Chapter 3: Connecting power
TT-98-129218-A
Power cable selection
21
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.
Example:
Ship supply voltage: 12 V DC
Ship source impedance (measured): 50 m
Extension cable type: 4 mm
2
(AWG 11)
Max. cable extension =
In this case, the power cable can be extended with up to 3.12 m.
If you need more length, you can double the maximum allowed length by connecting two cables in
stead of one, or you can use a cable with a larger diameter.
0 5
85
m
50
10
m
+
–
4
m
m
-----------------------------------------------------------
3 12
m
=
