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Topic Title: 4mm Radial t+e Current Carrying capacity
Topic Summary: Clarrification on situations where 4mm t+e can be supplied by 32A Breaker
Created On: 02 November 2017 09:06 AM
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 02 November 2017 09:06 AM
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Philgb4

Posts: 18
Joined: 18 October 2017

Hello Again Guys,

i'm sure this is a topic that has come up many times and it is something i am consistently coming across.

Everyone loves the opportunity to wire a 4mm radial instead of a 2.5mm Ring it seems.

When completing EICR's i seem to be finding lots of 4mm Radials that have been installed as additions/alterations.

My understanding of 4mm Radials is this:

The CCC given in Table 4D5 for clipped Direct is 37A - Assuming ambient temperature is low and little grouping factors this would be sufficient.

However, many of the sites i am reporting on have huge runs through trunking and Large grouping factors.

Though tables 4D5 does not supply a ref method B - I then tend to use 4D2A for reference method B only.

Of course though, 4D2A states 30A as Maximum CCC before any other factors have been taken into account.

My question is, why are so many Contractors installing these 4mm Radials in these circumstances protected by a 32A Circuit Breaker?

This is another issue i have been putting down as Insufficient Overload Protection.
The thign that really bugs me is in 95% of instances they could have quite easily dropped down to a 25A Circuit breaker, where the extra load isn't required and it would be sufficient.

Anyone care to shed some light?
 02 November 2017 09:49 AM
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Christofer

Posts: 11
Joined: 11 September 2017

Hi Phil

Grouping factors are really for cables of similar size and loading, something that in a domestic installation is pretty unlikely.

One way to look at is, lets say you have ten circuits leaving the Consumer unit, the sum of all the CCC of the cables could be 200 amps. The probable maximum demand of a domestic will be much lower, say 40 amps, so most cables will have have plenty of thermal capacity to soak up that heat without need to derate.

Don't forget that certain circuits may be omitted dependent upon load, probably all lighting circuits and in reality most power.

There are a few publications which cover adhoc loading and sizes with regards grouping factor, BS7671 doesn't really go in to much detail.
 02 November 2017 09:57 AM
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BPRC13

Posts: 11
Joined: 23 April 2017

Hi Phil,

My understanding is as follows with regards to grouping factors:

Table 4C1, note 9 states 'if, due to the known operating conditions, a cable expected to carry not more than 30% of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group'. This may explain why, despite a large number of grouped cables, that a reduced grouping factor (or none at all) has been applied.

Say 6no. 4mm2 cables grouped together.
CCC of a 4mm2 (clipped direct) = 37A
Grouped rating (ref method C): 0.3x0.72x37=8A
If each cable is drawing less than 8A then grouping may be discounted.

When considering trunking I would expect to see singles (rather than T&E). So;
Say 6no. 4mm2 circuits (singles) in trunking.
CCC of a 4mm2 (table 4D1A), ref method B = 32A
Grouped rating (ref method B): 0.3x0.57x32=5.472A
If each cable is drawing less than 5.472A then grouping may be discounted.

It does appear that trunking & T&E @ 4mm2 and a 32A protective device are incompatible at the get go!

Interested to see what others thoughts are too.

Regards,

BPRC
 02 November 2017 10:17 AM
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mapj1

Posts: 9553
Joined: 22 July 2004

Mainly because it is a recognised final circuit configuration and mentioned in things like the OSG - and like the ring in 2.5mm, the occasional overload that is possible in theory seems after many years of experience to either never occur, or when it does to do no harm.

As such an awful lot of the time, very little thought is applied, the cable is just rolled out.
And before we get all upset about such an approach, by and large, things actually work just fine.

A similar thing with the cables in a ring, when 2 pieces of 2.5mm go down the wall to a socket, they are in reality grouped, in terms of heating each other, but by dint of not being another circuit, seemingly magically exempted from the official de-rating calculation.

Personally I prefer a centre fed radial, as the load is likely to not be single sided, but other folk tend to assume I was trying for ring and somehow failed,or they then try and join the ends up...

-------------------------
regards Mike
 02 November 2017 10:32 AM
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broadgage

Posts: 2432
Joined: 07 August 2007

I would summarise by saying that under ideal conditions, that 4mm twin with earth is fine on a 32 amp circuit.

Under most real world conditions, de-rating due to grouping, ambient temperatures, thermal insulation, and voltage drop limitations are likely to make 4.00 mm unsuitable for a 32 amp circuit, especially in non domestic situations.

De-rating for grouping is often less important than it sounds on a domestic or similar sized install, remember that the cut out fuse limits the total sustained current to 100 amps, so you cant have more than 3 fully loaded 4mm cables on a 100 amp DNO supply.

Similar arguments apply in SOME large installations if sub-mains are fused at 100 amps or less.

In other cases though there is the chance of significant overheating.
I have for example seen dozens of 4mm 32 amp circuits emanating from a 3 phase board with a 200 amp supply.
With the load spread evenly and the full 200 amps being utilised, the load per circuit was under 10 amps.
If however the load was concentrated on say a dozen circuits then it could have ended badly.
 02 November 2017 10:43 AM
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AJJewsbury

Posts: 15990
Joined: 13 August 2003

I agree about grouping in simple domestic situations (excluding things like night storage heaters) is usually totally ignored in practice - with the average draw (according to DNOs) of about 10A per house, and most high power domestic appliances (cookers, showers) drawing full load for a relatively short length of time (compared to the warm up time of their cables) it's relatively easy to justify in most cases.

