KVH Industries TracPhone FB250 Installation Manual page 63

Chapter 3: Connecting power
Calculating the maximum power cable extension
For 24 V DC operation, the total impedance must be max. 500 mΩ, including
the source impedance in the ship.
For 12 V DC operation, the total impedance must be max. 85 mΩ, including
the source impedance in the ship.
The total impedance is made up of the following:
• the source impedance in the ship
• 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 mΩ (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 as shown in Measuring
the ship source impedance on page 118.
2. Then find the resistance per meter for the cable type you are going to use.
For 4 mm
For 1.5 mm
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 impedance + impedance of the supplied cable).
4. Then calculate the max. extension cable length as follows:
Max. length = 0.5 x
The length is multiplied by 0.5 above because there are two conductors in
the cable.
50
2
/AWG 11, the value is 4 mΩ/m at 20°C
2
/AWG 15, the value is 10 mΩ/m at 20°C
Max. impedance in extension cable (from step 3)
Power cable selection
impedance/meter (from step 2)