Abstract of NEC for Size of Cable for Single or Group of Motors

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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About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

11 Responses to Abstract of NEC for Size of Cable for Single or Group of Motors

  1. Anil sangwan says:

    good one

  2. enigma says:

    Thanks for the email update, man..Am loving the topics. Am still requesting you to send me all your post on the blog in PDF, I wand to be studying offline.

  3. enigma says:

    Great work, man..thanks

  4. willem says:

    i see that PF is not used in any of the calculations. why is this not part of the parameters?

  5. lalit says:

    Dear Jigesh
    Please guide me how to design ais or gis ss for 66/110/220 kv voltages.

  6. Anoop says:

    Its good for electrical engineer

  7. nikhil says:

    sir plz suggest me cable size sizes for equalent motors……

  8. Mohit Jain says:

    How can we calculate the current carrying capacity of cable?

  9. please I need it in pdf

  10. Ajeet says:

    good one Please let me know how to calculate Cable size to Amps Means i have 10 hp motor
    as per calculation 7x2x0.125= 17.5 Amps take it 19 Amps please tell me how to calculate the cable size.

  11. Aftab says:

    Mr.Jignesh please tell how to select cable size for three phase motor having 8.75 Amp ?

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