Breaker Size Calculator

The fuse size calculator tool converts the load in a circuit to the rated current of the device that protects this circuit. By entering the load, voltage and degree of utilization of the circuit it gives back the nearest standard fuse size which is bigger than the calculated current.

The calculations are simple. The decisive factor is to set the prerequisites correctly as many incorrect results result from this. Incorrect voltage values, wrong phase configurations or calculating continuous loads as short-term loads can lead to this.

What a fuse actually protects:

There is often a tendency to think that fuses protect electrical appliances but they do not. A fuse protects the wiring, connections and path of electricity to the appliances connected to the mains supply.

Once you understand this point, almost every rule that is mentioned later becomes clearer. It's not true that the bigger the fuse, the safer it is. A larger fuse can cause more current to flow through wires that are not capable of handling that much current. The wires will heat up and the insulation will deteriorate while the fuse does not blow.

There are three main reasons why a circuit breaker trips: an overload occurs when the current exceeds the allowable value of the circuit; a short circuit occurs when a live conductor connects directly to the neutral; and a ground fault occurs when a live conductor connects to a conductive object that should not normally carry electricity.

Calculating the size of the fuse

The calculation is done in two steps: First the value is converted into amperes, then a buffer for operating load is added and rounded up.

First step: converting the load to amperes

The required formula depends on the type of circuit. In DC circuits, current is equal to power divided by voltage.

In single-phase AC power, the power factor must be taken into account because the waveforms of current and voltage are not in phase.

For balanced three-phase AC power, measured as the voltage between lines, a factor of approximately 1.732 is required.

If the given value represents the voltage between phase and neutral conductor, this factor is 3.

If the rated output of a device is given in kVA instead of watts, then the power factor does not have to be taken into account at all since the product of volts and amperes already contains apparent power.

Second step: apply a safety factor for operating load and round up.

Multiply the load current by a factor that fits the mode of operation for the circuit and select the next highest standard size which is greater than or equal to the result.

For continuous loads a factor of 125 percent is applied. This is the most common case. A continuous load is one that runs at full current for at least three hours. Adding in a quarter ensures it stays within its thermal limits without tripping the breaker due to heat building up inside the breaker itself.

The 80 percent rule is a reversed representation of the same rule.

You will likely hear two different phrasings of this: Select the size with a 125 percent continuous load or keep the continuous load below 80 percent of the breaker's amp rating. Since the inverse of 1.25 is .80, both statements mean the same thing.

So a standard 20A breaker is suitable for continuous loads of about 16A, while a 30A breaker is suitable for about 24A. The exception to this are units marked 100 percent duty; these are properly ventilated and have the appropriate connection ratings so that they can carry the maximum current indicated on the nameplate continuously. Such units are intended for special applications; make sure you actually use one of those.

Selecting the size factor:

The default options of this tool cover most common cases.

The factors for motors, air conditioners and welding machines are starting points in planning and not results of standards. Motor branch circuits have their own rules for sizing, and equipment with MCA and MOCP on the nameplate must be selected based on those values, not general formulas.

If your local regulations or design requirements dictate other safety factors, select Custom and enter the percentage directly.

Example calculation:

A continuous load factor of 125% is used in each row and a two stage calculation is performed to reduce the result down to a standard size.

It is good to remember the lines for EV chargers. A 7.2 kW charger at 240 V draws 30 A. Since charging takes several hours, the rated current is 37.5 A. Therefore a circuit breaker of 40 A is used. This circuit breaker has a maximum continuous current rating of 32 A, which adequately covers the actual current drawn by the charger of 30 A.

Standard fuse sizes:

Since the nominal rating of a circuit breaker is fixed, the calculated value of 23.44A will never be used directly as a 23A circuit breaker. Instead it would be rounded up to the next highest available size.

Common sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A. For feeders and service equipment the size range extends to 600 A. Not all ratings are available in every region. In particular for residential distribution boards 15, 20, 30, 40, 50 A are commonly used more often.

Fit the fuse holder and the wires together.

Fuses and wiring must be selected together; they are not determined separately. The conductor must be designed for the rated current of the fuse. Otherwise, the cable will become the weakest link in the circuit that would never trip.

These are not specifications but only guidelines to check adequacy. The actual allowable current depends on the type of insulation, temperature rating of connectors, ambient temperature, number of conductors in the same conduit and length of cable run.

If the wiring is too long then other problems arise that cannot be solved by fuses. As voltage drop depends on the size of conductor and distance there may be unwanted voltages at the furthest points even if the circuit is correctly protected.

Other rules apply to labelling of HVAC systems.

Air conditioners, heat pumps and similar appliances are usually marked with two values: MCA and MOCP. The minimum current capacity value (MCA) determines the smallest allowable conductor size. The maximum overcurrent protection value (MOCP) indicates the size of the largest circuit breaker or fuse that can be installed.

These two values are neither operating current values nor can they be derived from general formulas. If these values are indicated on the type plate, use them as a reference. This tool can be used for loads where this data is not available.

Common mistakes:

Many errors when choosing the size of a circuit breaker are not due to arithmetical mistakes but rather false assumptions. If you want to enter 7.2 kW for example and instead enter 7.2 in the field for watts, then the result is wrong by a factor of thousand.

The choice of voltage can also make a big difference. A load with the same power rating of 4,500 W will draw 37.5 A (at 120 V) while only drawing 18.75 A (at 240 V). This difference determines whether a 50 A or 25 A circuit is required.

And there are situations that can actually lead to a fire. If a circuit breaker is constantly tripping, you should not simply try replacing it with a larger one. A tripping circuit breaker indicates a problem. First check for overloads, inrush currents, faulty appliances or inadequate wiring.

What this tool does not take into account:

This is a tool for preliminary testing of power load capacity and it is far from complete protection design.

It does not check the tripping capacity or available fault current. It also does not coordinate with upstream devices. The conductor cross-sections based on ambient temperature and conduit fill are not considered, nor do it consider requirements for AFCI or GFCI protection systems, compatibility with panel boards, or regional codes.

This tool is for planning and training purposes only. Electrical installations are subject to regulations and can be dangerous. The final selection of circuit breakers and wiring must follow applicable codes, equipment markings, and manufacturer's instructions and should be inspected by a qualified electrician or technician.