Single core cable selection

[Newbie2000]

Hello,

I have 690V network, 3 phase system, no neutral, with 2400Amps load current.

I end up with 8 runs of 3 core 240mm2 cable size.
When I select 1 core cable, I can manage with 12 runs. It means only 4 runs per phase.

3 core cable in general has low current capacity than single core.

Is that possible? or am I doing something wrong?

regards
Avi.

 

[7anoter4]

In my opinion you are speaking about BS7671 Table 4DA1 Reference Method F single core cable copper conductor PVC insulated 70oC insulation horizontal laying in free air one diameter spaced[615 A] compared with Table 4DA2 3 core cable in conduit -Reference Method B [297 A]
However, the method E is the method corresponding to F and the carrying capacity will be then 430A.
The air between cores facilitates cooling of cores by enhancing convection.
A conduit hinders heat evacuation by introducing a temperature drop in the chain of temperature drops.

 

[krisys]

If the cable is 240 mm[sup]2[/sup] three core is easy to handle. Normally, conductor size of 630 mm[sup]2[/sup] and above will be single core.

With single core it becomes easy for bending and termination. In the single core you don't need the break out space inside the cable box.

So if you want to increase the no. of cables, better not to do that and go for the single core 630mm[sup]2[/sup] conductor cable.

 

[ScottyUK]

240mm² copper cable is not easy to handle, certainly not in the small termination boxes typically provided on modern gear. Modern cable is rarely annealed properly and it very awkward to work with in confined spaces.

 

[Newbie2000]

Thanks for your reply.
I was looking for an answer on, if I can use 12 runs of 1x120mm2 (4 runs per phase) instead of 8 runs of 3 core 240mm2.
regards

 

[ppedUK]

Single core cables are capable of carrying more current than an equally equivalent size 3 core cable.

120mm2 single core cable (Cu/XLPE) is not capable of carrying 600A per phase (2400A load/4). Did you mean 240mm2 single core?

Assuming you are using copper conductor XLPE insulated cable, then the "in Air" rating of a 240mm" is around 600A in Trefoil and in the horizontal plane, but that is without de-rating for grouping etc. If the trefoil groups of cables are spaced >2x cable O.D. then de-rating for grouping need not usually be applied, but you must factor in any other de-rating factors for high ambient temp. etc.

You don't say what the length of the circuit is, but at 2400A you will have to also give consideration to volt drop and earth loop impedance constraints.

 

[jghrist]

Now you're changing the question. Is it 4 - 120 mm² per phase or 4 - 240 mm² per phase? What standard are you using for cable current capacity? I can't imagine that 4 - 120 mm² has the same current capacity as 8 - 240 mm², no matter what standard you use.

 

[Newbie2000]

Hi,

Jghrist - You are correct, my bad.

I intended to say is 8 runs of 3 core 240mm2 cables as per calculation for 2400Amps, 3 phase current.
But when I select 1 core 120mm2 cable, then I see that 4 runs 1 core 120 per phase should be sufficient. So it makes 12 runs of 1 core 120mm2 cables.
The cables are laid in cable tray, open air. So my figures and sizes are based on the ambient derating + bunching factor in cable trays.

The cable manufacturer I used has
1 core 120mm2 cable with ampacity of 320 Amps
3 core 240mm2 cable with ampacity of 344 Amps

regards

 

[ppedUK]

Am I missing something? 4 x 320A does NOT give you a 2400A feeder!

Suggest you get someone experienced in cable sizing to do the calculations for you.

 

[jghrist]

I think you are confusing amps per phase and 3 phase current. Do not think in terms of 3 phase current. Your 3 phase circuit has 800 amps in each phase.

 

[Newbie2000]

@jghirst,

Correct. When I do calculation its 2400 Amps. So considering balanced 3 phase, it is 800 Amps per phase.
So it means 4 runs of 120 mm2 should be good enough per phase (344 amps per run). Total 12 runs of 1 core 120 mm2 cables.

 

[jghrist]

Correct. When I do calculation its 2400 Amps. So considering balanced 3 phase, it is 800 Amps per phase.
So it means 4 runs of 120 mm2 should be good enough per phase (344 amps per run). Total 12 runs of 1 core 120 mm2 cables.

I don't know what your derating is, but if there is none, then 3 - 120 mm² per phase (total 3·320=960 > 800) would be good enough. 3 - 3/c 240 mm² would also work (total 3·344=1032 > 800). Again, forget about the 3 phase 2400 amps - there is no run of conductor that has to carry 2400 amps. This just confuses things.

 

[Newbie2000]

Thanks @jghirst.
The problem is solved for me.

 

[Marmite]

Is there any particular reason why you have only considered two cable sizes? The installation costs of 4 parallel sets of single core cable, or 3 sets of 240mm 3 core would be more than one set of 500mm Cu single core.
Regards
Marmite

 

[Newbie2000]

@Marmite,
I agree but per project requirements, It is not allowed to run more than 240mm2 cable.