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Topic Title: cable rating
Topic Summary: appraisal appreciated
Created On: 02 September 2013 08:42 AM
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 02 September 2013 08:42 AM
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psychicwarrior

Posts: 220
Joined: 18 October 2010

might seem a basic question, but I would appreciate some comments from folks that have a better understanding on cable ratings and temperature effects and protective device ratings.

taking a domestic situation and the [unfortunate perhaps] employ of Ref. Method 'B' for a couple of short lengths (light-weight trunking) on a kitchen radial in 4mm2 pvc/pvc t&e:

- 'ordinarily', one could protect this by using a 32A MCB
- Ref.Meth. B in Regs. suggests a cable rating of only 30A (but it is suggested that cables ratings may be increased in certain circumstances)

so, to be compliant as such and if Ref B cant be avoided:

- is the only recourse to reduce the MCB to 20A and potentially lose some capacity (for want of a better phrase) ?

- or increase to 6mm2 which seems a little impractical and perhaps a waste of resource for a standard socket circuit ?

i think im asking whether one could compliantly (with regard to Regs and certification etc) use a 32A breaker in this 'Ref. B' scenario - perhaps because in reality, diversity, expected use and cable rating temperatures mean there isn't an issue - or is it that to be compliant it is as it is (or choose a different method etc) ?

as an aside, the intervals of MCB ratings mean that a 25A for some manufacturers is not available, or that may be a reasonable compromise.

what drew this to my mind was the seemingly somewhat 'inbetweeness' that Ref. Method B creates - as if avoid it.

i had posted a version of this earlier, but it was longer winded and so i've tried to simplify it.

thank you
Habs
 02 September 2013 09:47 AM
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AJJewsbury

Posts: 11251
Joined: 13 August 2003

I know exactly where you're coming from. I had the same problem justifying using 4mm2 on a 32A radial using BS 8436 cable. With T&E you usually get an extra amp or two using table 4D5 rather than 4D2A, but method B is conspicuous by its absence from 4D5 - which makes you think there might be a bit of a margin there if you're using T&E, but it's hard to prove.

In practice (domestic, intermittent loads) the occasional slight overload of the cable is unlikely to make much significant difference - if the lifetime of T&E cable starts off at 70-odd years the occasional 10 minute overload of a couple of amps isn't going to make a significant difference.

In my case, in the end I proved it (at least to my own satisfaction) by starting with the 90-degree rating (the BS 8436 cable I'm using has XLPE insulation) and de-rating it by calculation to 70-degrees - which came out at 32A rather than 30A for method B. (I think that's justifiable as XLPE has a lower thermal resistance than PVC.) I can't help feeling it shouldn't be that difficult though.

- Andy.
 03 September 2013 07:25 PM
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psychicwarrior

Posts: 220
Joined: 18 October 2010

thanks Andy. i'd already spotted the 'extra amp possibility ....... i just feel its a rather daft situation. as you say..... feeling that it shouldn't be that difficult...... its a big waste whichever way i look at it.

so....... i wonder if any one else has any views !?
 03 September 2013 11:44 PM
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peteTLM

Posts: 3119
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It a margin.
Anyway your 32A MCB is 32A at 30'c, at 20'c its 33.5A.........

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----------------------------------------
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Every man has to know his limitations- Dirty Harry
 04 September 2013 09:05 AM
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psychicwarrior

Posts: 220
Joined: 18 October 2010

@peteTLM

i am presuming you have applied a correction factor to the ocpd rating in that...... i wasn't aware/hadn't considered that effect - is that how it works with ocpd's ? as in same as current carrying capacity ?

if that is the case, then just as the temp comes down and current carrying goes up for cables, so it does for ocpd's.....no gain there then

as anyone can see, Ref.M.' B ' states 30A at 30/70 Deg. C. - its only 2 Amps........ but the consequences are rather unsatisfactory looking at other options (assuming Meth B is required to be maintained). Shame really. I think I would put in a 20A and see what happens..............using 6mm2 is just well......... !

thats why i asked if any one had any better technical appraisal of ccc of cable in these circumstances to be able to compliantly use a 32A on a 4mm kitchen radial
 04 September 2013 10:07 AM
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iie62478

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How many socket outlets are there on this radial?

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Are we all in this together???
 04 September 2013 10:37 AM
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Parsley

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A couple of year ago I was asked to quote for remedials on a PIR carried out by an I&T specialist.
4.5KW over door fan heater wired in 4mm T&E in mini trunking, 32Amp type B MCB.
Code 1 observation by the specialist.

Regards
 04 September 2013 02:18 PM
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AJJewsbury

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If it's of any help this thread: http://www.theiet.org/forums/f...adid=36741&STARTPAGE=3 was my & OMS's discussion about using 90 degree cables at 70 degrees & the formula for calculating.

I suggest using 90-degree 4mm2 T&E (XLPE) - you've then got some wiggle room in terms of applying a 0.8 factor to 90-degree tables, or you could claim that the cable will at worst only run above 70-degrees in the enclosed (method B) parts of the run - while at the terminations where the sheath is removed it's like singles in conduit/trunking - table 4D1A applies and it won't exceed 70 degrees even at the full 32A.

- Andy.
 04 September 2013 04:29 PM
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psychicwarrior

Posts: 220
Joined: 18 October 2010

@Andy

thanks, i'll have a read

@iie62478

there are 6.

@Parsley

that i dont understand at all (i must be thick)
 04 September 2013 04:51 PM
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Parsley

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Sorry I probably haven't explained very well.

Single phase 4.5KW fan heater wired in 4mm T&E in mini trunking very short run, protected by a 32 Amp type B MCB. The I&T specialist had given it a code 1.

Regards
 04 September 2013 10:40 PM
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psychicwarrior

Posts: 220
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@parsley

sorry also i didnt mean i didnt understand you.............i couldnt sort out why the I&T person gave it a code 1 !
 05 September 2013 10:16 PM
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davezawadi

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Well we are at that point again. How much over 70 degrees could this cable be at 32A? Maybe 4 degrees which may shorten the life slightly but is of no realistic risk to anyone. So why the code one? Well the guy could be an idiot, or totally ignorant or looking for a pricey remedial or whatever. But he should certainly be barred from I&T!

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David
CEng etc, don't ask, its a result not a question!
 05 September 2013 10:38 PM
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Parsley

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Another code 1 was the lighting circuit that had 2 conductors terminated at the MCB and the lack of bonding to vent duct work at high level that was not extraneous not sure if that was a 1 or 2.

The company do not do remedial work only I&T.

Regards
 10 September 2013 10:46 AM
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psychicwarrior

Posts: 220
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just one further addition to my original posting:

unlikely perhaps, but if a cable manufacturer advised that their cable could carry more than that tabbed in the Regs, would that suffice ?
 10 September 2013 03:16 PM
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AJJewsbury

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unlikely perhaps, but if a cable manufacturer advised that their cable could carry more than that tabbed in the Regs, would that suffice ?

I don't see why not - provided they follow one of the methods listed in 523.3.
- Andy.
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