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Topic Title: Cable calculations to BS7671:2008
Topic Summary: Anyone like them, want to practice some?
Created On: 24 March 2008 03:18 PM
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 24 March 2008 03:18 PM
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bakey1959

Posts: 767
Joined: 24 October 2006

I would appreciate some reviews of the following cable selection procedures taken from BS7671:2008, and if anyone is so inclined, your results of three examples at the end.

If anyone wants it in word format private email your address and I will forward (better layout than this pasted version).

CURRENT-CARRYING CAPACITY AND VOLTAGE DROP FOR CABLES AND FLEXIBLE CORDS


The two basic calculations that determine the size of a cable to be used for a particular purpose are:

1. the current rating ( Iz ) of the cable under defined installation conditions

2. the maximum permitted voltage drop as defined in BS7671:2008 by Section 525 and Appendix 12.

Appendix 4 details the current carrying capacities,
correction factors and voltage drop for cables and flexible cords.




The factors which influence the cable selection include:

1. the design current ( Ib ) or rating of the equipment to be connected, and

2. the rating of the circuit protective device ( In ) , and

3. the tabulated current carrying capacity ( It ) of the cable, and

4. the installed current carrying capacity ( Iz ) of the cable, taking into account any correction factor(s) (C)



BS7671:2008 requires that: Iz ≥ In ≥ Ib






Procedure for determining the size of cable to be used.

Current carrying capacity

Divide the rating of the selected protective device ( In ) by any correction factors (C) and select a cable from Appendix 4 that is not less than this value ( It ).

It ≥ Current rating of protective device (In)
Any applicable correction factors (C)

CORRECTION FACTORS REFERENCES
Symbol Description BS7671:2008

Ca
The ambient temperature correction factor Appendix 4

Table 4B1 ( in free air )
or
Table 4B2 ( buried )

Cg
The grouping correction factor. Appendix 4

Table 4C1, 4C4 or 4C5 (in free air)
or
Table 4C2 or 4C3 ( buried or in ducts )

Cc
The protective device correction factor. Appendix 4

Section 4 gives 0.725 for
BS3036 'rewireable' fuses.


Ci
The correction factor for cables embedded in thermal insulation Regulation 523.7 - Table 52.2
Length in insulation Divide by
50mm 0.88
100mm 0.78
200mm 0.63
400mm 0.51
500mm or above 0.50


Voltage drop calculation

For 230v installations fed from a public supply the maximum permissible voltage drops are 3% (6.9v) for lighting and 5% (11.5v) for other uses.

Voltage drop = mV x Ib x L where: mV is the millivolts per amp per metre taken from tables 4D1A to 4J4A, and
Ib is design current of the circuit, and
L is the cable length in meters.

Anyone interested in working out the minimum cable sizes for the following examples?

1. A circuit has a 14kW heating load fed at 230v. The cables are thermosetting (90 degrees C) singles having copper conductors enclosed in conduit, fixed on a wall and run over a length of 36m. Overcurrent protection is provided by a BS88 fuse.
The ambient temperature is 40 degrees C and the circuit is grouped with two others of the same size, all circuits being loaded above 30%. The circuit does not run through thermal insulation.

2. A 2kW commercial lighting load is supplied at 230v. The cable length is 40m and the cable is PVC/PVC flat twin (thermoplastic 70 degree C). Overcurrent protection is provided by a Type B BS EN 60898 MCB. The ambient temperature is 30 degree C and the cable passes through and is surrounded by thermal insulation for a distance of 200mm. The cable is grouped with one other cable of the same size throughout the run and both circuits are loaded above 30%.

3. A 6.5kW water heater is fed from a domestic 230v supply using PVC/PVC flat twin (thermoplastic 70 degree C). The cable is 20m long and is protected by a BS3036 fuse. The ambient temperature is 35 degrees C and there is no thermal insulation or grouping.
 24 March 2008 03:59 PM
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GaryMo

Posts: 1213
Joined: 09 May 2007

1 - 16mm
2 - 2.5mm
3 - 6mm

Preparing to be shot down!

[EDIT] Keep up the good work. I for one would like to see more examples like this if people have any spare time.

Edited: 24 March 2008 at 04:08 PM by GaryMo
 24 March 2008 05:17 PM
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bakey1959

Posts: 767
Joined: 24 October 2006

Originally posted by: GaryMo

1 - 16mm
2 - 2.5mm
3 - 6mm

[EDIT] Keep up the good work. I for one would like to see more examples like this if people have any spare time.


Spot on 3 out of 3. If you care to write up your working outs and email me, I'll check against mine. As others may want to have a go it is probably too early to post solutions.
 24 March 2008 06:19 PM
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GaryMo

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Sent you a PM for your email address.
 24 March 2008 07:44 PM
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bakey1959

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Just PM'd back.
 25 March 2008 04:13 PM
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bakey1959

Posts: 767
Joined: 24 October 2006

Hi Gary

Very well done!

Just one minor disagreement...

Questions 2 and 3.

