Triplex Cable vs 3 conductor cable

[tinotoro]

Hi, what exactly is the difference between triplex and 3C cables in terms of ampacity. IEEE 835 refers to triplex cables and it could be interpreted that they would be applicable to 3C cables.

 

[waross]

In the fields that I work in triplex often is used to describe 3c neutral supported cable. The ampacity is based on aluminum conductors and free air rating. Triplex is two insulated conductors and a bare neutral. The neutral may have a steel core.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

In my opinion, if the cables run in a conduit or duct the difference is not significant. However, in direct buried and in free air run cases the difference is important.
In IEEE 835/1994 there are different tables for direct buried separate for single core 3 cables and for triplex. However , three-core cable is considered as triplex.
The distance between cable center-lines is important too. See 3.3.2 Direct buried.
If the cables run in free air it seems to me if the cables are not in direct contact
each cable could be considered as single[ and no influence from the other].
If we shall take IEC 60287-2-1 Table 2 [Remark]:
1)Values for a "single cable" also apply to each cable of a group when they are spaced horizontally with a clearance between cables of at least 0,75 times the cable overall diameter.
That means for a less clearance the cables will be considered as grouped.

 

[waross]

In the Canadian code, Tables 36A and 36B give the ampacitys for different sizes and configurations of Neutral supported cables.
The tables list three configurations:
Duplex,
Triplex,
and
Quadruplex.
Neutral supported cable is used overhead in free air.
Comparison, aluminum conductors:
Table 36A #2 Triplex NS 75,- 140 Amps
Table 3 #2 RW75 single conductors in free air,- 135 Amps.
(Based on the 2006 CEC, my copy of the 2015 CEC is at another location.)
Three conductor cables are generally suitable for installation in conduit. Some are suitable for wet locations, some are suitable for direct burial.
Triplex is a configuration of neutral supported cable that is always used overhead in free air and is common for service drops.
Texcan refers to it as Triplex Utility Distribution Cable.

https://www.texcan.com/triplex-utility-distribution-cable-2-awg-600v

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

Do the ampacity calculations for triplex assume two or three current carrying conductors? Maybe it doesn't matter much in free air.

 

[ScottyUK]

In the UK 'triplex' describes a cable formed from three single-core cables loosely twisted into a rope-like form. It's commonly used in medium-voltage distribution at 11kV and all three conductors are current-carrying phase conductors. It has a slightly better thermal performance than a single 3-core cable of similar cross-section and is arguably easier to terminate.

 

[tinotoro]

Thanks for all the inputs. Would it be correct to assume that the ampacity tables in IEEE835 for triplex cables are applicable to 3C cables?

 

[waross]

Better find out which triplex construction the tables refer to.
In North America, triplex is two hots and a neutral so the calculations may be for two conductor.
Quadruplex is three hots and a neutral and will be equivalent to the triplex that Scotty mentions.
Also, North American triplex is generally limited to 600 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

At my last job (a utility in the US), triplex was generally used to describe two hots and a neutral for both overhead (e.g. service drop) and underground (e.g. service lateral) cable. In the case of the underground cable the neutral was not capable of being utilized as a messenger. I and my past coworkers, colloquially I suppose, just considered a 'triplexed' cable as one that's bound together like Scotty described and comes on the spool that way. Now I see the distinction may be important when talking about ampacity. Interestingly, the other day I was looking at a specification submitted by my project's owner's engineer with a section looking for direction on if we wanted our medium voltage cables of a particular size range to be either triplex or 3 conductor cable (for use on a system without a neutral). This in the US.

 

[tinotoro]

Thanks for the feedback. I am referring specifically to the IEEE 835 ampacity tables for 'triplex' cables. Are they approved to 3C AC or TECK cables?

 

[rterickson]

A brief Google implies that IEEE 835 concerns itself with power cable as opposed to service cable.

Back when I was doing factory QA on T&D cable, triplexed meant 3 individual power cables which were twisted together (triplexed) onto the customer's reel so that they could then be installed that way (in conduit or DB). I can't recall if the ones we were buying were 5 kV or 35 kV, but definitely not OH service cable.