motor confusion

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misterbill1972

misterbill1972

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I have (2) motors @ 480V 3PH 50HP 60A. These are being fed from a VFD cabinet with a 175A main breaker. Each motor is being fed with a 80A main breaker inside the cabinet. There is also an adjustible overload switch which is set at 80A. My question is do you size the main breaker feeder wire to the overload switch (80A x 2 = 160A) or (60A x 125%=75A+60=135A)
 
Jim Dungar,
I agree,
Start by reading all of 430., especially the surrounding material.
Really interesting reading for a change.

I found, by reading 430.71,82 that we had been instructed to connect
the 'contactor & overload set' in a reverse (and unsafe) sequence.

Specifically, the 120 volt contactor / overload set, of a polyphase motor,
must still have the overloads on the hot side of the sequence,
with the 'contactor coil' on the neutral side.

In our applications,
if the overload (which is physically on the bottom of the set)
is connected to the neutral,
then a fault above could energize the motor!!!

Your comments please.

...
 
glene77is said:
Specifically, the 120 volt contactor / overload set, of a polyphase motor,
must still have the overloads on the hot side of the sequence,
with the 'contactor coil' on the neutral side.
Click to expand...
I do not see your quoted sections prohibiting this.

OL relays contacts may be installed on the "grounded conductor" side of the starter coil in many instances. The first that comes to mind is when the OL and contactor are factory installed into a single unit. In general the NEC is not applicable to the "inside" wiring of industrial control panels, machines, and processes. And, 430.74 (NEC2008) acknowledges that contacts may be in the grounded conductor and care must be taken that they are not bypassed. This "care" usually includes not mounting the OL contact in a remote enclosure. Other "care" is often simply the listing of contact location by UL or other testing agency.
 
iwire said:
If this feeder is supplying the VFD(s) it must be sized based on the VFD input current listed on the VFD.
Click to expand...

Thank you, I wire. I thought that was the case but was to tired to go there:D Would you size it per nameplate with no adjustment (125%) ?
 
No problem Gus, :smile:

This is one of those code rules I do not forget because I had to eat crow about it. :D

Would you size it per nameplate with no adjustment (125%)
Click to expand...

'not less than 125 percent of the rated input to the power conversion equipment.'

In the 2005 take a look at 430.122(A).

So if you run a 5 HP motor from a spare 25 HP VSD your stuck supplying the VSD with some very large conductors. But the conductors from the VSD to the motor are sized on the NEC tables just like normal.
 
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