Laying of Multiple Cables for Each Phase

Go for option "B".

Option A will create eddy Current.

Dear Sir,

May know how it induce eddy current? since it is supposed to install with PVC conduit, also do we need to have ground cable in every PVC conduit?

Regards,

asitha

http://en.wikipedia.org/wiki/Eddy_current

In your optional "A" Is to use single PVC conduct to run all the same phase cable in to the PVC conduct.

This means that you have 3 length of PVC Conduct and each of the PVC conduct has "RED Phase", "Yellow Phase" and the "Blue Phase" separately. The ground cable does not have any function to oppose the eddy current. Eddy current is created when your cable carry the current flowing into it.

This principle is the same principle with your tong tester.

You should lay a mix of all three phases together in each conduit, preferrably bound together in triad formation.

If you have large inrush currents with separated phases, physical deformation and whipping of the cables is possible.

You can actually see the deflection effect on large generator busbars as they accept full load.

Option B is better. Radiates less energy, and keeps the impedance down , while helping to control seperation of the phases on fault conditions.

PVC? I would prefer to see armoured cable or steel conduit.

Insulate one end of the conduits. Bring your phases through the cabinet walls with a non magnetic material or insulating board to prevent eddy current heating at those points.

Both CSA and the american electrical code have sample installations in the code books.

Dear Sir,

Could you kindly give me the Article No. of NEC you are referring to (having multiple cables for each phase)?

For the Canadian code CEC 2006

4-008

4-022

4-030

12-108

12-3022

figures: 12-34, 12-35, 12-36

Diagrams: B4-2, B4-3

Use both the code book and the Handbook with explanations.

Sorry, I don't have access to NEC at this work location, but the are very similar.

You shouldn't seperate the phases this will cause them to overheat and they supposedly cancel out the eddy current or whatever. If you use pvc this is not a parallel path for the electricity so you shouldn't need seperate grounds should you.

????

Please explain ground comment

GW,

In the NEC, when the cables are run in parallel with each conduit containing conductors for all the phases, then the ground must be run in parallel with each conduit containing one of the ground conductors.

Back to your original question--

• The use of conductors in parallel is to combat the "skin effect" that causes a larger onductor's ampacity to be more closely related to its diameter than its area. This therefore is a significant reduction in material expenses.
• A second purpose for using conductors in parallel is to reduce the difficulty encountered in their installation due to stiffness, weight, size, etc.
• When conductors are run in parallel, magnetic heating effects "eddy currents" must be avoided whenever they enter a steel enclosure or are run through a steel conduit. For enclosure entries, the typical method is to cut slots between all the conduit entries or to install a non-magnetizing plate (with all the conduit entries) into a single larger opening.
• When conductors are run in parallel, care must be taken to ensure that the current is equally divided between them--all cables for each phase must be the same length, same construction type, same insulation type, terminated at the same location, etc.
• Others have commented about radiation of magnetic fields and how this is reduced by grouping the cables with all phases in each group. This is true, but its importance is determined by the installation conditions.
• Grouping by phase makes the parallel conductor rules easier to achieve, because length is the hardest to control.
• Grouping by phase is more likely to be confusing to electricians doing maintenance or repairs in the future, because this is less commonly done.
• Grouping phases together is easier when there are more than three sets of parallel conductors.
• Grouping by phase is probably cost effective only for 3, 6, or 9 sets of parallel conductors (with three conductors in a conduit). For 5 or 7 sets of parallel conductors there is no way to run more than one per conduit and meet the rules for paralleling. The temperature derating rules cost you, if you have more than three conductors in any single conduit.
  

Generally, then, I would agree with those who say grouping all phases in each conduit is the better approach. However, either way is acceptable to the code authorities I use (NEC).

--JMM

Dear Sirs,

Thank you very much for all your valuable advise on my query. It really help me to understand steps we have to follow when using multiple cables for each phases. Is it correct me to understand that if we install several same phase conductors in PVC pipes, additional precaution has to be taken to avoid eddy current formation at entries to metal cubicles?

