Measuring Phone Wire - Black Box 10BASE-T Manual

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3.7 Measuring Phone Wire

1. Use a cable scanner or an ohmmeter to measure the lengths of phone wire

that will connect the Extenders. When using an ohmmeter, short-circuit one

end of the phone wire and measure the resistance between the two wires at

the other end (see Figure 3-8).

Figure 3-8. Using an ohmmeter to measure the length of phone wire.

2. Multiply the resistance by 20 to find the length of the wire in feet. For

example, 26.4 ohms would indicate a cable length of about 525 feet: 26.4 x 20

@ 525 (+/-). This formula works for 24-gauge phone wire. For a maximum of

600 feet of wire the resistance must measure 30 ohms or less. Refer to Table

3-1 for maximum acceptable resistance values of other wire gauges.

Ohm Meter

54.6

OHMS

Punchdown Block

10BASE-T EXTENDER

Short Pair

Together

Punchdown Block

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