480 Volt grinder

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littlerice40

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Hi I am an electrical apprentice and Im looking for a little guidance. I was at a job with my journeyman this last Friday adding a 480 V panelboard. It was a quick install and we had time left so they asked us if we could move and wire a small 480 V-4 Amp grinding table they had. So we moved it and realized there were no 480 V recepticles on that wall. So my journeyman noticed there was a 480 V bussplug near by that was feeding another small machine so he used it to feed this grinder. He ran the conductors through the same conduit to the same 4 square box as the other machine. Then, he ran the conduit from that box to the disconnect for this new grinder. He wired up the line side and then hooked a recepticle below the disconnect to the load side of the disconnect switch. The fuses in the Bussplug were 20 amp fuses existing and the old machine was about 8 amps I believe. He didint change those and he put 7 amp fuses in the disconnect for the grinder. He energized the grinder after restoring line power and it blew the fuses. He said it was the grinder and we just left it, he told the people it was a bad grinder so he unplugged it and the other machine still worked so we left. Is that really the case, could there be a short in the grinder? I mean when he checked incoming v in the grinder discoonect before plugging the grinder in the fuses held but once he plugged it in, boom. Should he have ran a seperate conduit for the grinder and not double tapped the lugs in the bussplug? I am curious, he doesnt explain things well to me, maybe because he doesnt know. Any help or info would be great. littlerice40

2-7 amp fuses opened and 2-20 amp fuses in the buss plug opened.
 
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I assume it was the 7 amp fuses that opened?

If that is the case it is entirely possible there is a problem with the grinder itself.
 
grinder

grinder

simply another opinuion here...not seeing the job I wouldn't question your journeyman..as far a "double tapping" the buss plug, I see no problem if the lugs are rated to accomadate two conductors. As far as the "short", performing test on the grinder with a megohm meter might be the most surefire way to determine that. My experience with grinders, do to the inertia, is the 7 amp fuses might be undersized. After checking the actual horsepower of the grinder, I would make sure the fuses were "time dealy"..if the 7 amps was correct according to 430.52, I would use Except. 2 and try increasing my fuse size (after checking the motor for a short)
 
grinder

grinder

I didn't see your response to I-wire, as I was typing my own. If the 20 amp fuses opened also, I would lean toward the "bad grinder" decision.
 
So it is ok to run two set of 480 V conductors in the same raceway? I am an apprentice so dont laugh. I though you had to seperate 480 V sets, is this ever the case??
 
The fact that the two 7 amp fuses opened places the problem any point between these fuses and the load (the motor windings).

The fact that two fuse blew suggests a line to line short circuit as opposed to a ground fault.

The fact that two 20 amp fuses opened as well also suggests a short circuit as opposed to simply a slow to start motor.

Per the NEC you can run as many 480 circuits in the same raceway as you would like. You can have 120, 208, 277 and 480 volt circuits all in the same raceway. The number of circuits in the raceway would not cause the fuses to open.

It is likely the 'double tap' in the bus plug is in fact a NEC violation but it is not one which would cause any of the fuses to open.
 
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