Breaker Size Calculator — load amps to a standard rating

Sizing an overcurrent device starts with one number — the load current — and applies one rule that catches most people out: a load running for three hours or more is treated as continuous, and the circuit must be sized for 125 percent of it. This breaker size calculator takes a load described in watts, amps or horsepower, works out the current, applies the continuous factor, and identifies the next standard breaker rating at or above the result.

Arb Digital builds free calculators that show their reasoning, and this one is deliberately narrow in what it claims. It performs the arithmetic set out in the National Electrical Code for the minimum circuit ampacity of a branch circuit. It does not select a conductor size, it does not apply the derating factors for ambient temperature or for multiple conductors in a raceway, and it does not know your local amendments. A licensed electrician sizes the final circuit. This page tells you what the arithmetic gives, so you can arrive at that conversation understanding the numbers.

Which Rule This Uses and Where It Applies

The 125 percent continuous-load factor comes from Article 210 of NFPA 70, the National Electrical Code, which requires the rating of a branch-circuit overcurrent device serving continuous load to be not less than the non-continuous load plus 125 percent of the continuous load. The list of standard ampere ratings the tool snaps to — 15, 20, 25, 30, 35, 40, 45, 50, 60 and upward — is the list of standard inverse-time breaker ratings given in Article 240 of the same code.

That means this page describes United States practice, and the practice of jurisdictions that adopt the NEC. It does not describe installations built to IEC 60364, the international standard for low-voltage electrical installations, or to national codes derived from it such as BS 7671 in the United Kingdom. Those use a different framework built around the relationship between design current, protective device rating and cable current-carrying capacity, with correction factors applied in a different order. The arithmetic here will not give you a compliant answer under those rules.

Local amendments matter as well. Cities and states adopt specific NEC editions and modify them, and an adopted edition may be several cycles behind the current one. Whichever number this page produces, the authority having jurisdiction over your installation decides what is acceptable there.

How to Use It

1. Take the figures from the nameplate. The rated power or full-load current on the equipment label is what the code calculation uses, not a clamp-meter reading taken while the machine idles.
2. Set the voltage to the nominal system voltage. 120 V or 240 V single phase, 208 V or 480 V three phase. Use the nominal figure, not the measured one.
3. Decide honestly whether the load is continuous. Three hours or more of operation at its maximum makes it continuous. Lighting in a shop, a heater in winter and most commercial refrigeration all qualify.
4. Enter power factor for inductive loads. Motors, transformers and electronic drives draw more current than their real power suggests. A power factor of 0.85 raises the current by nearly 18 percent.
5. Read the minimum circuit ampacity, not just the breaker. That figure is what the conductor must be able to carry after all derating — and it is the number to take to an electrician.

The Formula: How It Is Calculated

For a single-phase load described in watts, current is power divided by voltage and power factor: I = P / (V × PF). For a three-phase load it is I = P / (√3 × V × PF). A horsepower rating first converts to watts at 745.7 W per horsepower and is then divided by the motor's efficiency, because the input power a motor draws is larger than the mechanical output it delivers.

The minimum circuit ampacity is that current multiplied by 1.25 when the load is continuous, or taken as is when it is not. The tool then finds the smallest standard rating that is greater than or equal to the minimum. Working the default through: 1,800 W at 120 V with a power factor of 1 gives 1,800 ÷ 120 = 15.0 A. Treated as continuous, 15.0 × 1.25 = 18.75 A minimum. The smallest standard rating at or above 18.75 A is 20 A, so that is the answer, and the actual load then sits at 15 ÷ 20 = 75 percent of the breaker rating.

That 75 percent figure is the other half of the same rule seen from the opposite direction. A standard breaker is not intended to carry more than 80 percent of its rating continuously, and sizing at 125 percent of the load is exactly what guarantees the load stays at or below 80 percent of the device. The two statements are arithmetic inverses of each other — 1 ÷ 1.25 = 0.8 — and the tool reports both so the relationship is visible.

Why the Breaker Does Not Protect the Appliance

The most persistent misunderstanding in domestic wiring is that a breaker protects whatever is plugged in. It does not. A branch-circuit breaker protects the conductor behind it from carrying more current than it can dissipate safely. Appliances have their own internal protection, and the breaker is far too coarse and far too slow to substitute for it.

