Two ring mains in a room

[binky]

… just have to see if I can find a like minded plumbers forum now!

Good luck with that .

This forum was originally started by a fella known as Weish Wizard, RIP, as a direct result of the abuse on the Screwfix forum. We have worked hard to keep it friendly on here, failed occasionally, but generally it works.

 

[Humourme]

Good luck with that .

This forum was originally started by a fella known as Weish Wizard, RIP, as a direct result of the abuse on the Screwfix forum. We have worked hard to keep it friendly on here, failed occasionally, but generally it works.

I remember the name

 

[Andy™]

Weish Wizard, RIP,

WTF did i miss?

 

[binky]

WTF did i miss?

Thought he had died, I could be wrong, my wife tells me I'm usually wrong

 

[Stevie G]

Mcb manufacturers have seen this trend as well hence more making 25a mcb for 2.5 radials.

Hi Mort2376
I'm now retired, but we were alway's taught that the MCB is to protect the cable, from drawing over current ,and cause a risk of fire .
2.5mm current rating was 18amp max in my day , so why would i use a 25amp MCB ? Has the IEE reg's changed .Can you give me the new regulation that allows this .
Thanks Steve

 

[Murdoch]

Hi Mort2376
I'm now retired, but we were alway's taught that the MCB is to protect the cable, from drawing over current ,and cause a risk of fire .
2.5mm current rating was 18amp max in my day , so why would i use a 25amp MCB ? Has the IEE reg's changed .Can you give me the new regulation that allows this .
Thanks Steve

IIRC depending on the installation method 2.5mm T&E can go up to 27 amps

 

[binky]

Hi Mort2376
I'm now retired, but we were alway's taught that the MCB is to protect the cable, from drawing over current ,and cause a risk of fire .
2.5mm current rating was 18amp max in my day , so why would i use a 25amp MCB ? Has the IEE reg's changed .Can you give me the new regulation that allows this .
Thanks Steve

it's in the cable rating tables.

 

[SPECIAL LOCATION]

2.5mm current rating was 18amp max in my day , so why would i use a 25amp MCB ? Has the IEE reg's changed .Can you give me the new regulation that allows this .
Thanks Steve

Current regs:
BS7671:2018 Amd 2022 Table 4D5, page 456, Clipped Direct, Method C, 27amps.

Older regs:
BS7671:1992 Amd 1997 Table 4D2A, page 202, Clipped Direct, Method 1, 27amps.

Interested to know when was you day and what regs you are refering to that state max 18amps?

 

[mort2376]

I was thinking that. I qualified under 15th edition and it was 27a I was always taught.

 

[Stevie G]

Current regs:
BS7671:2018 Amd 2022 Table 4D5, page 456, Clipped Direct, Method C, 27amps.

Older regs:
BS7671:1992 Amd 1997 Table 4D2A, page 202, Clipped Direct, Method 1, 27amps.

Interested to know when was you day and what regs you are refering to that state max 18amps?

Cheers for that ,as they say every days a school day . It would have been 15th edition yellow book i think , but thanks for enlightening me .
The Cables i would probably have fitted ,had been bunched together, and covered in glass wool etc.
Why would Mort2376 fit 20amp Mcb, if cable the is rated at 27amp ,so saying Mcb manufacturers are pushing 25 mcbs not knowing how the cables are installed ,why not 15a mcb !

 

[Andy™]

2.5 on a RFC needs to be 20a after any derating, maybe thats where the 18a came from?

 

[Stevie G]

2.5 on a RFC needs to be 20a after any derating, maybe thats where the 18a came from?

I knew i had got 18amp from somewhere in the recesses of my brain LOL .I would have probably erred on the lower side also .

 

[SPECIAL LOCATION]

Cheers for that ,as they say every days a school day . It would have been 15th edition yellow book i think , but thanks for enlightening me .
The Cables i would probably have fitted ,had been bunched together, and covered in glass wool etc.
Why would Mort2376 fit 20amp Mcb, if cable the is rated at 27amp ,so saying Mcb manufacturers are pushing 25 mcbs not knowing how the cables are installed ,why not 15a mcb !

