IEC cable group derating factors for cables in air or underground 1

[duki]

Does anyone know where I can get the IEC recommended cable derating factors for underground feeders and cables in cable racks? I would also like to know which IEC spec in particular addresses this. I've read that 60287 addresses the topic but it consists of several different parts. I do not have access to it. Thanks for any help.

 

[cuky2000]

The enclose the German standard, DIN 57 /VDE 0298 may be use as an alternative to the IEC. Check also if there is any CENELEC standard in this matter.

See more detail in the enclose link

http://www.hitechcontrols.com/catalog/T_technical_info/T016_cable.htm

http://www.hitechcontrols.com/catalog/T_technical_info/T019_cable.htm

 

[KJvR]

There is an excerpt available on the Wiley site of the following book:
"Rating of Electric Power Cables in Unfavorable Thermal Environment" by George J. Anders
The excerpt is Chapter 1: Review of Power Cable Standard Rating Method. This will give you a summary of IEC60287.
See:

http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471679097.html

and click on "Read Excerpt Chapter (PDF)"
(Sorry, I am not sure how to insert a direct link)

 

[ScottyUK]

BS 7671, the UK Wiring Regulations, has a similar set of derating factors. It's at work (and I'm not - yippee!) so I can't verify that the DIN standard is the same as the BS. The numbers look very familiar though.

If you are over in IEC territory, many European countries still have their own requirements for electrical installations. For example, the UK hasn't adopted the harmonised EN document, hence we still have a British Standard denoted by 'BS 7671' rather than the 'BS (EN) 6****' we would otherwise have. You should consider using the standard of the country you are working in - the pure IEC document doesn't, by it's nature, accommodate local variations.

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I don't suffer from insanity. I enjoy it...

 

[singpower]

ScottyUK
How do you define "system" for above table?
For example, a system has 8 nos of single core cables, in other words, 2 cables per phase plus 2 cables for neutral,do I need to use derating factor?

scenerio 2:
There are two system and each system is the same as above,then what will be the derating factor?

Regards

 

[ScottyUK]

It's not my table or choice of word, but I interpret 'system' to mean a three phase group of cables associated with one circuit. The illustrations appear to show 3[Φ] 3-wire examples and omit the neutral.

There are good reasons for using the trefoil group rather than a flat installation, even though it takes more effort to achieve.

Taking the trefoil method as an example, from table 2 the derating would be 0.98 for two parallel trefoil groups with separation of 2x cable diameter. It is common to run the neutral alongside the trefoil as a parallel single conductor.

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I don't suffer from insanity. I enjoy it...

 

[singpower]

Hi ScottyUK,
Thank you for your reply.
Again appreciate it if you can explain what one circuit mean.Because I have confussion about "one circuit".

Let me repeat my above question, a system (which supply to single load) has 8 nos of single core cables, in other words, 2 cables per phase plus 2 cables for neutral,do I need to use derating factor?
I mean, is it one circuit or two circuit?

 

[davidbeach]

If the two parallel sets of conductors originate and terminate on the same busses, protected by the same protection, they are one circuit. If sources, terminations, or protection differ then they are different circuits.

But, I don't think that is relevant to the table. For the table, it would be the number of cables; I think that the word "system" in the English translation is a mistranslation. The reason I say that is because the derating is due to heating of the cables, and heating is independent of whether or not the cables are of the same circuit or of multiple circuits. The heating is a function of the current carried.

 

[davidbeach]

Looking at it again, I think I see what the cable/system notation is referring to. In both cases, I believe that it is the number of groups of three cores. Notice that in the first three conditions there are six cores in each, sometimes two cables, sometimes six cables, but, I believe for the chart each is two systems. Likewise, the last five conditions each have nine cores, sometimes three cables, sometimes nine cables, but in each case three systems.

 

[singpower]

Hi davidbeach,
Are you trying to say that 2 single core cable per phase,in 3 phase system can be called 'one circuit'? And it doesn't need derating factor?

Likewise, 3 single core cable per phase,3 phase system also can be called 'one circuit',right?

Becasue I still do not have clear picture of 'one circuit' unless someone answer my qestion.

Regards

 

[davidbeach]

I think that three single core cables in a tray, with spacing, require no derating. More cables, or no spacing, and derating is required. I'm not aware of derating requirements that are based on the number of circuits, but all consider the number of current carrying conductors. The more current carrying conductors in close proximity, the more derating is required. It all has to do with how well the heat from the cables can be dissipated.

 

[waross]

Hello singpower
Try replacing the word system with the word cable arrangement.
In describing the cable arrangement in the tables, the word "system" seems to be used to describe sometimes a three core cable, sometimes a single core cable, and sometimes a group of three single core cables.
The designation of circuit does not matter. As davidbeach points out, the reason for derating is mutual heating of the cables. The mutual heating depends on the current whether it is one circuit from one circuit breaker or 2 or more circuits from 2 or more circuit breakers. I believe that if there are two circuits with different sizes combined in the same cable run, that it is prudent to apply the derating factor to all cables in the group.
For instance, in both the NEC and CEC the derating of conductors in a conduit depends soley on the number of current carrying conductors, without regard to conductor size, or circuit arrangement. You may have single phase circuits, circuits with two phases and a neutral, three phase circuits, and/or parallel conductors on one breaker. The derating factor will depend only on the number of current carrying conductors and must be applied to all current carrying conductors and neutrals.
(Some neutrals do not need to be counted as current carrying, but should be derated.)
respectfully

 

[singpower]

Hi davidbeach and waross,
Thank all of you for enlighting me.
I think I got the point that you explained.
Regards

 

[duki]

Thank you everyone for your informative dialog, even though it may have drifted to the left somewhat.

Cuky200 offered a DIN equivalent of what I was asking for which is what I was asking about. My company has since purchase the 60287 standards and except for the cable in air factors it would take a math genius to understandstand the rest.

KJvr has a similar post concerning the ERA report 69-30 part 3 comparison to 60287. No one has responed to it. Does anyone have a copy of ERA 69-30 part 3? I would also like to know why there is such a big difference in the derating factors and why the cable companies seem to refer to the ERA report rather than IEC 60287.