How full Cable Tray is full

[pristava]

How full Cable Tray is full ?

The fundamental rules are mechanical forces defining Cable Tray Support and percentage of the cable fill (40-50% by NEC).

Here in Canada is very important to reduce Cable Ampacity depending from the space between installed cables. ( CEC and OSEC) That is my question.

a) if I am calculating only large cables from my present cable tray, 4/0 and larger, a minimum space between cables is satisfied – NO correction factor and NO reducing of the cable ampacity

b) if I am calculating all cables, minimum space between cables is not satisfied –and I have to install much larger cables what costs much more money

Can 3/0 and other smaller cable be neglected in our calculation ?

 

[GordS]

I would say no, you need to include ALL power and lighting conductors. CEC 12-2210(4) refers you to 4-004(7) when deciding which conductors are to be included in your calculation.

 

[MikeDB]

In the US, the National Electrical Code does restrict how many cables can be placed in the cable trays or wireways. This code does not apply to all areas and I have seen a lot of older installations that did not follow it, especially in power plants and substations.

As I recall, cables over a certain diameter can only be one layer thick, the tray can only be partially filled, and that all cables in the tray must be included. If followed, the NEC is very restrictive for trays and wireways.

 

[jghrist]

If the ampacity tables depend on the space between cables, this space needs to be air, not smaller cables. The smaller cables will heat the larger cables if they have any load. Even if unloaded, they will restrict air flow.

 

[cflatters]

Spot on jghrist -

The space should be free air for ventilation

There is also an inherent problem with leaving cables spaced on trays and ladders - inevitably some bright spark will come along and think that the space was left as spare for future and will put a cable in it !

We have the self same problem with the IEC/BS7671 regarding spacing.

Personally I always allow cables spaced for the obvious reasons but the end result is inevitable. Somebody will put a cable in the space.