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Install a new circuit breaker in a panel.

Electrical Panel Projects:

Installing A Circuit Breaker

Adding A New Circuit Requires Adding Another Circuit Breaker

In This Article:

The power is turned off. A cable clamp is installed and new 14-2G cable is brought into the panel. The ground wire is connected to the ground bus bar, the neutral wire is connected to the neutral bar, and the hot wire is connected to the new breaker, which is then pushed into its slot.

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Skill Level:
3 (Intermediate)
Time Taken:
About 30 Minutes
, Editor
Project Date:
March 2000
Start >>



First: Turn Off The Power.

Installing a basic single-pole circuit breaker involves 4 steps:

  1. Feed the cable into the breaker panel.
  2. Connect the ground wire.
  3. Connect the neutral wire.
  4. Connect the hot wire to the breaker and snap it in place.

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Step 0: Preparations

Turning off the power to the sub-panel by switching off the supply breaker at the main panel.

In this case, the power was controlled by a 100 amp breaker on the main breaker panel.

I turned off the breaker.



Then I removed the cover from the sub-panel that was getting a new circuit breaker.

Removing the cover on a circuit breaker panel.


This situation is a little different from most houses. This house has a 200 amp main breaker panel, which feeds 3 subsidiary breaker panels. This is a good technique for house wiring, although it does cost a little more.

This method allowed me to shut off the power at the main panel, so the 100 amp sub-panel had no live wires anywhere.  But there was still power to some parts of the house.

Most houses have just one panel, which contains the main breaker (typically 100 to 200 amps) and a large collection of single pole (120 volt) and double-pole (240 volt) breakers. In these cases, installing a breaker can be done safely, BUT...

There are still live wires present.  The big wires that enter the panel and feed the main breaker.  Avoiding contact with these wires is very important. (Worse would be accidentally shorting the service entrance wires with a metal tool, because they have a huge current capacity, and short-circuiting these wires would create a massive arc, like a big industrial welder, which would continue until something in the circuit was melted.  I know of no circuit breaker (other than the main breaker) that protects your service entrance wires from shorting.  I have seen 240 volt service entrance wires short together (during a windstorm) and the sparks are spectacular. I was surprised to learn that most step-down transformers that supply houses do not have short-circuit protection.


Circuit Breaker Panel Anatomy:

Labeled image of Square D brand of electrical sub-panel breaker panel.

1, 2. Incoming Hot wires. There is 240 volts between these wires, or 120 volts between either hot wire and the neutral line.

3. Neutral wire. This is at the same electrical potential as the ground. The neutral is connected to ground, but only at the main breaker panel.

4. Ground Bus Bar. This strip of metal has a row of screws for connecting the ground wires of the various circuits.

5, 6, 7. Neutral Bus Bars. This panel has 3 short bus bars for neutral wire connections. Some panels have only one long bar.

8. Circuit Breakers. Each single-pole breaker connects to one of the two hot bus bars. Each double-pole breaker connects to both of the bus bars (thus providing 240 volts between hot wires).

9. The last available space in this panel. Our new breaker will go here. 


Note in the above photo that there is no main circuit breaker. This is a main lug type of panel, used as a subsidiary panel (sub-panel).  The breaker that feeds this panel is upstream, at the main panel.


Verify The Power Is Off:

I touched the probes of my electrical tester to the incoming hot wires where they connected to the main lugs...

Measuring voltage at the breaker panel incoming wires.


Voltmeter shows zero volts.

...No voltage, just as planned.



For Example...

This is the volt-meter reading with the power turned on... 248 volts between the two incoming hot wires.

That could really hurt.

Voltmeter measuring incoming line voltage shows 248 volts when power is on.


Step 1: Run New Cable Into Panel

Inserting an NM-B cable clamp in knock-out hole in breaker panel.
A Cable Clamp Is Necessary:

I installed a new 3/8" cable clamp in a knock-out on top of the panel.

See Installing A Cable Clamp for more info.



After I installed the cable clamp, I ran the new cable into the panel.

I always pull the cable a good distance into the panel... I want plenty of extra cable available so it's easy to route the wires inside the panel.

The red arrow points to the location of the new breaker.


Bringing new electrical cable into breaker panel.


Tightening cable clamp in breaker panel after new cable has been pulled into box.
Secure The Wires:

Once I had enough electrical cable threaded into the panel, I tightened the cable clamp with a cordless drill-driver on a low torque setting.



I used a sharp knife to CAREFULLY slit the cable jacket.

Cutting plastic NM-B cable sheathing with sharp knife.


NM-B cable with sheathing cut away.

And then I cut away the jacket, all the way back to the cable clamp.



Step 2: Connect The Ground Wire

Ground Wiring:

A closer view of the ground bus bar.

Ground bus bar in circuit breaker panel.


Bending bare ground wire to fit neatly inside breaker panel.
Ground Wiring:

The new ground wire was snaked through the maze of wires.

I made some 90-degree bends so the wire would lay in an orderly fashion.



Ground Wiring:

I loosened a screw and inserted the ground wire...

Inserting bare ground wire into empty slot in ground bus bar, in circuit breaker panel.


Tightening screw on bus bar inside breaker box.
Ground Wiring:

...and I tightened the screw firmly.



