Extra long meter tails (10m) at least.

Far too many variables to give you sound advice.

Are you qualified in anyway to be working with electrics? I only ask so advice can be structured to suit.

 

If the cable is to be concealed in a wall in the house, you need to consider SWA to prevent the requirement of 30mA RCD protection to the submain.

If it was me, I would opt for an 80amp/100amp metal clad switch fuse adjacent to your garage CU, SWA (if the cable is to be flush) or Split Concentric if surface the whole run. A 17th edition dual split load metal clad board in the house.

This sort of thing needs designing and installing correctly. Are you using an electrician for the build?

 

He may not need the s/w, I thought split con was a no no on the Holy Bible?

 

Split con is treated like T+E. You can no longer bury it, but it's fine as a cable, just like T+E. It's no longer mentioned. It used to be suitable for burial in the ground and walls outside zones, but the mention "cable of centric construction" was removed.

Some DNO's do not care what tail length you have, others do. I prefer to have a switch fuse if at all possible.

 

Unless you have the right test gear to measure your Ze your gonna be a bit hamstrung right from the start as the Zs compliance on a run that long is gonna be tight on a fuse.

 

I did a job the other week where they moved the meter and wanted the mains unit re-connecting, (should have moved the mains unit looking back but that's another story).

From the meter box outside, up the cavity, along a porch loft space and into the mains in the hall at ceiling height. All in all about 7M of 25mm tails. Isolator supplied by DNO, no sw fuse, I'm not worried about it in any way.

Its all very well saying use SWA but it needs glanding into something, usually a metal switch fuse and if the meter box is external then you cant fit one inside it (not that you should anyway) especially if there is a DP isolator and/or a set of henleys. So you need a second external box for the sw fuse. I say just use PVC tails as long as you have a route where they don't have to be buried.

 

Ok so broadly speaking I think the best way to do this job is split the tails within an enclosure and from that enclosure install two switch fuses. One for CU1 and 1 for CU2. The switch fuses will need to be of a slower amperage than your main head fuses to achieve some discrimination. RCD protection is not acceptable for a sub-main so the cable would either have to be installed surface or if buried be an SWA.

I would recommend however seeking the services of a fully qualified electrician so as to be sure the advice I give is appropriate.

 

They will need to be half of the value of the cut out to have any chance of discrimination. If it's a 60a cut out that will be 30a, well below what would be required for the house.

 

One I did a while back from what I can remeber. 100A single phase incommer, 25mm tails up into a plastic enclosure in the garage with henleys inside, out of the henleys with short tails to a garage mains unit to do garage power/lighting/boiler etc. another pair of short tails to a 80A switch fuse and then out with a 25mm SWA, across a ceiling to the mains cupboard in the house. - I'm not sure why I put the henleys in an enclosure really, maybe to neaten it up.

Anyway the other end of the SWA was glanded into another sturdy plastic enclosure, into another set of henleys and then a further 2 sets of tails out into 2 x 10/12/? way boards. This time the enclosure was used because the SWA would have had single insulated cables out of it.

The job needed 3 phase really but as always, tightarses. 7 or 8 years later the house is still standing so it looks like its jobs a goodun.

 

I don't follow. I am talking overload?

 

In order for the lower rated device to go first in a fault it needs to be half that of the higher rated device. You know this already I'm sure.

 

To be honest. No I did not. I assumed a 63A BS88 would blow before a 100A BS88 if over load was over the 63A but lower than the 100A.

Maybe I am wrong.

 

The I2t let-through energy time difference between a 100a and 63a bs88 is tiny, fractions of a second, so small as to be anyone's guess which will pop first. I'll see if I can find some diagrams some where.

 

It's very simple. For complete discrimination (selectivity) the total I(sq)t of the smaller fuse must not exceed the pre-arcing I(sq)t of the upstream fuse.

I believe (without looking up charts) that it might be achievable with the 100A and 63A fuses stated.

 

Doesn't really matter what fuse blows though does it, if the SWA or tails are 25mm then they are both covered by the 100A cut out fuse anyway. Its more for overload anyway, if you use 16mm SWA then it needs a smaller fuse so a 63A would go eventually if 95A were being drawn and the 100A will never go.

 

I can't find anything about it in 7671 so I would think the only way to find out would be to contact the fuse maker.

 

regs state you can not use the dno mains switch and tails have to be protected by a fuse if they are longer than 3 meters your not allowed to use the dno fuse as protection . also if you bury them in to a wall or go through a wall they need to be rcd protected.