UK Running a cable for home Welder

[Stuart Lane]

I am planning to run a cable from my house consumer unit to a new consumer unit in my garage. I then want to run a medium duty welder from said garage CU. I am thinking:

SWA cable from 32A MCB in main CU in 4mm or 6mm ran under floor through adjacent loft to garage in / not in metal conduit.

Garage consumer unit with 63A main switch. Cable from 32A MCB in said CU (not sure what size) to one of those 32A socket and 110V blue plug arrangements.

I'm still planning and so just want a general opinion / advice. It's a standard question that's probably been asked a hundred times before. I may be way off so I am just fishing for advice on the best way to proceed.

Thanks

 

[Spoon]

First of all, it's a new circuit so this falls under Part P and is notifiable.
If the welder requires a 32A breaker then that is what size you should have in the garage CU. You need selectivity between the breaker in the garage CU and the house CU, so wiring it back to a 32A in the house CU will not do this.
Then size the cables according to the installation method.

 

[Stuart Lane]

OK. That's a good start. By selectivity do you mean 2 different size breakers e.g. a larger breaker in the house CU so under fault only the garage breaker will trip?

Thanks

 

[Spoon]

OK. That's a good start. By selectivity do you mean 2 different size breakers e.g. a larger breaker in the house CU so under fault only the garage breaker will trip?

Thanks

Yep mate.
Also, can your house CU take the extra 32A without going over the main fuse rating?
You also need a socket rated for the voltage & current you are using.

 

[Lucien Nunes]

Depending on what other loads you want to run on the garage CU, and the length of the distribution circuit, you might need to size it on voltage drop based on the peak current of the welder rather than the average (thermal rating) current. Otherwise you might find the lights flicker or things malfunction when you strike the arc. Not really an issue with inverter welders but can be a nuisance with conventional transformers. Inrush might also dictate the use of a C or D curve MCB, which will put more onerous requirements on the loop impedance and hence have an impact on cable size. OTOH if it is an inverter welder, take care when selecting the RCD as there could be significant HF residual current. On the subject of RCDs, if you are going to run a heavy-ish submain without RCD protection, it is good to check your external fault loop impedance beforehand to make sure you have the necessary oomph.

 

[pc1966]

Cable from 32A MCB in said CU (not sure what size) to one of those 32A socket and 110V blue plug arrangements.

The commando style plugs (round 16A/32A/63A/etc) for 230V are blue, 110V ones are yellow and usually wired as 55-0-55 for construction site safety reasons (400V are red and 24V SELV typically violet).

As well as the Part P stuff, you need to be quite sure of what voltage/current the load really has!

 

[pc1966]

As above on MCB choice, or if that is not suitable then you would be looking at splitting the tails and having a switched-fuse feed.

With SWA you don't need RCD protection for the sub-main, unless the Zs is too high to clear a fault in under 5s on over-current protection alone, but you should have it on socket outlets.

Also loss of supply in a workshop plunging you in to darkness is potentially very dangerous, so you might want to consider some emergency lighting just in case. You can get LED baton lights with it built in, so no major difficulties other than the need for both permanent live and switched live to them. Or a separate one, of course.

 

[Stuart Lane]

Thanks very much for all your replies - this is getting complicated but I'm learning all the time. I better do plenty more studying up before I undertake this project.

Cheers guys

 

[Diytim]

First of all, it's a new circuit so this falls under Part P and is notifiable.
If the welder requires a 32A breaker then that is what size you should have in the garage CU. You need selectivity between the breaker in the garage CU and the house CU, so wiring it back to a 32A in the house CU will not do this.
Then size the cables according to the installation method.

Don’t forget you will need an Earth stake outside the garage connected to your new CU.