Guidelines for Calculating Motor Loads and Sizing Cables in a Large Sawmill

[BrutusYates]

Hello everyone,

I'm currently working on a large-scale project involving a sawmill with over 400 motors. These motors are primarily powered through Eaton LV Switchgears. I have some specific queries related to ensuring the correct voltage at the motors and appropriately sizing the cables.

System Configuration:

• Voltage Source: 480V from the transformer, with the same voltage at the Switchgears.
• Control Infrastructure: The Switchgears feed MCCs, where we have FVNR starters and VFDs.
• Additional Components: Some motors are connected via feeder breakers from the MCCs to their respective drive cabinets.

Main Questions:

1. Voltage at Motor Terminals: Should I aim to maintain at least 460V at the motors to account for voltage drop, or should I use the 480V figure from the transformer and Switchgears for my voltage drop calculations? If the voltage were to drop to 460V once at the motor, How would I account for the increased current?
2. Cable Sizing: Given the configuration and voltage considerations, what methodology or standards should I follow to size the cables appropriately?

Any guidance, references, or best practices would be highly appreciated. Thank you in advance for your time and expertise!

 

[Jraef]

1. Motors are designed with the voltage drop in mind, so a motor for use on a 480V system is actually designed for 460V. The FLA on the motor nameplate will reflect the current at 460V. That’s all you need to use. As an aside, NEMA designed motors (those designed for use in North America) will be designed for +-10% from that nameplate rating. IEC (European) designed motors are not made to that same standard so all bets are off on those. Read the nameplates.

2. Conductor sizes are taken from tables in the NEC (article 430) based on the motor HP, not the amps. You then adjust if necessary for distance from the CONTROLLER to the motor and conduit fill if necessary. For the conductors feeding VFDs that are remote to the MCCs, those are required by 430.122 to be sized at 125% of the VFD input current rating, which you often must get from the manufacturer. You can also generally use the output current rating, but it is usually higher than the input current (due to power factor). YMMV though so it’s best to check because if a VFD is oversized for some reason, the input current may be higher.