Buy a new cord and you'll know for sure, if it's 15 years old it just
may be weathered and brittle. Do as you may see fit .
Almost no home owner will have a meter that can measuer the differance in
the resistance of 10 feet of # 12 and # 14 wire. It would be the differance
of about .0159 and .0252 ohms for 10 feet.
Good thought , but impractical for almost every one.
It would almost take lab grade equipment to do that.
Few people have a single meter with a sufficiently low-range ohms scale
at home, but you can still do it with a bit of ingenuity. If you pass
a constant current through the wire, you get a voltage across the ends
that is proportional to the wire resistance, and many DMMs (even some
of the cheapest ones) have a 200 mV scale. For example, with a 1 A
current source, the two 10-foot pieces of wire in the example above
would have a voltage drop of 16 and 25 mV, enough to be told
apart by a DMM on even a 2 V range.
The most convenient current source for this would be a lab supply with a
few amps of output and a constant-current mode. But a 6 V or 12 V
battery (or power supply) plus the appropriate resistor to give an amp
or two of current will also work. Just measure the actual current and
the voltage across the wire and divide to get resistance. The resistor
needed will be a few ohms, so inserting a meter set to its 10 A range
will have negligible effect on the current.
I once had a problem figuring out how a particular circuit was wired in
a previous home. I opened the breaker, then connected a lab supply set
to 1 A constant current mode to one outlet that I thought was probably
near one end of the string. I shorted hot to neutral at another outlet
that I though was near the other end. Then I measured the voltage at
all of the other outlets on that circuit. There was an easily
measurable voltage drop (tens of mV) between boxes along the circuit,
so I could tell what sequence they were wired in, and where an outlet
was fed from a "tee" along the main path. (It turned out that some
previous owner had routed the circuit through a junction box that was
completely buried inside a wall, with no access from any side).
Note that this method only works when power is completely disconnected
from the house panel. If you try it on an energized circuit, you'll
probably destroy your power supply!
That is the basic gist of a 4 wire probe. With one pair of leads, put some
current through the resistance under test. a 10V supply and a 100ohm resister
will put 100ma through and be fairly immune to the resistance of the first
set of probes. Measure the current going through the first pair of leads
(or it has to be regulated). With the second set of probes, measure the
voltage drop across the resistance under test. R=E/I.
That's more or less where I got the idea. The difference is that the
pair of measuring probes could move along the "resistor" and observe
the voltage drop. The amount of voltage drop between two outlets is
reasonably correlated with the length of wire between them.
If you're measuring resistance between outlets, you could plug in a small
resistive load like a 25W bulb and slap on a clamp on amp meter to
know the current through the circuit. Then you could measure the
voltage drop between outlets and know the resistance. To get from
there to the wire gauge, you'd have to know the length of the wiring
between the outlets.
Measuring resistance of wire is difficult with a common multimeter. The
resistance of one's probes tends to be greater than the resistance of the wire
and most 3 1/2 digit meters are very inaccurate for milliohm readings.
You either need to measure at least 100' of it, or use a 4 wire
Why wouldn't the table saw already have an appropriate line cord?
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