Sub Board Conundrum

[securespark]

Hello folks.

I went to see an install the other day:

A standard 60A 6 way Wylex board feeding a 4 way garage sub board at 45A and an adjacent 6 way 60A Wylex board in turn feeding (amongst other circuits) a 45A cooker circuit.

All 45A circuits are fed in 6mm2 T&E.

Yuk!

So I'm quoting for a clean-up: I'm installing one CU to replace the two 6 ways.

But what would you do with the sub main to the 4 way in the garage?

It currently feeds a 30A circuit, a 20A circuit and 2 5A circuits.

Would you:

Feed it from the main board?

Run a separate sub board from henley blocks to feed it?

Or extend the tails back to the main board (on the other side of the wall)?

If the first two, how would you satisfy discrimination?

[ricicle]

You will struggle to provide any discrimination between MCBs in a sub-board and an MCB feeding the sub-board in a short-circuit condition or possibly from inrush if any welding equipment were to be used in the garage.
You might be better feeding via henleys with a switch-fuse although that depends on your interpretation of the single point of isolation regulation for the whole installation.

[dannyboi2003]

Are the cartridge fuse holders for consumer units still available, MEM springs to mind? Could you feed it from the main board (non rcd way depending on cable) protected by a fuse to get around the mcb discrimination?

[holmslaw]

..

[securespark]

Thanks rice, that's as I thought.

danny, it's a possibility.

holms, that's my preferred route, by far.

The board is in a semi-recessed cupboard in the hall that's too small, but maybe I can persuade the customer to resite or open the cupboard up.

Thanks guys.

[ban-all-sheds]

securespark wrote:

Hello folks. I went to see an install the other day: A standard 60A 6 way Wylex board feeding a 4 way garage sub board at 45A and an adjacent 6 way 60A Wylex board in turn feeding (amongst other circuits) a 45A cooker circuit. All 45A circuits are fed in 6mm2 T&E. Yuk! So I'm quoting for a clean-up: I'm installing one CU to replace the two 6 ways. But what would you do with the sub main to the 4 way in the garage? It currently feeds a 30A circuit, a 20A circuit and 2 5A circuits. Would you: Feed it from the main board? Run a separate sub board from henley blocks to feed it? Or extend the tails back to the main board (on the other side of the wall)?

[holmslaw]

..

[sparkiedave]

one isolator, henley blocks then tails to each board would be my preferred option, if something is tripped in the garage there's no effect on house wiring, and no need to go into the house to re-set a breaker/RCD

[sparkiedave]

with a fuse on tails to garage depending on cable type

[ban-all-sheds]

holmslaw wrote:

ban-all-sheds wrote: securespark wrote: [/color] extend the tails back to the main board (on the other side of the wall)? How does that address discrimination problems?

The original question was badly worded. As it included "If the first two, how would you satisfy discrimination?" I took it to mean that the 3rd option had no discrimination issues because it was describing extending the tails to the board in the garage, on the other side of the wall.

[mfarrow]

dannyboi2003 wrote:

Are the cartridge fuse holders for consumer units still available, MEM springs to mind?

Not sure whether the new Memera 3 stuff will work in their domestic boards (???). Otherwise I believe they're still in Wylex's catalog, hager up to a certain size (32A I think) and of course the industrial sets like Merlin Gerin will work if a little pricier.

The usual problem is that manufacturer's data states you can't pull fuses to isolate circuits live, so you will either have to knock the whole lot off when you want to work on the sub board or install a separate isolator in the board just for that fuse/circuit.

[ban-all-sheds]

I've noticed as well that sometimes even within the same make, fuse carriers and MCBs have the feed tunnels in different places making mix'n'match on a busbar a little tricky...