What Is a Tandem Circuit Breaker?

A tandem breaker combines two separate 120-volt circuits to fit into one position. An addition of a tandem breaker is the least expensive way to add circuits to a completely full panel and one of the most frequently misused devices in residential wiring, as it can only be used if the panel is manufactured and rated to conduct properly.

Below is an explanation of its operation, limitations, and advice on how to check if your panel is suitable for a tandem breaker installation.

Briefly: A tandem breaker (sometimes referred to as twin, duplex or half-width breaker) contains two individual single pole breakers within one casing which provides two 120V circuits from a single socket. It should not be confused with a double pole breaker because it cannot deliver a voltage of 240V. Tandem breakers are only allowable in panels that are certified for their use and only in the locations stated on the panel label. If the mentioned conditions are met, it is a legal, safe method for making the space, when it is not followed – there is a risk of fire at the bus stab.

What It Is, and What It Is Not

There are three devices that are often mistaken for one another but they actually differ in functionality.

• Let’s start with single-pole breaker: a device consisting of one switch that provides one circuit.
• Then there’s tandem breaker: a device that creates two independent 120 V circuits that use one switch with two circuit stay apart from one another.
• And finally, there’s double-pole breaker: a type of switch allowing one circuit operating at 240 V with two switches using one circuit and the same trip for both.

Since the only similarity comes with the two-pole structure of a tandem and a double-pole breaker people tend to confuse them. Otherwise, the two are very different devices. One creates two 120 V circuits, and the other provides one 240 V circuit.

Thus, they can’t be interchanged. A single body of the type used throughout a residential panel, such as this single-pole 32 to 63 A MCB, occupies one module and has one handle; a tandem packs two such mechanisms into the same width with two handles and one shared bus clip.

Why the Panel Has to Be Listed for It

This section is the most critical section in the entire article.

A panelboard features a particular bus configuration, the number of spaces, and the hardware devices to be used. Some models can receive tandem breakers in each available space, others cannot allow this at all, and many accept the breakage only in some clearly specified positions like the ones in the lower half or the outer sites as stated by the label inside the door.

If a tandem is put in a position which is not specified for it, the current at the bus stab would be double as much as required. As a result, the stab starts heating up and plastic covering gets melted, and it is the same failure type as the one associated with poor connection, which is however expected to occur not in twenty years but right from the moment of installation.

Information regarding this could be obtained from the panel label or from the bus itself. Many models have a physical rejection feature, such as a small notch at the station, that eliminates the chance of a tandem being installed incorrectly. When this feature fails, the damage will be visible.

Tandem, Double-Pole, and the 240 V Question

The common trip of a double-pole device is far from a convenience. In a 240-V circuit if one pole is open and the other is closed, the device is still connected to one live wire whereby anyone who works on that equipment runs the risk of being exposed to full voltage across the load. That risk is eliminated with common trip. The correct construction, with a single handle operating both poles, is shown in this two-pole miniature circuit breaker.

A quad breaker, or a quadplex, is something else. It comprises two tandem pairs in a two-space element, and certain variants have a middle double-pole pair in addition to two 120 V ranges. Although these devices exist and are legal, they are most frequently installed in the wrong panel by someone who merely checked the possibility for installation.

The Real Downsides

• No increase in capacity. Tandems utilize the same capacity of the service panel without bringing anything new into the mix. The only thing that happens is that it creates some space, but it does not bring in any more current. For example, a 100 A service panel with tandem circuit breakers is still a 100 A service panel.
• Heat concentration. Two devices installed in one space operating in a service panel that had never been intended to be so crowded will be operating at higher temperatures than two devices in two different spaces.
• Limited product variety. There are AFCI and GFCI tandem devices, but the availability of these devices is limited, while there are virtually no AFCI/GFCI combinations at all. If a circuit should have protection against the arc, then it will be difficult to find the tandem that suits the task.
• Labelling confusion. The tandem occupies only one space on the load chart, and since the labeler was in a hurry, neither of the two circuit is described properly.
• Problems with resale and inspection. In case the home inspector registers the use of non-listed tandem devices in panels, he will write that in his report, and the electrician hired after the purchase should evaluate the panel.

