Breaker Sizing and the Continuous-Load 125% Rule

The breaker protects the wire

The most important idea in overcurrent protection is counter-intuitive: the breaker is sized to protect the conductor, not the appliance. Its job is to open before the wire overheats from too much current. That is why you can never simply “upsize” a breaker to stop nuisance trips — a 20 A breaker on 14 AWG wire would let the wire carry current it cannot handle, creating a fire risk. The breaker, the wire and the load must be matched as a set, and the conductor’s ampacity sets the ceiling.

Continuous vs non-continuous loads

The NEC distinguishes continuous loads — those expected to run for three hours or more, such as lighting, EV charging or HVAC — from non-continuous loads. For continuous loads, both the overcurrent device and the conductor must be sized at 125% of the load current. Equivalently, a standard breaker may be loaded continuously to only 80% of its rating (because 1 / 1.25 = 0.80). This margin keeps the breaker and wire from running hot for hours on end.

The rule in formula form:

required amps = continuous load × 1.25 + non-continuous load

Then round up to the next standard breaker size.

Standard breaker sizes

Overcurrent devices come in standard ratings, not arbitrary numbers (NEC 240.6). The common sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 amps, continuing upward from there. When your calculated requirement falls between two sizes, you move up to the next standard rating — provided the conductor can support it.

Worked example: a 16 A continuous load

Suppose you have a continuous load of 16 amps (an EV charger set to 16 A, for instance):

required = 16 × 1.25 = 20 A → the next standard size is 20 A.

So a 16 A continuous load needs a 20 A breaker and a conductor rated for at least 20 A (12 AWG copper at 75°C, subject to derating). If the load were given in watts, convert first: a 1,500 W load at 120 V is 1,500 / 120 = 12.5 A; as a continuous load that is 12.5 × 1.25 = 15.6 A → a 20 A breaker (15.6 rounds up past the 15 A size). The Breaker / OCPD Size calculator applies the 125% factor, rounds to a standard size and suggests the minimum copper conductor.

Matching the conductor

A breaker is only valid if the wire behind it can carry the breaker’s rating. The small-conductor rule (NEC 240.4(D)) caps the breaker regardless of the ampacity table: 14 AWG to 15 A, 12 AWG to 20 A and 10 AWG to 30 A. So a 20 A breaker pairs with 12 AWG (or larger), never 14 AWG. On long runs, voltage drop may force a still-larger conductor even though the breaker stays the same — sizing the wire and sizing the breaker are related but separate steps. Use the Wire Size calculator for the conductor and the Ampacity calculator to confirm it carries the breaker rating after derating.

Why the 125% margin exists

Thermal-magnetic breakers and conductor insulation are rated for continuous operation at a temperature that assumes some headroom. A device run at 100% of its rating for hours can drift toward its trip threshold and run its terminations hot. The 125% sizing (80% loading) builds in the margin that makes continuous operation safe and stable. It is one of the most frequently missed rules in DIY work, and it is exactly why an EV charger or a long-running heater needs a breaker and wire a size larger than the raw current suggests.

What counts as a continuous load

The whole rule turns on whether a load is continuous, so it pays to know what that means. The code defines a continuous load as one where the maximum current is expected to run for three hours or more. Store and office lighting qualifies because it is on all day; an electric vehicle charger qualifies because a full charge takes hours; electric heat and many commercial appliances qualify as well. A household receptacle circuit, by contrast, is generally treated as non-continuous because its loads come and go. When a load mixes continuous and non-continuous parts, you apply the twenty-five percent uplift only to the continuous portion and add the non-continuous portion at its face value, which is why the formula keeps the two terms separate.

The eighty percent way of saying the same thing

The same rule is often stated from the other direction: a standard overcurrent device may be loaded continuously to no more than eighty percent of its rating. The two statements are mathematically identical, because dividing by one and a quarter is the same as multiplying by eight tenths. A twenty amp breaker may therefore carry a sixteen amp continuous load, which is exactly the example worked above read backwards. This eighty percent framing is handy in the field when you are looking at an existing breaker and want to know how much continuous load it may legally carry, rather than starting from a load and sizing up. Both forms describe the same margin that keeps the device and its terminations from running hot for hours.

Breakers and conductors rise together

It is worth stressing that the twenty-five percent uplift applies to the conductor as well as to the breaker, not to one alone. If you size the overcurrent device for a continuous load but leave the wire at its bare ampacity, the wire can run hot under the same sustained current the rule was written to guard against. So both the device and the conductor are sized at one and a quarter times the continuous current, and then the conductor is checked against the small-conductor cap and any derating. The breaker and the wire move up as a pair, which keeps the protection and the thing it protects in proper proportion across the whole circuit.

Why oversizing a breaker is dangerous

A tempting but hazardous fix for a breaker that keeps tripping is to swap it for a larger one. Because the breaker exists to protect the conductor, fitting a bigger breaker on the same wire lets the wire carry current beyond its safe rating, and the breaker will no longer trip before the conductor overheats. The correct response to repeated tripping is to find the cause, which is usually a genuine overload or a fault, and to address it, or to upsize the wire and the breaker together if the circuit legitimately needs more capacity. Treating the breaker as a nuisance to be silenced rather than as the conductor protection it is remains one of the most dangerous shortcuts in amateur electrical work.

Putting it together

To size overcurrent protection: classify the load as continuous or not, multiply the continuous part by 1.25 and add any non-continuous part, round up to a standard breaker size, then confirm the conductor can carry that rating after the small-conductor cap and any derating. The result protects the wire for the life of the circuit. These are planning estimates: final breaker and conductor selection, terminal temperature ratings and all derating must comply with the NEC edition adopted by your jurisdiction and be verified by a licensed electrician.