SWA, T&E or tails??

[HappyHippyDad]

Evening all..

I am currently doing my first full rewire. The customer is having the wall knocked down with the meter/cutout and CU on it. the plan was to move the CU under the stairs and get DNO to move the supply to the same place. They have said they cannot move the cutout to where the CU will be. The approx length from cutout to CU will now be around 8m so I need to add a fused switch. My questions are about which type of cable to use:

1. I could use 2 core 25mm SWA (seperate 16mm earth) but I am unsure of how easy this is to bend through the route? It will be going straight up from the meter box and then 90degress into the house wall entering directly under the first floor floorboards. What is the best way to get it through the wall without putting too much strain on it (with regards bending angle)?

2. BS7671 states 25mm to be used, however I am fusing down to 80A, could I not use 16mm SWA as CCC is 87A?

3. Could I use tails as at no point will the tails be buried in the wall. They will be going directly under the floor boards from outside the house (suitable protected outside) and then down from the ceiling directly behind the new CU (there will be a cavity behind new CU as I will bring out from the wall). Do they need to be protected if they are not getting buried in the wall?

4. Which is easier to move around/use T&E (16mm or 25mm), SWA (16mm or 25mm) or tails?

5. Which is the best fused switch to use? KMF, wylex metal? Can I gland SWA into a KMF or does it need an adaptable box?

Cheers chaps:biggrin:

 

[Plonker 3]

Excuse me? So your suggesting using dno full capacity in one post then slating use of it in another.? Why is it bad practice fusing your submain at same as dno cutout on a capable ccc cable,? Can u explain?

You never heard of discrimination?

 

[Leesparkykent]

You never heard of discrimination?

How would you go about achieving full discrimination between the DNO fuse and the one in the switch fuse then on a domestic setup such as the OP's? IMO it will be nigh on impossible.

 

[Plonker 3]

How would you go about achieving full discrimination between the DNO fuse and the one in the switch fuse then on a domestic setup such as the OP's? IMO it will be nigh on impossible.

Maybe so, but at least design it to allow for it.

 

[Leesparkykent]

The total let through energy of the downstream fuse will have to be less than the pre-arcing energy of the upstream fuse to achieve full discrimination.

 

[Midwest]

Interesting discussion on discrimination, a question I've asked myself before, and received conflicting replies.

The meter box belongs to the owner of the property, but the DNO has certain requirements; BS 8567 (gas is BS 8499).

I asked my DNO (SSE) before on what they would expect to see in such a situation, as here. I asked DNO what would they say about extended tails, 'We would expect a switch fuse, as it is a submain'. Could I put equipment in said box, answer 'No not really, what are you thinking of? Switch fuse? 'Ahhh, okay'.

Considering size of fuse in switch fuse (i.e. DNO 100a, switch fuse 80a), I asked Elecsa tech support for advice;

'Without referring to the time current characteristics of each device for the exact information we believe that the fault protection characteristics of each device will be similar but not the same.

They do provide fault and overload protection for the whole installation. Therefore there will certainly some discrimination with overload protection.

The final decision regarding the discrimination of the two devices would be down to the designer of the installation'.

 

[telectrix]

might be an idea to use a MCCB instead of a sw.fuse. cost permitting. this should ensure the MCCB tripping before the \DNO fuse. discuss?

 

[bigspark17]

might be an idea to use a MCCB instead of a sw.fuse. cost permitting. this should ensure the MCCB tripping before the \DNO fuse. discuss?

Mccb would clearly take out the supply before dno fuse. Domestically 80A is usially suffiecent but with some customers adding multiple electric showers and u/floor leccy heating they are demanding alot more. Had a call out last winter where customers off supply, dno fuse on the pole had gone 60A on pole, 80A in dno head 80A switch fuse! They replaced it with 80.

 

[jackhammerJIM]

Waw have you really installed another meter box next to dno box! If you have that is shocking imo. Put ur switch fuse in the dno box, if its on. The owners property its theirs not dno so you may add your sw fuse in there! Flobbogasted at that pic!!

Box on the right houses the gas meter you wally !

 

[davesparks]

Just to stir the pot a little bit,

Why does the submain fuse need to discriminate with the service fuse?
Why shouldn't you have 2x 100A fuses in series?

 

[Plonker 3]

Just bad design really. Why bother fitting smaller BS1362 fuse in spurs or plug tops?

 

[Midwest]

But in the case of fault protection, could you be certain that an 80 would go before a 100, and in terms of supply, restricting owner to 80amps?

 

[Plonker 3]

But in the case of fault protection, could you be certain that an 80 would go before a 100, and in terms of supply, restricting owner to 80amps?

Seriously?

come on now, how often have you seen over 80 amps being pulled on a supply for a long enough for it to blow the fuse under normal load conditions?

 

[Midwest]

Seriously?

come on now, how often have you seen over 80 amps being pulled on a supply for a long enough for it to blow the fuse under normal load conditions?

As I said, posted the same question a while back, and this was one of the replies. I'm willing to be convinced either way.

 

[grumpyjohn01]

https://www.tlc-direct.co.uk/Products/CGFS100.html
fitted a couple recently, solid metal box with 32 and 25mm holes on top and bottom. value for money.

I've used a few of these to supply submains etc & I agree top value and a decent quality unit,but I've been thinking...what about amendment 3? they don't comply eh?

 

[Midwest]

Do at the mo though. Bit small for 25mm tails IMO.