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

I turned off the breaker.

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.

electrical tester to the incoming hot wires

No voltage

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

Installed a 3/8" cable clamp, cable into the panel.

Sharp knife to CAREFULLY slit the cable jacket.

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. Some bends so the wire would lay in an orderly fashion.

Loosened a screw and inserted the ground wire

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

Neutral line feeding the panel is supposed to be marked white (this one was covered with white electrical tape, which is OK).

wire neatly and made some bends.

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

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.

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.

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.)

firmly until the breaker was seated. The left end was still not connected.

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:

Knock-out was removed from the panel cover.

breaker turned off until the rest of the circuit is finished.

Reinhart Electricians

Reinhart Electric is a Washington state Electrical service companies and required to have a contractors license. Typically, you can find information on contractors at the Labor & Industries web site.

Reinhart Electricians are required to serve an apprenticeship lasting from 3 to 5 years under the general supervision of a Master Electrician and/or a Journeyman Electrician. Schooling in electrical theory and electrical building codes is required to complete the apprenticeship program. Reinhart Electric Journeyman electrician is a well rounded craftsman trained in all phases of electrical construction installation in various building styles and maintenance of equipment after installation. Reinhart Electric Journeyman is permitted to perform all types of electrical work.