I knew i had got 18amp from somewhere in the recesses of my brain LOL .I would have probably erred on the lower side also .

Method 4, enclosed in an insulated wall was 17.5amps back in 1997,
or Method 101 above plasterboard, buried under insulation is currently 17amps...

So if you were installing cables surrounded by insulation then 18amp could of been a relevant figure??

As a side note:- 2.5mm VoltDrop.. 18 mV/A/m.
You weren't thinking of volt drop calc's on a 2.5mm radial were you??

 

[Stevie G]

Method 4, enclosed in an insulated wall was 17.5amps back in 1997,
or Method 101 above plasterboard, buried under insulation is currently 17amps...

So if you were installing cables surrounded by insulation then 18amp could of been a relevant figure??

As a side note:- 2.5mm VoltDrop.. 18 mV/A/m.
You weren't thinking of volt drop calc's on a 2.5mm radial were you??

No i don't think it was volt drop ,but a quick rule of thumb of 18amps /2.5 mm T&E in the day , but thanks for the refresher

 

[Stevie G]

Method 4, enclosed in an insulated wall was 17.5amps back in 1997,
or Method 101 above plasterboard, buried under insulation is currently 17amps...

So if you were installing cables surrounded by insulation then 18amp could of been a relevant figure??

As a side note:- 2.5mm VoltDrop.. 18 mV/A/m.
You weren't thinking of volt drop calc's on a 2.5mm radial were you?

 

[mort2376]

There was always a large gap between 20a and 32a hence why 25a fills this gap and still complies with 2.5 cable rating under the correct installation method.

In the majority of older installs clipped direct is the method for socket circuits. And the 25a allows for failed rings to be converted into 2 radials which even in a kitchen can still operate without nuisance tripping.

 

[binky]

No i don't think it was volt drop ,but a quick rule of thumb of 18amps /2.5 mm T&E in the day , but thanks for the refresher

there's lots of confusion caused by the Onsite Guides, which quote 80% figures, something I've always hated as it's part of dumbing down our trade.

Your memory does help explain why I see so many 16A radial circuits, I've often wondered why some people dont seem to like fitting 20A MCBs

 

[Andy™]

there's lots of confusion caused by the Onsite Guides, which quote 80% figures, something I've always hated as it's part of dumbing down our trade.

and then some manufacturers (i.e schneider) can have a higher max Zs that the table in the regs. so just because Zs is slightly over BS7671 doesn't mean its not within the manufacturers spec so it might still be good

 

[SPECIAL LOCATION]

I've often wondered why some people dont seem to like fitting 20A MCBs

I think one of the reasons is that most manufactures who supply any fully / partially equipped CU bundles, typically never include a 20A (or 25A) MCB.. (or any 10A either!?)...

So 'Bob-The-Builder'... 'DIY-Derek'... 'Kevin-The-Kitchen-Fitter'... 'Larry-Loft-Converter'... 'Clive-Conservatory-fitter' etc...

[Or for the PC amongst us... 'Barbara-The-Builder'... 'DIY-Debbie'... 'Katrina-The-Kitchen-Fitter'... 'Lucy-Loft-Converter'... 'Carol-Conservatory-fitter' etc... ]

Who say they will "do the electrics" as they "know what to do"..
Go to B&Q / Wickes / Homebase / Screwbits / Trollstation / etc ...

And ALL they see are CU's with.. 6amps (for lights), 16amp (for the immersion heater if still have one), 32amps for the sockets/cooker, 40amp for the shower!!

20A, 25A, 10A.. simply do NOT exist in this world of electrical work...
and if they saw one, they wouldn't have a clue what you use them for!!!!

Full and partially equipped CU bundles, sold by DIY sheds, have created a huge quantity of very poorly designed electrical installations IMHO!