Page 2 of 2

Have you looked at the cord in good light with your glasses on? The U/L marking guide says this should be labeled every 24"

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 .
Jerry

http://community.webtv.net/awoodbutcher/MyWoodWorkingPage

http://community.webtv.net/awoodbutcher/CARWRECK

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!

Dave

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.

Dave

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 probe.

Why wouldn't the table saw already have an appropriate line cord? s

#### Site Timeline

- posted on September 2, 2008, 11:19 pm

Have you looked at the cord in good light with your glasses on? The U/L marking guide says this should be labeled every 24"

- posted on September 3, 2008, 11:00 pm

http://community.webtv.net/awoodbutcher/MyWoodWorkingPage

http://community.webtv.net/awoodbutcher/CARWRECK

- posted on September 5, 2008, 2:16 am

blueman posted for all of us...

Bite the cord blueman

Bite the cord blueman

--

Tekkie - I approve this advertisement/statement/utterance.

Tekkie - I approve this advertisement/statement/utterance.

- posted on September 5, 2008, 2:43 am

blueman wrote:

Measure the resistance. 14 gauge is 60% higher than #12 so it should be easy to do.

Measure the resistance. 14 gauge is 60% higher than #12 so it should be easy to do.

- posted on September 5, 2008, 4:26 am

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.

- posted on September 5, 2008, 8:27 pm

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!

Dave

- posted on September 5, 2008, 9:53 pm

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.

- posted on September 7, 2008, 7:33 am

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.

Dave

- posted on September 7, 2008, 3:17 pm

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.

- posted on September 5, 2008, 12:49 pm

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 probe.

Why wouldn't the table saw already have an appropriate line cord? s

- Kitchen faucet chatter
- - next thread in Home Repair

- paint problem- mismatch
- - previous thread in Home Repair

- Anyone seen this documentary?
- - newest thread in Home Repair

- What is this switch for on the furnance
- - last updated thread in Home Repair

- mouting a handrail to the top edge of a 4 x 2 beam.
- - the site's newest thread. Posted in UK Do-It-Yourself Forum

- What is this switch for on the furnance
- - the site's last updated thread. Posted in Home Repair