How do you calculate a wire's length by measuring its resistance?

Question

i've seen this done when purchasing wire at the hardware store. I'm sure there's a formula/chart they reference, but as an electrician i was wondering how they do it??

Answer 1

It can be done if you know the resistance per unit length of the wire. For average sized electrical wires it would only be accurate for very long lengths, because copper wire has such a low resistance.

Eg. standard stranded equipment wire (7x 0.2mm) has a resistance of about 0.1 ohms / metre. A typical digital multimeter measures resistance with an accuracy of about 0.1 ohms. That means that if you used this method, the length you get could be out by at 1 metre because of the inaccuracy of the meter, plus a percentage of the total length due to variation in the resistance / metre value of the wire.

Answer 2

I've never heard of this being done, but it's possible.

Wire tables give the resistance per 1000', so you can measure the resistance, and if you know the gauge, you can calculate the length from that.

But you would need a 4 terminal ohmmeter more accurate than the ordinary ones.

eg, #10 wire is about 1 ohm per 1000', so if you wanted the length accurate to 1 foot, you need to measure to within 0.001 ohm.

wire table:

http://www.powerstream.com/Wire_Size.htm

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Answer 3

No when you are just purchasing a wire you need  a measuring tool like the measuring steel tape.

And for the required diameter of wire they are also using a  InternationalWire Gauge it is also a measuring tool.

**Hope This Helps**

**Good Luck**