We have just bought a house and I need to rewire it, and wire the garage to suit the needs of a MIG welder. These sometimes can cause mcb's to become weak, and  I have been told it is the done thing to have a socket wired direct off the supply side of the fuseboard.  Is this correct? The fuseboard is some way from the garage, and I don't much like the idea of wiring from the supply side without fused protection, especially as the feed wire would be running under a bedroom floor.

Wouldn't it be better to wire from a 40 amp mcb at the consumer unit via an isolating switch in 6 mmm cable from the supply side of the fuseboard to 32 amp and 6 amp MCB's within garage so that the system was fully protected and at least if any tripped they could be easily reset?  

All advice welcome.

Thanks

John

We generally use a 16amp socket, on a D20 breaker in the board (if its a submain, you have to be careful about discrimination).

Like plug says, get a spark to calculate the cable size to ensure the Zs is low enough.  He will need to know the cable length, and the Ze at the origin of your supply (he could take 0.35 as the Ze, as this is the max the REC will give, but usually it is a lot lower, and this 'rule of thumb' figure may lead to un-necessarily large cables.

Anyone on here could calculate for you if you give us the distance, and if the cable proves to large, then we could recalculate if we knew the Ze.

Confussing??

sorry, yea, 0.8 for TN-S, 0.35 for TN-C-S, making asumptions

If TN-C-S(PME) or TN-S, then you could use a D20 MCB with 4mm/4mm cable(4mm L and N with 4mm earth) over a distance of 46m maximum.  This is assuming the welder is fixed equipment with a disconnection time of 5sec's.

If 0.4sec's is required, a D20 MCB with TN-S would not be possible, but the above spec on TN-C-S would be ok(just).

T+E with its 1.5mm earth in the 4mm size would be too small, a seperate earth would need to be run.

Sorry folks,

What exactly does TN-C-S

PME

and

TN-S


actually mean?

Thanks

John

so on a TN-C-S arrangement the earth is combined with the N - how does it get separated at the CU

From looking at a cutout, sometimes you cant figure out if it is TN-S or TN-C-S.  The rec's use concentric for TN-C-S, and sometimes split concentric for TN-S down here.  The only way to tell is with a loop tester, and this isn't a deffinate answer!

i suppose the other option is to open the cutout and find out for sure

or ask the rec directly

iirc pme should have a warning label but often doesn't

Chubby - that sort of thing may go down well over at Screwfix but we do try to avoid it over here. Do try and remain civilised on this Forum not a prat - save that for SF coz they deserve you over there and you them.

Many Thanks

RR