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Mapping Electrical Circuits

Materials List
Tools List (click item to shop)
Circuit tester
A scratch pad of paper
Carpenter's pencil
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IMPORTANT: Read this before you start


Let there be light! Unfortunately, many older homes have “outgrown” their electrical systems. When these homes were originally built, no one anticipated that we would be using as many electric devices as we do today. Over the years, new light fixtures, outlets and appliances have been added to homes that are not adequately wired to handle the load.

If you are planning to add any new outlets, lights or appliances, it is helpful to have a map (or schematic drawing) of your electrical circuits. This will help you determine if you can tap into an existing circuit or if you need to run a new circuit from the breaker panel. In this tutorial, you will learn how to map out your home's electrical system, which will help you plan for future needs or possibly alert you to an existing circuit overload. Also, many municipalities require that your circuit box be completely labeled before reselling your home.

Skill Level & Time To Complete
• Beginner - 1 to 2 hours
• Intermediate - 45 to 60 minutes
• Advanced - 30 to 45 minutes

You will be turning the breakers in your house off and on, or removing fuses to map your circuits. Make sure you are not turning off anything critical, like a computer.

Helpful Tips

If you don’t have a circuit tester, you can use a small electric lamp to test outlets.

“Gauge” is a measurement of wire thickness. The lower the number, the thicker the wire and the higher the amp rating. For instance, 14-gauge wire has an amp rating of 15 and is thinner than 12-gauge wire, which has an amp rating of 20.

1. Before you map out your electrical circuits, it is helpful to have a basic understanding of electricity. Electricity is the flow of electrons through a conductive material. The force with which electrons are moving is measured in volts. The speed that the electrons are moving is measured in amperes or amps. If you multiply volts x amps, you determine the number of watts that the circuit can safely provide. Watts are the amount of power required to run anything electric.
2. Plumbing and electricity have very similar operating principles. Volts (the force electrons are moving) are similar to the water pressure in a pipe. A thicker wire can carry more electricity (measured in amps), just like a pipe with a greater diameter can carry more water. Finally, watts measure total output of electricity, similar to measuring total gallons of water flowing through a pipe. The greater the water pressure and the wider the pipe, the more water you will get. Electricity has these same types of relationships, except they are measured in volts, amps and watts.
3. Here is an example of the relationship between volts, amps and watts. Most of the outlets in your house are probably wired with 14-gauge wire. 14-gauge wire has an amp rating of 15. Standard voltage through most of your house is 120 volts. If you multiply 15 amps x 120 volts, you get 1,800 watts. If you have one circuit that services 9 outlets, you can have eighteen lamps plugged into those outlets - each with a single 100-watt light bulb. Anything beyond that exceeds the capacity of the circuit, which can lead to a blown breaker or possibly greater damage to the electrical system.
4. Light bulbs use a relatively low amount of watts. However, a hairdryer uses about 1,200 watts. A refrigerator uses about 3500 watts and a large microwave uses about 1,800 watts. An average central air conditioning unit uses 5,000 watts and an electric range uses 12,000 watts! The point here is that each and every circuit in your house should be constructed to handle the watts required by the lights or appliances using it.
5. Every circuit should be built with each component having the same amp rating. As discussed above, 14-gauge wire has an amp rating of 15. Thicker wires can carry more amps. For instance, 12-gauge wire has a rating of 20 amps, providing a capacity of 2,400 watts at 120 volts. Breakers, fuses, switches and outlets all come in varying amp ratings to match the wiring. When installing a circuit with a specific amp rating, you should make sure that you use wire, a breaker or fuse, switches and outlets that all have an equivalent amp rating.
6. Now that you have an understanding of how to calculate the load and the requirements of a circuit, you can map out and interpret the electrical system in your house. Begin by drawing a floor plan of each floor of your home, then mark the location of each outlet, switch, light fixture and appliance.
7. Turn off the first breaker in your breaker panel or remove the first fuse in your fuse box. Make a note of the amp rating for that breaker for future reference. Using a circuit tester (or a small lamp) test the outlets in your house to determine which outlets are on that circuit. If the tester does not go on, then the outlet is on that circuit.
8. Mark the breaker number on your drawing next to each outlet that doesn’t have power. Turn light switches on and off to determine if any lights are on the circuit. Test appliances to see if they are receiving power. Mark on the drawing all of the electrical items that are on that circuit. Flip the breaker back on and turn off the next breaker. Continue the testing process until you have determined which breaker or fuse controls every light, outlet and appliance in your house.

To assess the load requirements for each circuit, check the watt requirements of all appliances and lights in your house. (Most appliances have this marked somewhere on the back or underside.) Add up the watts required by the lights and appliances on each circuit, then you can compare the total watts required on a circuit to the watts that the circuit can safely deliver (volts x amps). In figure 9 you can see that many major appliances require a separate circuit to safely provide the watts that are required to run each appliance.

Now that you have a map of your electric circuits, you can safely determine which circuit you can tap into, to add an extra receptacle. You might also determine that you need to split a single circuit into two circuits to safely handle the load it presently requires.



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