4
ELECTRICAL CONNECTIONS
22
OPTITEMP TT 53 C/R
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06/2018 - 4006523501 - MA OPTITEMP TT 53 R01 en
4.8 Cable length
In order to ensure reliable HART
®
communication, the maximum cable length of the output
circuit must be observed.
To calculate the maximum cable length for the output circuit, determine the total resistance of
the output loop (load resi approximate cable resistance). Find out the capacitance of the
cable being used. In the following tables you can find the maximum cable length based on the
typical values for 1 mm
2
cables. CN is the abbreviation for "Capacitance Number" which is
multiple of 5000 pF present in the device.
For multiple connections (Multidrop mode), the following formula shall be used:
L = [(65 x 10
6
) / (R x C)] x (Cn x 5000 + 10000) / C
with
L: cable length [m or ft]
R: load resistance (incl. the resistance of any Zener barrier) + cable resistsance [
Ω
]
C: cable capacitance [pF/m or pF/ft]
Cn: number of transmitters in the loop
Field device
Cable insulation
PVC
Polyethylene
Polyethylene foam
[m]
[ft]
[m]
[ft]
[m]
[ft]
1 (CN = 1)
600
1969
1100
3609
2000
6562
10 Multidrop (CN = 1)
500
1640
900
2953
1600
5249
10 Multidrop (CN = 4.4)
85
279
150
492
250
820
Table 4-1: Maximum length for typical 1 mm
2
cables
Insulation
Capacitance [pF/m]
PVC
300...400
Polyethylene
150...200
Polyethylene foam
75...100
Conductors
Resistance [
Ω
/km]
(both conductors in series)
Area [mm
2
]
Diameter [mm]
AWG
2.0
1.6
14
17
1.3
1.3
16
28
1.0
1.15
17
36
0.8
1.0
18
45
0.5
0.8
20
70
0.3
0.6
22
110
0.2
0.5
24
160
Table 4-2: Cable parameters
