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Topic Title: protective device and cable size
Topic Summary: 2.5mm onto 6mm - omitting overcurrent/fault current or not
Created On: 02 November 2012 11:00 PM
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 02 November 2012 11:00 PM
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Posts: 388
Joined: 18 October 2010

hello - its been a while and i fear i am going over old ground (having read previous posts) - even so i feel it is better to seek clarification and advice and hopefully then be better off for it and folks here do usually help me unravel my 'demons' :-)

situation from some work on an existing circuit thus described:

30A type2 MCB in DB protecting 6mm t&e to an immersion heater rated 3kW (oh and its flex connected on to the 6mm by a FCU with 13a fuse present).

I spied connected to the supply side of the FCU some 2.5mm t&e which ran in trunking for ~2m length to one single [13a] socket.

Then, another 2.5mm (0.5m length) came out of said single socket to a FCU (with 5a fuse) which ultimately, via 1.5mm, supplied a low powered extractor fan; all delivered via trunking.

This set up I am led to believe has been like this many years, was installed and signed off etc (though there isnt any paper work available - awol during the passing years). Zs @ fan was 0.56.

I got 'possessed' by the issue of the 30a mcb and 2.5mm combo and the regs with regard to this seemingly being a case of omitting suitable overload protection for the 2.5mm. Fault protection being covered.

In my mind I had been able to 'exorcise' the 1.5mm from the equation as the FCU takes care of that. I 'exorcised' the 6mm obviously and was left with the 2.5mm/30a overload demon due to the single socket it supplied.

however, I walked away from the 'halloween horror' convinced that it was indeed ok (if not the best design arguably) as anything plugged in is not likely to cause a dangerous overload and would have its own protection (max 13a) in any case.

Is my assessment reasonable and if not, please correct/advise?

Thank you all and regards
 02 November 2012 11:28 PM
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Joined: 19 April 2005

Hi Chris
I see what your getting at, but I dont think its a major safety issue.
Easiest option would be to change the mcb for a 20A. The 6mm will have been nicked from the unwanted electric shower no doubt by the plumber.

Resistance is futile.
 03 November 2012 10:14 AM
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Joined: 15 April 2005

The easiest option would be to do nothing and stop worrying.

You've answered your question already in your penultimate sentence

The 2.5mm cable is load-limited by a 13A socket.

S George
 03 November 2012 03:01 PM
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Posts: 388
Joined: 18 October 2010

its always good to consult one's peers. thank you for the replies.

im not always the brightest spark (if ever lol), but do try to give some careful thought to what goes on and understand why etc.

read a few threads on here and saw a few others seemingly 'confused' by this scenario - well actually it was about the 2.5 spur at the origin of a 32A ring etc.

my trouble is, i always overthink perhaps! duh.

all the best

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