How to Size a Circuit Breaker?

Choosing the right size for a circuit breaker is not merely based on the rating of the equipment but the wire gauge and the load characteristics. Thus, being able to understand the process the right way enables being able to carry out the calculations in a straightforward manner. Thus, if not carried out in the right sequence, one might end up with a circuit breaker of 30 Amps that protects 14 AWG cable.This is the right order and some numbers.

In short: Find out the load, apply a 125 percent factor to continuous loads, obtain a conductor sufficient for the resulting current and find the lowest available circuit breaker rating protecting from faults the conductor. The available ratings may be 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A. The 80 percent rule is the other way round: no continuous load has to be above 80 percent of the rating of a circuit breaker.

The Order of Operations

• Determine the load in amps at the circuit voltage.
• For continuous loads, multiply by 125 percent.
• Then determine the conductor ampacity to cover the load after any adjustment for ambient and installed temperature conditions.
• Determine the breaker ampacity rating. For safety, the ampacity of the breaker must be less than or equal to that of the conductor.
• Make sure that the interrupting capacity of the breaker is greater than the fault current available.

Step four is the surprising step: a breaker protects cables, not devices. Thus, the 40 amp cable with 8 AWG and a 50 amp device is properly designed, while the 40 amp cable with 8 AWG and 60 amp device is not.

Standard Ratings and What They Protect

This table is based on a termination temperature of 60 °C, which is the conservative assumption and the one relevant when the breaker terminals are rated 60 °C. Conductors rated at 75 °C or 90 °C have larger ampacity, but using a termination temperature thus limits the effective ampacity.

The device itself is the last variable. A single-pole MCB covering 32 to 63 A spans several rows of that table, and the marking on the front is what distinguishes a 40 A device from a 63 A one.

The 80% Rule, Stated Properly

A continuous load refers to any load that is likely to run for three hours or more at a time. For such loads, a circuit must be built to handle 125% of the load, which just means that the load must not be more than 80% of the circuit rating.

In terms of watts at 120 V, this means that a 15 A circuit would carry 1,440 W while a 20 A circuit would handle 1,920 W. At 240 V, a 30 A circuit carrying 5,760 W constantly can be compared to one with 50 A handling 9,600 W of continuous load.

What makes something continue is a question of usage not the appliance characteristics. The water heater is continuous while the oven is considered as demand load with separate factors. The EV charger is considered continuous in this case, thus a 40 A charger requires a 50 A circuit while a 48 A charger asks for 60 A circuit.

Thus, the rule has to be used for calculating the load, noting that the method of calculation consists of using first general lighting, then small appliance circuits and laundry before proceeding with the named appliances using their demand factors. Our FAQ on circuit ratings covers the common cases, and the arithmetic is worth doing before ordering anything.

Motors and the Two Numbers on the Nameplate

Motor circuits bend the simple rule intentionally. On the nameplate of a motor, it mentions the minimum circuit ampacity and maximum overcurrent protection value, while the two values differ.

The conductor size is determined based on the minimum circuit ampacity. The value for the breaker is based on the maximum value. This value is usually greater than the ampacity of the conductor, since the circuit breaker must be able to absorb the start current. This contradiction is resolved by means of the overload protection that is built into the motor starter, taking away the burden from the panel.

Hence, the motor circuit contains two protection devices: short-circuit protection device in the panel and thermal overload device in the starter. The latter provides the protection from the overload especially, and the value is selected according to the service factor current. The difference between the two is set out in the specification of this adjustable thermal overload relay.

Thus, determining the size of a motor circuit by dividing the number of watts by the voltage results in a circuit breaker tripping each time it is started. Namely, the figures on the nameplate are there because the way of getting them is different for a motor than for a resistive load.

Feeders and Subpanels

The sizing of a feeder is done according to the calculated load of the entire circuit with applied demand factors and then protected at or below feeder conductor ampacity. The machine on the upstream end can be either double pole/single pole rated device and its rating must coordinate with the highest rated device downstream thus ensuring that the fault trips the nearest machine instead of the feeder.

