Advantages / Disadvantages of Parallel Feeder Conductors

[Senselessticker]

I'm supplying 480/277 3PH 4Wire power to a new MCC (600A). I'm still a "rookie" engineer so please bear with me. What are the advantages/disadvantages of routing parallel phase conductors rather than a single phase conductor? Is it simply less expensive? Or is it some how "better" than routing one conductor per phase? Thanks in advance for any advice.

Sense

 

[tinfoil]

As soon as you have more than one cond/phase, you have to manage the issue of current sharing, with special attention paid to wire lengths and connection impedances.

However, it is much easier (i.e. cheaper) to achieve required ampacities with parallel runs. The limiting factor in cable ampacities is heat dissipation, and doubling the gauge of a wire will often only add 25% or so to its ampacity, as heat loss is a function of surface area, not cross-sectional area.

Look at the NEC (or CEC) tables for ampacities, and you'll see what I mean.

 

[jnims]

Another reason to select multiple conductors rather than a larger single conductor is ease of training the cable. A large conductor can be more difficult to bend. For example, bare conductors used to connect substation equipment to bus.

 

[rbulsara]

Upto a certain size its a matter of economics. Generally electricians do not like to handel larger than 500kcmil or 600kcmil condcutors and 4 inch conduit,as jnims said. So it is constructibility and ease of handling issue.

Other fact is the ampacity does not increase proportinal to the size. A condutor with a cross sectional doule than other does not necessarioy carry double current but less than that. Which due to skin effect, for large diameter solid conductor most current will flow in outer area and not the center. This also has to do with heat dissipation capacity. as tinfoil suggested.

 

[busbar]

 
From a practical aspect, two conductors/ø would do for 600 amperes.

 

[Wellermh]

Remember if you are going to run parallel phase conductors in three different conduits, i.e. underground duct bank, do not run all of your A-phase in one conduit, then B-phase in the next conduit etc. You must mix the phases, i.e. ABC conductors in one conduit, then the next set of parallel conductors should be ABC, etc. (for three conductor parallel runs).

 

[buzzp]

Someone told me that the lengths of paralleled cable is very critical. They mentioned that the lengths of the parallel runs needs to be with mm of each other. I did not question this but seems this is an exaggeration. Anyone care to comment?

 

[dpc]

buzzp,

I think the shorter the run, the more critical the length is. In very short runs, the difference in length, flux linkage, etc between the conductors becomes a much larger part of the total impedance of each conductor.

For long runs, the slight differences in length become much less significant.

Also keep in mind that if one of the conductor draws more current than the rest, it will increase in temperature and this will increase its resistance. This effect does help mitigate **some** the length difference.

 

[Bung]

Wouldn't there be a tendency for parallel conductors in close proximity to try to balance the current due to mutual coupling effects? Of course, if they are that close, you would lose out on the thermal capacity gains.


Bung
Life is non-linear...

 

[buzzp]

dpc,
I understand the logic behind equal lengths (equal sharing) but do you suppose mm's (say 3mm) difference in length would make any noticeable difference with cables ran, say 50-100 ft.?

 

[resqcapt19]

The current in each conductor of a parallel set it very closely related to the percentage change in length. If you have one at 49' and another at 51', the 49' length will carry about 51% of the load and the 51' length will carry about 49% of the load.
Don

 

[buzzp]

Okay, so the millimeters making a difference on even a 5' parallel length is hogwash, as I suspected.

 

[dpc]

I don't think a difference of millimeters is going to make much difference at all in the distribution of current. Most problems develop from contact resistance at terminations. If one termination is a little loose or corroded, the other conductors will see a big increase in current.

 

[ruggedscot]

when using parallel conductors and carrying large currents supporting critical loads I see circuits now use CT tecnology to monitor the currents in each conductor so that any unbalance between like conductors is picked up quickly and delt with. A large imbalance could over stress a termination and lead quickly to premature failure.

CT's feed an electronic module and the module reads out % imbalance on conductors.

Generally not used but the option would be there if it was needed to configure.

Rugged

 

[Bung]

I think I've seen a system somewhere using some kind of back to back connetion of CTs to force current sharing - I don't remember the details. It was one of those ideas that seemed so obvious at the time that I thought I would not forget it. And now I have. But maybe this will jog somebody else's memory?


Bung
Life is non-linear...