How long can a telephone extension-cord be?

[Floyd L. Davidson]

If the limit were 25 feet, you would have a problem connecting your
phone to the telco which may be several miles away.

They usually use 48 volts to supply 20mA of current in series with your
phone. The phone is about 600 ohms and drops 12 volts at 20mA which
leaves 36 volts that can be lost in the line. If you use 22 gauge
copper wire, the resistance is about .016 ohms per foot, and 36 volts
at 20mA is about 1800 ohms, so the maximum length would be 1800/.016 =
112500 feet or 21 miles. A larger gauge will increase the distance.

600 Ohms is the nominal *impedance* of a telephone set. The DC
resistance is necessarily less than 350 Ohms. Common values are
roughly about 200 Ohms.

Loop current is supplied through a resistance though, which is
typically 400 Ohms. (For extended length loops the voltage may
be higher than 48 VDC and/or the resistance may be less than 400
Ohms.)

And it is also true that the line has to work when the
battery voltage is low! So while it is a "48 Volt Battery",
the typical voltage is 52 VDC and the range that may be seen
is 42 to 56 VDC. So maximum loop length would need to be
calculated using 42 VDC, not 48 VDC.

Also the common wire sizes are 26 gauge and (on rare occasions)
24 gauge.

While 20 mA is the current at which the line is considered
"offhook", the actual steady state loop current must be 23 mA or
higher. Typically it is 27 mA or higher (and is current
limited).

Hence you have the right idea for determining maximum loop
length, it is actually slightly more complex and requires
different numbers.

 

[Bill Janssen]

600 Ohms is the nominal *impedance* of a telephone set. The DC
resistance is necessarily less than 350 Ohms. Common values are
roughly about 200 Ohms.

Loop current is supplied through a resistance though, which is
typically 400 Ohms. (For extended length loops the voltage may
be higher than 48 VDC and/or the resistance may be less than 400
Ohms.)

And it is also true that the line has to work when the
battery voltage is low! So while it is a "48 Volt Battery",
the typical voltage is 52 VDC and the range that may be seen
is 42 to 56 VDC. So maximum loop length would need to be
calculated using 42 VDC, not 48 VDC.

Also the common wire sizes are 26 gauge and (on rare occasions)
24 gauge.

While 20 mA is the current at which the line is considered
"offhook", the actual steady state loop current must be 23 mA or
higher. Typically it is 27 mA or higher (and is current
limited).

Hence you have the right idea for determining maximum loop
length, it is actually slightly more complex and requires
different numbers.

And in addition to the above there is resistance in the Central Office
equipment
that was about 400 Ohms. Don't know what it is in modern CO equipment.

Bill K7NOM

 

[Floyd L. Davidson]

^^^^^^^^^^^^^^^^^^^^

And in addition to the above there is resistance in the Central
Office equipment
that was about 400 Ohms. Don't know what it is in modern CO equipment.

Bill K7NOM

Yup. That's what I was referring to as "Loop current is
supplied through a resistance". I didn't make it clear enough
that it is indeed part of the line card in the switch.

Originally that was the resistance of the relay (two balanced
windings, one for tip and one for ring, each with 200 Ohms
resistance) that pulsed with the loop current to detect dialed
digits.

Today that is usually replaced with a current regulator, and
the effective resistance is whatever is required to limit the
current to whatever that particular equipment uses. Common
values are everything from 25mA to 45mA.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

The phone is not 600 Ohms DC resistance. A tone dialer phone is 330 Ohms
DC.

Not only that, but that's 21 miles round trip, or 10.5 miles loop length
since it's a pair. And the average phone works with much less than
12VDC. Usually they work with 6V, but a true Bell 2500 set should work
down to even less. The current is typically around 30mA. and for those
of you who have AoE, there is a good chart of this on page 936.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

And in addition to the above there is resistance in the Central Office
equipment
that was about 400 Ohms. Don't know what it is in modern CO equipment.

Umm, that's what he just said, above (I quote):