A 125 A two-pole device feeding a small subpanel is a common residential arrangement, and the kA rating matters here more than at a branch circuit because the fault level at the main is higher than at the end of a branch.

Coordination is the aspect which tends to be ignored. Two devices with identical ratings may trip in an erratic manner. If the feeder is too small, the entire subpanel will also go off every time a branch trips. As a general rule, there should be a ratio of at least two to one between upstream and downstream ratings, which should be confirmed using the manufacturer’s selectivity tables instead of being assumed.

Fault Level and Interrupting Capacity

The final figure to consider is one that people ignore when figuring at home—the available fault current at the device’s location. This is about 10 kA near a transformer but may be as low as 2 kA at the end of a long branch.

If a device rated at 6 kA is installed where 10 kA is available, it may fail to trip, which means that it ventilates, fuses, or otherwise fails to clear. This is a practical concern for densely populated urban systems served by big transformers in close proximity to buildings.

Where the fault level is unknown, it is prudent to opt for a device with a higher kA rating, and the difference in cost would be slight. A device rated 10 kA, such as this single-pole miniature circuit breaker, is the sensible default for a modern residential board.

Three Worked Examples

There is an electric heater that consumes 4,500 watts on 240 volts. The heater draws current of 18.75 A which is a load. The load is continuous and, according to the National Electric Code for continuous loads, it is rated at 125% which gives us a current of 23.4 A. Therefore, a breaker of 25 A rating will be enough for such a load as mentioned. However, a more common selection is a 30 A breaker installed using wire of 10 AWG gauge.

A car charger has to deliver current of 40 A on 240 volts circuit. Since the load is continuous, it is 125% of the load which gives us 50 A. Thus, a 50 A breaker needs to be installed on a wire of 6 AWG gauge and connected directly to the car charger. Getting the charger set to work with 48 A instead of the required 40 A will create a risk of melting down the power terminal.

A third example is a 3-ton air conditioning system. The specification sheet of the air conditioner shows the lowest circuit ampacity which is close to 19.6 A and maximum overcurrent protection you can have that is about 30 A. The way to determine the size of the conductors here is to consider the lowest fra, meaning in this case, the circuit ampacity method gives trouble.

FAQ

How do I calculate the size of a circuit breaker?

The circuit’s load is expressed in Amperes multiplied by 125% for loads that are considered continuous. Then make sure the breaker is of the same current rating or less as the conductor that transports the load current. The breaker will serve as protection for the wire.

Is a 30 amp 220 breaker 30 amps on each leg?

Definitely! A 30 amp double pole breaker can carry a load of 30 amps (30 amps at a voltage of 240 volts equals 7.2 kVA and not 60 amps).

Should I use a 40 or 50 amp breaker for my electric range?

The rating plate of a freestanding oven determines the classification of the appliance. Generally, freestanding ovens function at 40 A on 8 AWG current cables, although certain more complex appliances utilize 50 A on 6 AWG wires.

What is the 80% rule for breakers?

Continuous load must not surpass 80% of the breaker’s rating; hence, the circuit must have a 125% capacity of the load. It applies only to the loads that are expected to run for three hours or longer.

References

• Cerrowire — Ampacity Tables and Derating Factors
• Southwire — Conductor Sizing and Voltage Drop
• NEMA — Standard Circuit Breaker Ratings
• Rewiring America — Load Calculations for Electrified Homes
• U.S. Department of Energy — Appliance Load Planning

Conclusion

Load, 125%, conductor, breaker, fault level. These five considerations must come first and their importance in the selection process continually decreases. Therefore, the last of these matters, concerning the device being used, is the least significant point in the order of selection.

Simply send us the nameplate rating of the device and the conductor size and we will supply you with the device’s rating, the curve and pole configuration of the circuit.