English (GB)
10
5.4 Cable sizing
Make sure that the submersible drop cable can
withstand permanent submersion in the actual liquid
and at the actual temperature.
The cross-section (q) of the cable must meet the
following requirements:
• The submersible drop cable must be sized to the
rated maximum current (I
n
) of the motor.
• The cross-section must be sufficient to make a
voltage drop over the cable acceptable.
Grundfos supplies submersible drop cables for a
wide range of installations. For correct cable sizing,
Grundfos offers a cable sizing tool on the USB stick
supplied with the motor.
Fig. 8 Cable sizing tool
The sizing tool provides a accurate calculation of the
voltage drop at a given cross-section on the basis of
the following parameters:
• cable length
• operating voltage
• full-load current
• power factor
• ambient temperature.
The voltage drop can be calculated both for direct-
on-line and star-delta starting.
In order to minimise operating losses, the cable
cross-section may be increased. This is only cost-
efficient if the borehole provides the necessary
space, and if the operating time of the pump is long.
The cable sizing tool also provides a power loss
calculator that shows the potential savings of an
increased cross-section.
As an alternative to the cable sizing tool, select the
cross-section on the basis of the current values of
the given cables.
The cross-section of the submersible drop cable
must be large enough to meet the voltage quality
requirements specified in section 5.1 General.
Determine the voltage drop for the cross-section of
the submersible drop cable by means of the
diagrams on pages 20 and 21.
Use the following formula:
I = Rated maximum current of the motor.
For star-delta starting, I = rated maximum current of
the motor x 0.58.
Lx = Length of cable converted to a voltage drop of
1 % of the nominal voltage.
q = Cross-section of submersible drop cable.
Draw a straight line between the actual I-value and
the Lx-value. Where the line intersects the q-axis,
select the cross-section that lies right above the
intersection.
The diagrams are made on the basis of the formulas:
Single-phase submersible motor
Three-phase submersible motor
Submersible motor cables are
dimensioned for submersion in the liquid,
and will not necessarily have sufficient
cross-section to be in air.
TM05 8770 2613
Lx =
length of drop cable
permissible voltage drop in %
L =
U × ΔU
I × 2 × 100 ×
L =
U × ΔU
I × 1.73 × 100 ×
L = Length of submersible drop cable [m]
U = Rated voltage [V]
ΔU = Voltage drop [%]
I = Rated maximum current of the motor [A]
cos φ =0.9
ρ = Specific resistance: 0.02 [Ωmm
2
/m]
q = Cross-section of submersible drop cable
[mm
2
]
sin φ = 0.436
Xl = Inductive resistance: 0.078 x 10
-3
[Ω/m].
www.motralec.com / service-commercial@motralec.com / 01.39.97.65.10
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