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Topic Title: Sub-Mains Cables in Parallel
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Created On: 02 February 2013 12:30 PM
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 02 February 2013 12:30 PM
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sonic

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Running two SWAs in parallel as a sub mains over around 90mts at 630 Amps I see no problem so long as the same length etc any other things I should look for ?
 02 February 2013 05:54 PM
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Martynduerden

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Originally posted by: sonic any other things I should look for ?


A designer to sign the EIC ?

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Martyn.

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 02 February 2013 07:54 PM
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jcm256

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Below is a chart that shows current rating Air, Ground or Duct are you going to use 2x 70 or 2x 95 in parallel or leave a good margin (just in case). Only posting because often had to decide with inspections, should I be asking for a external earth wire run alongside the cable. Mostly let the hare sit because of disruption and cost, anyhow earthing CPC may be done with the armour up to a certain size, and no rule that I know that armouring is inadequate to serve as a CPC under existing wiring regulations and you have double. But don't take my word for it look it up yourself although expensive you could use a spare core or 90 mm earth wire.

http://www.prysmian.co.uk/expo.../attach/pdf/BS5467.pdf

(Note; should have, but never consulted any regulations on the above comments, some others may have all facts from the top of the head and will correct as required).
 02 February 2013 11:46 PM
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Martynduerden

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Originally posted by: jcm256

Below is a chart that shows current rating Air, Ground or Duct are you going to use 2x 70 or 2x 95 in parallel or leave a good margin (just in case). Only posting because often had to decide with inspections, should I be asking for a external earth wire run alongside the cable. Mostly let the hare sit because of disruption and cost, anyhow earthing CPC may be done with the armour up to a certain size, and no rule that I know that armouring is inadequate to serve as a CPC under existing wiring regulations and you have double. But don't take my word for it look it up yourself although expensive you could use a spare core or 90 mm earth wire.



http://www.prysmian.co.uk/expo.../attach/pdf/BS5467.pdf



(Note; should have, but never consulted any regulations on the above comments, some others may have all facts from the top of the head and will correct as required).


I'm prepared to be surprised and I will be if 2x95 will do 630A over 90m in most common installations?

As for the swa as cpc well 95mm is complient with 54.7 for CSA, a 95mm is c 140mm armouring on a 3 core

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Martyn.

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 03 February 2013 12:01 PM
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OMS

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They could quite easily Martyn - from a switchboard to a switchboard with 90C (or even 105C) copper spills, buried direct in material with a suitable thermal conductivity and appropriate spacing at a low(ish) ground temperature. I've installed 2 x 95mm2 on 630A protection in a number of cases. It would be a common approach to providing ring main systems in a variety of facility types.

And you are assuming that the 630A protection is providing overload protection - it may well not be depending on the local protection at a switchboard - the 630A protection may well be offering SC protection only, and is derived from a discrimination (or selectivity) analysis.

95mm2 run to 90C, at a ground temp not exceeding 20C with a thermal conductivy of bedding of 1.2W/m2.K. - Will give you 319A (if you apply all the relevant factors from 4B2,4B3, and reference to BS 7769 (ie note 2 to 4B3) to table 4E4A then you should get the same number(ish))

Two in parallel spaced by say 2 x de (and remember this is a single circuit) wouldn't normally attract a grouping factor.

So 2 x 319 = 638A >630A(In)

Given that the ground isn't really 20C - it's usually less, and very few loads aren't cyclic then I guess you've been suprised

Regards

OMS

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 03 February 2013 12:41 PM
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jcm256

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I'm prepared to be surprised and I will be if 2x95 will do 630A over 90m in most common installations?

As for the swa as cpc well 95mm is complient with 54.7 for CSA, a 95mm is c 140mm armouring on a 3 core
Martyn.

Martin, what size would you recon then, I did not look any further in formatting a reply other than to give a chart of volt drop etc. Because the original post had to much white space, what is one to make of a sentence ( Running two SWAs in parallel as a sub mains over around 90mts at 630 Amps) Obviously a loading of that size other work would be in the background liaisons with networks supplier, upgrading the main switchboard etc.

Regards
jcm

Edited: 03 February 2013 at 04:05 PM by jcm256
 03 February 2013 04:36 PM
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John Peckham

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Am I reading my BGB and the manufacturers sheet correctly. The BGB says 197A for 95mm XLPE in the ground in table 4E4 but Prysimion quotes 319A?

I have punched the numbers in to my Amtech and get a lot bigger cables depending what correction factors (ground temp and depth of lay etc) you put in with 630A at 0.9PF.

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John Peckham

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 03 February 2013 05:02 PM
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OMS

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Need to be slightly careful there John - the manufacturers data is based on the current carrying capacity derived from ERA report 69-30 part V - Sustained current ratings for cables with thermosetting insulation.

It's a world away from BS 7671 tabulated CCC

Regards

OMS

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 03 February 2013 05:13 PM
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John Peckham

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OMS

So where are you getting your 319A for 95mm in ground at 20 degrees and 1.2K from?

