Breaker Size Calculator

To size a breaker, divide the load in watts by the circuit voltage to get amps, divide by 0.8 for continuous loads, then round up to the next standard breaker size. A 3,000-watt continuous load on a 240-volt circuit draws 12.5 amps. Dividing by 0.8 gives a required ampacity of 15.6 amps, so the next standard breaker is a 20-amp breaker. The calculator above does this instantly and also shows load current, required ampacity, and the safety headroom left on the breaker.

Circuit breakers are not sized to match the load exactly. The National Electrical Code (NEC) requires a continuous load — one expected to run for three hours or more — to be served by an overcurrent device rated at least 125% of that load. Because 125% is the same as dividing by 0.80, electricians call it the 80% rule: a breaker should carry no more than 80% of its rating continuously. Sizing to this rule keeps the breaker cool, prevents nuisance tripping, and satisfies inspectors.

The equation this calculator uses has three steps:

Each step matters:

• Watts ÷ Volts converts power into current. On single-phase circuits, use 120V for standard receptacles and lighting, 240V for large appliances, and 208V for many commercial single-phase circuits fed from a three-phase panel.
• Divide by 0.8 only for continuous loads. This applies the NEC 125% factor. Non-continuous loads (running less than three hours, like a receptacle circuit) can be sized to the load current directly, though good practice still leaves margin.
• Round up to a standard size. Breakers are only manufactured in fixed ratings. NEC 240.6(A) lists the standard sizes; you must select the next size at or above the required ampacity, never a smaller one.

The conductor must also be rated for the breaker: a 20A breaker needs 12 AWG copper, a 30A breaker needs 10 AWG, and so on. Verify wire sizing with the wire size calculator before finalizing any circuit. For whole-panel planning, use the electrical load calculator for house.

These examples show the full three-step method with the arithmetic spelled out.

Example 1 — 3,000W continuous heater on 240V: Amps = 3,000 ÷ 240 = 12.5 A. Continuous, so required = 12.5 ÷ 0.8 = 15.63 A. The next standard breaker above 15.63 is 20 A. A 15A breaker would be undersized because 15.63 exceeds 15.

Example 2 — 1,500W space heater on 120V: Amps = 1,500 ÷ 120 = 12.5 A. Continuous, so required = 12.5 ÷ 0.8 = 15.6 A → 20 A breaker on 12 AWG wire. This is why space heaters are notorious for tripping shared 15A living-room circuits.

Example 3 — 4,800W electric range element on 240V (non-continuous): Amps = 4,800 ÷ 240 = 20 A. Cooking is treated as non-continuous, so required ≈ 20 A → a 25 A breaker would cover it, though ranges are usually on 40–50A circuits because of the combined nameplate demand of all burners plus the oven.

Example 4 — 1 HP motor at 240V: A 1 HP single-phase motor has a full-load current near 8 A. Motor branch-circuit protection is sized higher — often 250% of full-load current for inrush — so a motor may legally sit on a 15–20A breaker even though its running current is only 8 A. Motors follow NEC Article 430 rather than the plain 80% rule, so treat this tool's motor output as a running-load starting point and confirm with the motor nameplate.

The single most misunderstood part of breaker sizing is the difference between continuous and non-continuous loads.

A continuous load is any load expected to operate at its maximum current for three hours or more. Examples include electric water heaters, baseboard heat, EV chargers, commercial lighting, and HVAC. NEC 210.20(A) requires the breaker (and the conductor) to be rated at 125% of a continuous load — equivalently, the continuous load may not exceed 80% of the breaker rating. That is why a 20A breaker is limited to 16 continuous amps and a 30A breaker to 24 continuous amps.

A non-continuous load runs in shorter bursts: kitchen appliances, power tools, receptacle circuits, and cooking equipment. These can be sized to the actual load current, though most electricians still leave headroom. When a circuit carries both continuous and non-continuous loads, the required breaker is 125% of the continuous portion plus 100% of the non-continuous portion.

Getting this wrong is a real hazard. A breaker loaded to 100% continuously runs hot, its thermal element ages, and it may either trip randomly (a nuisance) or fail to trip when it should. Sizing to 80% keeps everything within its thermal design margin.

