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Topic Title: Distribution Boards with 20% Spare Capacity
Topic Summary: What does it actually mean?
Created On: 12 February 2019 01:34 PM
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 12 February 2019 01:34 PM
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adammid

Posts: 153
Joined: 02 November 2010

Been having a debate at work with what the term 'all distribution boards shall have a minimum of 20% spare capacity' actually mean.

A) if 10 ways are currently used, would it be an additional 2 ways spare?(20% of 10 = 2)

or

B) does it mean 20% of the total capacity of the board is spare. For example if it was 12 way distribution board would there need to be minimum of 3 ways spare (20% of 12 = 2.4 therefore round up to the nearest whole number which is 3).
 12 February 2019 01:50 PM
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AJJewsbury

Posts: 17795
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The way it's phrased, I would have taken it to mean 20% of the board's capacity - i.e. at most 80% of the board's outgoing ways can be used initially.

But as it's not regulation, but presumably something from a contract or specification, you could always ask whoever originated the document to clarify.

- Andy.
 12 February 2019 02:04 PM
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mikejumper

Posts: 2810
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I have had architects plans in the past that stipulate 25%.

I agree with this approach, it seems sensible, but not always practical on a CU change due to space limitations.

I wouldn't want it as a reg. but I think strong guidance in this direction in the OSG would be good (if it's not already there).

Think of the grief it would save when additional circuits are needed.
 12 February 2019 02:25 PM
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geoffsd

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Of course, if at a later date, new circuits are added, you would presumably have to change the CU to maintain the 20% spare capacity.

Some guidance notes are just unnecessarily ridiculous..
 12 February 2019 02:44 PM
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broadgage

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Joined: 07 August 2007

I would say that it means 20% spare capacity in the number of ways, rounded up to the nearest whole number.

12 circuits needed=at least 15 circuits to be provided.

Beware two slightly different ways of calculating this, which could be clarified meaning either "20% of ways shall be left unused" or alternatively "20% additional ways shall be provided"

If an installation calls for 100 ways, then to provide 20% extra means a 120 way board.

Alternatively if the requirement is that 20% of the provided ways are to be left unused, then a 120 way board must have 20% left unused, that leaves only 96 ways available for use, and not the 100 ways that are needed. A 126 way board with 26 ways unused would be needed
A 126 way board might cost a lot more than 120 ways, though in many cases the same standard size will meet either requirement.

(improbably large numbers used for clarity)
 12 February 2019 03:23 PM
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dustydazzler

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I always try to leave atleast one spare way on each rcd / or 2 spares ways if rcbo board


This method has served me well
 12 February 2019 06:14 PM
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OMS

Posts: 22864
Joined: 23 March 2004

For me, I tend to consider it as 20% spare capacity based on any particular board or switchboard actually defined ways

So for example, I would expect it be the physical number of ways available for future use (anything beyond that are "blank" not "spare" )

It is as important that you also capture the spare "amps" required for those ways

One method is to sum up the design currents of all the designed ways, divide that by the number of designed ways - this gives you an average load per way - which you then add to the diversivied design current of the DB to size the capacity of the supply cable and upstream protection

The cable containment systems also need to be able to accept cables for those spare capacity ways - so again, that needs to be factored in to the design

Regards

OMS

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Let the wind blow you, across a big floor.
 12 February 2019 06:44 PM
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chrispearson

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Originally posted by: OMS

It is as important that you also capture the spare "amps" required for those ways ...


Yes, I don't think that it is as simple as leaving 20% of the ways empty, but I think that this would be a good starting point.

Clearly it is a sensible approach, but it may be best to supply MCBs because when the occupier wants to pop in a new circuit, the DB may have become obsolete.

If you have a fairly big property, it may be difficult to find a DB that has sufficient ways to leave 20% spare short of installing a double row one.
 12 February 2019 06:52 PM
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OMS

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The trouble with actually adding the MCB's is that potentially you don't know what you might need - although a design approach that has separate lighting, clean power and dirty power boards might tend towards you being able to leave a number of 10A MCB's in in the lighting boards and a mixture of 16A, 20A and 32A ways in the power boards

In a commercial situation, that might mean scavenging from Board A to have the correct ampacity for board B

This also helps with "load per way" which tend to become much closer to Ib when you are adding more power circuits

Spare capacity is just that, an ability for the installation owner to make reasonable changes (akin to the typical churn experienced in commercial installations and growth perhaps in a healthcare setting)

For a domestic, then you are probably starting from an assumption that the installation can deliver say 80A, so it really is probably only spare ways you want, rather than spare capacity

Horses for Courses, really

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 12 February 2019 08:24 PM
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mikejumper

Posts: 2810
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Originally posted by: chrispearson
If you have a fairly big property, it may be difficult to find a DB that has sufficient ways to leave 20% spare short of installing a double row one.

Duplex boards work quite well for this.
Very common in France where they need lots of ways for all their radials.
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