gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
201212-1158 EST
The thread titled Intresting situation with 3 way switches and junction box mystery was closed because it was seeming to be going nowhere.
I think one problem was lack of clarity on the meaning of words, and an accurate description of what was found. There were many good troubleshooting tips provided.
My point in this post is that it is possible to use certain techniques that will allow you means to trace circuits.
First, use of a 120 V 1500 W space heater can be useful as a test load. This calculates to 12.5 A. My present test heater runs about 12 A at 113 V after warm up. Somewhat more current at start of heating.
Second, a #12 copper wire at room temperature has a resistance of about 1.62 ohms per 1000 ft or 0.0016 ohms per foot. At 10 A the voltage drop for 6 ft is about 9.6 mV. If you put a 10 A load at the end of a 100 ft #12 copper wire, then the voltage drop over that 100 ft is about 1.6 V. At intermediate points it is proportionally less. No problem to make these measurements with a good digital VOM and an extension cord.
Third, a long straight single conductor wire carrying a current produces a measurable magnetic field about the wire. With a 10 A current, a 7000 turn coil, and a millivolt meter I can get a relatively useful reading, 20 mV or more, from a wire behind a plaster wall. The signal level is far too low if the return current path is adjacent to the current we want to trace. Thus, the extension cord is used for the return current path.
With this information you can do a lot of hidden wire tracing.
.
The thread titled Intresting situation with 3 way switches and junction box mystery was closed because it was seeming to be going nowhere.
I think one problem was lack of clarity on the meaning of words, and an accurate description of what was found. There were many good troubleshooting tips provided.
My point in this post is that it is possible to use certain techniques that will allow you means to trace circuits.
First, use of a 120 V 1500 W space heater can be useful as a test load. This calculates to 12.5 A. My present test heater runs about 12 A at 113 V after warm up. Somewhat more current at start of heating.
Second, a #12 copper wire at room temperature has a resistance of about 1.62 ohms per 1000 ft or 0.0016 ohms per foot. At 10 A the voltage drop for 6 ft is about 9.6 mV. If you put a 10 A load at the end of a 100 ft #12 copper wire, then the voltage drop over that 100 ft is about 1.6 V. At intermediate points it is proportionally less. No problem to make these measurements with a good digital VOM and an extension cord.
Third, a long straight single conductor wire carrying a current produces a measurable magnetic field about the wire. With a 10 A current, a 7000 turn coil, and a millivolt meter I can get a relatively useful reading, 20 mV or more, from a wire behind a plaster wall. The signal level is far too low if the return current path is adjacent to the current we want to trace. Thus, the extension cord is used for the return current path.
With this information you can do a lot of hidden wire tracing.
.
