Fire Hazard? 2.5mm 20A Cable Changing To 4mm 25A Cable

[Tozzy]

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• Tozzy
• 9 Aug 2012
• 1

Am I right in assuming that in this particular radial circuit, the 25A cable would be more than capable of taking the load of the two electrical radiators except that the 20A cable would sizzle as the radiators are at the end of the circuit after the 20A cable?
If the circuit was wired as shown below would it be better?:

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• Tozzy
• 9 Aug 2012
• 1

Would love to know.

Please note: This is just a question, not an existing installation or a future installation about to be carried out!! Intentionally haven't added the MCB to the diagram as not necessary; it's just a diagram.

Thank you!

 

[securespark]

A 20A radial on 2.5 is OK as long as the area served by it does not exceed 50m².

A 30A radial on 4mm² is OK as long as the area served by it does not exceed

 

[Tozzy]

A 30A radial on 4mm² is OK as long as the area served by it does not exceed

75m². I know. But you didn't answer my question .

 

[EFLImpudence]

Yes you are correct...

...but no one would design a circuit like your first diagram.


Also - 4mm² T+E is capable of carrying a maximum of 37A (depending on the installation method) so is normally on a 32A mcb.

Your flames under the 2.5mm² cable hopefully would not happen because the 20A mcb, which should have been used, would disconnect the supply.
Although with the maximum current shown of 22.3A (@240V) neither would actually happen.

 

[securespark]

Sorry, I missed a bit off!

Yeah, I was a bit confused because 2.5 is rated at a max of 26A in ideal conditions.

Therefore it would not start sizzling!

 

[ban-all-sheds]

This is just a question, not an existing installation or a future installation about to be carried out!!

Why?

Given that you are also curious about cable installation routes....

What re you planning?

Intentionally haven't added the MCB to the diagram as not necessary

But as you see, it is.

 

[bernardgreen]

Intentionally haven't added the MCB to the diagram as not necessary; it's just a diagram

In which case replace both the green ticks with the red flames, or have you forgotten the possibility of a short circuit occuring ?

 

[Tozzy]

Sorry for late reply sparkies (Diagram was based on 230V supply)

Also - 4mm² T+E is capable of carrying a maximum of 37A (depending on the installation method) so is normally on a 32A mcb.

Depending on the installation method?? I assume you mean clipped?? I read 4mm² was 25A? Is there anywhere on the net I can get exact figures?

Now a 32A mcb would be useless in this diagram as it would not protect the 2.5mm² cable, only the 4mm².

Your flames under the 2.5mm² cable hopefully would not happen because the 20A mcb, which should have been used, would disconnect the supply.

Therefore, you'd never get the benefit of the full capacity of the 4mm² cable.

So basically what I'm saying is, you can't uprate the circuitry eg. 2.5mm² to 4mm². Is this correct? It has to be consistent yep?

Yeah, I was a bit confused because 2.5 is rated at a max of 26A in ideal conditions.

I thought 2.5mm was minimum of 18.5 to 20A?? How's it gotten to 26A?

...but no one would design a circuit like your first diagram.

It's merely to get an answer to my question and what could be easier than showing someone a diagram

Given that you are also curious about cable installation routes....

What re you planning?

A career in electrics?

 

[RF Lighting]

Sorry for late reply sparkies (Diagram was based on 230V supply)

Also - 4mm² T+E is capable of carrying a maximum of 37A (depending on the installation method) so is normally on a 32A mcb.

Depending on the installation method?? I assume you mean clipped?? I read 4mm² was 25A? Is there anywhere on the net I can get exact figures?

Installation method includes clipped direct, in metallic conduit, in non-metallic conduit, in metallic trunking, in non-metallic trunking, on cable tray, in thermal insulation, buried in the ground, and many more besides.

Installation method can reduce how much current a cable can safely carry by up to 50%

So whilst 4.0mm² 6242Y cable can carry 37A in optimum conditions, it would only be able to carry 18.5A in the worst case conditions.

All the current carrying capacities, and information about installation methods, derating factors and much more can be found here

Now a 32A mcb would be useless in this diagram as it would not protect the 2.5mm² cable, only the 4mm².

Correct, and for that reason it is not permitted by BS7671

Your flames under the 2.5mm² cable hopefully would not happen because the 20A mcb, which should have been used, would disconnect the supply.

Therefore, you'd never get the benefit of the full capacity of the 4mm² cable.

So basically what I'm saying is, you can't uprate the circuitry eg. 2.5mm² to 4mm². Is this correct? It has to be consistent yep?

Correct. You have to rate the protection to the weakest part of the circuit.

You can, and indeed may need to increase the conductor size if there is a long run and volts drop becomes too great for the cable, or, if the cable runs through thermal insulation for part of the circuit, you may want to increase the size of that cable to maintain the overall circuit capacity.

Yeah, I was a bit confused because 2.5 is rated at a max of 26A in ideal conditions.

I thought 2.5mm was minimum of 18.5 to 20A?? How's it gotten to 26A?

It's always been 13A to 26A depending on the installation method.

...but no one would design a circuit like your first diagram.

It's merely to get an answer to my question and what could be easier than showing someone a diagram

Given that you are also curious about cable installation routes....

What re you planning?

A career in electrics?

I think you've still got quite a bit of learning to do yet my friend.....

 

[Tozzy]

I think you've still got quite a bit of learning to do yet my friend.....

But in life we never stop learning . RF Lighting thanks very much! Very informative; clears everything up.

EDIT:

if the cable runs through thermal insulation for part of the circuit, you may want to increase the size of that cable to maintain the overall circuit capacity.

Ah, just reread this and wondered, so you'd make a fused spur in 4mm2 off the 2.5mm2 cable? If you get chance to see this and reply, thanks in advance

 

[ban-all-sheds]

I thought 2.5mm was minimum of 18.5 to 20A?? How's it gotten to 26A?

To be added to your basket:

http://www.amazon.co.uk/exec/obidos/ASIN/1849192693 or http://electrical.theiet.org/books/regulations/17th-edition-amd1.cfm

http://www.amazon.co.uk/On-Site-Guide-Regulations-incorporating-Amendment/dp/1849192871 or http://electrical.theiet.org/books/regulations/on-site-17th-ed-amd1.cfm

http://www.amazon.co.uk/Electrical-Installation-Design-Guide-Calculations/dp/0863415504 or http://www.theiet.org/publishing/books/wir-reg/electrical-installation-design-guide.cfm

 

[Tozzy]

Thanks ban-all-summerhouses, *cough cough*, I mean ban-all-sheds They are something I'd be very interested in reading, although I'll probably buy them off ebay as they might be a little cheaper and will get them used. One of them is £70! That's crazy

I'd get this one instead:

http://www.amazon.co.uk/Electricians-Guide-Building-Regulations-Wiring/dp/0863418627/ref=pd_cp_b_2

 

[sparkybird]

what rating of circuit breaker would you use in both those situations? that's the key really.....

 

[EFLImpudence]

Misleading?

 

[JohnW2]

Misleading?

Indeed so - I presume that 'we' have no control over those pages - so to whom do we complain?

Kind Regards