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Tuesday, 24 April 2012

Cable size for motor

Abstract of National Electrical Code for Size of Cable for Motors:

  
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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