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Adding a 16a spur to garage distribution box for arc welder

Postby oxford49 » Mon Nov 05, 2007 1:21 pm

I want to install a small inverter (DC) arc welder into my garage. The arc welder is rated at 5a - 125a max and instructions state that it should be connected to a 16a/240v power supply via a 16a socket and plug. At the moment, I have a distribution box (DB) in the garage which only has two circuits, one is a lighting circuit (5a ‘type B’ mcb) and the other a 13a power circuit (this is protected by a 20a ‘type C’ mcb). This DB is connected to a spare slot on the house DB via 2.5mm SWG cable. Both DBs have RCD protection.
The problem is that I have no spare circuit on the garage DB. Can I connect the 16a socket to a spur taken from the back of a 13a socket or should it connect directly to the DB? If it has to be wired direct, can I split the power circuit into two (one to the 13a ring and the other to the 16a socket) via a junction box next to the distribution box. Would this have to be fused separately? What would be the normal/ best way of adding the 16a circuit to the power supply in the garage?
Any help would be greatly appreciated.
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Postby sparks » Mon Nov 05, 2007 9:55 pm

Hi how i would do it is to reduce the MCB rating to 16 amps and spur off the socket ,however you must use a C or D type MCB as the surge current could trip the MCB ,with the 16 amp socket being rated at 16 amps the MCB must be 16 amps to,the cable could then be a 2.5mm
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Postby sparx » Mon Nov 05, 2007 10:18 pm

you could run another 2.5 from 20A mcb as unlikely to be using 13A socket at same time, or if easier loop from existing 13A socket, what you have is a type A3 radial so would comply.
My worry would be more practical than theoretical ie a SWA of only 2.5mm will have a very high volt drop at 20A if the run to house consumer unit is very far, this will result in poor 'striking' of arc and rod 'sticking'.[ 18mV / Amp/Metre ] or put another way, a 20M run would drop 7.2V if no other load, so your nominal 230v supply would drop to 222.6V,
Now for the record 'you do know this is part-p notifiable work under bldg regs?'. regards SPARX
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Postby oxford49 » Tue Nov 06, 2007 8:57 am

[b]Hi Sparx[/b]
Thanks for your comments - great help - especially re. voltage drop to welder. I am going to change the mcb to a 16a type C and run a spur from the back of a 13a socket as suggested (the supplier of the welder has since said that 16a is a recommendation but they run them from 13a sockets anyway!)
It's meant to have non-stick/ easy strike features and is an inverter (DC) type, so I will see how it goes with the 2.5mm SWG cable. I do have a 20m run to the garage, so your comments are especially useful - if I have problems I will replace the 2.5 with 4mm SWG from the house to the garage DB and reduce voltage drop. Cheers mate!
(oh, yes, I do know about notifiability, so at the moment all of the above is theoretical, but point taken!)
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