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Breaker size calculator

An 18.75 A load that runs for three hours or more carries the continuous-load margin, so the design current is 23.4 A and the smallest standard rating above it is 25 A. The size below, 20 A, would trip on a healthy circuit; on a 30 A conductor the chosen breaker uses 83.3 % of what the cable can carry.

Your circuit

From your own table, already derated for grouping and temperature. Leave it empty if you do not have it.

Heating, EV charging and lighting run for hours, so the rating carries a 25 % margin.

Result

Breaker rating
25 A
Design current
23.4 A
Rating against the cable
83.3 %

The rating below and the one above

  • 20 A-3.4 A · 66.7 %Trips on this load
  • 25 A1.6 A · 83.3 %Protects the cableyour breaker
  • 30 A6.6 A · 100.0 %Protects the cable

NEC 240.6(A) · Rating against the cable: 83.3 %

How the calculation works

Two conditions have to hold at once, and they pull in opposite directions:

design current  ≤  breaker rating  ≤  conductor ampacity

The left inequality keeps the circuit working: a rating under the design current trips on a load that is doing nothing wrong. The right one keeps it safe: a breaker larger than the conductor can carry lets the cable run hot indefinitely, because from the device’s point of view nothing unusual is happening.

The design current is the load current, multiplied by the continuous-load factor when the load runs for hours. That factor is a code rule rather than physics, so it is an input here and it travels with the result, and the calculator rounds the outcome up to the next rating that exists in the series you picked.

What the tool adds to the single number is the two ratings either side of it, each judged the same way - one shows what you lose by going cheaper, the other shows exactly where “one size up, to be safe” stops being safe. If you leave the ampacity field empty the sizing still works, but nothing on the page has checked the cable, and the panel says so rather than implying a pass.

Worked example

Your circuit

Load current (A)
18.8 A
Type of load
Continuous - three hours or more
Rating series
NEC 240.6(A)
Ampacity of the cable (A)
30 A

Result

Breaker rating
25 A
Design current
23.4 A
Rating against the cable
83.3 %

Protects the cable

Formula
Ib · k ≤ In ≤ Iz
Standard
NEC 240.6(A), NEC 210.20(A)
Limits of the model
Overload only: a breaker also has to clear a short circuit, which depends on the prospective fault current and on the device’s breaking capacity.

Computed by Voltacalc with the same engine as the tool above.

What applies in the United States

In the United States the 125 % rule for continuous loads is written down twice – in 210.20(A) for branch circuits and 215.3 for feeders - and it is the step most DIY sizing skips, which is why a 16 A EV charger keeps tripping a 20 A breaker that looks generous on paper. The standard ratings themselves are the 240.6(A) list, and the ampacity your conductor is allowed is the 310.16 table after the 90 °C column has been knocked back to the temperature rating of the terminals - usually 75 °C. The AHJ in your city is the one who decides how that reads on your job.

Who decides
NFPA / the authority having jurisdiction (AHJ) in your state or city
Standard
NEC (NFPA 70), National Electrical Code
Ampacity table
NEC Table 310.16, with the correction and adjustment factors of 310.15(B) and (C), and the small-conductor rule in 240.4(D)
Permitted drop
The NEC does not make voltage drop a rule for most branch circuits: 210.19(A) and 215.2(A)(1) carry Informational Notes recommending no more than 3% on the branch circuit and 5% for feeder plus branch combined. Your AHJ, or a specific article such as 647.4(D), can make a lower figure mandatory. (Checked: September 12, 2026)

Questions electricians ask

What counts as a continuous load?

One whose maximum current is expected to run for three hours or more: lighting in a store, an EV charger, electric heat, a well pump on a long duty cycle. The device is then sized for 125 % of that current, because the breaker itself heats up over those hours and its trip curve moves with it.

Why does the calculator ask for the cable's ampacity?

Because that is the number the breaker is actually protecting. The load decides the smallest rating that will not nuisance-trip; the conductor decides the largest rating that is legal. Choosing between them without the second number is how a 12 AWG circuit ends up behind a 30 A breaker and overheats without ever tripping.

The calculator rounds up. Is that always right?

For the overload side, yes: a rating below the design current trips on a healthy circuit, and an installer who has to reset a breaker every evening will eventually put in something far too large. Rounding up stops at the conductor, though - the next size up is only available if the cable ampacity allows it.

Does this size the breaker for a short circuit too?

No. This is the overload side only. Interrupting a fault depends on the available fault current at that point in the system and on the device's interrupting rating, which is a separate calculation and a separate label on the breaker.

Standards and sources

  • NEC (NFPA 70), National Electrical Code
  • NEC Table 310.16, with the correction and adjustment factors of 310.15(B) and (C), and the small-conductor rule in 240.4(D)

Last updated: September 13, 2026

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