6mm t&e domestic on 40a MCB ??

[zsoltimatrix]

Hi guys,

I can not decide to fail this or not:

6mm t&e in domestic environment, cables in mini trunking, on a 40a MCB

I would use 10mm of course, but am seeing this in many flats so is it acceptable or its a fail on the EICR?

Tank you!

 

[mark28118]

Taking it its a twin and earth cable ? No it can only carry 34A mate. MCB needs down-rating or cable up-grading to a 10mm to comply.

Page 276, BS 7671, reference method B

 

[zsoltimatrix]

This was installed 5 years ago, they did the same in about 50 flats. NICEIC company and all...

 

[telectrix]

Taking it its a twin and earth cable ? No it can only carry 34A mate. MCB needs down-rating or cable up-grading to a 10mm to comply.

Page 276, BS 7671, reference method B

38A to be exact. but OP doesn't say what the load is. if a cooker, i would fot a 32A MCB. if shower, would have to check the KW.

 

[zsoltimatrix]

Does the fact that it is on an RCD changes anything?

 

[Edd]

Does the fact that it is on an RCD changes anything?

No as RCDs wont trip on over current, so you could quite easy drag 40amps through the 6mm and the RCD wont care

 

[telectrix]

nor would a 32A MCB . need to know the load current to make informed comment.

 

[zsoltimatrix]

nor would a 32A MCB . need to know the load current to make informed comment.

Load current is around 35A.

But the load current can change, somebody can replace the shower with a bigger one.

You would not put a 2.5 mm to a 32a breaker even if it would only supply one single socket, would you? Load current can change anytime.

 

[telectrix]

true. 35A is close to the max. for 6mm in trunking. i'd be inclined to recommend a 32A (type B). that would never trip at 35A. designer has probably used ref. method C , giving him 46A for the 6mm. end of the day, it's your call.

 

[malcolmsanford]

It should really be coded a C2 on an EICR and so regarded as an unsatisfactory as a report

 

[Paul.M]

"But the load current can change, somebody can replace the shower with a bigger one."

Anybody changing a shower over to a larger load must check if the mcb and cable and iso switch is appropriate before any works commence, therefore not your problem at the time of you doing your work.

If its pulling 35A at flc its a 8kw shower. 6mm cable, put it on a 32A mcb because it will never be run at full temp for an hour to trip the mcb.

 

[telectrix]

my thoughts exactly . 32A MCB is the cheapest option, but, as malc. said, a code 2 will CYA. however, BS7671 does not allow a ref. method B for T/E ( enclosed in trunking), so designer has probably used ref. method C.

 

[Paul.M]

my thoughts exactly . 32A MCB is the cheapest option, but, as malc. said, a code 2 will CYA. however, BS7671 does not allow a ref. method B for T/E ( enclosed in trunking), so designer has probably used ref. method C.

Table 4D2A or am I wrong? (38A btw)

 

[telectrix]

table 4D5 for flat sheathed cable

 

[Paul.M]

table 4D5 for flat sheathed cable

Yes it is but as you've already said flat t+e method B isn't listed? If you compare the values from both tables, flat t+e is the same throughout all ref methods (apart from clipped direct, t+e disperses heat better).

Because table 4D5 doesn't list method B I'll take it from table 4D2A as the closest comparison and I'd stand in a court room to defend it. The IEE must think that no one puts t+e in trunking lol.

Just thought I'd explain why I said what I said so it doesn't mislead anybody by looking at the wrong table.

 

[telectrix]

yeh. i'd probably use 4D2A and ref. B to be on the safe side. wonder what the IET's take is.

 

[zsoltimatrix]

It should really be coded a C2 on an EICR and so regarded as an unsatisfactory as a report

Yes, I did exactly that. Somebody will have to back and change the mcb.

Thanks guys!

 

[G_FUNK]

Went to look at a fuse board change today the is a 8kw shower on a old 6mm t & e cable run is just under 10 mtrs its on a 30 bs3036 now. When i change the board i shall put it on a 32amp type b 60898 as the shower wont be ran at full load for long if ever ran at full load at all. The cable are surface in the airing cupboard and then under the floor above a plaster board ceiling with no thermal insulation. I shall make a note on the eic that if the shower is changed for a larger load then the cable will need to be up sized. If i came across a 6mm cable on a 40 amp breaker then i would be inclined to change it for a 32amp breaker. I would treat it the same as 1.0 or 1.5mm domestic up stairs lighting circuit on a 16 amp breaker.