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Selecting a Loop
Calculating DC Loop Resistance
For a Meridian digital telephone, the loop must be without bridge taps, less
than 175 ohm DC resistance, and less than 12.0 dB loss at 256 kHz. For
single-gauge 22 and 24 AWG cable, and D inside wiring, the length limit is
914.4 m (3000 ft). For single-gauge 26 AWG cable, the length limit is
640.08 m (2100 ft).
The allowable loop length assumes there is no under-carpet cable. If there is
under-carpet cable that is a different type than Western Electric (WE) 4-pair
cable, reduce the allowable loop length by using the following equation:
LM = [12 – (UC x UL)] /LL
where:
LM = loop length limit in km (kft) (excluding the length of the
under-carpet cable)
LL
= loop loss in dB/km (dB/kft) at 256 kHz
UC = length of the under-carpet cable in km (kft)
UL = loss of the under-carpet cable in dB/km (dB/kft) at 256 kHz
(see Table 77 on
Use Procedure 63 to calculate the DC loop resistance.
Procedure 63
Calculating DC loop resistance
1
Calculate the DC loop resistance by adding the resistance of each cable
section. Calculate the resistance of each cable section by using the
following formula (cable resistances are given in Table 75 on
LRi = CRi x SLi
where:
LRi = DC resistance for cable section i
CRi = conductor resistance per unit length for the cable section i
SLi = length of cable section i
Digital telephones line engineering
page 512
for dB values)
Telephones and Consoles
Page 503 of 532
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504):
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