NEC Cable trays cable ampacity 1

[7anoter4]

NEC ampacity tables are based on IEEE 835/1994 calculations. However, for cable tray it is not mentioned in NEC how the table here is calculated. It is not any connection with ANSI/NEMA WC1/ICEA P-54-440, for instance.

 

[7anoter4]

The main problem with cable tray cable ampacity in NEC it is the reduction factor for filling different as per art. 392.22. NEC does not recon ANSI/NEMA WC51/ICEA P-54-440 Ampacities of Cables Installed in Cable Trays and calculating as per this standard the result is far from NEC. If we compare NEC with VDE/DIN 276-1000 or with IEC 60364-5-52 it is a huge difference for multiple conductor cables of more than 2 awg for instance. By the way, no multiple conductor cable dimensions is indicated in NEC ,never-the-less Table 310.15(16) is for up to 3 conductor cable. In the ANSI /NEMA WC1 standard the overall diameter is indicated.

 

[waross]

In the CEC, the derating table for more than 3 conductors in a raceway or cable gives the following values.
7-24 conductors, correction factor 70$
25-42 conductors, correction factor 60$
43 or more conductors, correction factor 50$

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Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

Thank you, waross. I have with me only the 22nd edition of CEC [of 2012] where at Rule 12-2210 it is a wrong sentence but I think on 23rd edition was corrected.
However, I calculated the ampacity for 3*1/c 2/0 copper XLPE insulated according to NEC,CEC, ANSI/NEMA WC51and IEC 60364-5-52.
Since 2/0 awg cross section area it is 67.42 mm^2 it is close to 70mm^2 so I may do this comparation. The number of cables permissible according to NEC Art.392.22 in a ladder-type cable trays it is 16 [corresponding to 21 sqr.inches maximum admissible in an 18 inches wide tray].No such permission is according to other standard. According to CEC maximum is 6 cables in 18” tray and then according to Table 5D a reducing factor of 0.82 it has to be applied. The same according to IEC 60364-5-52.

 

[waross]

It depends.
For over 100% diameter spacing the ampacity of single conductor cables may be determined from the free air tables in the code.
For over 100% diameter spacing the ampacity of multi conductor cables may be determined from the "Three or more conductors in a raceway or cable" tables in the code.
The worst case is over 42 conductors with no spacing between cables at 50% ampacity.
The maximum number of cables in conduit or tubing is 200 but I see no such limit for cable trays.

Ampacity for single conductor cables is determined from the free air tables.
Ampacity for multi conductor cables is determined from the three or more conductor tables.
There is a big difference between free air ratings and raceway or cable ratings.

Typical feeder installations. Spaced, single conductors are often used. The saving in copper more than offsets the extra cost of tray and labour.
Branch circuit power, lighting and control cables. The cables are typically derated 50% and are not spaced. There are often too many cables in an industrial plant for any other method of installation.

I suggest that for general planning the choices are spaced cables at 100% ampacity or unspaced cables at 50% ampacity.
In special cases you may consult tables for 25% to 100% spacing and for 4 to 42 conductors.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!