System calculations
B.4 EST3 Installation and Service Manual
Calculating maximum wire capacitance per foot
The capacitive property of twisted-pair cable varies and the cost
of cable generally increases as the capacitance per foot
decreases. Following is a sample calculation for determining the
maximum capacitance per foot that a cable can have for a given
network length.
The maximum capacitance per foot equals the maximum
cumulative capacitance divided by the total network length. In
equation form:
MCPF = MCC / TNL
Where:
MCC = Maximum cumulative capacitance, from the table given
in this topic
TNL = •Total network length, the sum of the lengths of
individual copper runs in the network
Here’s an example. The total copper distance of a network is
26,000 feet. Calculate the maximum capacitance per foot that
can be used for 18 AWG twisted-pair cable at 38.4K baud.
MCPF = MCC / TNL
MCPF = 1.4 μF / 26,000 ft
MCPF = 53.8 pF/ft
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