Most household circuits that power standard outlets and light fixtures in your home are 120-volt circuits controlled by single-pole circuit breakers at the main service panel, but major electric appliances, including electric ranges, water heaters, clothes dryers, furnaces, and air conditioners are usually powered by 240-volt circuits that are controlled by double-pole circuit breakers.
Connecting a 240-volt circuit breaker for a newly installed circuit, while not a difficult job, is normally done by a licensed electrician due to the inherent danger that comes with working in the main electrical service panel. It's even possible that your local code authority will not allow this work to be done by an amateur. But a DIYer with a good working understanding of electrical circuits and experience with electrical repairs may well be capable of installing circuit breakers.
Watch Now: How To Install a 240 Volt Circuit Breaker
240-Volt Circuit Basics
A pure 240-volt circuit is categorically different than a 120-volt circuit. In a 120-volt circuit, there is one hot wire carrying 120-volts of power, while the white circuit wire is a neutral wire. In a pure 240-volt circuit, however, both insulated wires carry 120 volts of live current and there is no neutral wire at all. These pure 240-volt circuits are sometimes described as "3-wire circuits," since they contain two hot wires plus a bare copper or green insulated grounding wire. Baseboard heaters, for example, are wired with pure 240-volt circuits that have no need of a neutral wire. The 30-amp 240-volt circuit demonstrated in the project below is of this type—it includes two 10-gauge black hot wire conductors and a 10-gauge green ground wire conductor for a 3-wire system with no neutral.
Some 240-volt circuits, however, are described as 120/240-volt circuits. With these, two black or red wires each carry 120 volts, and there is also a white wire that serves as a neutral. A 120/240-volt circuit is used with appliances like stoves and dryers, where the timers or controls may operate on 120-volt current, while heating elements make use of the full 240-volts. These circuits are often described as "4-wire circuits" since they include two hot wires (red or black), a neutral wire, (white) and a bare copper or green insulated grounding wire.
The same double-pole circuit breakers are used for both types of circuits, and they are installed the same way. The only difference is that in a 120-240-volt circuit, there is also a white neutral circuit wire that must be connected to the neutral bus bar in the main service panel.
Before You Start
Circuit breakers are proprietary to the manufacturer of your electrical service panel, so make sure to chooose a new breaker that is designed for your panel. For example, don't try to install a Square D breaker in an Eaton electrical panel.
Also, make sure the amperage of the breaker matches the amperage of the circuit wires and the appliance being powered. A 20-amp 240-volt circuit calls for 12-gauge wire; a 30-amp circuit calls for 10-gauge wire; a 40-amp circuit calls for 8-gauge wire; and a 50-amp circuit calls for 6-gauge wire.
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Installing a circuit breaker involves working in your electrical service panel (breaker box), so you must be familiar with the parts of a panel and how it works to ensure a safe installation. Unless you are familiar with wiring concepts and have experience doing electrical repairs, you should hire a professional electrician to connect circuits breakers or do any other work at the main service panel.
This project may also require a building permit, depending on the rules in your area. You're required to follow local regulations, even as a DIYer.
Be aware that switching off the main breaker in the service panel shuts off the power to the panel's bus bars and all household circuit breakers, but it does not turn off the power cables coming from the utility service lines. The cables and the terminals they connect to in the service panel remain live and carry deadly current—even when the main breaker is switched off. Never touch these conductors or terminals while working in the panel.
Equipment / Tools
- Voltage tester
- Wire strippers
- 30-amp, 240-volt double-pole circuit breaker
Turn off the Power
Open the door to the electrical service panel. Turn off the power to the panel's breakers and hot bus bars by switching off the main breaker.
Remove the cover plate over the breakers (called the dead front panel), without touching any wires inside the panel.
Confirm that the power is off by testing the branch circuit breakers (not the main breaker) with a voltage tester. If you detect any voltage at any breaker, call an electrician for help.
Do not touch the two hot service cables coming into the panel or the terminals (called service lugs) where the cables connect near the main breaker. These have power at all times unless the utility company shuts down your service feed.
Remove Panel Knockouts
Rotate the dead front cover so you can work from the back side. Remove two adjacent knockout tabs for the new double-pole breaker, as needed. Knockouts either are loosened with a screwdriver blade and then removed with pliers or are simply grabbed with pliers and bent until they break off.
Secure and Strip the Circuit Wires
If necessary, feed the circuit wires for the new 240-volt circuit into the panel. If NM cable has been used, you may need to strip away the outer sheathing of the cable so that only about 1/2 inch of sheathing extends into the panel through the cable clamp.
Strip about 1/2 inch of insulation from each end of the two hot wires and the green ground wire, using wire strippers. If you are installing a 120/240-volt circuit, also strip the insulated white (neutral) wire.
Connect the Hot Wires
Connect each hot circuit wire to one of the terminals on the new 240-volt circuit breaker. Depending on how the circuit has been installed, these might be two black wires, a black and a red wire, or a black wire plus a white wire marked with black tape to identify it as a hot wire.
Make sure to insert the wires fully into the screw terminal, then tighten the terminal screw securely using a screwdriver. Connect only one wire per terminal.
Install the Breaker
Install the breaker into the panel by tilting the breaker so that its tabbed end hooks into the slots or mounting bar on the panel housing, then pivot the other end down until it snaps into place on the two hot bus bar knife blades.
Check for a Proper Fit
Confirm that the breaker is secure and completely snapped into place. It should be flush with the other breakers in the panel.
Connect the Wires
Connect the circuit ground wire to the grounding bar on the service panel by inserting the stripped end of the wire into an open slot on the bar and securing the wire tightly with the terminal screw. The ground wire must use an open slot on the bus bar; do not connect more than one wire to a single slot.
Now, connect the neutral circuit wire. For 120/240-volt circuits, connect the neutral circuit wire to the neutral bar on the service panel by inserting the stripped end of the wire into an open slot on the bar and securing the wire tightly with the terminal screw. The neutral wire must use an open slot on the bus bar; do not connect more than one wire to a single slot.
Complete the Installation
Complete the wiring connections and installation of the receptacle, following the manufacturer's directions.
Reinstall the dead front cover and the door on the service panel. Switch on the main breaker to restore power, and test the new circuit for proper operation.
Special Circumstances: GFCI Circuit Breakers
Your electrical code may require a special type of GFCI circuit breaker for certain situations, such as for a 240-volt circuit that powers a pool or spa heater. These breakers have an attached coiled white pigtail wire. They are installed in exactly the same way as a standard breaker, except that this extra white pigtail wire must be attached to one of the screw lugs on the neutral bus bar in the service panel.
GFCI circuit breakers offer extra protection, and they may be required in situations where there is an increased risk of shock, such as with circuits that are used around water sources.