grundfos data booklet

8SP A, SPCable sizing8. Cable sizingCablesGrundfos offers submersible drop cables for allapplications: 4-core cable, single leads.Cables for Grundfos 4" submersible motors areavailable with or without plugs. The submersible dropcable is chosen according to application and type ofinstallation.Standard version:Max. liquid temperature +70 °C, for short periods up to+90 °C.Tables indicating cable dimension in boreholeThe tables indicate the maximum length of drop cablesin metres from motor starter to pump at direct-on-linestarting at different cable dimensions.If star-delta starting is used, the current will be reducedby 3 (I x 0.58), meaning that the cable length may be3 longer (L x 1.73) than indicated in the tables.If, for example, the operating current is 10 % lowerthan the full-load current, the cable may be 10 %longer than indicated in the tables.The calculation of the cable length is based on amaximum voltage drop of 1 % to 3 % of the ratedvoltage and a water temperature of maximum 30 °C.In order to minimise operating losses, the cablecross-section may be increased compared to what isindicated in the tables. This is only economical if theborehole provides the necessary space, and if theoperational time of the pump is long, especially if theoperating voltage is below the rated voltage.Formula designationsU = Rated voltage [V]∆U = Voltage drop [%]I = Rated current of the motor [A]cos φ = Power factorρ = Specific resistance: 0.025 [Ω mm 2 ]q = Cross-section of submersible drop cable[mm 2 ]sin φ = 1 – cos 2 X L = Inductive resistance: 0.078 x 10-3 [Ω/m].ExampleMotor size: 30 kW, MMS 8000Starting method:Direct on lineRated voltage (U):3 x 400 V, 50 HzVoltage drop (∆U): 3 %Rated current (I):64.0 APower factor (cos φ): 0.85Specific resistance (ρ): 0.025Cross-section (q): 25 mm 2sin φ: 0.54Inductive resistance (X L ): 0.078 x 10 -3 [Ω/m]L =64.0 x 1.73 x 100 x (0.85 xL = 120 m.400 x 30.025 + 0.54 x 0.078 x 10 -3 )25The table values are calculated on the basis of theformula:Max. cable length of a single-phase submersiblepump:L =U x ∆UI x 2 x 100 x (cos φ xρ + sin φ x XL )qMax. cable length of a three-phase submersible pump:[m]U x ∆UL =I x 1.73 x 100 x (cos φ xρ + sin φ x XL )q[m]94