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Topic Title: Cable size
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Created On: 13 May 2013 07:00 PM
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 13 May 2013 07:00 PM
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Avatar for mrlondon.
mrlondon

Posts: 240
Joined: 20 July 2006

Hello
Can any help me with cable calculation im a bit rusty.

Customer has 100A supply/cutout (TT) coming into small unused outbuilding and wants it moved 60metres to new build.


or if any one can help with formula i will look up values

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Mr_London

 13 May 2013 07:05 PM
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Avatar for OMS.
OMS

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Joined: 23 March 2004

What cable do you fancy using ?

regards

OMS

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Failure is always an option
 13 May 2013 07:08 PM
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mrlondon

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i was thinking 25mm SWA.

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Mr_London

 13 May 2013 07:25 PM
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alanblaby

Posts: 342
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Table 4E4A gives 116A for 2 core single phase, and 96A for 3 or 4 core 3 phase if buried, more if clipped direct.
Table 4E4B shows the voltage drop, and depending on what cable you use (2, 3 or 4 core?) then you may have problems with volt drop.
More information required - single/three phase?
Demand?
Cable run?
 13 May 2013 07:30 PM
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mrlondon

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Ill use whats ever cable is suitable, i was thinking 3core.
its single phase and its 60m buried.

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Mr_London

 13 May 2013 07:30 PM
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Avatar for OMS.
OMS

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OK - you can squeeze 99A down it and lose about 9Volts doing so.

25mm2 3C XLPE/SWA/LSF run to 70C buried direct.

Source end protction 100A or 80A BS 88 or BS 1361

regards

OMS

Edit to change volts drop from 4 to 9 as my typing is crap

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Failure is always an option

Edited: 13 May 2013 at 07:45 PM by OMS
 13 May 2013 07:32 PM
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mrlondon

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what difference would using 35mm2 3C XLPE/SWA/LSF make?

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Mr_London

 13 May 2013 07:44 PM
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Avatar for OMS.
OMS

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Cable runs cooler, less volts drop - about 4 volts

regards

OMS

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Failure is always an option
 13 May 2013 09:13 PM
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Avatar for slittle.
slittle

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I'm sure you've thought of it, but........

Bear in mind if it's TT you're going to need something at the front end to protect it against earth faults.

Time delay RCD springs to mind before the metalclad switch fuse


Stu
 14 May 2013 08:17 AM
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UKPN

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"cust has 100amp/supply/cut-out and wants it moved 60m".
I read that as he wants a service alteration.
the DNO will be be happy to quote for this and advise on
mains/loadings/meter tails/bonding etc.

Regards
 14 May 2013 09:11 AM
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AJJewsbury

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Why 3 core for TT? Seems like an awful lot of extra copper... (thinking of the polar bears)
- Andy.
 14 May 2013 10:44 AM
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Avatar for OMS.
OMS

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Originally posted by: AJJewsbury

Why 3 core for TT? Seems like an awful lot of extra copper... (thinking of the polar bears)

- Andy.


Depending on the set up, we don't know where the MET is.

25mm2 2 core has a steel armour CSA of 42mm2 - depending on how we evaluate the equivalent conductance (and I'm not saying that it has to be copper equivalence as for PME) then it could be shown that we can't achieve a minimum of 6.0mm2 of copper

Equally, service alterations change and once the cable is in the ground, the big cost is digging - so 3C up front would be a sensible choice as it would address both the TT and future PME bonding conditions.

Regards

OMS

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Failure is always an option
 14 May 2013 04:40 PM
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mrlondon

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Thought i would go down and check the measurements, turns out its more like 65-66 metres, so you think 35mm is a better option?

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Mr_London

 14 May 2013 05:04 PM
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AJJewsbury

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It depends how much voltage drop you can afford to loose in the SWA after taking into account the downstream circuits.

e.g. if you're working to BS 7671's suggestions of 3% (lighting) and 6% (other) limits

Say your downstream lighting circuits lost 2% (4.6V) you'd have 1% (2.3V) left to loose in the SWA.

If the MD really is 100A that would allow you 2300mV/100A/65m or about 0.354mA/V/m ... which would put you into rather larger territory according to 4D4B (about 150mm2).

.. so you might want to re-juggle the figures a bit! (e.g. 1% for the final lighting circuits, so allowing 2% in the SWA, 60A max demand suggests about 1.18mV/A/m - closer to 35mm2.

I'm sure that some will suggest that the v.d. limits can be further 'interpreted' to your advantage

- Andy.
 14 May 2013 08:25 PM
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UKPN

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"service alterations change"
why would anyone want to supply a "new build" from a unused/disused point of supply? the best way here as I see it is to fit a new service in the new house???? discon the old DNO service and pot end at the boundary. if the old building is ever needed a sub main can the be fed from the new point of supply.

Regards
 14 May 2013 08:35 PM
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mrlondon

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The existing house is to be extended/renovated, plus he doesn't want the existing power line poles going across his front lawn
the new CU will be located about 66m from the stables.

link to the drawing sent from the DNO

[drawing here

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Mr_London

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