How to determine the length of a cable using a ohmmeter

Short-circuit one end of the phone wire pair and measure the resistance between the two wires at the other end.

Multiply the resistance by 20 to find the length of the wire in feet. For example, a reading of 54.6 20 1,100). For a maximum of 1,500 ft., the resistance must measure 75 ohms or less. This formula works for standard 24-gauge phone wire.

Cable Lengths and Gauge DC Resistance Chart

18 Gauge

78 X ohms = resistance 51 32 25 12 6 4 2
feet 4,000 2,500 2,000 1,000 500 300 200

20 Gauge

50 X ohms = resistance 80 50 40 20 10 6 4
feet 4,000 2,500 2,000 1,000 500 300 200

22 Gauge

31 X ohms = resistance 129 80 64 32 16 9 6
feet 4,000 2,500 2,000 1,000 500 300 200

24 Gauge

20 X ohms = resistance 200 125 100 50 25 15 10
feet 4,000 2,500 2,000 1,000 500 300 200

26 Gauge

12 X ohms = resistance 133 208 166 83 41 25 16
feet 4,000 2,500 2,000 1,000 500 300 200

Note: 27 Gauge and above is not recommended.