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Digital telephones line engineering Page 299 of 504
Telephones and Consoles Description, Installation, and Operation
Selecting a Loop
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:
Calculating DC Loop Resistance
Use Procedure 46 to calculate the DC loop resistance.
Procedure 46
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 41 on page 300):
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
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 43 on page 306 for dB values)