How long can a telephone extension-cord be?

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.

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Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com

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.

--
Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com

your

volts

=

Ohms

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.

your

volts

=

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