32A Radial on 4mm cable? Surface mounted in trunking?

[PrometheusTwingo]

Hi there,

I need to fit a couple of double sockets and oven (not hob) in a new kitchen. When the electrician fitted the consumer unit he put in a double socket on its own dedicated radial circuit just beneath the CU. It's a 32amp fuse using 4mm wire clipped direct.

I intend to extend this radial to the new sockets. Can I use 4mm wire for this if its surface mounted in plastic trunking? It will be in the trunking with a 6mm cable for the hob.

I am asking because of the BS for 4.0mm stating for this current, the cable must be clipped direct. I guess what I'm really asking is does surface trunking count as clipped direct?

As a side note, does surface mounted cable have to be in trunking at all? Can I just use cable clips if the cable is in a relatively safe place such as low down at the back of a brick cupboard, horizontally near the ceiling etc?

Many thanks,

M

 

[ricicle]

Only you can decide whether or not trunking is required to suit your aesthetics and to afford (limited) mechanical protection. Cables in pvc mini trunking do not count as clipped direct and appropriate factors will have to be applied - especially if you intend to run a hob circuit in there also.

 

[JohnW2]

I intend to extend this radial to the new sockets. Can I use 4mm wire for this if its surface mounted in plastic trunking? It will be in the trunking with a 6mm cable for the hob. ... I am asking because of the BS for 4.0mm stating for this current, the cable must be clipped direct. I guess what I'm really asking is does surface trunking count as clipped direct?

Strictly speaking, no. Cable in surface-mounted trunking counts as Reference Method B, which only gives you 30A current-carrying capacity for 4mm² (even without taking into account the presence of the other cable). When you say 32A fuse, do you mean fuse or MCB? If the latter, if 25A were enough for you and if a 25A MCB were available for your CU, you could have it changed to that, if you wanted to remain compliant with regulations.

As a side note, does surface mounted cable have to be in trunking at all? Can I just use cable clips if the cable is in a relatively safe place such as low down at the back of a brick cupboard, horizontally near the ceiling etc?

As you surmise, provided it is not judged to be at appreciable risk of mechanical damage (and, frankly, plastic trunking does not provide much protection, anyway!), trunking is not required - i.e. 'clipped direct' can be just what it says.

Kind Regards, John

 

[davelx]

Look at table 4A2 (since you mention the "BS for 4.0" I'm assuming you have a copy of the regs ) Your proposed installation method is ref. method B. Clipped direct is method C.

Then look at table 4D2A for ref method B and you will find the CCC for 4.0mm is only 30A, so technically a 32A MCB cannot be used, you would need to go up to 6mm cable, or reduce the MCB to <=30A.

doh!

 

[PrometheusTwingo]

32A vs 25A fuse: Yes, I mean MCB. 32A does seem a bit overkill for a single double socket! But once I've added on the double sockets which will probably be the kettle etc, and oven, 32A would be good.

Ref Method: Yes, I just wanted to clarify that in surface conduit is B and not C.

Aesthetics are not an issue as it's running along the back a brick built cupboard, and wardrobe just above the skirting and. This is about as protected as it gets at floor level, so I would be happy with clipping.

BUT, does passing through an insulated plasterboard wall for 4 inches and masonry wall for 5 inches mean take me out of reference method C?

Working with 6mm at the socket end would be a real pain. But if needs be.

M

 

[JohnW2]

32A vs 25A fuse: Yes, I mean MCB. 32A does seem a bit overkill for a single double socket! But once I've added on the double sockets which will probably be the kettle etc, and oven, 32A would be good.

Fair enough.

Ref Method: Yes, I just wanted to clarify that in surface conduit is B and not C.

Yes, as I said, if in trunking it is B - so you know whatthe strictly correct answer is. However, people will probably vary in how excited they want to get about the difference between 30A and 32A!

Aesthetics are not an issue as it's running along the back a brick built cupboard, and wardrobe just above the skirting and. This is about as protected as it gets at floor level, so I would be happy with clipping.

Fair enough.

BUT, does passing through an insulated plasterboard wall for 4 inches and masonry wall for 5 inches mean take me out of reference method C?

Others may disagree (and cover me in flak!), but I personally would be inclined to apply common sense! If the cable were completely enclosed in insulation for 100mm, then, strictly speaking, Table 52.2 of the regs would call for a de-rating of 0.88 - which would take you down to 31.68A (which sounds very close to 32A to me!). I don't think that going through the masonary wall would call for any de-rating - certainly not if you 'plastered it in', in which case it would count a Method C!

Working with 6mm at the socket end would be a real pain. But if needs be.

I agree, particularly if it were only being done just to achieve strict compliance with regs!

Kind Regards, John

 

[PrometheusTwingo]

I want to play it safe. I'll probably clip direct but then downgrade the MCB.

Many thanks.

 

[JohnW2]

I want to play it safe. I'll probably clip direct but then downgrade it to a 30A MCB.

OK - but, unfortunately, you'll almost certainly not find one. If you're lucky, there might be a 25A one available for your CU, but otherwise you'd be down to 20A. ... or, of course 6mm² cable. Others will probably disagree, but you are probably being more 'cautious' than I would

Kind Regards, John

 

[securespark]

I would clip direct and fit a 32A MCB.

 

[JohnW2]

I would clip direct and fit a 32A MCB.

FWIW, as I've more-or-less implied, so would I, personally, and I would not lose any sleep over 4" going through an insulated wall.

Kind Regards, John