Thank you,

Asitha

PVC pipe has nothing to do with the eddy currents in the gland plate. They occur because a current passes through multiple holes in a steel magnetic plate, and that acts like an induction heater. Using aluminum is non magnetic and fields are much less, so heating is less. Cutting slots between the holes breaks the current path, using an insulating gland plate is often structurally stronger and also prevents the induction heating effect.

I have seen the paint blistered off a steel cabinet when the phases were brought in individually, so this is real, not imagined problems.

Grouping all three phases through each hole helps cancel the magnetic fields. Using individual cables for each phase aggravates the problem. Get the picture?

Dear Sir,

When we are bring individual phase cables separately, can we avoid the heating of enclosure by not having any gland plates at the entry of cables to the cubicle? So all the phase cables will enter to cubicle through a one opening (Maximum current per phase is 2000 A).

Regards,

Asitha

Technically yes.

BUT, you have to support your cable terminations and close the opening. It helps keep water, rodents, insects, and other vermin out. (Yes and the bipeds).

If you have a poor connection and things catch fire it helps contain the fire spread.

I don't think your local inspectors would allow it.

I agree with using three phases in each hole to reduce the eddy in a metal cabinet.

This is why I was very surprised to see a whole drilling platform having its metal electrical cabinets wired with one cable per hole. Those were hundreds of amps per cable with plenty of harmonics (DC drives AC and armatures). I took temperature readings but couldn't see any effect on surface temperature.

Is it possible that we make a bigger deal that it really is?

In my experience I have found only small heat rise with an aluminum gland plate.

However, using mild steel ~1/8 inch thick, Current /phase ~1000Amps, 3 conductors, 3 cables ( 1 per phase), and the paint blistered off the surrounding gland plate. Non magnetic stainless also works OK. Or use 1/4 inch "Glastic" also works.

Dear Sir,

Thanks you very much for valuable help on this. I have another doubt about induction heating. Could you kindly let me know whether there is a possibility of heating the PVC conduit due to induction heating if we use separate conduits for separate phases?

Regards,

Asitha

No. Wrong frequency.

The current flowing in a single line will induce a current in any nearby conductor. This includes other lines and the ground itself. For long runs of high current under water DC lines actually have lower losses than AC lines.

Assuming this installation is bellow ground but not under water and the lines are of fairly ordinary lengths then mixing the phases together in the conduit will cause the induced eddy currents to mostly show up in the other lines. The phasing of three phase power tends to cause these currents to cancel out so the losses don't appear as heating but do tend to appear as a peak voltage trimming effect on each line.

Running all the same phase in a single conduit reduces the self-induction in each line but maximizes the physical stresses on each line as they now all strongly repel each other when fully energized.

Eddy current losses to the surrounding ground are likely to be the greatest source of loss in a typical underground line installation, they can be calculated based on radio frequency transmission losses.

Running the phases together each in its own conduit, each conduit spaced in a larger pvc conduit should produce the lowest energy transmission losses, but all the physical and inductive heating problems mentioned by the other commentators could be present.

Matching the three phases each in a single conduit with no ground line present should prove to be the most practical solution. Safety ground for the system would then be a very substantial ground rod installation at the point of power consumption.

Sincerely,

Mr. Gee

Dear aindatissa

Option B as referred by you is correct. Also Mr.Simon wan, expressed the same view.

In the option A referred by you, the cable gets heated beyond permissible level - due to inductance effect (also) and hence leads to problem.

RAJESWARI.

Dear Mr.aindatissa,

The RED Core of all the 8 cables to be grouped and connected to R Phase, the YELLOW Core of all the 8 cables to be grouped and connected to Y Phase, the BLUE Core of all the 8 cables to be grouped and connected to B Phase.

This will nullify the EXCESS HEATING EFFECT of the Cable due to CUMULATIVE EFFECT of CAPACITANCE is NULLIFIED.

DHAYANANDHAN.S