swa fuse rating

[thespook]

Hi
I have 2 cables running overhead to my garage both 2.5 twin and earth,its a straight run from my CU to the garage CU. I have now decided to do away with the overhead part and put 4mil SWA under ground. My question is, at the moment the overhead cable (2 x 2.5) is fused at 32amps at the main CU, would this fuse be ok for the 4mil SWA or would I need to down size it to a smaller rating.
Thanks

 

[electronicsuk]

More info please. PVC or XPLE insulated? Length of cable? Number of cores? Installation methed? (besides the part of the run that's buried)

 

[thespook]

PVC insulation, the SWA cable is 25 meters but the total length is 50 meters, its 3 core. Apart from the buried SWA cable the rest iof the 2.5 cable is going between the floor and ceiling of the house.
In the garage its a 20amp ring main plus lights.
Thanks

 

[Taylortwocities]

its then basically a radial with 4mm cable. No apparent derating concerns.
You can use the existing 32A fuse to protect the cable.

I'm assuming there's an RCD either at the house end or at the garage end.

As a query: are those two lengths of 2.5 the two legs of a ring circuit out to the garage?

PS: Dont forget this is notifiable work......... P P P P P P P P P P P BLAH BLAH

 

[thespook]

Not sure what you mean by "two legs of a ring circuit" The two pieces of 2.5 run direct from my indoor CU on its own 32amp fuse to the garage CU. I have a rcd CU fitted in the garage and a separate rcd fitted on the cable going to the garage.
Thanks

 

[Taylortwocities]

Ahh I see. So both pieces of T&E start at the CU and both terminate at the CU in the garage giving u an effective 5mm csa?

 

[RF Lighting]

Better known as a ring circuit , although in this instance it is only feeding one outlet.

 

[Taylortwocities]

The single outlet "appears" to be the garage CU so its not a ring. More like two radials

 

[plugwash]

well a radial wired with doubled up cable.

 

[ricicle]

well a radial wired with doubled up cable.

ie parallel supply

 

[ColJack]

there's no rule that says a ring has to have more than 1 point on it is there?

 

[ricicle]

there's no rule that says a ring has to have more than 1 point on it is there?

Or that it has to be a socket circuit, or is there ?

 

[ColJack]

they quite often use 2 or 3 cables in large mains installs simply because they either don't do cable large enough, or that if they did it's not flexible enough to be bent to fit..

I always wondered why they don't do this anyway since 2 cables can take more current than the equivalent size can..

ie 10mm² can take 46A so that's 92A for double.. but 25mm² can only take 80A

____________________________________

please excuse any typos, I'm typing with 2 plasters on my fingers..

 

[Taylortwocities]

Interesting - I guess that is only if the cables are suitably spaced. if they are grouped you may well get back to a number that is more like 2xcsa rating?

 

[ColJack]

From table 4D1A..

reference method 11...
3 or 4 cables, three phase AC flat and touching or trefoil

25mm² = 112A.. x2=224A
50mm² = 172A

I'm assuming that as the method states touching or trefoil then it already takes grouping factors into account... ?