A breaker is not the on/off switch the homeowner thinks it is — it is a calibrated thermal/magnetic device sized to protect the wire downstream from overheating. Get the breaker right and the wire never runs warm. Get it wrong and either the breaker nuisance-trips or, worse, the wire cooks before the breaker notices.

Breakers come in standard sizes only

NEC 240.6(A) lists the standard breaker sizes:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A

Anything else is non-standard — they exist (35, 45 A are stocked but unusual) but selecting one outside this list means specifying a custom breaker. The breaker calculator always rounds up to the nearest standard.

The 125% rule for continuous loads

A continuous load operates 3 hours or more — lighting in a retail store, an EV charger, electric baseboard heat in January, the freezer in your garage, the LED sign over your shop. NEC 210.20(A) requires breaker (and conductor) to be sized at 125% of the continuous load:

  • 16 A continuous → 16 × 1.25 = 20 A breaker
  • 32 A EV charger → 32 × 1.25 = 40 A breaker
  • 48 A EV charger → 48 × 1.25 = 60 A breaker

The 80%/125% rule comes from the breaker thermal curve. Standard breakers are calibrated for 80% sustained operation at their rating; push past that and the bimetal strip drifts, then nuisance-trips, then doesn't trip when it should.

Watts to amps the right way

For resistive loads (heaters, incandescent lights, ovens):

amps = watts ÷ volts

For 3-phase: divide by V × √3 × power factor. For motors: use the FLA (full-load amps) on the nameplate, not watts — motor power factor and inrush mess up the math.

Motor circuits play by NEC 430

Motors get sized differently because of inrush. A 12 A motor drawing 60 A for half a second at startup would trip a 15 A breaker every time it cycled. NEC 430.52 lets the inverse- time breaker on a motor circuit run up to 250% of the FLA (typical small motors), with the overload relay at the starter handling the actual thermal protection of the motor windings.

A 12 A continuous-rated air handler motor commonly lands on a 30 A breaker — fully code-legal, with a 12 A overload at the contactor doing the protection. The calculator flags motor mode and applies the 2.5× multiplier.

Wire and breaker have to match

Sizing wire to one rating and breaker to another gets people killed. Pairs that always go together (residential, NEC 310.16 75°C):

  • 14 AWG copper ↔ 15 A breaker
  • 12 AWG copper ↔ 20 A breaker
  • 10 AWG copper ↔ 30 A breaker
  • 8 AWG copper ↔ 40 A or 50 A breaker
  • 6 AWG copper ↔ 60 A or 65 A breaker
  • 4 AWG copper ↔ 85 A breaker
  • 3 AWG copper ↔ 100 A breaker
  • 2/0 AWG copper ↔ 175 A breaker
  • 3/0 AWG copper ↔ 200 A breaker

Run the load through the breaker calculator, then verify the matching wire on the wire gauge calculator.

Common breaker mistakes

Upsizing the breaker to stop nuisance trips.A 15 A circuit on 14 AWG that keeps tripping is telling you the load is too heavy for the wire — not the breaker. Going to 20 A on 14 AWG turns the breaker into kindling protection instead of wire protection. Add a circuit, don't upsize the breaker.

Putting an EV charger on a 50 A breaker for a 40 A car. A 40 A continuous load is 50 A breaker only if the EVSE is hardwired and listed for 80% — a plug-in 40 A charger needs 50 A breaker, but 50 A continuous needs a 60 A breaker. Read the EVSE spec sheet.

Forgetting the AFCI / GFCI requirements.Bedrooms, kitchens, and bathrooms have specific breaker-type requirements that the size calculator does not check.

Estimate only. The breaker size calculator gives the next-standard NEC 240.6 size with the 125% continuous multiplier and a basic motor flag. AFCI, GFCI, series-rated panels, and selective coordination are not modeled. Confirm breaker selection with a licensed electrician and the local AHJ.