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Topic Title: Spare capacity on electrical loads?
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Created On: 04 January 2017 01:08 PM
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 04 January 2017 01:08 PM
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Matt01

Posts: 211
Joined: 25 July 2008

Can i ask does anybody know if there are any guidance docs out there that state spare capacity figures?

For example the 20% to 25% gets used alot as standard but does this come from anywhere?

I have a contractor trying to reduce the amount of spare capacity on a Transformer which would enable them to use a lower rated TX hence saving them money, but affects its future proofing. The clients not up to speed with this and to close it off i would like to state a guidance doc.
 04 January 2017 01:21 PM
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di515223

Posts: 337
Joined: 08 July 2010

A little more information would be useful, Is this a Hospital, Data centre, Office block, Hotel, Factory or Airport?
The issue of spare capacity depends on your definition of Spare.

Dave
 04 January 2017 01:43 PM
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AJJewsbury

Posts: 15853
Joined: 13 August 2003

I don't think you'll find a hard-and-fast figure somewhere (certainly not in BS 7671) as so much will depend on the individual circumstances and how long the installation will be expected to last for. Certainly figures like 20% 'spare ways' in a DB for instance have been long said to be good practice - but that could well be insufficient for spec-built industrial unit that's initially only equipped with basic lighting and a few sockets, but at the same time would be wasteful for perhaps a chain-store shop that goes through a complete 'refresh' (including DBs) every few years and little if anything alters in between.

I'd suggest that usually it's a matter of contract with the customer - If things are so tight that the customer would have to pay for a transformer upgrade before plugging in the Christmas lights next year, they'll soon see the economic advantage in planning ahead a bit. Likewise the customer should know what future plans they have - and how much 'spare' cash they can find at the moment.

As for contractors - they should be doing exactly what the contact states - certainly not providing less just in order to boost their own profits.

- Andy.
 04 January 2017 01:43 PM
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Matt01

Posts: 211
Joined: 25 July 2008

Its a high-rised residential building. We have applied the appropriate diversities through the design but tis the spare capacity figure we add thats raised the question.
 04 January 2017 08:35 PM
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sparkingchip

Posts: 9974
Joined: 18 January 2003

What do you think you have may have left out of the specification that may need to be fitted later?

Andy
 04 January 2017 10:43 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

Diversity, spare capacity and how much allowance for the day it all lines up wrong, is a tricky one - do you expect the building use to change much over time?

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regards Mike
 04 January 2017 11:41 PM
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alancapon

Posts: 6883
Joined: 27 December 2005

Originally posted by: Matt01
. . . I have a contractor trying to reduce the amount of spare capacity on a Transformer which would enable them to use a lower rated TX hence saving them money, but affects its future proofing. . .

What was written in the Contract Document that the contactor is working to? If there is nothing about how to size the transformer, and specifically the amount of spare capacity that should be left, you may be a bit stuck. In the DNO world, it would be usual to work out the maximum load including diversity, then choose the next largest standard size.

Regards,

Alan.
 05 January 2017 10:07 AM
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peteTLM

Posts: 3715
Joined: 31 March 2005

Also, unless you own the transformer, any spare capacity you may have allowed for will be flogged off to the customer down the road by the DNO to boost profits at your cost.

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

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