How to Read a Circuit Breaker?

The face of a miniature circuit breaker contains six pieces of information, all in an area less than a postage stamp, and function as the six factors to determine whether this circuit breaker is suitable for an application. None of them mentions “20 A,” for example, therefore it takes a minute to properly identify a breaker and years to guess.

This is the manual with all the inscriptions in order of their appearance.

In summary: Make sure to check the ampere rating (which should be the highest rated), the trip curve letter (B, C, or D), the rated breaking capacity in terms of kA, the voltage and frequency, the no. of poles, and what standard it has been tested for. The ampere rating indicates the current, while the trip curve indicates the speed of the device in case of a surge. The kA is the measurement of fault current beyond which the device will be able to trip safely.

The Six Fields on the Front

The breaking capacity is the most frequently neglected field that is frequently misapplied in the actual location. The equipment that has a rated breaking capacity of 6kA cannot break a 10kA fault safely; this could vent, weld or not break. Devices for residential and light commercial boards are commonly rated 6, 10 or 15 kA for exactly this reason, and the rating is moulded into the case rather than buried in the datasheet, as this 10 kA single-pole miniature circuit breaker shows.

What the Big Number Means, and What It Does Not

The ampere rating describes how much current a device can indefinitely conduct under its reference ambient temperature, which is often 30 °C for IEC (International Electrotechnical Commission) devices and 40 °C for many UL (Underwriters Laboratories) devices. From this, two conclusions can be drawn.

• First, rating depends on the temperature of the device. For example, a 20 A rated device situated in a panel whose temperature rises to 50 °C will carry current less than 20 A since its bimetallic element will be close to the trip point of the device at this temperature. Derating diagrams are issued by manufacturers so installing a panel in a hot garage will reveal the significance of these diagrams.
• Second, rating is not the same as the trip point. A 20 A thermal-magnetic device is constructed to hold 20 A and can be allowed to operate at a value equal to 1.13 times the rated current for an hour. It has to trip at 1.45 times the rated current in less than 1 hour. There exists a range of values between the two above mentioned thresholds that corresponds to false tripping events which is a normality in the very design of the device.

B, C and D Curves

The letter indicates the level at which the magnetic trip is activated, which is the immediate part of the response.

• B curve: works at three to five times nominal current, it is appropriate for resistive loads, lighting and circuits with no inrush.
• C curve: works at five to ten times of nominal current, it is the general choice for circuits connecting sockets, small motors and mixed loads.
• D curve: works at ten to twenty of nominal current, suitable for loads with high inrush – transformers, bigger motors, compressors, welding equipment.

If the device is chosen incorrectly, then it will cause one of two failures. For example, a B curve circuit breaker will trip on every compressor start and someone will “fix” the problem by installing a more powerful device which will deprive the cable of protection. A D curve in a lighting circuit will allow for a fault current to run long enough to heat the wiring before it trips out. Both happen frequently in panels modified by more than one person.

Multi-pole devices have the same magnetic curve for all poles, which is important given that mixes of poles do not exist in normal catalogs. The correct construction is visible in this two-pole 32 to 63 A device rated 10 kA, where the curve letter applies to both poles.

Reading the Handle and the Window

Each breaker has a clear indicator of its position, and the recent ones have a discernible window that lights red when closed and green when opened. The handle shows the status of the breaker whereas the window acts as a confirmation of the handle status.

The middle position of the breaker can cause some ambiguity as it represents the state of the breaker acting thermally and tripping in the middle position, thus it is required to push the handle to OFF position as the state of being ON cannot take place. This is in fact intended to indicate that the switch has been tripped and not turned ON. Trying to force the switch to ON position leads to damage to the handle.

In addition, the handle usually has the scope of current added on most household devices, which can be implemented either via molding or printing. Some models show the current only when put in OFF position and thus, if the breaker is on, this information can be obtained from the schedule of the panel or its inside label.

Reading the Panel Schedule

An unmarked breaker may refer to a breaker without a directory. The schedule on the inside door of the panel informs you of the protected circuit. A panel with a handwritten, outdated schedule is worse than a blank panel because the information on it is definitely incorrect.

Change the schedule each time there is a circuit change. The right information must consist of device number + amperes + description that would enable the identification of the load: “kitchen counter, north wall, GFCI” instead of “kitchen”.

What to do when a panel has no schedule at all is one of the more common questions we get, and the practical method, switching one device at a time and confirming with a tester at each outlet, is described in our panel and breaker FAQ.

When the Markings Are Gone

Old devices lose their support for printing for a variety of reasons, such as cleaning, heat, or simply time. There are three methods available to identify them.

• First is the panel label, which states which device’s series the panel can accept; thus, the search can be quickly narrowed down.
• Second is the physical shape. Bus clip geometry, terminal placement, and the width of the modules all belong to a specific series and can distinguish the item fairly easy for someone familiar with the brand.
• Lastly, there is the datasheet.Our technical download library holds the dimension drawings and ratings for the KNB and KNL ranges, which is usually faster than measuring a device in a live panel.

What definitely isn’t an option, however, is relying on appearance. A device that fits into a slot perfectly might not belong to the same series, thus leading to overheating of the bus stab, and lack of certification and safety features.

What the Number Means on a Real Circuit

After reading the device, assess if the breaker is compatible with the gauge of wire. A rating of 20 A means that a 12 AWG copper wire is being protected while a rating of 15 A means use of 14 AWG wire. A 30 A breaker will need a wire of 10 AWG. If you have a large breaker on a small wire, then the wire actually becomes the fuse.

For a standard branch device the ampere number is also visible at a glance on the product, which is why choosing by number works for a like-for-like replacement: a single-pole device rated 32 to 63 A covers the larger branch circuits in the same format as the 15 A version beside it, and the marking is what distinguishes them.

FAQ

What is the meaning of numbers on circuit breakers?

The large number refers to the capacity in amperes. The letter indicates the type of trip curve. The number followed by kA tells us about the breaking capacity in kilo-amperes. Together with the pole count and voltage, we will have an idea of what the device carries, how fast it reacts and what kind of fault it can stand.

What does the B circuit breaker and the C circuit breaker mean?

With the B, the magnetic mechanism releases at 3 to 5 times more than the rated current. For C-purpose one should expect this at 5 to 10. When talking about D-brakers the numbers go to 10 to 20 times. With the help of the letter one can match the device for the load.

How do you turn the switch on?

With the majority of residential devices, the uppermost position means ON while the lowest means OFF, with the closed state of the business concerning the handle position. The indicator shows red when closed and green when opened. If the switch is in the middle position, we shall press it down in order to reset.

How the circuit breaker functions?

This is a switch consisting of two trip elements. When the current is too high, the bimetallic element bends and releases the latch. The solenoid opens the latch in cases when there is too much current in the system. When the latch is opened, the spring operates the switch contacts and the arc is cooled down in the arc chute.

References

• NEMA — Circuit Breaker Marking and Application
• UL Solutions — Circuit Breaker Standards and Listings
• Siemens — Miniature Circuit Breaker Technical Data
• ABB — MCB Trip Characteristics and Selectivity
• This Old House — Understanding Your Breaker Panel

Conclusion

The computation of amperage, arrangement, kA, electricity and the number of poles as well as additional characteristics, will take no longer than thirty seconds to distinguish between a safety device and a compliance device.

While changing the existing trip device, remember to snap a photo of the face of the device and to send it in case the sticker is not legible. Using a photo to identify the model would save you an extra trip to the supplier for the purposes of field identification event.