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Topic Title: Re: Cable installation
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Created On: 02 May 2014 12:04 PM
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 02 May 2014 12:04 PM
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SreejithK

Posts: 11
Joined: 14 November 2012

Hi Guys
Working on a project and have got an issue. We have a feed serving a kiosk protected by a 100A MCCB. The length of the run was around 40m and amtech results were 25mm cable with no cpc.
Now the client wants to go for a bigger kiosk and the ducts are about 200m away from the switchboard protected by a 160A MCCB with a 125A trip unit and Installation method is buried in duct, design current 115A and I am trying to size the cable to prevent any major costs.
I have tried the BICC data buried in ground method with 3 or 4 cable touching each other and I am initially only going one size up to 35mm with a 25mm separate cpc but I have a volt drop of 6.11%.
Is there any other way around it. Has anyone come across such a situation Any ideas would helpful..
Regards
Sree
 02 May 2014 12:29 PM
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peteTLM

Posts: 3120
Joined: 31 March 2005

So whats the max Zs for the device protecting the cable?

I think the cable sizes need to go up a bit to say the least.....the increase from 40m to 200m distance is going to be of huge significance.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 02 May 2014 12:35 PM
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John Peckham

Posts: 7375
Joined: 23 April 2005

I am a bit confused about the length of run is it 40 or 200. What is the Zs at the board? Do I understand that the cable in question is grouped with 3 or 4 other cables? What is the actual MCCB you are using, make and model? What volt drop are you wanting? What soil resistivity and premature are you using? What power factor?

I am getting between 70-120mm depending on the length (40 or 200) on my Amtech with my guesses at the other data.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 02 May 2014 01:27 PM
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Jaymack

Posts: 4587
Joined: 07 April 2004

Originally posted by: John Peckham
I am getting between 70-120mm depending on the length (40 or 200) on my Amtech with my guesses at the other data.

No problem there with VD then!

Regards
 02 May 2014 01:54 PM
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SreejithK

Posts: 11
Joined: 14 November 2012

Hi guys

The length is 200m. Yes the cable is grouped with another 3 cables which makes it 4 cables. The MCCB is a schneider nsxn 160A mccb with a 125 a trip unit.. volt drop less than 5%,, soil resistivity default amtechs values.. PF 0.9.. Max Zs of the cable is 0.4.

I do understand that the cable needs to get a bit bigger .. but has anyone tried to used the option BICC option in Amtech?? Some detail on the implication would be helpful..
 02 May 2014 02:19 PM
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SreejithK

Posts: 11
Joined: 14 November 2012

If we actually use the BICC methods of installation we could reduce the cable sizes. But could that be taken into consideration ? If so how to validate the design??
 02 May 2014 03:13 PM
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Avatar for OMS.
OMS

Posts: 19471
Joined: 23 March 2004

If you want to use the BICC data then you need to be reasonably certain that the ground conditions are at least aproximately the same as your site.

You might also want to consider any limitations of conductor operating temperature (ie 90C XLPE run to 70C max)

So if you want to validate the design, then use data from site (if you have it).

That said, BS 7671 tabulated values for soil temperature, thermal resitivity etc are quite onerous - you rarely find ground that bad, and presuming that Ib is not a constant load, you can easily show that a BICC approach is not unreasonable in the UK climate - but you may have to demonstrate that.

So, what is the backfill material around the ducts and what depth are they at. If you have ducts at say 800mm and significant sand/stonedust backfill then BICC will be appropriate compared to worst case BS 7671.

Just a point to note however - the cable data in BS 7671 and BICC is identical, all BICC have done is apply the allowable factors in BS 7671 to provide thier data

Regards

OMS

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Failure is always an option
 02 May 2014 04:17 PM
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SreejithK

Posts: 11
Joined: 14 November 2012

Hi Guys

Thanks for the replies. Are there any BICC cable manufacturers in the UK? And would anyone know how expensive they would be to a BS7671 SWA 25mm cable roughly?
Cheers
 02 May 2014 05:32 PM
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OMS

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You haven't grasped this have you.

BICC data refers to a method of calcuating XLPE/SWA/PVC Cables based on defined criteria - which differ from BS 7671

So if you take the BS 7671 data for default and then correct depth of lay, ground temperature and soil resistivity you'll get the same CCC as the BICC Data.

In Amtech, if you select BICC Data, it will default to the relevant ground conditions that BICC prepared thier data for.

BICC is a cable manufacturer, there is no such thing as a BS 7671 cable.

If you have Amtech, then you need to select an armoured cable to BS 6724, decide if you want to run it at 90C, correct for depth of lay, ground temp and thermal resistance as defined in the BICC Data and then press the go button.

If you want to select from BICC Data, then you'll get the same answer as above

Just be warned that BICC data runs XLPE to 90C - which might not be acceptable to you

Regards

OMS

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Failure is always an option
 02 May 2014 05:46 PM
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SreejithK

Posts: 11
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Oms

Thanks for that. Just did some look into it and arrived at what you have mentioned above. We have got the site conditions and have tried to adjust it to suit the resistivity, depth of lay and temp and the size has gone down from 70 to 50 mil, which is good news for us. Thanks a lot for the reply guys.
Cheers
 02 May 2014 07:09 PM
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OMS

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OK, no drama

Just be a bit wary of overdoing the economic cable sizing bit - the cost here is the pit and duct system.

The cost involved in changing a cable size is small in relation to the level of risk you absorb by undersizing it.

Grouping won't be helping you either - if you are trying to put 4 cables of around 50mm or 70mm CSA 4C into ducts, then they'll need to be big ducts to allow that lot to be dragged in over 200m.

You could get this down to say a 35mm2 4C if you can get away from the grouping factors - but you would need to be very sure of the load profile to attempt it with any certainty

Regards

OMS

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Failure is always an option
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