You selected the cable from Table 4D2A, Table 4D5 is the one intended for 'Flat' PVC Sheathed/ PVC Insulated cables (Twin & Earth).

Thanks for pointing out missing installation method in question 2, it would help wouldn't it

Edited: 25 March 2008 at 04:14 PM by bakey1959
 25 March 2008 05:22 PM
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GaryMo

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Cheers Chris.
I'm a little embarrassed to say but I've always used 4D2A for twin and earth
Not any more though
 25 March 2008 07:55 PM
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bakey1959

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There see, you've now gained an extra amp with your 6mm T&E
 25 March 2008 09:48 PM
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hertzal123

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Bakey,
Is it true,for a fixed load such as heater not subject to overload,factor for 3036 fuse need not be used.Also if all factors do not apply along the whole cable route,ie thermal insulation,do you only apply the factor that gives the highest current carrying capacity.(from Trevor Marks Guide)
Regards,Hz
 25 March 2008 10:22 PM
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bakey1959

Posts: 767
Joined: 24 October 2006

Originally posted by: hertzal123

Bakey,

Is it true,for a fixed load such as heater not subject to overload,factor for 3036 fuse need not be used.Also if all factors do not apply along the whole cable route,ie thermal insulation,do you only apply the factor that gives the highest current carrying capacity.(from Trevor Marks Guide)

Regards,Hz


So here what you would now be referring to in Q3 is 433.3.1 (ii) in BS7671.2008, Yes. So apply the 0.725 factor to socket circuits, lighting circuits, cooker circuits, etc., but not an immersion, ashower, or such fixed loads that "because of the characteristic of the load...is not likely to carry overload current...".
On the second point 523.7 and Table 52.2 gives the derating factor to be applied to the situation.
 26 March 2008 09:54 PM
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AJJewsbury

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BS7671:2008 requires that: Iz ≥ In ≥ Ib

Strictly speaking, I don't think that's literally true.

In ≥ Ib - yes

Iz ≥ Ib - yes

but Iz ≥ In ≥ Ib would mean that you're requiring Iz ≥ In which is only required if the device is required to protect the cables against overload - sometimes this isn't needed - for example - industrial motor circuits where overload protection is at the motor end, or even the humble unfused spur in a domestic environment - both of which BS 7671 permits.

- Andy.
 27 March 2008 06:11 PM
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bakey1959

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Yes Andy, your right. I should really be using It instead of Iz (as in Appendic 4), and then consider the Iz/In relationship if overload protection is afforded by the device.
 24 September 2015 03:59 PM
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mrshr

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Joined: 24 September 2015

Hi,
Appreciate this is an old thread but would be grateful of solutions.

I've got 1 and 3 correct but struggling with number 2.
Wouldn't mind seeing the solutions to all of them though, just to make sure I did use the right method/tables etc. for 1 and 3.

Many thanks,
shr.
 18 October 2017 11:12 AM
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Philgb4

Posts: 18
Joined: 18 October 2017

Hi Guys,

Hope you don't mind me jumping in to this one and starting it up again.
wonder if you could help point me in the right direction.
i'm currently completing EICR's for a large commercial retailer and am constantly finding issues with i believe is insufficient overload protection for circuits.
the most consistent one is 70c pvc/pvc 1.5mm t+e protected by a 16A device - I've been ruling these as unacceptable on the basis that:
Most of these circuits are supplied via trunking with multiple circuits (20-30) also fed via the same trunking route (the question of the 25% spare capacity is nearly always apparent!). ambient air temperature is around 25 c.

please feel free to slander my calculations below, i'm looking for any observations or advice here.

so based on 20 circuits grouping, my calculations are as follows

16
------------
(1.03x0.38)

therefore 'It' is more than or equal to 40.87

which would mean i should be using 10mm cable (based on table 4D2A, yes i know i should be using 4D5 - but this does not give a ref method B)

is this correct? this sounds insane.
Am i doing something wrong here?
 18 October 2017 11:46 AM
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AJJewsbury

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Joined: 13 August 2003

is this correct? this sounds insane.
Am i doing something wrong here?

Mathematically you're probably on the right track. There may be more to the problem though...

Is In really Ib? Unless all the circuits are feeding uncontrolled loads such as socket outlets Ib could be significantly less than 16A - which could reduce your numbers significantly (presuming the circuits aren't liable to simultaneous overload)

Do all the circuits regularly draw the full 16A simultaneously? If not your calculations may be over-estimating the problem, perhaps significantly. If you've got 30 16A circuits running flat out originating from the same DB, it would need to be rated at at least 160A/phase if evenly spread over 3-phases (or 480A single phase!) - if you're DB or its supply is rated less than that (and isn't overloaded) then the problem can't be as big as your numbers would suggest.

- Andy.
 18 October 2017 01:02 PM
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Philgb4

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Thanks for the help Andy.

