Calculate Motor Pump Size


  • Calculate Size of Pump having following Details
  • Static Suction Head(h2)=0 Meter
  • Static Discharge Head (h1)=50 Meter.
  • Required Amount of Water (Q1)=300 Liter/Min.
  • Density of Liquid (D) =1000 Kg/M3
  • Pump Efficiency (pe)=80%
  • Motor Efficiency(me)= 90%
  • Friction Losses in Pipes (f)=30%

Calculations:

  • Flow Rate (Q) =Q1x1.66/100000 =300×1.66/100000= 0.005 M3/Sec
  • Actual Total Head (After Friction Losses) (H) = (h1+h2)+((h1+h2)xf)
  • Actual Total Head (After Friction Losses) (H)=50+(50×30%)= 65 Meter.
  • Pump Hydraulic Power (ph) = (D x Q x H x9.87)/1000
  • Pump Hydraulic Power (ph) = (1000 x 0.005 x 65 x9.87)/1000 =3KW
  • Motor/ Pump Shaft Power (ps)= ph / pe = 3 / 80% = 4KW
  • Required Motor Size: ps / me =4 / 90% = 4.5 KW
  • Required Size of Motor Pump = 4.5 HP or 6 HP

Calculate Size of Cable for Motor (As NEC)


NEC Code 430.22 (Size of Cable for Single Motor):

  • Size of Cable for Branch circuit which has Single Motor connection is 125% of Motor Full Load Current Capacity.
  • Example: what is the minimum rating in amperes for Cables supplying 1 No of 5 hp, 415-volt, 3-phase motor at 0.8 Power Factor. Full-load currents for 5 hp = 7Amp.
  • Min Capacity of Cable= (7X125%) =8.75 Amp.

 NEC Code 430.6(A) (Size of Cable for Group of Motors or Elect. Load).

  • Cables or Feeder which is supplying more than one motors other load(s), shall have an ampacity not less than 125 % of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A).
  • For Calculating minimum Ampere Capacity of Main feeder and Cable is 125% of Highest Full Load Current + Sum of Full Load Current of remaining Motors.
  • Example:what is the minimum rating in amperes for Cables supplying 1 No of 5 hp, 415-volt, 3-phase motor at 0.8 Power Factor, 1 No of 10 hp, 415-volt, 3-phase motor at 0.8 Power Factor, 1 No of 15 hp, 415-volt, 3-phase motor at 0.8 Power Factor and 1 No of 5hp, 230-volt, single-phase motor at 0.8 Power Factor?
  • Full-load currents for 5 hp = 7Amp.
  • Full-load currents for 10 hp = 13Amp.
  • Full-load currents for 15 hp = 19Amp.
  • Full-load currents for 10 hp (1 Ph) = 21Amp.
  • Here Capacity wise Large Motor is 15 Hp but Highest Full Load current is 21Amp of 5hp Single Phase Motor so 125% of Highest Full Load current is 21X125%=26.25Amp
  • Min Capacity of Cable= (26.25+7+13+19) =65.25 Amp.

 NEC Code 430.24 (Size of Cable for Group of Motors or Electrical Load).

  • As specified in 430.24, conductors supplying two or more motors must have an ampacity not less than 125 % of the full-load current rating of the highest rated motor +  the sum of the full-load current ratings of all the other motors in the group or on the same phase.
  •  It may not be necessary to include all the motors into the calculation. It is permissible to balance the motors as evenly as possible between phases before performing motor-load calculations.
  • Example:what is the minimum rating in amperes for conductors supplying 1No of 10 hp, 415-volt, 3-phase motor at 0.8 P.F and 3 No of 3 hp, 230-volt, single-phase motors at 0.8 P.F.
  • The full-load current for a 10 hp, 415-volt, 3-phase motor is 13 amperes.
  • The Full-load current for single-phase 3 hp motors is 12 amperes.
  • Here for Load Balancing one Single Phase Motor is connected on R Phase Second in B Phase and third is in Y Phase.Because the motors are balanced between phases, the full-load current on each phase is 25 amperes (13 + 12 = 25).
  • Here multiply 13 amperes by 125 %=(13 × 125% = 16.25 Amp). Add to this value the full-load currents of the other motor on the same phase (16.25 + 12 = 28.25 Amp).
  • The minimum rating in amperes for conductors supplying these motors is 28 amperes.

 NEC 430/32 Size of Overload Protection for Motor:

  • Overload protection (Heater or Thermal cut out protection) would be a device that thermally protects a given motor from damage due to heat when loaded too heavy with work.
  • All continuous duty motors rated more than 1HP must have some type of an approved overload device.
  • An overload shall be installed on each conductor that controls the running of the motor rated more than one horsepower. NEC 430/37 plus the grounded leg of a three phase grounded system must contain an overload also. This Grounded leg of a three phase system is the only time you may install an overload or over – current device on a grounded conductor that is supplying a motor.
  • To Find the motor running overload protection size that is required, you must multiply the F.L.C. (full load current) with the minimum or the maximum percentage ratings as follows;

Maximum Overload

  • Maximum overload = F.L.C. (full load current of a motor) X allowable % of the maximum setting of an overload,
  • 130% for motors, found in NEC Article 430/34.
  • Increase of 5% allowed if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15.

Minimum Overload

  • Minimum Overload = F.L.C. (full load current of a motor) X allowable % of the minimum setting of an overload,
  • 115% for motors found in NEC Article 430/32/B/1.
  • Increase of 10% allowed to 125% if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15

 

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