New supply tails in trunking or SWA?

[ChevyKevv]

Hi, doing a garage conversion into habitable room and will have to move the CU.

Currently it's at high level above the entry where the meter tails come through the wall.

The CU needs to be moved to make way for a doorway and will be fitted approx 6M from existing position on the other side of the doorway.

The floor is to be raised 100mm to make level with existing house floor level and the walls are to be battened out and plasterboarded.

Question, would it be best to run the new supply in SWA down the wall and under the new floor and back up to new CU, or use new tails in 50mm X 50mm trunking down the wall and under the new floor.

I'm fitting a 100A fused switch as the tails will be greater than 3M long.

just looking for pro's and con's for each method.

Thanks

 

[Silly Sausage]

Personally I'd go for SWA, through a duct under the floor. I hate trunking!

 

[Darkwood]

Fit fused isolator where tails come through from meter cupboard assuming no longer than 3m tails, use metalclad isolator and swa underfloor, this way any leaks flooding etc wont fill trunking up with water leaving tails/trunking method submerged long term. If 1ph then use 3core swa using 3rd core for earth as well as earthing Armour... jobs a good un'

 

[ChevyKevv]

It's trying to get the bends done in the SWA that are making me think trunking.

 

[Darkwood]

what size is this swa even at 35mm you should be ok to keep bends within limits, if not because of shallow void just bend sideways first as it enters under flow then bend forward to go across room and vice versa at other end.

 

[ChevyKevv]

25mm 3 Core. So should bend ok I suppose.

 

[telectrix]

80A fused iso. not 100A. to give discrimination and protect DNO fuse.

 

[ChevyKevv]

Thanks, switch came with both sets of fuses so 80A will be fitted.

 

[Darkwood]

Good point raised Tel, i assumed as i shouldn't really on here that OP had reduced already for discrim' but reading it again it does sound like domestic, so yes 25mmSWA 80amp fuse up front is by far the quickest easiest and cheapest option to name but a few advantages.

 

[Guitarist]

Just a point about discrimination...
Whilst we all acknowledge that it is ideal to provide this when using a sw/fuse unit to extend a supply, why do many on here suggest that we need 80A on a 100A supply, but it's ok to leave 60A on a 60A supply, thus giving no discrimination at all? I realise that for most modern houses, fusing down to say, 40A, would probably be impossible due to the power requirements at certain times (shower/cooker etc), whereas it's unlikely that anyone in a "normal" domestic environment would need more than the 80A suggested.
What I'm trying to say is this... Do we only bother with discrimination when it suits us to do so?

 

[telectrix]

good point. IMO discrimination should be used where possible. however if the DNO fuse is a paltry 60A, then it's time it blew and got uprated.

 

[brightspark11]

You say the garage is to be converted into a habitable room, just wondering what the room is to be used for. 80/100A seems a lot for one room.

 

[Top Cat]

^^^^^^^^^^^^^^^^^^^^agree^^^^^^^^^^^^^^^^^^^Cannabis?

 

[Des 56]

No need to downgrade to a 80 amp fuse,there is no current using equipment between the fuses,therefore discriminatiing would achieve zilch
If either fuse blows the effect is exactly the same,the powers gone

 

[Guitarist]

No need to downgrade to a 80 amp fuse,there is no current using equipment between the fuses,therefore discriminatiing would achieve zilch
If either fuse blows the effect is exactly the same,the powers gone

Agree Des. I suppose the only benefit from a fuse we fit blowing, is that we can replace it instantly.

 

[Engineer54]

I realise that for most modern houses, fusing down to say, 40A, would probably be impossible due to the power requirements at certain times (shower/cooker etc)

The average domestic installation with gas central heating, without electrical shower unit, would rarely see 40A at the meter. My parents house had a 60A cut-out fuse and a 40A meter. They were both changed (80A fuse and 100A meter) in 96, but had never had a problem with either!! ...lol!!!

 

[GaryM]

I've seen 1.6:1 and even 2:1 mentioned for fuse discrimination. Based on that you wouldn't achieve discrimination using an 80A fuse with a 100A main fuse, you'll be needing a 63A or even a 50A fuse.

 

[Guitarist]

Personally, provided the cabling is protected, I would go for the maximum fuse to match the main incomer. Let's face it, how often does the 100A or 60A main fuse actually blow? Why should we restrict the customer's supply potential for the chance at providing the possibility of genuine discrimination on the rare occasion that a fault would take the mains out anyway?

 

[GaryM]

Personally, provided the cabling is protected, I would go for the maximum fuse to match the main incomer.

So would I.

 

[ChevyKevv]

So would I.

It turns out the main fuse is only 60A anyway, this on a house built in the 1990's so i'm going for a 60A in the fused switch.