That's a good point - of course the grouped circuits are varying in load.
e.g. lighting circuits, socket radials and rings, and other equipment.
how would i then calculate this baring in mind the time it may take me to calculate the design current of all the grouped cables is ridiculous.
Of course Bs7671 table 4C1 Note 1 states ' these factors are applicable to uniform groups of cables, equally loaded'
though are there any provisions taken of other circuits which may not be uniformed or equally loaded? should other 'non-uniformed' or 'unequally loaded' circuits then be disregarded?

As a rule of thumb i have been suggesting that any 1.5mm t+e cables protected by a 16A breaker is insufficient and have recommended changing the protective device to a 10A (if design current allows) or rewiring to 2.5mm t+e.

but of course this calculation completely throws this off.
 18 October 2017 01:46 PM
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Parsley

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Are there any signs of overheating?Are the cables in the trunking hot to touch?
If you're that concerned maybe suggest some thermal imaging is carried out, or install some max temperature stickers on the T&E in the trunking.
Suggesting the installation needs rewiring or protective devices could be embarrassing and costly if you're proved incorrect.
I guess you've already asked the client for the original design information.
As Andy stated for fixed loads you can use the design current instead of the PD rating.

Regards
 18 October 2017 02:49 PM
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AJJewsbury

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Of course Bs7671 table 4C1 Note 1 states ' these factors are applicable to uniform groups of cables, equally loaded'
though are there any provisions taken of other circuits which may not be uniformed or equally loaded? should other 'non-uniformed' or 'unequally loaded' circuits then be disregarded?

No don't disregard grouping for non-uniform groups, just bare in mind that the maths behind the grouping factors assumes cables are all the same - if they're not the results in theory would be a little different - but no-one could say how much precisely without knowing exactly what cables you have and in what arrangement (of which there could be an almost infinite number of combinations, so they couldn't tabulate them even if they wanted to). I suspect that most people just use the published table and as long as they're a little leeway left somewhere (e.g. by having to go up to the next standard cable size or ambient being a bit below 30 degrees) don't worry about it too much.

how would i then calculate this baring in mind the time it may take me to calculate the design current of all the grouped cables is ridiculous.

The correct way would of course be to reckon up each individual load - or better still refer back to the original design (cough cough). You might get some 'back of the envelope' figures by looking at the supply to the DB though - either by rating of its supply protective device or by clamping and logging over a typical day for instance - on the basis that you can't have more current coming out than going in that might give you a rough idea of how many '16A equivalent' circuits you should be thinking about. E.g. if it was a 63A single phase supply - grouping shouldn't be any more onerous than for 4 fully loaded 16A circuits, or one 32A and two 16A or whatever combination seems most sensible in the circumstances. It won't give you an answer accurate to two decimal places or anything like that but it might give you an idea of whether to panic or relax.

- Andy.
 18 October 2017 04:05 PM
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Philgb4

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Thanks for the help Guys.

Fortunately the client isn't too interested in checking up on our recommendations - that being said it is in my interest to be as open and as accurate as possible.
I am unable to confirm if any cables are hot to touch - as of yet my electricians have not reported this, so i would have to say no the for time being.

that is a good way to look at it Andy - of course clamping the cable would give me a good idea of what current is being used on the circuits - the only difficult bit would be that many of the stores we are looking at are so poorly maintained that there are so many light fittings not currently working for various reasons and even light hidden away in ceiling voids where a refurb has been completed on top of the current installation - of course i could work out the difference but to do with for every store in every instance would be incredibly time consuming for me as well as the electricians completing the tests.


I suspect that most people just use the published table and as long as they're a little leeway left somewhere (e.g. by having to go up to the next standard cable size or ambient being a bit below 30 degrees) don't worry about it too much.



I would say that most of these installations do just seem to take into consideration tabulated value rather than there being evidence of any calculations completed at all.

of course my biggest concern is ensuring that the cables are sufficiently protected.

Generally, i am observing points served, with the type of fittings installed which allows we to calculate an estimated load of that circuit - in regards to grouping factors becomes complex though as we have observed but with an estimated guess i am going under the tabulated values and generally ensuring all 1.5mm t+e is protected by a max 10A device and 2.5mm t+e is protected by a max 16A device.

Once again, thanks for your advice.
 18 October 2017 10:41 PM
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mapj1

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The other factor to bear in mind is that even if the copper cores in the cable for a few metres in the middle of a tight part of the bundle does indeed get hotter than 70 degrees from time to time, yes, it is indeed non regs compliant, but not always immediately a safety disaster.
not sure if you have seen this but the gist is that running hot ages the cables faster, but short duration overloads are pretty benign.
To an extent how serious such overheating actually is also depends on the design life - cars for example where the wiring only needs to last for a few years and a few hours a day, permits much hotter cable ratings for the same sorts of plastic insulation, and the equivalent of 35 A or more on 2.5mmsq, and tightly laced bundles of many cables.
As commented above the thermal stickers are a good investigative technique, and can inform where the real hot spots are, allowing a targeted response, rather than a lot of rip up and rewire than may not all be needed.

-------------------------
regards Mike
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