Safety Note: Installing a double pole circuit breaker is not an amateur project. Don't attempt it if you're not an experienced electrician.
Most home appliances operate at 120 volts, which can be serviced by a single breaker switch, but some large appliances like car chargers, stoves, and washing machines need 240 volts, which requires combining two breaker switches into one double pole switch.
Double pole switches flip together if they get tripped. They can provide power to two 120 volt circuits or one 240 volt circuit.
The voltage provided to power your home originates at a local transformer, probably housed on a nearby post, where thousands of incoming volts are stepped down to a more manageable 240 volts. This figure illustrates the secondary winding of a transformer energized with 240 volts between two powerlines (A and B).
The line identified as C is a center tap added to the secondary winding of the transformer that if checked for voltage levels between either A & C or B & C would give you 120 volts reading either way.
Wires A, B, and C in Figure 1 are the same wires seen in Figure 2, where the two black wires are connected to the black wires coming out at the top of the mast in Figure 2, and where the Neutral (usually a bare wire) is connected to the white wire at the mast.
At the other end of the mast, past the electricity meter, inside the electric service panel, the two black wires hook up to the large double pole service disconnect breaker, commonly referred to as the “Main.” The white Neutral hooks up to the Neutral bus bars as shown in Figure 3.
At this point, if you measure the voltage between the two black wire terminals at the top of the Main, your reading would still be 240 volts, and 120 volts across the Neutral and each one of the black wires known as the Hot lines.
As mentioned earlier, the Main is a double pole breaker, with the other side of each pole connected to their respective bus bars that both run vertically at the full height of the panel, where all the breakers from every circuit are connected.
Step 1 - Safety Precautions
1.1 - When or if you’re considering opening up the electric panel, get help from an experienced electrician if it’s something you’re not familiar with doing. Working at or altering anything inside the panel can be extremely dangerous, so never rely on guesswork. This is not an area for trial and error.
1.2 - Before you start, you must first get a trouble light that is battery-powered and securely set up to illuminate your working area.
1.3 - If you’re replacing a circuit breaker, make sure to positively identify the right breaker by shutting it off and testing the appliance for power. Mark that breaker with tape or a marker of some kind—its tag on the cover will be gone with the cover removed.
1.4 - Never attempt to remove the front cover of the panel box and expose all its wiring without making sure that the “Main” is turned off, shutting all the lights off around the house. Make sure all preparations are done before turning it off as the rest of the house will then be in the dark.
Step 2 - What Type of Breaker Will You Need?
All service panels are equipped with several bus bars designed to carry a series of wires or circuit breakers. The bars line these breakers up vertically from one end to the other—like people sitting in a “bus.”
Figure 4 shows the two Hot bus bars, laid flat at the back of the box with a series of blades bent perpendicular to project outwards and onto which the breakers can be clamped.
The blades are wide enough that two breakers inserted side-by-side facing each other can hook up to the same blade (same Hot line). Because of the alternating zig-zag design and positioning of the two bus bars, the next blade directly underneath or above gets its voltage from the other Hot line.
The picture also shows two different breakers—the one on the left is only half the thickness and with a smaller toggle lever than the one on the right with also a much larger lever.
The smaller breaker is called a single-pole breaker because of its single slot (left breaker in Fig. 5) that can slide and clamp itself on a blade, thus tapping voltage off one side only of the Hot lines.
This means that once the breaker is turned ON, 120 volts will flow into the black (or colored) wire secured to the breaker’s terminal (right breaker in Figures 5 and 6) and leading to the circuit.
But since it needs a return path to the power grid, the white wire from that circuit’s cable needs to be connected to the Neutral bus bar inside the panel, thus completing the circuit.
The larger breaker on the right in Figure 4 has two slots that when inserted onto the Hot bus bars connect to two successive blades of the bus to provide 240 volts through the breaker once activated.
This one is called a double-pole circuit breaker and it has two terminal screws onto which two colored wires from the circuit’s cable—one black and the other usually red—will be connected (Figures 4 and 6).
Step 3 - The Right Make and the Right Size Circuit Breaker
Electrical panels and their circuit breakers are available from many different manufacturers worldwide. Each one has a proprietary system for securing breakers to a bus. So when shopping for breakers, make sure that you check the make of the panel first and buy the type of breaker that will fit inside.
Step 4 - Determine the Size of the Breaker Needed
If you’re installing a double pole circuit breaker, you must need 240 volts at an appliance or tool that consumes a lot of power such as a range, a dryer, a hot water heater, etc.
The more power you need, the larger the wire gauge size you need, so check the appliance’s nameplate to find out just how much amperage it will draw from the power grid. This will determine the size of your circuit breaker.
Step 5 - The Right Wire Gauge for the Job
The more power the appliance is rated for, the larger the gauge size for the wires needs to be. If you’re not familiar with wire gauge sizes and amperage, you should consult with a reliable electrician to determine for you the size of the cable and figure out whether it needs a Neutral wire, as described in Step 6.
Step 6 - Make Sure You Use the Proper Cable
When wiring a 240 volts circuit, you must always use the proper cable with at least two colored wires (one black and one another color besides white—usually red). A white wire must never be connected to a breaker terminal, but strictly to the Neutral bus instead.
Once the double-pole breaker is powered-up, the power flows through one of the two colored wires from one terminal, passes through the load, and returns through the other wire to the other terminal, completing the 240 volt circuit back into the grid.
There are some situations, however, where the appliances, such as an electric range, require 240 volts at the elements, but also 120 volts to power the fans, the PC control boards, and the lights.
Appliances such as these need a 3-wires cable—as described in Step 5—with the white Neutral wire connected to the Neutral bus (Figure 7) and the two colored wires connected to the two circuit breaker terminals inside the panel, while at the other end they will go to a terminal block inside the appliance.
This way, anything connected across the two colored wires will be powered with 240 volts while 120 volts will be provided to anything connected across the Neutral white and either the black or the red terminal screw.
To finish, don’t forget that the GND wires (bare and green wires) always need to be grounded to GND terminals (Figure 7) and appliances’ chassis.
Step 7 - Installing the Circuit Breaker
7.1 - Once you’ve determined all the previous steps have been properly followed, turn on the trouble-light and switch off the Main. Then and only then, you can remove the front cover of the panel.
7.2 - If you’re replacing a defective breaker, identify which one is busted and remove the two colored wires connected to its terminals. You can then pry up the breaker with a flat screwdriver, or unscrew it from the panel, depending on the system used, and set it aside.
7.3 - Repeating Step 7.2 in reverse order, install the new circuit breaker in place, making sure it's properly secured, then connect the two colored wires to its terminals. Replace the front cover and secure it in place.
7.4 - Once the cover is back on, switch every one of the breakers off within the panel to prevent a power surge from everything going on at once. You can now turn the Main back on, then all the other breakers, one at a time. Go back to the appliance to see if it’s working.