KVH Industries FB250, Installation Manual

Page: 71 / 202

Chapter 3: Connecting power
58 Power cable selection
Calculating the maximum power cable extension
For 24 V DC operation
, the total impedance must be max. 500 m

, including
the source impedance in the ship installation.
For 12 V DC operation
, the total impedance must be max. 85 m

, including
the source impedance in the ship installation.
The total impedance is made up of the following:
• the source impedance in the ship installation
• the cable impedance of the supplied power cable, including the impedance
in the joint of the two cables. In the following example, the impedance of
the cable and joint is set to 10 m

(1 m power cable). Note that if the cable
length or type is changed, the impedance will change accordingly.
• the extension cable impedance.
To calculate the maximum cable extension, do as follows:
1. First measure the source impedance in the ship installation as shown in
Measuring the ship source impedance on page 54.
2. Then find the resistance per meter for the cable type you are going to use.
For 4 mm
2
/AWG 11, the value is 4 m

/m at 20°C
For 1.5 mm
2
/AWG 15, the value is 10 m

/m at 20°C
For other cable types, refer to the data sheet for the cable.
3. Calculate the maximum allowed impedance in the extension cable as
follows:
Max. allowed impedance in extension cable = max. total impedance -
(measured source imp impedance of the supplied cable).
4. Then calculate the max. extension cable length as follows:
Max. impedance in extension cable (from step 3)
Max. length = 0.5 x impedance/meter (from step 2)
The length is multiplied by 0.5 above because there are two conductors in
the cable.