🔘What size breaker do you need?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Breaker size calculator

Load on the circuit

One-way run

System voltage

Metal

Terminal rating

Ambient

°F

Current-carrying conductors in the raceway

25 amp breaker

protecting 10 AWG copper · 10 AWG ground · the load asks for 23.4 A

0.6% drop at 25 ft

Ampacity wants10 AWG
Voltage drop wants14 AWG

Current the conductor has to carry: 23.4 A
Usable ampacity after derating: 35 A (table 35)
Longest run before the drop passes 3%: 124 ft
In metric sizes: 5.3 mm²

12 AWG had the ampacity and 240.4(D) still rejected it, at 20 A.

This assumes a continuous load, so 125%. For short bursts read the current line rather than the breaker.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

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What size breaker do you need?

The AmpSizer breaker size for a 4.5 kW water heater on 240 volts is 25 amps, protecting 10 AWG copper. The heater draws 18.75 A, the continuous-load multiplier makes that 23.4 A, and the next standard rating in 240.6(A) is 25.

The interesting part is the conductor. 12 AWG copper carries 25 A in Table 310.16 and still cannot be used here, because 240.4(D) holds 12 AWG to a 20 amp device — and a 20 amp device does not protect a 23.4 amp load.

That is the rule that makes this a 10 AWG circuit rather than a 12 AWG one, and it is why a breaker calculator that only looks at ampacity gives the wrong wire.

The chart below pairs every common device rating with the conductor it is allowed to protect, computed from the same tables as the calculator. The 240.4(D) ceiling is already applied, which is why 20 A shows 12 AWG rather than the 25 A that Table 310.16 would suggest.

Breaker rating against the smallest conductor that carries it at 75 °C, with no derating and a short run. Computed from NEC Table 310.16, 240.4(D) and Table 250.122.
Breaker	Copper	Aluminium	Copper ground	What it usually feeds
15 A	14 AWG	12 AWG	14 AWG	Lighting and general receptacles
20 A	12 AWG	12 AWG	12 AWG	Kitchen, bathroom and garage receptacles
30 A	10 AWG	10 AWG	10 AWG	Dryer, small water heater, RV outlet
40 A	8 AWG	8 AWG	10 AWG	Range, 40 A EV charger
50 A	8 AWG	6 AWG	10 AWG	Range, welder, 50 A EV charger, hot tub
60 A	6 AWG	4 AWG	10 AWG	Sub panel, large hot tub, tankless heater
70 A	4 AWG	3 AWG	8 AWG	Sub panel, larger tankless heater
100 A	3 AWG	1 AWG	8 AWG	Sub panel, or a small service
125 A	1 AWG	2/0 AWG	6 AWG	Sub panel
150 A	1/0 AWG	3/0 AWG	6 AWG	Service, or a large sub panel
200 A	3/0 AWG	250 kcmil	6 AWG	Typical whole-house service

The standard ratings, and why they matter

240.6(A) is a list, and a device that is not on it effectively does not exist for sizing purposes:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 and upward.

Rounding always goes up, with one exception worth knowing: motor circuits under 430.52 are allowed to go up to a percentage ceiling and then take the next size down if the ceiling falls between two standard ratings. That is a motor rule, not a general one, and it is on the motor page.

Where the answer changes

A continuous load changes the multiplier, and with it often the wire. Electric heat and EV charging are the two that catch people, because both feel like ordinary appliances and neither is.

Derating changes the conductor but not the device. Six conductors in a pipe do not make the breaker smaller; they make the wire bigger.

A long run changes the conductor but not the device either. Voltage drop is a design target, not overcurrent protection, so upsizing for distance leaves the breaker exactly where it was — and pulls the ground up with it under 250.122(B).

Frequently asked questions

What size breaker for a 240 volt heater?

Divide the watts by 240 for the current, multiply by 1.25 because a heater is a continuous load, and round up to a standard rating. A 4.5 kW element is 18.75 A, so 25 A; a 5.5 kW is 22.9 A, so 30 A. Then check the conductor separately, because the small-conductor rule can force it larger than the ampacity table suggests.

Can I put a bigger breaker in if it keeps tripping?

No, and this is the most dangerous mistake in domestic electrical work. The breaker is sized to protect the conductor; a bigger one leaves the wire unprotected while removing the only warning you had. A breaker that trips repeatedly is either a circuit that is genuinely overloaded, or a fault. Both need finding, not silencing.

Why are there no 22 or 27 amp breakers?

Because 240.6(A) lists the standard ratings, and manufacturers build to that list: 15, 20, 25, 30, 35, 40, 45, 50, 60 and upward. Anything in between is a non-standard device, so a load that calculates to 23.4 A gets a 25 A breaker, and one at 26 A gets 30.

Which loads count as continuous?

Anything expected to run for three hours or more: general lighting, electric heating, EV charging, refrigeration in a shop, process equipment. A domestic receptacle circuit, a range and a dryer are not continuous. The distinction is a factor of 1.25 on the conductor and the device, so it is worth being right about.

Is the breaker sized to the wire or to the load?

Both, from opposite directions. It has to be big enough that the load does not trip it, and small enough that it protects the conductor. When those two cannot both be satisfied, the conductor has to get bigger — which is exactly what the calculator does when 240.4(D) blocks a size.

How we calculate this

The load current is watts divided by volts, times the square root of three for a three-phase circuit. Continuous loads are multiplied by 1.25 per 210.19(A)(1) and 215.2(A)(1). The device is the next standard ampere rating at or above that figure from the list in 240.6(A). The conductor is then chosen so that it satisfies both allowable ampacity (Table 310.16 corrected by Tables 310.15(B)(1) and 310.15(C)(1)) and the voltage drop target — and so that the small-conductor ceiling of 240.4(D) permits the device: 15 A for 14 AWG copper, 20 A for 12 AWG, 30 A for 10 AWG. A conductor with enough ampacity but too low a ceiling is rejected rather than paired with a device too small for the load.

Worked example

With Load on the circuit 4.5, One-way run 25, System voltage 240 V, Metal Copper, Terminal rating 75 °C — the usual breaker, Ambient 86, Current-carrying conductors in the raceway 3, this page works out 25 amp breaker. protecting 10 AWG copper · 10 AWG ground · the load asks for 23.4 A

Current the conductor has to carry23.4 A
Usable ampacity after derating35 A (table 35 )
Longest run before the drop passes 3%124 ft
In metric sizes5.3 mm²

Sources:

What a breaker is actually protecting

The wire, not the appliance. Appliances have their own internal protection. The breaker exists so that a fault or an overload on the conductor opens before the insulation is damaged.

80% is a consequence, not a rule. There is no code sentence saying “load a breaker to 80%”. There is a sentence saying a continuous load is taken at 125%, which produces the same number backwards, and it only applies to continuous loads.

A breaker is not a switch, and not a surge device. It is a thermal and magnetic overcurrent device. Using one as a daily on-off switch wears the mechanism that is supposed to save you.