1. Enter the load in watts. Use the nameplate wattage of the appliance or the total watts of everything on the circuit. If you only know amps, multiply amps × volts first.
2. Select the voltage. Choose 120V for standard branch circuits, 240V for large appliances and dryers, or 208V for commercial single-phase circuits.
3. Choose the load type. Select "Continuous" for anything that runs three hours or more (heaters, EV chargers, HVAC); select "Non-continuous" for intermittent loads.
4. Read the recommended breaker size. The primary result is the next standard breaker at or above the required ampacity. The secondary outputs show your actual load current, the code-required ampacity, and the percentage of headroom remaining.

After sizing the breaker, confirm the conductor with the wire size calculator and check the total circuit demand with the circuit load calculator.

• Ignoring the 80% rule. Loading a 20A breaker to a full 2,400W continuously (20A × 120V) will trip it. The real continuous limit is 1,920W.
• Upsizing the breaker without upsizing the wire. Putting a 30A breaker on 12 AWG wire is a fire hazard — the wire can overheat before the breaker trips. The breaker protects the conductor, so they must match.
• Rounding down. If the required ampacity is 15.6 A, you must go up to 20 A, not down to 15 A.
• Treating motors like resistive loads. Motors draw large inrush current and follow NEC Article 430, not the simple 80% rule. Use the circuit load calculator for resistive loads and a motor-specific method for motors.
• Forgetting derating. Multiple conductors in a conduit or high ambient temperature reduce a wire's usable ampacity, which can force a larger conductor even when the breaker size looks fine.

A frequent surprise for people new to electrical work is that you cannot buy a breaker in any rating you like. NEC 240.6(A) publishes a fixed list of standard overcurrent-device sizes, and manufacturers build to that list: 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, and 200 amps, with larger sizes continuing above. Notice the gaps — there is no 35A breaker, and the jump from 100A to 125A skips 110 and 115. Whenever your calculated required ampacity lands between two standard values, you must select the next size up, never the closer size down.

This standardization exists for good reasons. Coordinating protection across a system — the main breaker, subpanel feeders, and branch circuits — is far simpler when everyone works from the same rung of sizes. It also lets conductor ampacity tables, tap rules, and equipment ratings align cleanly with breaker ratings. The trade-off is that circuits are almost always slightly oversized relative to the pure calculation, which is exactly the safety margin the code intends.

There is one important exception built into the code. NEC 240.4(B) permits rounding up to the next standard breaker when a conductor's ampacity does not match a standard size — but only for circuits rated 800 amps or less, and only when the conductor is not part of a multi-outlet branch circuit supplying receptacles for portable, cord-and-plug loads. For ordinary receptacle circuits, NEC 240.4(D) instead sets hard ceilings: 14 AWG copper is capped at 15A, 12 AWG at 20A, and 10 AWG at 30A, regardless of the higher ampacities those wires show in the 60/75/90°C tables. These "small-conductor" rules are why a 12 AWG wire that can technically carry 25 or 30 amps is still limited to a 20A breaker on a normal branch circuit.

Understanding this hierarchy — calculate the load, apply the continuous factor, round up to a standard size, then confirm the conductor is legal for that breaker — keeps every circuit both code-compliant and genuinely safe. When in doubt, size conservatively; the small extra cost of the next wire gauge is trivial next to the risk of an overheated conductor.

Here are typical breaker sizes for common residential loads, useful as a sanity check against the calculator.

Lighting and general receptacles sit on 15A or 20A circuits. Kitchen small-appliance circuits are 20A by code (at least two required). An electric dryer is usually a 30A/240V circuit on 10 AWG. An electric water heater (4,500W/240V) needs a 30A breaker — see the dedicated water heater breaker calculator. An electric range typically uses 40–50A. A Level 2 EV charger at 32A continuous needs a 40A breaker (32 ÷ 0.8 = 40).

Central air conditioning and heat pumps list a "maximum overcurrent protection" (MOCP) value on the nameplate — always use that number rather than calculating from watts, because HVAC compressors have special inrush requirements. When adding several new circuits, verify the panel and service can carry the total with the electrical load calculator for house so you do not overload the main breaker.