Derating factor for LV Cables - IEC

[NickParker]

In the IEC std. 60364-5-52 tables, I could not find the derating factor for Low voltage Multicore cables which are laid in several layers on the same tray. How the derating factor for this case is arrived at?

 

[7anoter4]

The standard IEC 60364-5-52 recommends a formula which compared to NEC is impossible. If the cable tray filling is not taken into consideration but the cable are considered in a bulk mode the formula indicated in A.52.5.2 (523.4.3.2) Groups on trays/A.52.5.1 (523.4.3.1) Groups in conduits, cable trunking or cable ducting
it conduits to a non-sense result [compared with NEC Art 392].
For instance if the cable is XLPE insulated 3*35 mm^2 copper conductor and if the cable tray filling is 15% of the inner cable tray volume NEC allows 130 A for any cable tray width but IEC permits 96 A for 100 mm and 4.39 A for 900 mm.
An apologizing sentence is written:
The group reduction factor obtained in accordance with A.52.5.1 will provide a value which is on the safe side. This subject is under consideration.

 

[leur2011]

NickParker[/b]]I could not find the derating factor for Low voltage Multicore cables which are laid in several layers on the same tray.

Find below derating factor for Low voltage multicore cables which are laid in several layers on the same tray.

 

[NickParker]

Hi Leur,

1) Table A.52.17 item 1.(IEC 60364-5-52)
Can we use the worst reduction factor (0,38) for multilayer cable routing? It is very often that cables are laid on several layers laying each on other.
I would like to know how to derate cables in such situation?

2) If above is not accepted for multilayer laying of cables, look on IEC 60364-5-52 Table A.52.20 NOTE 2:
"NOTE 2 Factors apply to single layer groups of cables as shown above and do not apply when cables are installed in
more than one layer touching each other. Values for such installations may be significantly lower and has to be
determined by an appropriate method."

Which one I should consider? Another guy at work said the derating factor has to be taken as 0.73 in this case, I do not know he has arrived at this value?

 

[7anoter4]

At first I prefer NEC or NEMA/ICEA Standard [WC/P-54-44] Ampacity of Cables in Open-top Cable Trays. The awg cross section I translate to the square mm taken a larger one.
NEC and NEMA take into consideration the occupied volume - by cables- instead of number of cables.
Second, since- as you already said- IEC 60364-5-52 states:
"Values for such installations may be significantly lower and has to be determined by an appropriate method." you may use other "appropriate method".
The theoretical basis of NEMA/ICEA calculation is :
Ampacities for Cables in Randomly Filled Tray by J. Stolpe 1970 and
Ampacities of Multiconductor Cables in Trays by Ralph Lee 1971.

 

[7anoter4]

In order to calculate the allowable current carrying capacity [ampacity] of a cable in randomly filled cable tray [a multilayers single cable tray] it is recommended to read the:
ANSI/NEMA WC 51-2009/ICEA P-54-440-2009.
You have to calculate at first the depth of cables [in the cable tray].
The depth of cables in trays should be calculated as follows:
depth[inches]= (n1*d1^2+n2*d2^2+.....nn*dn^2)/w where:
d1 , d2 , ...dn = diameter of cables, inches.
n1 , n2 , ...nn = number of cables of diameters d1 , d2 ,...dn , respectively.
w = width of tray, inches.
After then to choose a corresponding Allowable Heat Generation from Table 2-1:
depth / heat generation
[inches] [watts/inches/feet]
1; 5.93
1.5; 3.56
2; 2.43
2.5; 1.78
3; 1.38
3.5; 1.1
4; 0.9
Then calculate-or take from a catalogue- Rc for the conductor permissible temperature including skin effect.
Finally:
I=d*sqrt(Allowable Heat Generation/n/Rc) where:
I = ampacity, amperes
d = cable diameter, inches
n = number of cable conductors[1,2 or 3]
Rc = conductor resistance, micro-ohms/ft [ohm/ft/10^6]

 

[leur2011]

Don't confuse the derating factor for multicore cables touching each other (as illustrated in your sketch) with the installation of multicore cables which are laid in several layers on the same tray, I mean does your standard in cable installation allows you to lay low voltage multicore cables in several layers on the same tray. Two completely different issues, and frankly I can't find Table A.52.17 item 1 in IEC std 60364-5-52 version 2009.

 

[ScottyUK]

You might find some guidance in one of these reports from ERA, now known as 'RINA Consulting'. When I worked there 20-odd years ago the cables group was very well-regarded, although the company has gone through massive change since those days and I have long since lost touch with my former colleagues.

PVC insulated
XLPE insulated

 

[NickParker]

Hi Leur,

PFA the tabe in IEC 2001.