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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