National Electrical Installation Standards

Standards as High as Your Own

Tuesday, April 3, 2018


I just replaced a bad contactor on a made in USA drive that came from the manufacture with bypass function. It has two 75 Horsepower contactors for the drive input and bypass but only a 60 Horsepower contactor for the drive output. I replaced this with a 75 HP contactor. I asked the service technician about this deliberate under sizing and he stated being the 60 HP contactor pulls in before drive starts its output that it is okay. Often these drives have to run all day @ 100 % speed so the ampere draw is up there. [They reduce speeds at night and weekends] I know that these companies get away with murder undercutting the NEC but even this does not appear to be right . When I infrared inside drive cabinets that are in an air conditioned spaces their conductors are 10 to 15 degrees warmer then then the properly sized supply and load conductors. You would think that with all of the energy codes they would look into it . I called one of these companies up and asked why do they have 2 cooling fans that are mounted to a six by 8 foot drive enclosure running 24/7. They were extremely upset that I questioned them on that on that. These cabinets supply power to 18 pulse 150 HP VFD'S for chilled water pumps that do not run in the winter but our company prefers to have drives energized all year. Some drive companies install a thermostat that only turn fans on at around 80 degrees. Thanks Tom Grabowski



Hey Tom thanks for your question. Ask a manufacturer's representative, ideally someone involved in the design of the product, why the contactor is sized that way. You don't mention if the drives are listed but if so and you believe there is a problem with them complying with the product standard requirements contact the listing laboratory and share your concern. Some have an online form you can fill out. The internal wiring and construction of equipment does not need to be inspected at the time of installation if it is listed as stated in the second paragraph of 90.7.

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