Electrical Panel Projects: Installing A Circuit Breaker

By Bruce W. Maki, Editor

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.

Step 1:

	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 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.

Breaker Panel Anatomy: 1, 2. Incoming Hot wires. There is 240 volts between these wires, or 120 volts between either wire and the neutral line. 3. Neutral wire. This is at the same electrical potential as the ground. At the main breaker only, the neutral is connected to ground. 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. 

	I touched the probes of my electrical tester to the incoming hot wires...
	...No voltage, just as planned.

This is the volt-meter reading with the power turned on. That could really hurt.

	After I installed a 3/8" cable clamp, I ran the cable into the panel. See Installing A Cable Clamp for more info.
	I tightened the clamp with a cordless drill-driver on a low torque setting.

	I used a sharp knife to CAREFULLY slit the cable jacket.
	And then I cut away the jacket, all the way back to the cable clamp.

Step 2: Connect The Ground Wire

	A closer view of the ground bus bar.
	The new ground wire was snaked through the maze of wires. I made some bends so the wire would lay in an orderly fashion.

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

Notes 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 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 purchase a good quality torque wrench and screwdriver attachments and become familiar with just how tight 20 inch-pounds feels like.

Step 3: Connecting The Neutral Wire

	The neutral line feeding the panel is supposed to be marked white (this one was covered with white electrical tape, which is OK).
	I routed the wire neatly and made some bends.

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

Step 4: The Hot Wire

	There was one empty spot in this panel.
	A Square-D brand of single-pole breaker, 15 amps.

	A view of the connection terminal on the breaker. Two wires can be attached.
	A view of the bottom. The metal U-clips on the right hold the unit to a plastic bar in the panel. The U-clips on the left (hard to see in this shot) are part of the circuit, and connect to the hot bus bar.

	The hot wire was stripped and secured under the screw.
	Note the metal bus bar. The circuit breaker grabs on here.

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

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.

Wrapping Up:

	The last knock-out was removed from the panel cover.
	I will leave this breaker turned off until the rest of the circuit 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, about 10 years ago.  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 biggest 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 motion that I figure might be the biggest risk. 

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.

The house in this article also has a disconnect box outdoors, just under the electric meter. This small panel has one circuit breaker (100 amp), although there is room for 4. A couple of years ago the we noticed that all the lights were flickering.  We discovered that the main breaker (in the basement, on the main panel) was arcing, which is like a small, constant lightning. The breaker was slightly warm, but not hot.  The next day we replaced the entire panel (the hot bus bar was eroded away), which took about eight hours. The neat thing was, by shutting off that 100 amp breaker outdoors, we could cut the power to the entire house, and turn it back on when done. The alternative (which applies to most houses) is to call the electric company and have them disconnect the power by removing the electric meter, which is, to say the least, inconvenient.

I have become a firm believer in organizing the house's electrical system by using subsidiary panels. My preference is too locate a sub-panel near the kitchen, in the garage, on the second floor, etc. I prefer to have all basic 120 volt circuits connected to one of several sub-panels, leaving the main panel to supply only the sub-panels and the larger 240 volt appliances (range, dryer, water heater, well pump)

Copyright © 2000, 2005  HammerZone.com

Written March 10, 2000
Revised January 7, 2005