32A mcbs protecting ring main

[keithsto]

I've been playing with the calculator here

Link

It seems that 2.5mm² cable, depending on where it is installed could melt at 13amps. I've got 32A mcbs in my consumer unit. Should I be worried?

Can you take advantage of the ring main and assume half goes each way so the mcb could be 26A or nearest equivalent?

Thanks

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Mod 9

 

[securespark]

The rating of 2.5 and the method of the installation (ring final) mean that it can be protected by a 32A MCB.

If insulation is involved, you may have to up the csa to 4mm².

 

[securespark]

Also, I think that calculator is for radial circuits.

 

[Spark123]

Yep, in a ring there are two conductors forming the ring. As a rule all the 2.5mm in the circuit needs to be rated at least 20A for it's installation method after correction for grouping, ambient temp, thermal insulation rewirable fuses etc.

 

[ban-all-sheds]

Keith - see these:

• http://www2.theiet.org/Publish/WireRegs/Amd1andRC.pdf
  
• http://www.theiet.org/publishing/wiring-regulations/updates/amend1-02.cfm?type=pdf[/list]

 

[keithsto]

Thanks for the document links.

So if I'm reading that correctly 2.5mm is good for 20A in the worst type of environment which is conduit in an insulated wall?

Is that calculator I found too conservative then?

 

[Zambezi]

Thanks for the document links.

So if I'm reading that correctly 2.5mm is good for 20A in the worst type of environment which is conduit in an insulated wall?

Is that calculator I found too conservative then?

"In conduit in an insulated wall" is not the worst case scenario, try the same with "in insulation not touching plasterboard".
The TLC calculator is for radial circuits, just because you have said there are 2 cables together (in the drop down menu) does not make it a ring circuit. That is for grouping, when you group cables together, you need to de-rate the cable because they don't dissipate heat as well when they are grouped together.

 

[davelx]

"In conduit in an insulated wall" is not the worst case scenario, try the same with "in insulation not touching plasterboard".
The TLC calculator is for radial circuits, just because you have said there are 2 cables together (in the drop down menu) does not make it a ring circuit. That is for grouping, when you group cables together, you need to de-rate the cable because they don't dissipate heat as well when they are grouped together.

Indeed, and if you set up the calculator with a single T+E cable, 25C ambient, 4.6Kw (ie 20A) load and "in insulation not touching plasterboard", it says you need 6mm^2 cable and rates that at 24A. So 2.5 is not going to meet the 20A requirement for a Ring Final under those installation conditions.