4mm radial circuit

[storyelectrical]

I've just been flicking through the regs to do some cable calcs for a job and noticed that a 4mm 2 core cable installed using method B (under floors etc) has a capacity of 30A. However it is ok to connect to a 32A MCB for a radial socket circuit. Don't know if its just me missing something (which I'm guessing it probably is!) but that doesn't sound right. I'm looking at table 4D2A. I just can't figure it out!

 

[Guest55]

yes its fine , crack on.

 

[storyelectrical]

Thanks for the reply. I know its fine but I'm just trying to figure out why it is regarding the regs.

 

[Heisenberg]

yes its fine , crack on.

As this seems to break the Ib =< In =< Iz rule, why is it fine?

 

[drew35]

Because Biff said so! Just put that on the cert...

 

[CRAIG FISHER]

I've just been flicking through the regs to do some cable calcs for a job and noticed that a 4mm 2 core cable installed using method B (under floors etc) has a capacity of 30A. However it is ok to connect to a 32A MCB for a radial socket circuit. Don't know if its just me missing something (which I'm guessing it probably is!) but that doesn't sound right. I'm looking at table 4D2A. I just can't figure it out!

If the cable is flat twin and earth how come you are using reference method B

 

[telectrix]

installed under floors and buried in non-insulating wall ( e.g. plaster ) then ref. method C. then a 32A MCB is fine.

 

[marshr02]

Hi there. I have exactly the same concern. I have 32A, 4mm radial circuit. The worst case reference method throughout the run is method B in conduit - more exactly I ran the cable in 20mm oval conduit chased into the brickwork - rather than capping (presumably capping not enclosing the cable is not termed as conduit). The tables showing 30A CCC (nearest table that the OSG is based on is multicore in BS7671, which makes me doubt how accurate the 30A figure is) I would expect that this fixing method is quite common. I'm not an experienced electrician so am interested what folk do in this case.

 

[telectrix]

there is no reference method B for T/E. you can't downrate to 100,101,etc. as it's not in thermal insulation. i generally use ref. method C if some of the run is in trunking etc.

 

[marshr02]

"there is no reference method B for T/E" ... yes I saw that. The OSG mentions this and claims the figures for 'multicore cable' are similar. OSG Page 52, Table 7.2 then gives method B for T&E @ 30A. Interesting to hear your experience though - so you run 4mm in conduit on a 32A CB? Cheers

 

[telectrix]

you can run 4mm cable at 32A till the cows come home. it won't get anywhere near 70deg.C.

 

[marshr02]

you can run 4mm cable at 32A till the cows come home. it won't get anywhere near 70deg.C.

Okay, but do you put 4mm cable in oval conduit on a 32A CB? Cheers

 

[telectrix]

i would as long as the oval tube is big enough to leave a decent airspace. not if i have to use lube to get the buggers in. LOL.

 

[marshr02]

That's interesting. I thought the airgap in conduit is what performs an insulating function. So I put the 4mm cables in 16mm oval conduit - not really tight, but not really loose,. Thanks for the reply...

 

[Guest55]

As this seems to break the Ib =< In =< Iz rule, why is it fine?

read posts #7 , 9 & 11.
and cables under floors are nothing like as derated as conduit values.
why is it not fine ?