By Mark Beatty, Design Engineer, Gardner Denver Inc.

Safety is the driving force in the determination of minimum recommended supply wire sizes for variable-speed compressor packages. With this in mind, all wire size recommendations should be based on meeting, or exceeding, the minimum requirements of the National Electrical Code (NEC).

In determining the proper minimum wire size recommendation, NEC 430.122 lists the preliminary requirement to obtain the drive-rated input ampacity which is listed on the drive data label, and in the appropriate drive manual. All drive input amps are added together, as applicable. Add the ancillary load amps from coils, controller, power supply, and other ancillary loads to the drive input amps. Then multiply this sum by 1.25 to obtain package amperage which can be used for wire sizing, at 30ºC ambient, per NEC Table 310.16. Please note that the 1.25 multiplier to the drive input amperage is required per NEC 430.122. Adding the ancillary load amperage total to the drive input amperage, prior to performing the 1.25 multiplier calculation, serves to add a small amount of increased safety factor for wire sizing. An example of this procedure is demonstrated as follows:

  • For a 460V, 40 kW compressor, the drive input amps are listed as 90A.
  • The remaining ancillary loads total 10A.
  • 90A + 10A = 100A x 1.25 = 125A at 30ºC ambient (Package Amps)

Based on the example above, if a wire run length of 100 feet of 75ºC copper wire is used and all terminal connection points are rated for a minimum of 75ºC, then copper wire size number 1 AWG can be used for this application as it is rated for 130A at 30ºC ambient which meets the minimum package conductor amperage requirement of 125A. Keep in mind that the length of the wire run to the compressor is important as it directly affects voltage drop.

Read the full article at Compressed Air Best Practices.