7.6 Cable sizing
Before you install the pump, make sure to use the
right cable size for the submersible drop cable.
The cross-section of the submersible drop
cable must be large enough to meet the
voltage requirements.
How to calculate the maximum cable length
If the power factor (PF) of the motor unit equals 1.0,
you can use this equation to calculate the maximum
cable length:
TM070522
Explanation of the equation
Symbol Unit Description
L
MAX
[m] Maximum cable length
U [V] Supply voltage
ΔU [%]
Maximum recommended volt-
age drop in percentage
I [A] Maximum motor current
ρ
[Ω
mm
2
/m]
Specific resistance of the cable
ϥ
[mm
2
]
Cross-sectional area of the indi-
vidual wires in the submersible
drop cable
Maximum motor current
The maximum motor current depends on the motor
properties and electrical installation. According to IEC
60364-5-52:2009 the installation and cable must
be dimensioned for a current that is higher than the
maximum motor current.
Maximum recommended voltage drop
• According to IEC 60364-5-52:2009 for installation
in domestic applications, the maximum
recommended voltage drop is 5 % for cable
lengths up to 100 m.
• For installation in industrial applications and in
regions where the IEC standard is not applicable,
local regulations may require that a different
maximum value for voltage drop must be used for
the calculation of the maximum cable length.
Specific resistance of the drop cables
The specific resistance of the drop cables supplied by
Grundfos for SQ and SQE pumps is 0.02 Ω mm
2
/m.
Maximum cable lengths for Grundfos MSF 3
motors
Calculation of maximum cable length for the different
motor sizes is based on a voltage drop of 5 % and a
supply voltage of 240 V.
If the calculation above cannot be used, go to
Grundfos Product Center for sizing.
7.7 Fitting the submersible drop cable
It is recommended to join the submersible drop cable
and the motor cable by means of a Grundfos cable
termination kit, type KM.
Cable termination kit, type KM
Cross-sectional area Product number
1.5 to 6.0 mm
2
96021473
For larger cross-sections, please contact Grundfos.
15
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