Motor Feeder Overload

[erickench]

Don, I would like to point your attention to NEC 675.7(A). It states:

The equivalent continuous-current rating for the selection of branch-circuit conductors and overcurrent protection shall be equal to 125 percent of the motor nameplate full-load current rating of the largest motor, plus a quantity equal to the sum of each of the motor nameplate full-load current ratings of all remaining motors on the circuit multiplied by the maximum percent duty cycle at which they can continuously operate.

As you can see the motors for an irrigation machine are fed from a circuit that pretty much resembles a feeder in that there is more than one motor included in the machine. There is no reference to NEC 430.52 although Stallcups references it in his book "Electrical Calculations Simplified" and performs the calculation in accordance with NEC 430.52 and NEC 675.7(A).
NEC 675.7(A) performs the calculation in a similar matter to NEC 430.24. It's pretty clear that this NEC rule is requiring an overload but no short-circuit ground-fault protection.

 

[CONTROL FREQ]

It may indeed have been acceptable 30+ years ago, it's theoretically still possible if all the stars align in terms of breaker ratings and motor sizes, but I have never seen it work. The rules have definitely tightened up over the years. You may be grandfathered in now but if you change anything, you may find yourself in a heap of hurt.


Aaaaand there you go. I guess the cost of down time is irrelevant to your company. Either that or they carry a stock of spare motors with quick disconnect plugs on them for swapping them out quickly. It might be worth investing in some proper motor protection...

I couldn't agree more. The example was just that. I actually tried to follow up but was booted off for some reason, and went to bed. The motors I spoke of are not critical to production e.g. a roof top exhaust fan (2H.P. cap start/cap run), I personally HAVE never, nor would I EVER recommend or perform an installation like I described. I actually had to defend refusing to, because the boss wanted it and I said no way.