Separation of Parallel Cables

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Gerry de Pedro

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We ran 2 sets of 4 ? 300 mm. sq. cable in a cable tray as a parallel feeder for a given Main Distribution Board. The tray is right above the switchgear. The height between the switchgear and the tray is 0.6 meter AND THE TOP OF THE SWITCHGEAR TO THE TERMINAL INSIDE WHERE THE sets of the cables are to be terminated is 2.5 meters. We are to land the cables but, from the tray, can we start to join the RED phase together, the Yellow phase together, the BLUE phase together and the NEUTRAL together down to the terminal inside the switchgear in a total length of 3.1 meters as it is not mentioned in the code when to group together same phase cables? Is this a violation of the code? Would it cause induction heating for that small distance? Please explain. Thanks a lot.
 
Re: Separation of Parallel Cables

Without looking it up, as long as they are in the same raceway, it is not required they be grouped. It is only when they are in seperate raceways they have to be grouped.
 
Re: Separation of Parallel Cables

I agree. Induction heating takes place when multiple runs of a single phase are in a raceway, without the other two phases nearby to cancel the magnetic fields. What you are describing should be fine.
 
Re: Separation of Parallel Cables

Make sure that there isn't any steel or iron such as a tray rung, a unistrut support or a brace between the conductors that could make an inductive loop. All of the cables need to come into the switchgear through the same opening. Or at least each hole has to contain an ABC & N conductor, which would defeat the reason for grouping them by phase.
 
Re: Separation of Parallel Cables

Since it appears you are using IEC rated conductors, it may be a bit hard to "identify" them; however, let's assume they are equivalent to Type TC. In that case, they will need some form of intermediate support (approximately every 1.8m) between the tray and the terminals.
 
Re: Separation of Parallel Cables

These 2 sets of 4 x 300 mm. sq. cable are single core cable,i.e. a total of 8 single conductors at 2 cables per phase.Help please.
 
Re: Separation of Parallel Cables

Induction heating is caused when there is a continuous circle of metal around any grouping of identical phases. This heating can occur when phase conductors enter the metal enclosure of the switchgear. To prevent this heating: do not group your phases before entering the switchgear; enter through a single non-metallic plate; or cut a slit (in the metallic enclosure plate) to join all of the individual openings.
 
Re: Separation of Parallel Cables

Cables are entering the SWGR thru its top metallic plate. Each cable has its individual metallic cable glands( like locknut and bushing). We can not even cut a slit between glands. But we can use CBG Plastic Glands for each cable but A and A Phase, B and B Phase, C and C Phase and N and N Phase are near each other before entering the top of the SWGR but different hole. Would this be Okay. Thanks.
 
Re: Separation of Parallel Cables

No it is not OK.

300.20 Induced Currents in Metal Enclosures or Metal Raceways.

(B) Individual Conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit.

Exception: In the case of circuits supplying vacuum or electric-discharge lighting systems or signs or X-ray apparatus, the currents carried by the conductors are so small that the inductive heating effect can be ignored where these conductors are placed in metal enclosures or pass through metal.
Click to expand...
 
Re: Separation of Parallel Cables

Cables are entering the SWGR thru its top metallic plate. Each cable has its individual metallic cable glands( like locknut and bushing).
Click to expand...
You can only do this if all of the metal is non ferrous.
Don
 
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