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How to Size a Breaker for a Continuous Load

Under the National Electrical Code (NEC) 2020, 210.20(A) sizes the overcurrent device at 100 percent of the noncontinuous load plus 125 percent of the continuous load, then rounds up to a standard rating from 240.6(A). A continuous load is one whose maximum current runs for three hours or more: EV chargers, electric water heaters, and commercial lighting are the usual ones. A 32 A continuous EV charger therefore needs a 40 A breaker, not a 35 A one. The worked examples below show the whole path from load to breaker to conductor.

Skip the longhand: the Breaker Size Calculator takes the continuous and noncontinuous portions separately, applies the 125 percent rule, rounds to the next 240.6(A) size, and returns the minimum conductor in the same step.

Why 125 percent, and what "continuous" means

A standard molded-case breaker is tested and listed to carry only 80 percent of its rating continuously inside its enclosure, because the breaker and the surrounding conductors heat up over a long run and a fully loaded breaker would drift toward its trip point. The 125 percent multiplier is that same 80 percent limit written from the load side: 1 ÷ 0.80 = 1.25. Size the breaker so the continuous load is no more than 80 percent of it, and you have the margin the listing assumes. NEC 210.20(A) states it directly, and the parallel conductor rule lives in 210.19(A)(1).

"Continuous" is defined in Article 100 as a load whose maximum current is expected to continue for three hours or more. It is a duration test, not a load type: office lighting that stays on all shift is continuous, while a 10-second garbage disposal is not. Some equipment is declared continuous by its own article regardless, such as fixed storage water heaters of 120 gallons or less under 422.13. The few loads with a larger multiplier, such as household electric ranges and dryers, are sized by their own demand tables (220.55 and 220.54) instead of this rule.

The procedure

  1. Split the load into its continuous and noncontinuous amperes. Convert watts to amperes first if needed: single-phase amperes = watts ÷ volts.
  2. Required protection = noncontinuous + (1.25 × continuous).
  3. Round up to the next standard rating in 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 and so on. Always up, never down below the requirement.
  4. Size the conductor to the same noncontinuous-plus-125-percent figure in the termination column, then confirm it still carries the actual load after any ampacity adjustment or correction. Take the larger conductor.

Example 1: 32 A continuous EV charger

Load: 32 A, continuous (charging runs for hours). Noncontinuous: 0 A.

Required protection: 0 + (1.25 × 32) = 40 A.

240.6(A): 40 A is already a standard rating, so the breaker is 40 A.

Conductor: minimum ampacity 40 A. From Table 310.16, 8 AWG copper is 50 A at 75°C and 40 A at 60°C, both at or above 40 A, so 8 AWG copper.

This is the number stamped on most 32 A hardwired chargers: a 40 A circuit on 8 AWG copper. Note what the rule prevents. Sized on the raw 32 A the charger would look fine on a 35 A breaker, but that leaves only 3 A of headroom against a load that runs at 32 A for hours, exactly the nuisance-trip case 210.20(A) is written to avoid.

Example 2: 4500 W / 240 V water heater

Current: 4500 W ÷ 240 V = 18.75 A.

422.13 makes a fixed storage water heater a continuous load, so all 18.75 A is continuous.

Required protection: 1.25 × 18.75 = 23.4 A.

240.6(A): round 23.4 A up to the next standard rating, 25 A.

Conductor: minimum ampacity 23.4 A. 10 AWG copper is 35 A at 75°C and 30 A at 60°C, both above 23.4 A, so 10 AWG copper.

A 25 A breaker on 10 AWG copper satisfies the code. The field commonly installs a 30 A breaker on the same 10 AWG, which is legal because 240.4(D) caps 10 AWG copper at 30 A and the conductor's 30 A ampacity in the 60°C column still covers the 18.75 A load. Either way, the mistake to avoid is sizing on 18.75 A and reaching for a 20 A breaker, which trips.

Example 3: a mixed load

Most real circuits carry both kinds of load, and only the continuous part gets the 1.25 multiplier. Take a circuit with 24 A of continuous lighting and 12 A of noncontinuous receptacle load:

Continuous: 24 A. Noncontinuous: 12 A.

Required protection: 12 + (1.25 × 24) = 12 + 30 = 42 A.

240.6(A): round 42 A up to the next standard rating, 45 A.

Conductor: minimum ampacity 42 A. 8 AWG copper at 50 A (75°C) covers it; 10 AWG at 35 A does not.

A 45 A breaker on 8 AWG copper. The noncontinuous 12 A enters the sum at face value while the continuous 24 A enters at 30 A, and it is the total, 42 A, that gets rounded, not each piece separately.

From breaker to wire

Once the 125 percent rule fixes the breaker, choosing the conductor is the 240.4 pairing: the wire's Table 310.16 ampacity in the terminal column has to be at least the breaker rating. A 40 A continuous load lands on a 50 A breaker (40 × 1.25 = 50), and a 50 A breaker takes 6 AWG copper. That hand-off, from a settled breaker size down to the smallest legal conductor, is worked in detail in what size wire for a 50 amp breaker, including where the 60°C termination default forces a larger wire than the 75°C column suggests.

Where this rule does not apply

Motors and HVAC equipment do not use the 125 percent continuous rule for overcurrent sizing. Motor branch circuits under Article 430 size the short-circuit and ground-fault device off the full-load-current tables and allow ratings far above the conductor ampacity, because the overload relay protects the motor separately. Air-conditioning and refrigeration equipment under Article 440 puts the answer on the nameplate as Minimum Circuit Ampacity and Maximum Overcurrent Protection; follow the nameplate. Feeding either through the general 210.20(A) math gives the wrong breaker.

NEC reference

NEC 2020. Continuous-load overcurrent sizing from 210.20(A) and the parallel conductor requirement in 210.19(A)(1). Definition of a continuous load from Article 100. Standard breaker ratings from 240.6(A). Small-conductor limits from 240.4(D). Water-heater continuous classification from 422.13. Conductor ampacities from Table 310.16. Range and dryer demand, which override this rule, from Tables 220.55 and 220.54.

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

Get the full guide

NEC Code Quickstart

This guide covers one calculation. NEC Code Quickstart walks all twelve exam calculations start to finish, each with a worked example and the exact code reference: ampacity and derating, breaker sizing, voltage drop, box and conduit fill, grounding, dwelling load, range and dryer demand, motor circuits, and transformer current. Written for the 2023 NEC with notes for the 2026 edition.

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