Sometimes you need more control of a motor than just on-off and adjustable speed drives allow for such control and to a degree of which is nothing short of amazing! Adjustable-speed drives can be variable-frequency speed drives (VFDs)”which refer to alternating-current drives only, or variable-speed drives (VSDs) which can be alternating-current or direct-current drives. The branch or feeder circuit runs through the adjustable-speed drive which in turn controls the speed of the motor(s) served. Do you know how to size these branch or feeder circuit conductors? How about the overcurrent protection device rating? Do we use the table values in the NEC based on the horsepower of the motor?

If you're studying for an exam, or you just don't know or aren't quite sure of the Code requirements for sizing these conductors [430.122] or the overcurrent protection for these circuits [430.130] you need to watch this video where these rules and overall concepts of adjustable speed drives are explained (in great detail by myself and a panel of experts) and review the included text, graphics, and example calculations. Doing so will help to ensure that you apply these rules confidently and accurately!

Click here or on the thumbnail to the left to view or download the PDF of this rule to follow as you watch the video. (3 page, 254 KB)