Disconect switch?

Consider this setup. Fuses, contactor, disconnect switch close to motor for service and then the motor.

Nothing strange about this, what I find strange though is six wires are coming out of contactor 2 for each phase to two different contacts in the disconnect (six contacts in the disconnect 2 for each phase) and then joined together to the apropriate phase in the motor.

Enyone has any idea why, any benefit of this kind of setup? Also I kind of rule out the possibility that the manufacturer did not have a disconnect with the proper rating since this is a well built and "rich" machine.

Any idea.

TyIA Eugen

I'm not quite clear on what your saying... But is sounds as if they doubled the wire going to the contactor, or the motor is wired Wye Delta with no isolation.

If the size of conductor is large ie 350MCM or greater, it is easy to pull two smaller wire per phase in a conduit.
Moreover bending large size (>350MCM) wire is extremly difficult whereas smaller wire is flexible.

I agree with shahn4.

We usually would pull two 300 amp cables for a 500 amp service.

Thank you guys!

There are two cables with 12 gauge wires, I guess they had only one size cable.

shahn4 said:

If the size of conductor is large ie 350MCM or greater, it is easy to pull two smaller wire per phase in a conduit.
Moreover bending large size (>350MCM) wire is extremly difficult whereas smaller wire is flexible.

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I would have to agree too. However, It isn't necessarily constrained to 350MCM. I've seen it done for wire as small as 3/0. I just put out a design today with (2) 250MCM per phase in order to supply a large motor.

As for the disconnect, I'm really not sure why it was designed that way. I would guess that it was just easier for the disconnect manufacturer to get the necessary ampacity by splitting each phase into two poles. This especially makes sense if the wiring would probably be done that way because of the reasons discussed above.

Sounds like someone wanted to be able to change the motor leads in the disconnect rather than directly at the motor. 6 pole disconnects are special order items. The NEC does not allow paralleling of #12 wire.

I read 2 contacts per pole, that is common on alot of disconnects, non-fused and fused.

The NEC does not allow paralleling of #12 wire.

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I hope you will clarify that statement please.

Since this is using 12 gauge wire then I dont see why part winding or wye/delta would be used but I guess that is possible.
What HP is this motor, whats its FLA?

Ron,

I disagree, 2 contacts per pole is not "common on a lot of disconnects". 6 pole disconnects are usually required for part winding, seperate winding, and wye-delta motors, but even these while not rare, are not common. And I have never seen any 6 pole fused disconnect.

The smallest paralled conductors, those that are connected on both ends, allowed by the NEC are #1AWG. Some UL Listed equipment may be built with smaller paralleled conductors, but not "building" wiring.

I am quoting the #1 size from memory.

ok

I was hoping you would state the Article for that 1/0 specification but I found it. Article 310.4 Conductors in Parallel

Exception #2 allows smaller sizes for control power but I cant see anything that would allow motor wiring be paralleled.
This is the kind of information that it is good to make a note of.

Tell the truth I am not sure I have ever seen a 6 pole disconnect but could see where they could be used. In this case I dont see where a 6 pole disconnect would be needed after a single contactor. I cant see wye/delta, part-winding, etc being applicable as stated especially considering the wire size.

It is possible that a contactor can be 2 step for wye/start or part winding use but with #12 wire I just cant see a need for either.

I am wondering if a braking system could be involved.

Guess this is a strange setup.

Is the disconnect a knife type or a manual motor starter type?

Maybe there used to be two motors and they've been replaced by a single larger motor.

The larger motor requires bigger wires, but the two-motor disconnect (6-pole) was already in place and paid for.

I've seen stranger things done.

(Editor's note: The above is purely speculation.)