A Note On Screw Tightness:

Most circuit breaker panels have instructions that dictate the proper amount of torque (twisting force) to apply to the screw when tightening.  This panel specified 20 inch-pounds for #12 and #14 wire.  I have never seen an electrician use any type of torque measuring device when installing electrical equipment.  But be warned, there is a correct amount of tightening, and it's quite firm.

Certain things can happen when the conductors are under full current load, such as heating, thermal expansion and distortion of the round copper wire. If a screw is not tight enough, a wire may begin it's career being tight and secure, but eventually become loose after repeated heating/cooling cycles.

Of course, if a screw is tightened too much, the threads will strip or the drive slot will strip. Anybody with doubts about tightness should acquire a torque wrench and screwdriver attachments and become familiar with just how tight 20 inch-pounds feels like.



Step 3: Connecting The Neutral Wire

Neutral feed wire connected to neutral bus bar inside circuit breaker panel.
Neutral Wiring:

The neutral line feeding the panel is supposed to be marked white.

In this panel, the neutral incoming line was covered with white electrical tape, which is accepted by the electrical code.



Neutral Wiring:

I routed the wire neatly and made some bends.

Bending neutral wire to fit neatly inside panel.


Placing neutral wire in neutral bus bar, inside breaker panel.
Neutral Wiring:

I stripped the insulation from the end of the wire and inserted the bare end into a connection terminal.



Neutral Wiring:

I tightened the screw on the neutral bus bar.

Tightening screw on neutral bus bar.


Step 4: The Hot Wire

One of two hot bus bars in Square D circuit breaker panel.
The Hot Wire:

There was one empty spot in this panel (red arrow).



The Breaker:

A Square-D brand of single-pole breaker, 15 amps.

This Square D panel uses their "QO" line of breakers, which are different from their "Homeline" products.

When a QO breaker is tripped, there is an orange indicator that appears just below the word "OFF" on the face of the breaker.

Also, QO breakers are narrower than Homeline breakers (and some other brands of breakers), so the panel is smaller than other brands for the same number of breakers.

Square D brand of circuit breaker, QO product line.


Electrical wiring connection point on Square D circuit breaker.
The Hot Wire:

A view of the connection terminal on the breaker. Two wires can be attached, one at each red arrow.

Note that there is no need to bend the end of the wire into a hook... the wire simply needs to be straight.



The Hot Wire:

I stripped about 5/8" of insulation from the end of the hot wire (black wire) and secured it under the screw.

I made sure the end of the wire poked through beyond the metal clasp.

Hot wire connected to terminal on Square D circuit breaker.


Underside of Square D QO brand of circuit breaker.
The Breaker:

A view of the bottom. The metal U-clips on the right hold the circuit breaker to a plastic bar in the panel. The U-clips on the left (hard to see in this photo) are part of the circuit, and connect to the hot bus bar.



The Breaker:

Note the hot bus bar (arrow 2). The circuit breaker grabs on here.

Arrow 1 points to the plastic "grab" bar mentioned earlier.

Square D breaker panel showing points where breaker connects.


Square D QO circuit breaker showing hold-down clips.
The Breaker:

The installation sequence: First the hold-on clip is pushed onto the plastic bar. (I angled the breaker so a photo could be taken. In practice the breaker is parallel to its neighbor.)



The Breaker:

With my thumb I pushed firmly until the breaker was seated. The left end was still not connected.

Pushing circuit breaker into place in panel.


Pushing live end of Square D breaker onto hot bus bar.
The Breaker:

The business end was pushed in firmly until it was seated.

There are no screws that attach this type of breaker to the panel, it is held in only by spring clips, and the panel cover.



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Before you hurt yourself, read our disclaimer.















Wrapping Up:

I removed the last knock-out from the panel cover, using a pair of Channel-Lock pliers.

You just twist the metal rectangle back-and-forth until it breaks away from the cover.

Note: You can buy metal "blanks" to fill in these spaces, if you ever remove a breaker and need to fill in the opening in the panel cover.

Removing knock-out from breaker panel cover plate.


Breaker: Off
Circuit breaker panel with cover installed, new breaker turned off for now.

I will leave this breaker turned off until the rest of the circuit wiring is finished.



A Few Thoughts On Fear And Electricity:

I studied electrical engineering in college, before switching into mechanical.  I studied electronics in high school.  For a long time I have understood the fundamentals of electricity, including it's potential to kill.

Since my teenage years I have been doing electrical repairs, but I never had the need to add a new circuit breaker until I bought my first house in 1990. I had quite a few reservations about working on circuit breaker panels.

A relative showed me how to replace a circuit breaker, and pointed out the dangers.  When the main breaker is turned off (in a typical one-panel house) there is still voltage in the panel, at the two big hot wires that feed the main breaker.

Touching these wires would require a lot of carelessness. Or stupidity. My only fear is accidentally touching the live metal parts with the wire I'm working on, possibly causing a short and a big nasty arc. Whenever I have shorted something, I've always jumped or jerked my hand away, and it's that involuntary reaction that I figure might pose the biggest risk of injury.

After I replaced all the wiring in my first house, I was comfortable with digging into a circuit breaker box.  But there is nothing as secure as having a panel with no live wires, such as the sub-panel in this article.


More Info:
Tools Used:
  • Cordless Drill/Driver
  • Screwdrivers
  • Hammer
  • Wire Cutters
  • Wire Stripper
  • Needle-nose Pliers
  • Electrical Test Meter
Materials Used:
  • Circuit Breaker, Single Pole
  • Cable Clamp
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