Cable Sizing - Grundfos SP Series Installation And Operating Instructions Manual

6.3 Cable sizing

Submersible motor cables are dimensioned for submer-
sion in liquid, and will not necessarily have sufficient
cross-section to be in free air.
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 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. A cable sizing tool is available on Grundfos Insite at:
https://www.grundfos.com/sp-system/download-sp-app-html.
Cable sizing tool
The sizing tool provides an 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 Electrical connection.
Determine the voltage drop for the cross-section of the submersible
drop cable by means of the diagrams in the Appendix.
Use the following formula:
I: rated maximum current of the motor.
For star-delta starting, I is equal to the rated maximum current of
the motor x 0.58.
Lx: length of cable converted to a voltage drop of 1 % of the
nominal voltage.
length of drop cable
Lx =
permissible voltage drop in %
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:
14
Single-phase submersible motor
L =
I × 2 × 100 × (cos φ ×
Three-phase submersible motor
L =
I × 1.73 × 100 × (cos φ ×
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: 0.025 [Ωmm
q
Cross-section of submersible drop cable [mm
sin φ 0.436
Xl
Inductive resistance: 0.078 x 10
Related information
6. Electrical connection
U × ΔU
ρ
+ sin φ × Xl)
q
U × ΔU
ρ
+ sin φ × Xl)
q
2
/m]
2
]
-3
[Ω/m].