Cable Sizing and Overload Protection for Motors

Abstract of National Electrical Code for Size of Cable for Motors

National Electrical Code 430.22 (Size of Cable for Single Motor)

The size of the cable for a branch circuit that has a single motor connection must be 125% of the motor's full load current capacity.

Example: What is the minimum rating in amperes for cables supplying one number of 5 horsepower, 415-volt, 3-phase motor at 0.8 power factor?
Full-load current for 5 horsepower = 7 amperes.
Minimum capacity of cable = (7 × 125%) = 8.75 amperes.

National Electrical Code 430.6(A) (Size of Cable for Group of Motors or Electrical Load)

Cables or feeders supplying more than one motor or other loads must 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).

Example: What is the minimum rating in amperes for cables supplying one number of 5 horsepower, 415-volt, 3-phase motor at 0.8 power factor, one number of 10 horsepower, 415-volt, 3-phase motor at 0.8 power factor, one number of 15 horsepower, 415-volt, 3-phase motor at 0.8 power factor, and one number of 5 horsepower, 230-volt, single-phase motor at 0.8 power factor?
Full-load current for 5 horsepower = 7 amperes.
Full-load current for 10 horsepower = 13 amperes.
Full-load current for 15 horsepower = 19 amperes.
Full-load current for 5 horsepower (single-phase) = 21 amperes.
Here, the capacity-wise largest motor is 15 horsepower, but the highest full-load current is 21 amperes for the 5 horsepower single-phase motor. Therefore, 125% of the highest full-load current is 21 × 125% = 26.25 amperes.
Minimum capacity of cable = (26.25 + 7 + 13 + 19) = 65.25 amperes.

National Electrical 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 plus 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 in 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 one number of 10 horsepower, 415-volt, 3-phase motor at 0.8 power factor and three numbers of 3 horsepower, 230-volt, single-phase motors at 0.8 power factor?
The full-load current for a 10 horsepower, 415-volt, 3-phase motor is 13 amperes.
The full-load current for single-phase 3 horsepower motors is 12 amperes.
For load balancing, one single-phase motor is connected on the R phase, the second on the B phase, and the third on the Y phase.
Because the motors are balanced between phases, the full-load current on each phase is 25 amperes (13 + 12 = 25).
Multiply 13 amperes by 125% = (13 × 125% = 16.25 amperes). Add to this value the full-load currents of the other motor on the same phase (16.25 + 12 = 28.25 amperes).
The minimum rating in amperes for conductors supplying these motors is 28 amperes.

National Electrical Code 430.32 (Size of Overload Protection for Motor)

Overload protection (heater or thermal cut-out protection) is a device that thermally protects a given motor from damage due to heat when it is loaded too heavily. All continuous-duty motors rated more than 1 horsepower must have some type of approved overload device. An overload shall be installed on each conductor that controls the running of the motor rated more than one horsepower. National Electrical Code 430.37 states that the grounded leg of a three-phase grounded system must also contain an overload. 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 full-load current by the minimum or maximum percentage ratings as follows:

• Maximum Overload: Maximum overload = full-load current of a motor × allowable percentage of the maximum setting of an overload, 130% for motors, found in National Electrical Code Article 430.34. An increase of 5% is 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 = full-load current of a motor × allowable percentage of the minimum setting of an overload, 115% for motors, found in National Electrical Code Article 430.32(B)(1). An increase of 10% is allowed to 125% if the marked temperature rise is not over 40 degrees or the marked service factor is not less than 1.15.