Options for a 32 amp supply to out building

[rebelwarrior]

Hello, are there any helpful sparks out there who will answer a question for me? I want to put a 2 way CU into a garage for lighting and a few 13amp sockets but I also want a 32 amp socket for occasional use of a welder. I'm doing the first fix myself as I have the ceilings down anyway.

My plan is to come off the non-RCD protected side of the house CU with a 40amp MCB. I'll run 6mm T&E to the outside via an IP66 junction box on the wall and then 6mm SWA to the garage (voltage drop is not a critical). As I understand it this should comply (even without RCD protection at source) so long as I protect the T&E with an earthed galvi conduit wherever it is less than 50mm deep (even if in a safe zone). Can anyone confirm this?

In the garage I will fit a 2 way RCD protected CU with 16 and 6 amp MCB's, which I understand should give reasonable discrimination with the 40 amp at source, but is it acceptable to fit a large switch before the garage CU to divert the entire sub main to a 32amp socket for the welder. I realize I would have no lighting or heater when welding, but would it be acceptable under the regs to do this?


Regards

Chris

 

[Heisenberg]

I take it you are planning on getting LABC in to give this the once over once you are finished? Because no spark worth his salt will come in and sign off another persons work.

Anyway, I'll just say 2 things...

1. Use a metal box to go from T&E to SWA. Glanding into metal is infinitely preferable to plastic.
2. Make sure you buy a 4 way garage unit, as the RCD will take up 2 ways on its own.

:yinyang:

 

[yellowvanman]

1. Use a metal box to go from T&E to SWA. Glanding into metal is infinitely preferable to plastic.
:yinyang:

shouldn't be a problem if you use earthing nuts.

OP - You can use a changeover switch to swop between CU and welder - can get various sizes to fit your needs. configure as 1 input/2 outputs

 

[Deleted member 26818]

Do you really want to lose the lighting when welding?

 

[rebelwarrior]

Do you really want to lose the lighting when welding?

Obviously it's not ideal but I can always run a light from an extension lead if need be. The problem as I see it is that if I have a 32 amp MCB in the garage CU (to run the welder off that) I need a 63 amp at source and then the cable will have to be rated at at least that and 10mm + cable = very expensive.

 

[Deleted member 26818]

Unless your going to have the welder on full blast, it's unlikely that the welder will draw that much current.
I'd change the enclosure to allow for three or four ways, and put the welder on a 32A, the sockets on a 20A and the lighting on a 6A.
Remember that when applying diversity, you would take 100% of the largest load, and 40% of the rest.
32A + 40% of 20A + 40% of 6A. About 42.4A.
I doubt very much that you would ever draw anywhere near that amount for any appreciable amount of time.
If neccessary, use a 45A MCB at the house CU, and 6mm² cable.
Of course, it's quite likely that the welder will have a maximum current rating below the 32A, just that 32A is the closest rating.

 

[imago]

I'm just wiring up a garage as part of a rewire, the way I'm doing it is not the only way but thought I'd give you the example.

40a RCBO in the board feeding 10mm T&E through galv conduit to the garage (existing conduit so re-using it) into a garage CU with 32a MCB for the four double sockets (2.5mm T&E ring), and a 6a for the lights (1.5mm).

 

[Heisenberg]

I'm just wiring up a garage as part of a rewire, the way I'm doing it is not the only way but thought I'd give you the example.

40a RCBO in the board feeding 10mm T&E through galv conduit to the garage (existing conduit so re-using it) into a garage CU with 32a MCB for the four double sockets (2.5mm T&E ring), and a 6a for the lights (1.5mm).

Assuming that you're using installation method B, I think that is a little over spec'ed. Using 6mm and a 32A breaker at the main board, I'd have gone with a 20A radial and 6A lighting circuit. I also try and avoid having the RCD/RCBO back at the house, as I'm a lazy B and don't like unnecessary exercise :tongue3:

But as you rightly said, 'not the only way' just my preferred method.

 

[imago]

Yep, and it sounds fine to me. For the one I'm doing I asked the customer what he's likely to use in the garage etc. 6mm would cover it fine as you describe, but I offered him the option of a little future or 'what if' proofing as it's a full rewire. He saw the sense in removing any need to upgrade the cable and protection if he decides to get himself a lathe or something when he retires in a couple of years.

The maths made sense cost wise as there's not that much in it at this stage. But pulling in new cable etc further down the line would be relatively expensive.

 

[Heisenberg]

Yep, and it sounds fine to me. For the one I'm doing I asked the customer what he's likely to use in the garage etc. 6mm would cover it fine as you describe, but I offered him the option of a little future or 'what if' proofing as it's a full rewire. He saw the sense in removing any need to upgrade the cable and protection if he decides to get himself a lathe or something when he retires in a couple of years.

The maths made sense cost wise as there's not that much in it at this stage. But pulling in new cable etc further down the line would be relatively expensive.

Isn't it nice to have a sensible customer who is actually prepared to take your advice and has belief in you as a professional, rather than automatically believe you're there to rip them off.
Unfortunately, they seem to be few and far between at the moment.

Hope it all goes well.

 

[rebelwarrior]

Thanks everbody for your responses.

If neccessary, use a 45A MCB at the house CU, and 6mm² cable.

...This would be ideal for me but would only be acceptable if using installation method C with my 6mm² T&E run, right? i.e. I thought the breaker current must be less than the circuit capacity (38 amps for 6mm² T&E in conduit ref. method B) regardless of the likely usage. I can't install as per method C unless I use an RCD at the source end, which I might end up doing just to get the extra capacity cheaply.

Thanks Again

 

[Deleted member 26818]

Sorry, I wasn't aware you intended running the T&E in conduit or trunking, I assumed you had a route, that allowed it to be clipped direct.
You could consider using high temperature SWA throughout, you wouldn't require an RCD then.

 

[Supernube]

Can I please just ask how much this sort of install by a qualified electrician is likely to cost ?
Im looking to run a welder in an outbuilding myself.
Thank you

 

[trev]

Can I please just ask how much this sort of install by a qualified electrician is likely to cost ?
Im looking to run a welder in an outbuilding myself.
Thank you

Too many variables to give a price mate, post up your location and one of us will be near you and come to your aid.

 

[Amp David]

SWA all the way. Why fart about with trying to get 6mm twin down a length of conduit?

 

[Supernube]

Too many variables to give a price mate, post up your location and one of us will be near you and come to your aid.

Its one of those things I may or may not do depending on the price Im afraid...but Im sure it will cost what it costs... full stop.
Im In Bovey Tracey, Devon.
Thank you