This is why replacing a breaker that keeps tripping with a larger one is dangerous rather than merely non-compliant. The wire in the wall has a fixed current-carrying capacity determined by its size, its insulation temperature rating, and the conditions it runs in. Raise the breaker above that capacity and the wire can now run hot indefinitely without anything intervening. The breaker will still be doing exactly what it was designed to do, which is nothing until its own threshold is exceeded.

The correct response to persistent tripping is to find out why the current is high. That may be a genuine overload — too much connected to one circuit — or a fault, or a failing motor drawing excess current as its bearings degrade. Each has a different fix, and none of them is a bigger breaker.

What This Calculator Deliberately Does Not Tell You

It does not give a wire gauge. Selecting a conductor requires the allowable ampacity tables in Article 310 of the code, and those figures depend on the insulation temperature rating, the ambient temperature, the number of current-carrying conductors bundled together, and the temperature rating of the terminals at both ends. Ambient correction alone can reduce a conductor's usable ampacity by a third in a hot attic or a boiler room. Publishing a gauge without those inputs would be worse than publishing nothing, because it would look authoritative.

It also does not handle several categories of load that have their own rules. Motor branch circuits are sized by a separate method using full-load current tables and permitted short-circuit protection percentages that can exceed 250 percent of the full-load current, because a motor's inrush is many times its running current. Air-conditioning and refrigeration equipment carries a minimum circuit ampacity and a maximum overcurrent protective device rating printed on its own nameplate, and those printed values take precedence over any calculation. Welders, transformers and capacitor banks each have their own articles.

Finally, it does not consider voltage drop. A circuit can be perfectly protected and still deliver too little voltage at the far end of a long run, which makes motors run hot and lighting dim. Voltage drop is a performance calculation rather than a protection one, and it frequently drives conductors larger than protection alone would require.

Reading the Loading and Headroom Figures

The loading percentage tells you how much of the chosen breaker's rating your load actually occupies. For a continuous load, anything at or below 80 percent is what the sizing rule is designed to produce. A figure well below that means you have landed just above a standard rating boundary and are carrying more spare capacity than you needed — not a problem, but worth noticing if conductor cost matters over a long run.

A loading figure above 80 percent on a continuous load means the arithmetic did not land where the rule intends, which normally indicates an input error: a power factor left at 1 for a motor load, or a load marked non-continuous that genuinely runs all day. The tool flags this case explicitly rather than letting it pass.

The headroom figure, in amps, is the difference between the breaker rating and the actual load current. It is the practical number for planning: if you are considering adding a second device to the same circuit, its current has to fit inside that headroom after the continuous factor is applied to the combined total, not to each load separately. For working out the running cost of whatever ends up on the circuit, the electricity bill calculator converts power and hours into money.

Common Mistakes to Avoid

• Using a measured idle current instead of the nameplate rating — the calculation is based on the load's rated maximum, and a machine that is not working hard tells you nothing about it.
• Leaving power factor at 1 for a motor — inductive loads draw more current than their real power implies, and ignoring that understates the required rating.
• Calling a load non-continuous because it cycles — a thermostat-controlled heater that runs most of the winter afternoon is continuous in practice, and the three-hour test applies to operation at maximum current.
• Fitting a larger breaker to stop nuisance tripping — the wire's capacity has not changed. Increasing the breaker removes protection instead of adding capacity.
• Treating this number as a conductor size — minimum circuit ampacity is the starting point for conductor selection, before ambient and bundling derating are applied from the code tables.

Related Free Tools From Arb Digital

Electrical work usually needs several numbers at once. Use the electrical power calculator to move between volts, amps, watts and resistance, the electricity bill calculator to price the consumption, and the watts to horsepower converter when equipment is rated one way and specified the other. The power converter handles other power units, and the percentage calculator is handy for checking derating factors by hand. The complete free online tools hub lists every calculator here.

Frequently Asked Questions

This tool is provided for educational use only. It performs a code arithmetic exercise and is not electrical design advice; a licensed electrician must size, install and verify the final circuit in accordance with the code edition and amendments adopted in your jurisdiction.