Single Core or 3 Core - Why ?

Generally, the current that can be carried by these cables is limited by the temperature that the insulation will be subjected to. Depending on the cable configuration and how many are placed together, the cooling efficiency will vary affecting the insulation temperature. Different insulation temperature rating will give different ampacity for the same cable cross section. Just look at the size of the wires used to wind a transformer. They are less than half the size that we use in to interconnect equipments simply because their insulation can take 150C instead of the 90C insulation that we use.

The other aspect of cable sizing is the voltage drop on long runs. Generally, the inductance is the major contributor. One is therefore advantaged to pack the cables as close as possible to each other but this reduce the thermal dissipation and the current capacity. A good compromise is to bondle three phases together in one cable which minimizes the inductive voltage drop and allows you to to add few other of the same in parallel with enough spacing to cool them down.

That is the main reasons for the different results.

Hi Marcot,

Thanks for responding.

I want to know whether I am right in the way I have used 3 core cable or single core cable.

1) When we say 3core carries 100A. Does it mean all 3 cores together carry 100A?

2) An is current the only criteria in going for 3 core or 1 core?

Thank you

I am not 100% certain that I understant exactly what you are trying to say and I might be confused with some of the therms used. Here is a list of the concepts that you might want to investigate.

1-You might want to revise your current calculation since you need some parenthesis. 1000 × 2000 / (1.73 × 690 × 0.8 × 0.85) = 2461.00A

2-I assume that a three core cable is made out of 3 isolated conductors with a ground conductor. This is normally used in a three phase / three wires connections. In this case, each cable would be rated for the 100A that you give as an example.

3-The 2461A that you calculated above is the phase current. Each phase carry this current if the load is balanced.

4-You seem to use some de-rating for parallel conductors. This is usually given in tables from your local electrical code. The final de-rating depends on the number of conductors in parallel, the type of insulation, and the type of trays used to run the cables.

5-With the size of this installation, you will need a certified master electrician and/or an engineer to make the final calculations for construction. There are many aspects such as the available short circuit, the protection coordination and the mechanical installation that are cover by laws. Be careful. This discussion is not legal advise.

Hi

1) 3-core carries 100amp means 100amps per phase. Thus in your original post, you would require 9 150mm2 cables per phase, plus neutral cables, plus earth. That's a lot of cables.

2) 3-core cables will be armoured, neutral screened, or sheathed. This increases their thermal insulation thus decreases the current carrying capacity.

Thus 1-core cables will carry more current for a given size. But as the installation is more difficult, especially if they are paralleled, volt drop can increase due to stray inductance.

1-core cables are also easier to deliver and handle/install.

I would not parallel more than 3 cables if I can get away with it. At 2500amps, I would prefer busbar trunking to cable. This gives you much better withstand of short circuit faults, is faster and cheaper to install, and can easily be re-used if the installation is changed. Easy also to use aluminium for the conductors.

almost all 3 or 3 1/2 core cables are armoured.Single core cables are not armoured as the armour surrounding a cable cable heated up due to induced currents which cancels in a armour surrounding the 3 core due to phase shift.So heat dissipation is less in 3 core cables and hence lower rating of current.

the reason for the derating will have to do with the 3 wires being wrapped/strrapped together their is less area to disipate heat as their is from a single cable.

Hi Marcot and Baby Guinness,

Sorry to bother you guys again on this.

Question 1a:

So that means for a 2000kW, 3 phase, 690 V, 50 Hz load, am I right to say the following:

I got
1000 × 2000 / 1.73 × 690 × 0.8 × 0.85 = 2461.00A

So are you saying that 2461 Amps is required to be carried by the cable PER PHASE or for all phases together?

Question 1b:

1000 × 2000 / 1.73 × 690 × 0.8 × 0.85 = 2461.00A

As 2461/293 = 8.4 runs

That means 9 runs of single core ,150 sq.mm per phase ? ( that is 27 cables of single core, 150 sqmm cables totally for the 2000kW load?)

OR

9 runs for all the 3 phases together?( that is 9 cables of single core, 150 sqmm totally?)

Question 2:

What does 3 core 150 sq.mm mean - Does it mean that in that single cable, each core is of (150/3 = 50) that is 50 sqmm or is each core in the 3 core Single cable is 150 sq.mm.

Thank you.

