What you need to do before you touch the panel

Adding a 20 amp circuit breaker means opening your electrical panel and working with live wires, even with the main breaker off. This is not a task for someone without electrical training. Most jurisdictions require a licensed electrician to install breakers, and many will not issue a permit for homeowner work inside the panel itself. Check your local building department or electrical inspector before you start — they will tell you whether you can do this work or whether you must hire a professional.

If you are permitted to do the work, you will need to turn off the main breaker, which cuts power to the entire house. Even with the main breaker off, the main lugs — the large terminals where power enters the panel — remain live and dangerous. A multimeter will not protect you here. This step requires respect for the hazard and a clear understanding of what you are doing.

Key Takeaways

  • Most areas require a licensed electrician to install a breaker inside the panel; check your local building department before starting any work.
  • The main breaker must be off before you open the panel, but the main lugs stay live even after the main is off.
  • A 20 amp breaker fits into an empty slot in the panel and connects to a hot bus bar with a single wire terminal.
  • The breaker must match your panel brand and voltage, and the wire running from the breaker must be 12 AWG copper or 10 AWG aluminum for a 20 amp circuit.
  • After installation, the breaker must be tested and the work inspected by your local electrical inspector before the circuit is used.

Matching the breaker to your panel

A 20 amp breaker will not fit every panel. Breakers are made for specific panel brands — Siemens breakers do not fit Cutler-Hammer panels, and vice versa. Open your panel door and look at the label inside. It will say the manufacturer name and the voltage (usually 120/240 volts for a residential panel). Write down both pieces of information.

When you buy the breaker, match the brand and voltage exactly. A single-pole 20 amp breaker is what you need for a standard 120-volt circuit. If you are adding a 240-volt circuit (for a dryer or air conditioner), you need a two-pole 20 amp breaker, which takes up two slots. The breaker will cost between $15 and $40 depending on the brand.

The wire that connects to the breaker

A 20 amp circuit must use 12 AWG copper wire or 10 AWG aluminum wire. Do not use 14 AWG wire — it is rated for 15 amps and will overheat on a 20 amp circuit. The wire runs from the breaker to the outlet, switch, or appliance you are powering. If you are running new wire, you will also need conduit (metal or plastic tubing) to protect it where it is exposed, and you will need to know the distance from the panel to the endpoint so you can buy enough wire.

The wire enters the panel through a knockout hole in the bottom or side. You will remove a small metal disc from the hole, feed the wire through, and then find it with a cable clamp inside the panel. The wire itself should be stripped about 3/4 inch at the end that connects to the breaker terminal.

Opening the panel and locating an empty slot

Turn off the main breaker. Wait a few seconds, then use a multimeter set to AC voltage to test the bus bars inside the panel — the metal strips that run vertically down both sides. If the meter reads zero, the panel is de-energized. If it reads anything else, stop and call an electrician.

Look for an empty slot in the panel. Breakers snap into place along the bus bars. A single-pole breaker takes one slot; a two-pole breaker takes two adjacent slots. If there are no empty slots, you may need to replace an existing breaker with a tandem breaker (which holds two circuits in one slot), but this also requires an electrician and an inspection. Many panels have a blank cover plate where a breaker could go — that is an empty slot.

Installing the breaker into the slot

The breaker has a hook or clip on one end that catches the bus bar. Line up the breaker with the empty slot so the clip end goes in first. Push firmly until you hear or feel a click — the breaker should snap into place and sit flush with the other breakers. The terminal end of the breaker (where the wire connects) should now be facing outward toward the edge of the panel.

Insert the stripped end of the wire into the terminal screw on the breaker. Tighten the screw firmly with a screwdriver — the wire should not move when you tug on it. Do not overtighten; you are looking for snug, not stripped. The wire should be secured to the panel with a cable clamp within 12 inches of where it enters the panel.

Testing and inspection

Once the breaker is installed and the wire is connected, close the panel door. Turn the main breaker back on. The new breaker should be in the off position. Flip it to the on position — it should stay on without tripping. If it trips when ready, turn it back off and call an electrician; something is wrong with the circuit.

Before you use the circuit, your local electrical inspector must examine the work. Call your building department to schedule an inspection. The inspector will check that the breaker is the correct type for your panel, that the wire gauge matches the breaker amperage, that the connections are tight, and that the work was done safely. You cannot legally use the circuit until the inspection passes.

When you should hire an electrician instead

If your panel has no empty slots, if you are unsure about wire sizing, if the main breaker will not turn off, or if you do not have a multimeter and do not know how to use one, hire a licensed electrician. The cost is usually $150 to $300 for a single breaker installation plus the cost of the breaker itself. That is far less than the cost of an electrical fire or a serious shock.

If your local building department says homeowners cannot do this work in your area, that rule exists because the hazard is real. Follow it.

Frequently Asked Questions

Can I install a breaker if my panel is full?

No, not without replacing an existing breaker or upgrading the panel. Some panels accept tandem breakers, which fit two circuits into one slot, but this requires an electrician and an inspection. If your panel is full and you need more circuits, you may need a larger panel, which is a job for a professional.

What size wire do I need for a 20 amp breaker?

Use 12 AWG copper wire or 10 AWG aluminum wire. Never use 14 AWG wire on a 20 amp circuit — it will overheat. The wire must be the correct size before you install the breaker.

What happens if I use the wrong breaker brand?

The breaker will not fit into the slot, or it will fit but not make a safe connection to the bus bar. Always match the breaker brand to your panel brand. The label inside your panel door tells you which brand you have.

Do I need a permit to add a circuit breaker?

Yes, in most places. Call your local building department before you start. They will tell you whether you need a permit, whether you can do the work yourself, and what inspection is required. Working without a permit can affect your home's resale value and your insurance.

What if the breaker trips as soon as I turn it on?

Something is wrong with the circuit — usually a short or an overload. Turn the breaker off when ready and do not use it. Call an electrician to find and fix the problem before you try again.