In our previous issue, we discussed the fact that you can exceed the maximum overcurrent protection device (OCPD) sizes provided by Table 430.52 if the table limits don’t permit starting the motor. But the exceptions have limits, too.
It’s unlikely you’ll get to a situation where the exceptions noted in 430.52(C)(1) aren’t enough to let you proceed. But what if you do? You’ve reached the limits permitted by the NEC and still your motor won’t start.
Probably, something is wrong. Some possible issues:
- Wrong NEMA Design motor. The application requires a NEMA Design B motor (high starting torque) and yours is a NEMA Design D.
- Bad motor. Has this motor been balanced and tested by a motor shop? Maybe it has a high starting current due to a bent shaft, winding problem, bearing problem, or other defect. Can you even turn the shaft by hand?
- Bad load. So you have it connected to the gearbox, but the gear lube smells like it has burned. Badly. Or maybe the gears are jammed.
- Alignment issues. If severe enough, these will require increased starting torque.
So inspect the motor system. Correct any deficiencies, and then try again using Table 430.52.
If you’ve done the preceding steps but still can’t start the motor using an OCPD within NEC limits, contact your electrical distributor about a soft starter or motor drive for it. Either one is a good investment that provides other advantages beyond just meeting OCPD requirements.