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Minimum Space Between Power & Instrument Cables

01/31/2016 2:50 AM

Hi,

I want to install power(600v) cable and instrument cables(110v) in a same cable tray of 600 mm, what shall be the gap provided?

What is the minimum gap shall be maintained between Instrument and power cable trays (Layer of trays)?

Thanks in advance!

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#1

Re: Minimum space between power & instrument cables

01/31/2016 5:04 AM

Dear Mr.dinesh0477,

You have not referred whether the Instrument Cable - is shielded type or not shielded type. If it is shielded type a gap of 300 MM is sufficient. The shield should be earthed on one end only and not at both ends.

If not shielded type pl. do not lay the Inst.Cable in the same tray because it will create severe Induction Problem, especially when the load is NON-LINEAR load.

DHAYANANDHAN.S

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#3
In reply to #1

Re: Minimum space between power & instrument cables

01/31/2016 9:39 AM

Thank you sir!

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#2

Re: Minimum space between power & instrument cables

01/31/2016 9:36 AM

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

Manufacturers specifications usually override local codes.

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#4

Re: Minimum Space Between Power & Instrument Cables

01/31/2016 3:48 PM

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?

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#5

Re: Minimum Space Between Power & Instrument Cables

01/31/2016 10:07 PM

None is required as long as the lower voltage conductors have insulation equal to or greater than the highest voltage conductor in the raceway, and the voltage on any conductor is below 1,000Volts, see NEC 300.3.(C).(1)

If the voltage exceeds 1,000V then they are not permitted to be in the same raceway regardless of the clearance between them, see NEC 300.3.(C).(2) et al.

Before anyone goes off on a tangent about induced noise, etc., they should remember that the NEC is a "safety" guide, not a "design" guide or a "best practices" guide. As long as your installation meets the minimum requirements then your AHJ should approve the installation ("should" can be very subjective).

The NEC is very clear that it is a minimum requirement, they really don't care about if it operates properly, that's the purview of qualified electrical engineers. From 90.1,Purpose (A) Practical Safeguarding: "...This code is not intended as design guide or training manual for untrained persons..." Use it wisely and it will save you many headaches, use it improperly and it will cost your clients time and money, their specifications and documents always supercede the codes unless they are in conflict with them.

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#6

Re: Minimum Space Between Power & Instrument Cables

01/31/2016 11:18 PM

Please, refer to manufacture's recommendation.

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#7

Re: Minimum Space Between Power & Instrument Cables

02/02/2016 2:17 AM

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.

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#8
In reply to #7

Re: Minimum Space Between Power & Instrument Cables

02/02/2016 8:56 AM

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.

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#9
In reply to #8

Re: Minimum Space Between Power & Instrument Cables

02/02/2016 12:35 PM

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.

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#10
In reply to #9

Re: Minimum Space Between Power & Instrument Cables

02/02/2016 2:32 PM

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.

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