1a) PER PHASE

2461amps each phase

1b) 9 runs PER PHASE = 27 cables in total (for phases only; you will also need additional neutral & earth cabling also)

2) 3 core 150mm2 means 3 cables bound together in a single, overall sheathed, cable. Each cable sized as 150mm2.

See http://www.generalcable.co.nz/newzealand/Products/Energy.aspx

Go to 'Multicore and circular TPS' and click on the various cable types; you will see pictures of their construction.

Hey guys,

1) Are you saying that for example if current carrying capacity of a single core 35mm2 cable is given as 138 , then that single conductor carries 138 A,

whereas if we say that a 3 core cable of 35 mm2 is carrying 149 A, it means that EACH CORE of the 3 core cable carries 149 A?

2) Are are you are also saying when I say 3 core 35mm2 , it means that there are 3 cores of cross-section area 35mm2 is packed to gether in one cable with the effective cross-section of the cores being 105mm2.

If so, why do they give in the data sheet that 3 core cable of 35mm2 carries only 149 A. Should it not be 3 x 149 A? BG - I am refering to the General cable data sheet, which you refered.

Thank u

A1) <...if...a 3 core cable of 35 mm2 is carrying 149 A, it means that EACH CORE of the 3 core cable carries 149 A...>

Not quite.

At that instant, T₀, one of the cores will be carrying <...149A...>, and the other two will be carrying 74.5A each.

The situation will be different a fraction of a second later: At T₁, being 1/(3 x [frequency]) of a second later than T₀, that same conductor will be carrying 74.5A, as will one of the others, and the third will be carrying <...149A...>. At T₂, being 2/(3*[frequency] of a second later than T₀...

A2) <...3 core 35mm2 ...means...there are 3 cores of cross-section area 35mm2 is packed to gether in one cable with the effective cross-section of the cores being 105mm2...> The effective cross-sectional area for carrying out cable sizing calculations remains <...35mm2...> so long as all three phases are being carried in one cable. If <...105mm2...> cable is called for by calculation, then a <...3 core...35mm2...> is the wrong cable to use: either it is too small or several of them will be needed where a single, larger cable is required.

If in doubt, consult a qualified local Electrician. This individual will be able to size the cables correctly according to the prevailing national standards having been trained and assessed as capable to so do, install, test and document the installation after testing. The document is the Electician's warrant to the user that the installation is safe to use.

BabyGuinness answer was correct. 3 core cabe means 3 cables arranged or bound with single. based on your current rating. motor take 6times a normal current at the time of starting. So should take that current also while selecting the cable. 2461 x 1.5 = 3692 Amps as per NEC. I can select the cable 10 Run, 400sqmm aluminium armoured PVC sheathed. the sqmm will be reduced if u lay the in ground.

Thanks

So is that a YES and YES to my last two questions.

Thank you.

Well, not to put too fine a point on it, YES & YES!

Note that in a 3-phase balanced system (with zero harmonics), the currents sum to zero (as they are each out of phase by 120 deg). So three cables each carrying 150amps, does not equal 1 cable carrying 450amps. Data sheets refer to the current per phase, in a balanced cable.

So, lets say we have a single phase system with 2-cores carrying 150amps. EACH cable, phase and neutral, carries 150amps. Therefore volt drop is doubled, as the current must make the to- and return-journies.

In 3-phase, the currents cancel. Thus the currents only make the to-journey. If you have a neutral (i.e. 4-core), it should carry no current at all. But only in an ideal world. In reality, there must be some allowance for out of balance. You can get away with a reduced neutral (normally half size). But this is a calculated judgement call for an experienced engineer, in conjunction with local regulations.

In any case, you shouldn't parallel so many cables. Each individual cable must be capable of withstanding the short circuit fault current from your source. Also the number of terminations you are creating is enormous, and again each must withstand the s/c current. So again, busbar trunking is the way to go. See

V Srini, I don't know what you are connecting and where. Be careful. Depending on the type of industry and the location in the world, there are many additional installation restrictions that will apply. Baby Guiness has given a lot of good info but you cannot use this as a complete design. From your questions, you are not qualified to do this design. If you are only asking general questions to get an idea of what this job involves, it is OK but do not go ahead on a design based on the info gathered here. Find a competent local engineer to finalize this. You might be surprised by the added costs if your application requires explosion proof cabling or the cable trays needed to support all this cabling.

Good luck.

I agree with your comments.Mr V.Srinivasan has to work under Design Engineer for couple of years and get his guidance before becoming full time designer.

Electrical Power system design is not easy as people think.