1/6 (16.6%) Concentric Neutral and other fractions

[qman5]

Can someone shed some light on why we would use a 1/6th (or other fractions) than 1/3rd (33%) or 1 (100%)?

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I understand that 1/3 (33%) concentric neutrals are used for 3 phase loads and 1 (100%) concentric neutrals for single phase applications. I'm not immediately aware of any applications that would require anything less than a 33%.

Any ideas or supporting information that you can share?

Thanks!

 

[davidbeach]

What voltage level? What loads and how are they connected?

 

[qman5]

Specifically, this is at the 25 kV level. This was a utility substation application for outgoing feeders and also switchgear-to-switchgear connections in the same station. The loads are unknown -- only the feeder ampacities are known. However, this is more of a general questions. I am in the business of quoting utilities and sometimes I need to select a cable with only the voltage level and ampacity required. It would be helpful to understand the why behind the questions and understand questions I need to be asking to the owners.

In the case, unfortunately, we don't have this information, but I would like to understand the whys so I can ask the right questions to get the answers (or to research what is needed on my own)

 

[davidbeach]

Well, it matters how the loads are connected.

 

[qman5]

The loads are connected Delta(HV)-WyeGnd(LV). The cables are on the LV side. (I edited my second reply, I'm interested in the theory).

 

[davidbeach]

So, you’re saying all of loads are connected phase-phase? What does the neutral need to do? Why wouldn’t 3/16 be sufficient?

 

[qman5]

The neutral is there to carry unbalance current and also capacitive charging current.

I would assume that if we are confident that our loads are balanced, and ideally have no neutral, bu code dictates that we need one. Practically speaking, we would like to minimize the extra cost of the concentric neutral, so we would reduce the sizing and include only the cable screen. However, to add some design margin, it's safer to go with some concentric neutral that without any.

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[cuky2000]

Although could be a cost-saving went large quantity of URD cables is procured. 1/6 rated neutral concentric is a rare application in the industry considered as a special design for some cable manufacturers such as Okonite and others.

The following are potential but rare application in the industry:
1) This can be used with a similar application as cable shielding with copper tape.
2) For cable with single end grounding with low circulating current, 1/6 concentric neutral could be an option.
3) It should be noted that 2 cables per phase with 1/6 concentric neutrals are equivalent to a single conductor with 33% concentric neutrals of a single conductor per phase good for 3 phase application.

 

[dpc]

Assuming no line-neutral loads, the neutral has to be able to carry ground fault current at a high enough value of current to clear the fault before the neutral melts. Similar to the equipment grounding conductor in LV circuits. Agree with cuky, that cost savings from reducing smaller than 33% probably don't justify the special cable. Stepping over dollars to pick up dimes. Sometimes engineers seriously overthink things.