Kidde VM-1 Technical Reference Manual

Appendix A: System calculations
P/N 3101890-EN • REV 006 • ISS 21JUN18 169
The overall length of data networks designed with twisted-pair cable is about twice as long as data networks
designed with shielded cable due to the additional capacitance resulting from the shield.
Calculating a maximum length
The maximum length of a data network can be calculated by dividing the maximum cumulative capacitance
allowed by the highest capacitance rating of the selected cable.
For example, say you wanted to determine maximum length of a data network using 18 AWG cable that is rated
at 25 pF per foot. The network will communicate at 38.4 Kbaud.
The maximum length equals the maximum cumulative capacitance divided by the capacitance per foot. In
equation form:
ML = MCC / CPF
Where,
ML = Maximum length
MCC = Maximum cumulative capacitance
CPF = Capacitance per foot
In our example, the calculation is as follows:
ML = 1.4 µF / 25 pF/ft
ML = 56,000 ft
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
For 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

Kidde VM-1 Specifications

General

Brand	Kidde
Model	VM-1
Category	Fire Alarms
Language	English