On a similar note I did struggle slightly when using 4mm2 BS 8436 cable on 32A radials installation method B (no sneaking off to table 4D5 for circular cables of course) - and ended up justifying it, as it was XPLE insulated, by taking the 90 degree ratings and downrating to 70 degrees - which conveniently (if I've done it right) comes out at 32A precisely.

- Andy.
 02 November 2017 10:54 AM
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Philgb4

Posts: 18
Joined: 18 October 2017

Originally posted by: Christofer

Grouping factors are really for cables of similar size and loading, something that in a domestic installation is pretty unlikely.


Sorry Guys, forgot to mention these Eicr's are taking place in large commercial units with an average of around 40+ circuits, with 90% of these circuits fed through trunking.

Originally posted by: broadgage

I would summarise by saying that under ideal conditions, that 4mm twin with earth is fine on a 32 amp circuit.

Under most real world conditions, de-rating due to grouping, ambient temperatures, thermal insulation, and voltage drop limitations are likely to make 4.00 mm unsuitable for a 32 amp circuit, especially in non domestic situations.


I would tend to agree with you broadgage.
i personally thinnk that most of the work in these units is poorly engineered and completed.

It seems to be a get in and get out job.

if in a domestic property and clipped i would say that a 4mm radial would be sufficient.

but in this instance i would recommend it would be 6mm or protected by a 25A circuit breaker.

When considering trunking I would expect to see singles (rather than T&E).


I would agree with you here, singles would be common practice for trunking, however i think the contractors completing these jobs are more interested in speed in the essence of keeping costs low to win the jobs.
 02 November 2017 12:02 PM
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mapj1

Posts: 9553
Joined: 22 July 2004

Sorry Guys, forgot to mention these Eicr's are taking place in large commercial units with an average of around 40+ circuits, with 90% of these circuits fed through trunking.


Then some real tests will be needed to see how hot it actually gets, and if the assumption that load diversity will sort it out, that may have been conscious design decisions, but probably in reality were not considered at all, are actually justified.

The maximum temperature non-resetting colour stickers are very good for waving under peoples noses in such case.

" at some point in the last 2 weeks the centre of this bundle of cables reached a maximum temperature over 80 degrees" is far more useful than ' the ducting looks a bit full for my liking'.
Or of course
"During tests over the last 2 weeks the highest temperature ever recorded was 40 degrees, we do not consider this to be at risk of heating due to overload"
Report conclusions changed depending on the colour they change to.

-------------------------
regards Mike
 02 November 2017 12:19 PM
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Philgb4

Posts: 18
Joined: 18 October 2017

Thanks Mike, i do try and stick very much to the CCC tables + correction factors rather than actually look at the temperature of the cables, none of my electricians on site have reported cables hot to touch as of yet.

i think the silly thing about it is most of the items they tend to supply would never need 32A, so why the need for the 32A Circuit Breaker? surely they should have just installed a 25A and this would allow the cable to be sufficiently protected anyway.
 02 November 2017 01:28 PM
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broadgage

Posts: 2432
Joined: 07 August 2007

25 amp MCBs are far less common than 32 amp units, and for some less experienced persons, anything other than 6 amp, 10 amps, 16 amps, 20 amps or 32 amps is outside their comfort zone and therefore avoided.
 02 November 2017 01:32 PM
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dustydazzler

Posts: 1445
Joined: 19 January 2016

Slightly off topic
But in recent years I am noticing a very common trend in domesticated new build
The 20a Rfc
 02 November 2017 01:34 PM
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geoffsd

Posts: 1756
Joined: 15 June 2010

R stands for Radial as well as Ring. Pet hate - sorry.
 02 November 2017 02:01 PM
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dustydazzler

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Joined: 19 January 2016

 02 November 2017 02:48 PM
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mapj1

Posts: 9553
Joined: 22 July 2004

well you could do both a ring and a radial in 2,5mm in mildly different conditions, so converting a 2.5mm RFC into a 2,5mm RFC is perfectly possible.


Perhaps it should be RiFC or RaFC, to avoid ambiguity.

Like Geoff B's use of TNCS-s and TNC-s to indicate if the DNO cable is pure CNE or mixed CNE/SNE, and almost everyone using Dy and Yy to denote winding patterns for HV to lv transformers.
We can make it up as we go along, if it sticks it will become the forum standard.

(as the word 'badger' is now a verb on this forum since the summer ,thanks in part to a spoof loo roll advert./)

-------------------------
regards Mike
 02 November 2017 03:20 PM
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Philgb4

Posts: 18
Joined: 18 October 2017

Well thing 'RFC' nonsense just got a whole lot more confusing....
 02 November 2017 03:22 PM
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Philgb4

Posts: 18
Joined: 18 October 2017

Originally posted by: broadgage

25 amp MCBs are far less common than 32 amp units, and for some less experienced persons, anything other than 6 amp, 10 amps, 16 amps, 20 amps or 32 amps is outside their comfort zone and therefore avoided.


I make you right mate.

everytime i put together a list from suppliers and request 25A circuit breakers - i always seem to get 32's.
really got the hump with it after a while.
then again when they dont have B types, they give me C types and vice versa.
it almost feels like the wholesalers are engineering the whole project themselves.....
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