Home Wiring LabUM Physics Demo Lab 07/2013
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Materials
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14-2G Wire
1 – 22” backbone
2 – 6” “Pigtail Packs”
1 “Junction Box” backbone with special plug for variac
1 Receptacle
1 Switch
1 Home Wiring Board
1 Multi-purpose wire tool
1 Needle Nosed Pliers
1 Gas Pipe Pliers
1 Screwdriver
5 Wirenuts
1 Multimeter
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Building the Home-Wiring Board
Reference the Home Wiring Manual as needed to understand each component of assembly. Appropriate page numbers are cited in the text.
Special Note
For completeness, the Home wiring manual and attached schematic include the wiring details for a light fixture. We will NOTwire the light fixture for today’s lab.
1: Prepare the Wire
Prepareaccording to the directions in the manual:
- sheathed wire backbones (p.4, backbone wire section of the analysis)
Do not strip the ends of the backbone until you feed it through the boxes.
- pigtails (p.5)
2: The Receptacle
Wire the receptacle (p. 9). We want the receptacle to be wired in parallel to the whole circuit. You are going to be connecting two components together here: the receptacle, and the backbone wiring that leads to the switch.
All three of the connections must be pigtailed so that the receptacle can be easily replaced if it fails. This would not be possible if it were connected directly to the backbone wire.
First, feed the loose ends of the backbone wire into the receptacle box. You will need three pigtails for this task. A quick reminder about the anatomy of a receptacle, the short slot is hot and the tall slot is neutral. The screws on the corresponding sides are meant to connect to hot or neutral as is appropriate.
Take the hot (black) pigtail, and the hot backbone lead, and twist these wires together with your needle-nose pliers (p. 8). Cap this with a wirenut, and make sure only insulated wire is showing (bare wire could cause a short).
Repeat that step with the neutral wire, twist the backbone lead and pigtail together and cap with a wirenut.
Repeat that step again with the ground wire. Twist the leads, and cap with a wirenut.
You now have three pigtail ends attached to the backbone with wire nut connections. Form the ends of the pigtails into neat, round hooks and attach them to the receptacle. The ground wire can either be screwed to the metal receptacle box or to the receptacle ground connection. The hot wire should be connected to the hot slot on the receptacle, and the neutral wire should be connected to the neutral slot.
3: The Switch
Moving down the line, it is now time to install the switch (p. 10). Here, you will be attaching the backbone from the receptacle, the switch, and the backbone from the junction box, which is wired with a special plug. You will not use pigtailing for the hot lead because the purpose of the switch is to interrupt the current, so you don’t want the current in the hot lead to bypass the switch as a pigtail set-up does.
Feed the loose ends of the two backbone sections you mean to connect into the switch box. The backbone with the special yellow plug simulates a connection to an electrical junction box and will be used to connect your completed wiring project to the electrical power provided by the wall socket. For safety this is a special plug that cannot be plugged directly into the wall.
Neutral Wire: Look at the schematic, and notice that the neutral wire doesn’t go to the switch. This is because the switch doesn’t need to be powered, its only job is to mechanically connect or disconnect the hot lead. This means that the two white backbone wires only need to be spliced together. So take needle-nose pliers or your multi-tool to grip and twist the wires together as shown in the wirenut photo in the manual. Twist on the wirenut, and now the wires are connected and bypassing the switch.
Ground Wire: The ground requires the use of a pigtail for safety.Take the three ground ends (one pigtail and two backbone leads), and twist them together with your needle-nose pliers and twist on a wirenut. The other end of the ground pigtail must be formed into a hook and attached to the metal box, or the ground screw on the switch.
Finally we must connect the hot leads. From the photo of the switch in the manual, you will notice two screws on one side of the switch. These are the two connection points for each backbonehot lead. For safety, the switch controls the hot leads.Form the hot leads into hooks and then attach them to the switch.
6: Testing your Connections
Attached is a schematic of all the points at which you need to test your board. Test the board with the switch in the “on” position.
We can verify that the board is correctly wired by testing the resistance between various test points. You are going to test resistances on your board with a multimeter. You will do this before you connect it to 120V AC from the wall.
What resistance readings mean:
If two points you are checking with your multi-meter probe are connected, then there will be almost no resistance between them because the current can flow freely in our low resistance wires.
If they are not connected, then there will be “infinite” resistance, and your multi-meter will read “OL”, as in “overload”. This is because the electrons cannot flow between disconnected points, and so there is unlimited resistance to the flow of electrons.
Examples:
Place the multi-meter probes on points A and B of the diagram. They are not connected because nothing is plugged in the receptacle and the light bulb is not in the socket. The resistance should therefore read “OL”.
The neutral slot of the receptacle (point E) should be connected to the neutral lead (point B). Place one probe in the tall, neutral slot of the receptacle, and the other on the neutral lead. They are connected, so the resistance should read 0.
Fill in the Excel chart with your measurements by testing the board with the multimeter. See if you observe resistances consistent with the correct resistances. If not, you have wiring problems and may need to re-work some of your connections.
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