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John Peckham

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 03 February 2013 05:24 PM
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OMS

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Same place as the manufacturer did - ERA report 69-30 part V - Sustained current ratings for cables with thermosetting insulation.

i don't have a copy readily available at the moment - I'll dig one out and post you a few details

Handy if you are getting into an argument regarding what a cable can actually carry bearing in mind the concept of thermally equivalent loads.

take a look at GN6 - protection against overload in section 2 and look for thermally equivalent curren - a quick lok at figure 2 should show you ccurve that might suggest that a substantial % increase in the CC may be practicable.

regards

OMS

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 03 February 2013 05:29 PM
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John Peckham

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OMS

Which figure 2? I need a second number ?

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John Peckham

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 03 February 2013 05:43 PM
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OMS

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Section 2.2.4 ? - not near my up to date versions here John, so guessing a bit

Section 2 - protection against overload - thermally equivalent current - test for a thermally equivalent load - you'll find it

In rough numbers, the value in BS 7671 is 197A

Apply factors of about 1.04 for 20C ambient, about 1.04 for 15c ground temp and about 1.43 for ground conductivity and you should see approx 320A anyway.


Regards

OMS

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Edited: 03 February 2013 at 05:52 PM by OMS
 03 February 2013 06:00 PM
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John Peckham

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OMS

I dont think you can apply the 1st 1.04 to table 4E4A as Method D ratings are for ground temps of 20 degrees anyway. I was using 15 degrees as per the ERA figures in my Amtech. I will go and have a look at my GN6. In the mean time for those of us who do not have a geology degree how do you measure/assess the soil thermal conductivity over the length of the cable run? I would guess over the 90 metres in this case there would be large varitaions?

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John Peckham

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 03 February 2013 06:30 PM
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OMS

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LoL - OK, omit the first 1.04 if you like, make the last factor for ground conductivity closer to 1.5 rather than 1.43 - we could call it 1.47

The simple way to ensure you know the ground conductivity is to insist on a quartz based backfill for the trench bedding and overfill - ie use sand or stonedust. for at least 100mm around the cable in every direction.

failing that measure at relevant positions along the trench at the anticipated depth. an analysis with Fouriers law, and single dimension will get you thermal diffusivity. Several typical method exist using a line heat source or thermo time domain reflectometry or simple measurement at depth with parallel needle probes

If all of that sounds too much of a faff, put in bigger cables - it's probably cheaper to chuck in a pair of 120mm or 150mm if you aren't certain about the ground. Measuring is expensive.

Just keep in mind most loads aren't steady state so thermally equivalent plays a big part in the margins

Regards

OMS

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 03 February 2013 10:13 PM
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mawry

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^sheesh. And I thought I was a geek!!!;-)
 03 February 2013 11:16 PM
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John Peckham

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OMS

I am going to need to see this piece of ERA magic that improves cable ratings as my calculations say 240mm or 300mm in parallel for a 630A continuous load at 0.9PF, 5% volt drop, 0.7m deep with a ground temp of 15 degress, spaced more than 2 diams. either at 1.5 or 2.5 soil resistivity when connected to a 630 MCCB?


Clearly the client would prefer your calculations but the cable salesmen would prefer mine!

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John Peckham

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 04 February 2013 11:38 AM
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OMS

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No magic John

First read 7.1 - ref method D - that should point you at the ERA report.

Then

Air temp = 25C Factor = 1.02
Ground temp = 15C Factor = 1.04
Depth = 0.5 Factor = 1.03
Soil K.m/W = 1.2 Factor = 1.4

From Table 4E4A - It = 197A

197 x 1.02 x 1.04 x 1.03 x 1.4 = 301A

Nearly there - now consider the accuracy of those factors, and that the dominant factor is ground thermal resitivity and note 1 to table 4B3

Adding the 5% tolerance (we would have worked this out as an actual factor in practice)

So 301 x 1.05 = 316A

Two in parallel atrracts no derating so = 316 x 2 = 632A > In of 630A

I'd say that complies wouldn't you

Now - volts drop - are you serious about 5%.

If so check your calc for parallel conductors ((630/2) x 90 x 0.45)/1000

= 7 volts.

As this is highly likley to be distribution then ESQCR at 10% would allow us 40Volts

I guess you are putting this through Amtech John ? - possibly why you are almost doubling the CSA size.

As I said, it was just an exercise to show it's not a problem -

I'm prepared to be surprised and I will be if 2x95 will do 630A over 90m


I've used that conductor size and protection combination several times on ring main run open applications - it's very rare that Ib would be an exact and sustained 630A

I'f I was designing it in a high risk application or one where growth was required and was likley to be actually used then I'd step up to 120mm or 150mm as a minimum.

If these rings were coming from either half of a switch board ie 630A MCCB at either end of the cable, then I would also look atthe board current rating and the number of rings deployed. Lets say it's 5 rings, assuming one off for fault and a supply failure into one half of the board rated at say 2500A - so I'd need 550A per ring to re supply side 2 plus the ring loads - in that case, I'd perhaps go to 185mm or 240mm - but that's an extreme case for say healthcare to provide dual unified or maybe financial to keep the dollars rolling. The normal presumption is ring run open of course

No wonder the cable sellers love you John

Regards

OMS

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Failure is always an option
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