Cable tray spare spacing requirements vs. fill requirements per NEC article 392

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HSB_007

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Good day All

I have a quick question regarding the requirements for cable tray spare space requirements and the relationship with article 392 in NEC. We have a company requirement to keep 20% spare space in addition to 20% spare cabling already included with the mutli-pair home-run cables.

If I correctly understand NEC article 392, it mandates that 50% of the total cross-sectional area of all the cables be provided as spare space. Of course, this is more stringent than our requirement because we only allow 20% spare space. Remember we in our standards refer to instrumentation cables which are typically very low power/voltage of less than 50 VDC (ITC or PLTC). I feel that 50% in NEC is due to power loading since we deal with high power cables, which is not what we use in Instrumentation applications.

The question here is: does mandating both requirements pose a contradiction?
 
The NEC does not mandate any spare space (or spare cabling) in cable trays. The spacing referred to in Article 392 for negating ampacity adjustment.
 
You can reduce NEC spacing to zero which will require ampacity derating... whereas spacing by one diameter achieves no derating but requires a tray twice as wide or greater for the same conductors. This has no correlation with leaving extra space for future conductor additions.

If you are not talking about extra space for future additions, please explain...
 
Cable tray spare spacing requirements vs. fill requirements per NEC article 392

I am actually talking about future additions of cables as you have mentioned. However, remember the 50% spacing is needed because of the amount of heat generated from high power cables (large values of amps). I am referring to instrument cables which generally do not dissipate lots of heat since the amount of current is fairly low (20 m A DC maximum for each pair).

Does the fact that ampaciy rating for process instruments are fairly low dictate adding more cables, and therefore more cable fill?

This is why we mandate 20% spare space whereas 50% spacing is required per NEC.

I hope the issue is now clear. I am debating that specifying 20% spare space and at the same time requiring to comply with NEC 392 would cause a conflict (because 20% is not 50%). My thinking is that I should probably just stick with 20% and remove the 50% requirements. What do you think?
 
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