 120V loads?
Executive summary
The National Electrical Code requires overcurrent protective devices for branch circuits to be sized so they can carry the noncontinuous load plus 125% of any continuous load, where “continuous” means currents expected to run for three hours or more (Art. 100 and 210.20(A)) [1] [2]. That 125% adjustment drives both breaker selection and conductor ampacity decisions unless the equipment and assembly are listed for operation at 100% of their rating, in which case different allowances apply [3] [4].
1. What the NEC actually says about continuous loads and the 125% rule
The NEC defines a continuous load as one where the maximum current is expected to continue for three hours or more, and mandates that the overcurrent protective device (OCPD) for a branch circuit be sized to handle the noncontinuous load plus 125% of the continuous load—commonly paraphrased as “size the breaker at 125% of the continuous load” [1] [2]. Sections often cited for this requirement include 210.20(A) for branch circuits and parallel feeder rules such as 215.3 for sizing of other OCPDs, which together require the OCPD rating to be not less than 100% of noncontinuous plus 125% of continuous loads [5] [4].
2. How that translates to a 120‑volt continuous load in practice
For a single‑phase 120 V circuit the math is straightforward: calculate the continuous current, multiply by 1.25, add any noncontinuous current, and then select the next standard breaker rating and matching conductor ampacity per NEC tables and 240.6(A) [4] [6]. Example: a 16 A continuous load would drive breaker selection to 16 A × 1.25 = 20 A, so a 20 A breaker and conductor sized accordingly are appropriate [7] [4].
3. The 80% vs 100% rated breaker nuance and listed equipment exception
Although breakers are tested to carry 100% of their nameplate rating, the NEC’s 125% rule has historically produced the so‑called “80% rule” practice: sizing a breaker so continuous load does not exceed 80% of the breaker rating unless the assembly is specifically listed for 100% operation [8] [2]. The NEC includes an exception allowing the use of OCPDs listed for continuous operation at 100% of rating—where the assembly, including breaker terminals and equipment, is listed for 100%—which can eliminate the 125% upsizing in some configurations [3] [9].
4. Conductors, terminals and practical constraints electricians must consider
Breaker sizing is only one part of the equation; conductors must be sized to carry the larger of the required load and the conductor protection requirement, meaning conductors are typically upsized to match the 125% continuous load calculation and must respect terminal temperature ratings, ambient corrections, and ampacity tables in Article 310 when selecting wire gauge [3] [9]. The NEC also requires selecting the nearest standard breaker rating per 240.6(A) after computing required ampacity for both breaker and conductor coordination [3] [10].
5. Conflicting interpretations, industry perspectives and hidden incentives
Industry sources and training authorities (e.g., Mike Holt, Schneider Electric, manufacturer blogs) all reiterate the 125% rule but emphasize different practical points: manufacturers explain equipment listing distinctions and why some breakers are marketed as 80% or 100% rated, while training and code‑interpretation sources stress conductor terminal temperature and real‑world conditions that drive conservative upsizing [2] [5] [8]. There is an implicit agenda from manufacturers and aftermarket vendors to highlight rating distinctions because it affects which breakers and conductors they sell, and from trainers who emphasize conservative sizing to avoid liability—readers should note that phrasing like “80% breaker” is shorthand for an NEC sizing practice, not a literal breaker limitation [11] [8].
6. Bottom line for a 120V continuous circuit
When designing or evaluating a continuous 120 V load, compute the actual continuous current, multiply that current by 1.25, add any noncontinuous load, then select a breaker and conductor that meet or exceed that computed amperage using NEC standard ratings and tables; consider whether equipment is listed for 100% operation to determine if the 125% upsizing can be waived [1] [4] [3]. Where nuance or local amendment exists, consult the current NEC text and the equipment listings because code text, terminal ratings, and listing exceptions determine the allowable final selection [9] [3].