Minimum Space Between Power & Instrument Cables

Thank you sir!

What do your local codes and more importantly the various manufacturers specify?

Manufacturers specifications usually override local codes.

Do read your client's specification, Mildred. I'm not going to do it for you, as that might make you lazy, and we don't want that, do we?

Please, refer to manufacture's recommendation.

At first, your cable has to be control cable (110 V) and not instrument cable. Except the cable from potential transformer other instrument cable voltage has to be less than 50 V-in my opinion. If it is control indeed-usually not shielded, except in a high-voltage switchyard-in our practice- and I think it is NEC spirit also-you may put it in a same cable tray with up to 600 V without any gap.

If it is instrument cable, in our practice and I think it is IEEE recommendation, you have to segregate it in a dedicated tray as per IEEE 525-for instance:

IEEE 525 Guide for the Design and Installation of Cable Systems in Substations

12.3.2 Tray system design

a) In general, vertical spacing for cable trays should be 12 in (30.5 cm), measured from the bottom of the upper tray to the top of the lower tray. A minimum clearance of 9 in (22.9 cm) should be maintained between the top of a tray and beams, piping, etc., to facilitate installation of cables in the tray.

I forgot something important:

the lowest control cable diameter has to be not less than a third of the largest power cable

in order to avoid mechanical abuse on control cable.

That would be a rather stringent requirement. Consider if the control cable was #14 THHN with an outside diameter of 2.77mm, that would preclude the use of any power cable whose outside diameter exceeded 8.31mm or #4 THHN (8.04mm OD). The situation would only get worse if thicker insulation and/or smaller control cable was used.

Since the original question dealt with laying cables in a tray as opposed to pulling them through conduit (where the mechanical forces during installation are much greater), I believe your one third requirement can be relaxed.

Yet another example of why only properly trained personnel should get involved in the design and engineering of cable systems.

I should agree with you if always they would lay the cable in a cable tray. However, usually one pull the cables in cable tray-mainly in a solid bottom cable tray.