hands on testing (apprentices)

[ibew501ed]

100210-1122 EST

Never worry about asking a question if you do not understand something.

Copper wire has a resistance based on the characteristics of copper, is different than aluminum, iron, stainless steel, etc. All these materials have a resistance that changes with temperature. Some over a limited temperature range may have a very small change with respect to temperature. For example Constantan. Manganin is even more stable. See http://www.allaboutcircuits.com/vol_1/chpt_12/6.html

If you have a circuit with no current flowing in the wires, then you can assume a 20 deg C temperature for the wire under normal conditions in a heated house. For #12 copper wire the resistance is about 1.588 ohms/1000 ft. A 1500 W heater has a resistance of about 10 ohms and on a 120 V circuit draws about 12 A.

Because of the thermal time constant of wire you can apply a 12 A load for a short time to a #12 copper wire with little change in resistance. If you leave the 12 A on for a long time you can see the copper wire resistance change. But to do this test you need to also be monitoring current as well as the voltage drop.

The resistance of a typical 1500 W heater will show a substantial resistance change from cold to hot.

Consider two duplex outlets connected by 10 ft of #12 copper. Put your meter between the hot terminal of one outlet and the hot terminal of the other outlet without removing cover plates. With no current flowing the voltage should read near 0 millivolts. Plug the 1500 W heater into the outlet furthest from the power source. Ideally you should measure the current, but we will assume 12 A. The voltage difference read should be 10*1.588*12/1000 = 0.191 V or 191 MV. Next move the 1500 W load to the outlet closest to the power source and the voltage difference should be near 0.

You should get essentially the same results on the neutral.

There is some small voltage drop in the termination of the wire to each outlet, but I eliminated the voltage drop of the heater plug to the outlet by using a connection point for the meter that did not include the heater plug.

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Thanks for the clarity and I will use this in class BUT I don't want to embarrass them.

I'm also thinking about a bell circuit for an apartment house (low volt, easy to staple wires, and rings the bell when correct) either they wire it or i wire it wrong and they troubleshoot it.(this part is being done at our picnic)

 

[wawireguy]

I wouldn't make it all mechanic work either. How about some of the thinking parts of being a real electrician.

 

[ericsherman37]

The apprenticeship I just graduated from has an annual competition similar to this. Ours is modeled after the Western States Regional electrical competition. We have the following events:

Code & General Knowledge test (including NEC, state codes, transformers, motors, electrical theory, calculations, etc.)

Motor control ladder diagram drawing (presented with a control scenario in prose and have to draw a ladder diagram of it)

Motor control wiring project (taking the ladder diagram and wiring it on a control set-up board)

Residential wiring project (lights, plugs, 3 and 4-way switches, doorbell circuit)

Material Identification (trying to identify some of the most obscure and weird electrical devices/objects in existence without touching them or reading labels)

Conduit Bending (each competitor gets one full stick of 1/2" EMT and a drawing of what the finished pipe includes: back-to-back 90s, three and four-bend saddles, offset, box offset on one end. A "jig" is built on a piece of plywood to determine if the finished pipe fits in correctly).

It's really fun and we usually wind up getting pizza or whatever for lunch. The winner gets to go on and represent our apprenticeship at the Regional competition.