None of those factors imply the ineffectiveness of the gadget. They just limit its use to narrow spheres of application making it different from the universal gadget.How the two device types are marked, and how to tell them apart in a panel, is covered in our device and panel FAQ.

When a Tandem Is the Right Answer

There are three circumstances under which one justifies the other.

• The panel has been listed as well as having genuine extra position of the required type. It is justified to install a lighting circuit into a 200-amphetamine panel that has free six tandem positions.
• The circuits are small and the load is light. It should be noted that two of five-amp lighting circuits can be accommodated without difficulty but it cannot be said about two kitchen circuits.
• The alternative solution is not appropriate. The panels can seem like viable options with the help of a sub-feed that creates an illusion of a long run through a finished ceiling.

In a different situation, the honest alternatives would be either installing a second sub-panel or replacing a panel altogether. A board of the size used for that purpose, such as this 4 to 24 module distribution board, costs less than a service upgrade and takes the pressure off the main panel permanently.

How to Check Before You Buy One

• Opening the panel door and inspecting the label for maximum number of tandem-capable positions. This count is usually larger than the number of physical spaces.
• Once the label is read, it also necessary to take a picture of both the label and the bus stabs. In the photo one could see the rejection feature and possible damages, if any.
• Finding out the panel brand and series is important, since you need to make sure that your tandem matches both.
• Now that you know the number of existing circuits and can find out their voltage, it is necessary to make sure that they are 120 V circuits. But if you find out to be a 240 V circuit, then it means that there is a double-pole device and none of the slots would be freed as a result.
• You need to find out whether the existing circuit is AFCI or GFCI and whether there is a tandem available in this series.

The sixth step is where lots of value is. A panel that is full because the house has expanded is, as a rule, also the one that is close to the capacity limit, and adding two more circuits just moves the limit line along.If the service is already 100 A and the loads are electric heat, an EV charger and a range, the answer is an upgrade, and our technical team will say so rather than sell a $25 part that postpones the problem by a year.

FAQ

What is the downside of tandem breakers?

They absorb heat only in one position, they may be installed only in certain designated-panel positions, no matter how much AFCI and GFCI versions exist, and all of them merely add circuits without additional functions. When properly used, none of that is significant, while misusing it in a wrong panel makes it matter.

What’s the difference between a tandem breaker and a regular breaker?

A normal single-pole breaker is used to work on one circuit of 120 V in one area. A tandem employs two separate circuits of 120 V in the same area. They are exactly the same in electrical terms; however, the tandem shares only one bus connection to connect the two loads.

Are tandem breakers allowed by code?

Yes, that is where the panelboard must be listed and marked as suitable for that use. The code requires the equipment to be designated for the use in that panel, that is why the question actually depends not on the breaker itself but on what is written as the label on the inside of your panel door.

What is the difference between a 2-pole breaker and a tandem breaker?

That two-pole circuit breakers serve two 120 V circuits represents the former of two circuit designs. The latter circuit design involves two circuit breakers feeding a single 240 V circuit via a common trip. Tandems provide no option for a 240 V solution, and double-pole solutions cannot be divided into two independent circuits.

References

• NEMA — Panelboard and Circuit Breaker Application
• UL Solutions — Panelboard Listing and Classified Breakers
• This Old House — Adding Circuits to a Full Panel
• Bob Vila — Tandem vs Double-Pole Breakers
• U.S. Department of Energy — Home Electrical Capacity

Conclusion

A tandem breaker is a legitimate device with a limited purpose: it is used in a panel that contains two 120 V circuits and has a designation of “one”. All other uses of a tandem breaker could have instead been achieved using a subpanel.

Take a photo of your panel designation and send it to us. We will tell you whether the panel allows the use of tandem breakers, how many may be used, and what positions can be occupied.