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Parallel cable sizing

Hi, I'm currently completing my project report for BEng based on an existing install at work. The install is between a 1000 kVA trans to main section board, the existing cable is 5x 240mm 4C SWA in duct buried. All ACB on the circuit are 1600A, I calc the max output of the trans at 1443A.

I'm trying to prove the existing install using the BS 18th regs as the installer hasn't supplied any, and this is where I'm having an issue.

From my understanding rating factors must be applied to In (protective device) and that value must be less than or equal to It (tabulated current carrying capacity of a cable). I've got a 'new' In value of 2962A which Ive divided by 5 (number of cores per phase) which gives 592A. Am I right in thinking that I then need to specify a cable using the 592A value? If that is correct than the existing install is incorrect?

In table 4E4A (18th regs) the max current for 240 SWA multi core, ref D (buried) is 324A.

Have I gone wrong at some point?

Any help is very much appreciated, thanks Adam.

0 Former Community Member over 5 years ago

Hi ARW182,

Would you be able to expand on this a bit more? How did you get to the max output of 1443A and then the new In value of 2962? I'm struggling to see how you've got from a 1000kVA Tx to those values, maybe because I've been out of the classroom for too long, but it'd be nice to have a refresher!

Regards,

S
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0 Former Community Member over 5 years ago

1443amps is based on 400V. 1MVA/400 x 1.73

We need to see the correction factors you've used.

You will struggle to make 5 x 240mm2 work using BS7671 ccc ratings.

I would have used a couple of large 1c AWAs per phase rather than 5 x 240mm2 4c SWAs.

Regards

Parsley
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0 mapj1 over 5 years ago

you may find it does not meet the ratings as tabulated in the regs, especially if you are using grouping factors to generate the higher effective current.

I agree with the 1440 amps per phase, s that is 1000VA /230V = 4.3kA , then split into 3 phases call it 1450A per phase as a round number.

If that current is split into n cores per phase then each core carries 1450 /n

If n is 5, then 1450 becomes 290A per phase per cable

Then if you apply the right grouping and environmental factors, you will see that you need a larger cable than the 290A would suggest, but please describe how the cables and cooling are organised, as it is unlikely to be as bad as the factor of 2 you seem to be using, and I'm not sure if you are rolling in a factor of 1.45 you do not need, or one of the C factors is counted twice..
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0 Alan Capon over 5 years ago

Are you sure you have the secondary voltage correct? The standard ESI transformers still have 433V secondaries with 100% primary volts on nominal tap.

Regards,

Alan.
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0 Alan Capon over 5 years ago

I should have added that in the UK, a standard ESI 1000KVA transformer has a rated output of 1333A. I am unsure where you have got your value of 2692A from, unless it is a setting from your protective device. It is a bit excessive to expect from a 1000KVA transformer for very long.

Regards,

Alan.
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0 ARW182 over 5 years ago

Morning all, thank you for your input.

So the process of getting to the 2900 A (ish) value was by using the protective device value (1600 A) and dividing it by the assumed rating factors, which were (I don't have my book on me rn) I think 0.6 (Cg) and 0.9 (Cc I think, the value is from a note in the regs). I'm not sure about the existing install so I've assumed it was installed to keep the effects on the cables to a minimum.

Using the tabulated current from my initial post of 324 A (T 4E4A ref D) that would give a total of 1620 A, which I understand is rated more than the protective device on paper but the ref d column in the table must be a ballpark figure?

I may be incorrect about the transformer voltage, I assumed the value. The cables are able to carry the transformers max operational current but Iz should be greater than In... which it is by 20 A.

Thanks, Adam.
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0 Former Community Member over 5 years ago

The op appears to be using a correction factor of 0.54??

It》In/correction factor

1600/0.54 = 2692 Amps

It/n = 2692/5 = 592 Amps

We need to understand how the correction factors have been selected.

The LV tails protective device Ir setting could be adjusted to match the transformers LV output.

Looking a data sheet for a Wilson 11000/415V 1000kVA Tx, full load current at normal tap is stated as HV side 52.5A LV side 1391A.

Regards

Parsley
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0 Former Community Member over 5 years ago

I suspect 2 key issues here:

1 - The cable sizing is based on the nominal setting of the 1600A device - it is likely that this is actually set (or should be) to a value around the TX FLC - ie 1333A

2 - The multicore cables are buried in ducts - so they will have a certain natural spacing - which suggests the grouping factor and buried direct factors may be a little less onerous - also the determination of depth of lay and ground resistivity will have a reasonable impact

If we take the FLC as being 1333A, and a reasonable correction factor for all conditions of grouping, ground temperature, depth of lay, spacing etc of say 0.75, then we need (for 5no multicore XLPE run to 90C) a conductor CCC of 1333A/0.75 = 1777/5 = 355A - which is just about credible for a 240mm2 conductor

regards

OMS
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Parallel cable sizing

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