6
N93-561-85 Issue 0
© Copyright Protec Fire Detection plc 2009
In order to select the required gauge of cabling for a given installation, the maximum cold resistance of
the cable must be calculated. Table 4.1 gives guidelines for cable selection, showing cable AWG
versus resistance per km.
Please note that the resistance of the actual cable being used on a particular installation must be used
to determine if the cable is acceptable or not. This data will be available from the cable data sheets,
available from the cable manufacturer.
Table 4.1 – Cable DC resistance guidelines
AWG
DC resistance (
Ω
per km)
12
5.5
13
7
14
9
15
11
16
14
17
17
18
21
19
27
20
34
Equations have been developed which allow the system designer to calculate a value of cable
resistance for a given installation type.
The equations assume worst case conditions where all the fittings are lumped at one end of a broken
loop (effectively a spur).
Equation 1 – Calculating the maximum cable resistance of a spur.
Loop resistance
(max)
= ( 8 / (
η
x 0.0004 ) )
Ω
(Equation 1)
Where
η
is the number of Digilite® fittings on the cable.
Worked Example 1
The following example shows a situation where 280 fittings are required on a 1.5km loop.
η
= 280.
Loop resistance
(max)
= ( 8 / ( 280 x 0.0004 ) ) = 71
Ω
This shows that the loop wiring must have a cold resistance of no more than 71
Ω
(or 35.5
Ω
per
conductor). So using table 3.1 20 AWG cable would be sufficient.
Digilite®
control panel
Number of fittings (
η
)
Loop broken
at one end