Sizing a Branch Circuit — Continuous Loads, Breakers, and the 1.25× Rule
The branch-circuit math people get wrong most often isn't the ampacity table — it's the 1.25× multiplier on continuous loads. Skip that step and you'll undersize the breaker, the breaker will trip on long-running loads, and the customer will call you back to "fix it."
What NEC 210.20(A) actually says
The overcurrent device (OCPD — your breaker) shall not be less than the non-continuous load plus 125% of the continuous load. In a formula:
Demand amps = non-continuous_amps + 1.25 × continuous_amps
Same rule applies to feeders under 215.3.
What "continuous" means
A load is continuous if it's expected to run at its maximum current for 3 hours or more. Some examples:
- Continuous: water heater on heavy use, EV charger, pool pump, hot-tub heater, baseboard heating, large fixed lighting in commercial space.
- Non-continuous: microwave, hair dryer, vacuum, small kitchen appliances, refrigerator (cycles on and off), tools.
The fridge confuses people. Even though it runs all day, the compressor cycles — it's not at maximum current for 3 hours straight. Non-continuous.
Why 1.25
Heat. A breaker rated 20 A continuously is actually designed to carry 20 A only intermittently. Run 20 A for 3+ hours and the breaker, the wire, and the terminations all heat up enough that the safety factor erodes. The 1.25 multiplier means a breaker rated for 80% of its label is the right size for a load running at 100% continuously. Or stated the other way: a 20 A breaker can carry 16 A continuously without nuisance-tripping over hours.
Then there's the wire
After you pick the breaker, the wire must have ampacity at least equal to the breaker rating, with the NEC 240.4(D) small-conductor caps baked in:
- 14 AWG copper: max 15 A breaker
- 12 AWG copper: max 20 A breaker
- 10 AWG copper: max 30 A breaker
These are hard caps regardless of what the temperature column says. The 75°C copper THHN ampacity column would technically allow 25 A on #12, but 240.4(D) caps it at 20.
A worked example
Kitchen circuit feeds a refrigerator (700 W, non-continuous), a microwave (1500 W, non-continuous), and a toaster oven (1200 W, non-continuous). All on 120 V.
Convert to amps: - Fridge: 700 / 120 = 5.83 A non-continuous - Microwave: 1500 / 120 = 12.50 A non-continuous - Toaster: 1200 / 120 = 10.00 A non-continuous
Total non-continuous: 28.33 A. Total continuous: 0 A.
Demand: 28.33 + 1.25 × 0 = 28.33 A.
Next standard breaker (NEC 240.6) at or above 28.33 A is 30 A.
Wire to match 30 A breaker on copper THHN at 75°C: 10 AWG (capped at 30 A by 240.4(D), exactly enough).
So this circuit needs a 30 A breaker on 10 AWG copper. In practice the kitchen rule is two 20 A small-appliance branch circuits split among the receptacles — you wouldn't put all three on one circuit — but the math walks the same way.
A continuous-load case
Same kitchen, but add a 7200 W EV charger at 240 V on its own circuit. Continuous.
Charger amps: 7200 / 240 = 30.0 A continuous.
Demand: 0 + 1.25 × 30 = 37.5 A.
Next standard breaker: 40 A.
Wire at 40 A on 75°C copper THHN: 8 AWG (50 A from the table).
Note that the breaker is larger than the load, by design. The 30 A continuous load on a 40 A breaker is exactly the 80% relationship — and the NEC 625.41 EVSE rule confirms it.
The mixed-voltage trap
If your kitchen circuit is 120 V and your EV charger is 240 V, they belong on separate circuits. The math still works on a single sheet — you sum demand in amps regardless of voltage — but the breaker / wire recommendation only makes sense when all loads share a voltage. Mixing 120 V and 240 V appliances on one calc gives a number that doesn't correspond to a real-world feeder.
Skip the math
The Circuit Load Calculator walks you through this. Pick appliances from a 38-item library (with the continuous flag pre-set per device), or type in custom loads. The tool sums non-continuous + 1.25 × continuous, picks the next standard breaker, picks the matching copper wire honoring the 240.4(D) caps, and warns you if you've mixed voltages on one calc.
Branch circuit, not service load
What this tool sizes is a single circuit under NEC 210.20. Whole-house service load uses NEC Article 220 demand factors — different rules, more complex, with allowances for the fact that not every appliance runs at the same time. Don't try to size a service panel with the branch-circuit calc.
The short version
Continuous load × 1.25, plus non-continuous × 1.0, rounded up to the next standard breaker, paired with wire that has at least that ampacity (with small-conductor caps applied). That's the whole rule.