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Hi all

New to this forum, looks like a hive of information.

I'm wanting to upgrade my garage electrics - I'm not a qualified electrician, hence my questions, but I do have some reasonable understanding of the fundamentals.

I'll set the scene, but basically I'm just looking for affirmation that my choice is design/components is within regs as I have not studied the regs in great detail.

CURRENT SETUP:
I have a feed coming into the garage consumer unit (garage is attached to the house), with a 40A RCD, and two MCBS: (1 x 6A (lights) and 1 x 20A Socket)).
Garage has 2 x 13A dual sockets in it, both wired as radials with 2.5mm2 off the single 20A MCB.

NEW REQUIREMENTS:
I need to run 2 x air compressors simultaneously for several hours continuously sometimes for my sandblasting setup. Each compressor is rated at 2.2KW 13A. I've run these many times before with a clamp ammeter to test their continuous running amps as I work in the compressor repair industry and they run anywhere from 7A to 12.5A dependant on the air pressure in the tank (startup amps around 20-45A dependant on a few factors). So basically I am wanting to draw an additional 25A from the garage and the current setup will not allow me to do this in its current form. These compressors tend to be recommended to be run on 16A blue plugs BS EN 60309, and I wish to run them on these. Whilst the compressors are running my other load requirements are just some lights and a radio.

PROPOSAL NO.1:
  • As the garage consumer unit has only two MCBs, it wont allow me to add another two, I am looking to replace it with a 4 way unit and retain the current 40A RCD.
  • I will wire the existing 2 x 13A sockets and the 20A MCB into the new 4-way garage C.U. as well as the Lights 6A MCB - so I now have the same as existing setup, but with two free MCB slots.
  • This leaves me with two MCBs free, which I propose to spec at 16A each (C-type).
  • NEW MCB No.1 (16A C type) Use 2.5mm2 twin+earth, at a total length of about 4m - should allow me up to 21A (installed as method 102), run to a dedicated 16A socket for compressor no. 1
  • NEW MCB No.2 (16A C type) Use 2.5mm2 twin+earth, at a total length of about 4m - should allow me up to 21A (installed as method 102), run to a dedicated 16A socket for compressor no. 2

PROPOSAL NO.2: (Probably not as good I admit, but simpler and cheaper).
  • Retain the existing garage C.U.
  • Install a new single 32A Socket (BS EN 60309)
  • Join both supply wires from each compressor to an appropriate sealed junction box (wall mounted probably) with a 4mm2 cable exiting it to a single 32A plug (wiring the compressors in parallel to single 32A plug.
  • Change the 20A MCB feeding the current 13A sockets for a 32A MCB C-Type
  • Run a 4mm2 twin and earth cable with total length of 4m to the new 32A Socket and wired to the new 32A MCB (shared with the existing 2 x 13A sockets).
FYI, each compressor is already fitted with a 15A thermal overload protection device, so I'm not worried about over-current on individual machines

I hope that all makes sense as I've put it. Please let me know if anyone sees anything unsafe / against regs or if you have an easier, cheaper, better solution.

With thanks
Frank.
 

westward10

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Starting those compressors together will almost certainly trip that immediately. I suggest you consult an electrician to see what you can and can't do with the existing supply. Any change of consumer unit or new circuits require notification under Part P off the Building Regulations.
 
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Starting those compressors together will almost certainly trip that immediately. I suggest you consult an electrician to see what you can and can't do with the existing supply. Any change of consumer unit or new circuits require notification under Part P off the Building Regulations.
Thanks. They will never start together though. I have run a twin setup for years at my workshop and have never started at exactly the same time as they both have mechanical pressure switches, which are adjustable and almost impossible to sync exactly due to their mechanical nature. So that is not a concern. Building regs aside, I'm still interested in the information if anyone knows.
 

westward10

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Right so I can up the mcb in the main house consumer to 40A if the cable is 6mm? I was thinking the same. I'll verify the cable tomorrow.
And if I can do that, do either of my two proposed design ideas hold merit?
That is not what I said, I said it is dependent on the cables installation method which can affect its current carrying capacity this is why you need an electrician to assess it.
 

SJD

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If I'm following the correct E from the supply cable on the photo, it is solid, so it will be 2.5mm2, meaning the cable is 6mm2 ?

EDIT: As seen at the earth bar (at the cable, it is not so clear).
 
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I've just verified a 6.0 debossed on the sheathing on the main house consumer side. So it's definitely 6mm2 t+e. I haven't verified if it's buried under loft insulation yet but I've been in the loft many a time and pretty sure it is. If I lift it from the insulation and clip it to the joists I could run 47a through it by my reckoning.
 
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Even if I keep the original 6mm feed with 32a MCB on the main feeding 40a RCD on the garage, surely I'd be able to run my 25a compressor load and some fluorescent lights concurrently and modify the garage consumer unit accordingly and still be within the regs? Do either of my proposed designs have any merit or are they floored in some way?
 
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This really isn't DIY territory as I have already said you need to consult an electrician who can if necessary notify Building Control

This really isn't DIY territory as I have already said you need to consult an electrician who can if necessary notify Building Control.

This really isn't DIY territory as I have already said you need to consult an electrician who can if necessary notify Building Control.
I appreciate your concern for my safety and the law of the land. However I'm not asking anyone here to vouch their advice or be responsible for it. I'm asking some simple questions on basic electrical regs with a lot of detailed information supplied, and I'll hopefully learn something along the way even if that may be to enlist the help of a professional. If nobody wants to share their knowledge, that's no problem, but 'speak to an electrician' is not helpful. I understand where you're coming from (and totally expected to get this response from at least one or a few people) but I prefer to do everything myself and learn from the experience. I like to think I do it in a responsible manner too.
 

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