Quick Help on number of wires 1

[KnicksJets]

Hello
We have a 350 HP pump running on 3 phase 440 Volts wire. Each phase has 2 wires in it.
Question is why are these 2 wires required generally (I can provide the rating if necessary)? Is that for load or redundancy? Is it possible to remove one wire and operate on single wire?

 

[dpc]

There are two conductors per phase to provide adequate cross-sectional area to carry the necessary current without overheating. The conductors are connected in parallel with each conductor carrying half the current.

So, no, you can't remove one conductor of the two.

 

[KnicksJets]

DPC thanks a lot, I think you answered the question.
Just to make sure, looking at the wiring diagram at the Motor Control Center (MCC), I could see three terminals (or contacts): T1, T2 and T3. Each terminal have two wires connected to it. The first wire from each terminal goes to one cable (field run) and second wire from each terminal goes to second cable. I guess, each conductor is carrying half the current.
You agree?
Thanks for such a quick response. Why is it that another pump with same HP has only one wire per phase then?
Pdshah.

 

[davidbeach]

Bigger wire? That is some pretty rudimentary electrical engineering there; wire ampacity is a very basic concept.

 

[VEBill]

One problem with using two wires in parallel to reach a particular current rating is over-current protection. In other words, what if one wire is accidently cut leaving just the other to carry the load? Without proper system design, you might end up with a single wire being 'protected' [not!] with a circuit breaker having twice the proper rating for one wire.

Ideally, use the right size wire.

If using two wires, consider if you need to use two paralleled and ganged breakers - one for each individual wire. (And in this case, probably also ganged across the 3-phases too - total of six ganged breakers - maybe, yikes.)

The same sort of thing applies to paralleled contacts (a very common technique in electronic systems). It leaves an obviously rare but potentially dangerous opening for Murphy and his horsemen to ride in through.

I assume that the NEC would disallow using parallel wires without attention to this aspect of the over-current protection issue.

It strikes me that there is a fine line between parallel wires and multi-strand conductors anyway.

 

[jraef]

I assume that the NEC would disallow using parallel wires without attention to this aspect of the over-current protection issue.

Um, no. The largest wire made (here in the US) is technically 2000kCMIL and that is good for at best 750A (assuming 3 conductors in a raceway). So how would you feed a 1000A service then? Not to mention the herculean task of attempting to bend 2000kCMIL cable! I know of no contractors who pull anything larger than 600kCMIL and most no larger than 500. It's just too stiff to work with. Especially when working in MCCs where space is often limited and snaking cable around corners is a given, many contractors prefer to run multiple smaller conductors rather than one large one.

The NEC calls for no special provisions to parallel runs of cable, it's an accepted fact.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[KnicksJets]

"Bigger wire", if that was the case would I even be asking that question? They both are same size and same rating load wise.
Very good point VE1BLL.
My question is answered, thanks a bunch to you all.

 

[waross]

If the motor is or was at some time a star delta start motor, you would have 6 wires, each carrying full load current.
Iif the motor was originally star delta and is now used as delta only, the connections may be made in the motor junction box instead of at the MCC an one set of cables may be removed.
Be careful, you have not supplied enough information for us to say with certainty, star delta is a possibility, not a certainty.
respectfully

 

[davidbeach]

For a 350hp 460V motor (so a 440V motor would have a higher current), the NEC requires conductors with a minimum ampacity of 518A. To get there using only a single conductor per phase would require 900kcmil conductors. I doubt that your motor with one conductor per phase actually has 900kcmil conductors, so the conductors feeding it a undersized, likely severely undersized. One might be tempted to run two sets of 250kcmil conductors and wind up being 8A short; to meet the code with two conductors per phase (at 460V) would require 300kcmil, an uncommon size, or more likely two sets of 350kcmil. If your other motor has only one set of 250 or 350, you will have a bad day sooner or later. Personally, I'd run three sets of 2/0 per phase for a 350hp motor.