I’m about to re-wire my garage to put some sockets nearer where I actually want them and add some outdoor sockets.

Circuit breaker for the garage is 20A, but there are spare 32A and 40A breakers. I want to use wiring suitable for the 40A if possible so it can be swapped over at some point. Otherwise I’d just match what’s there.

So what wire do I need to use for a 30m ring main around the garage (10m long, 1car wide) , its reasonable to assume I might want to put all the load at the furthest point (E.g. chainsaw and leaf blower both plugged in to the far outside socket).

At a guess the standard 2.5 mm but I’m no electrician. Also technically I don’t think you’re allowed to make connections to the distribution board.

https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

Not to be taken as an endorsement of the plan.

Errmmm – get proper advice.  A standard ring main can only use 32amp breakers.  What on earth are you going to be doing in there that you want 40 amps?

Do you have a secondary consumer unit in the garage?  What is that connected to if so – a breaker in the main consumer unit or direct to the supply?  What amperage is that supply to the garage rated to?  What size wire?

To me what you want to do seems like a recipe for burning your house down

40a is a a lot! Bearing in mind some houses only have a 60a main fuse.

I would get you local electrician in for some advice.

There is a lot more to think about than just what wire size I.e do you have rcd protection, exporting the pme to an outbuilding, discrimination between protective devices in the garage and house

Main consumer unit is in the garage, and has a 100A feed. Theres no problems there TJ!

Brewing beer is a lot easier on a 32A circuit (3 elements is 30A, and if I ever upgrade to a bigger pot it would be 16A), 40A would give some headroom. In the meantime 20A won’t even run the leaf blower (13A) and chainsaw (10A) simultaneously but I can live with that for now.

Besides, like I said the plan for now is just to upgrade the existing wiring and add a few more sockets to where I actually want them. Nothing is going to be underrated or be in any danger of burning anything down.

The question boils down to, is it worth the extra £20 for 4mm over 2.5mm to potentially allow for it to be moved to another circuit at a later date.

Well 4mm is rated up to 37amps (assuming installation method is ‘clipped direct’) so you’d have to run a 6mm radial from your 40amp breaker.

A standard ring final circuit is 2.5mm on a 32amp breaker which serves a floor area of up to 100m2. You could also run a 4mm radial circuit on a 32amp.

Whichever way you do it, would have to be rcd protected.

Pfft 40A is nothing, was looking at an 8KA breaker today (750V D.C.).

All sorts of questions that boil down to your intended appliances (and where they will plug in), the cable runs, diversity calcs, and the possibilty of “extras”, e.g. a space heater for working out there in the cold, that might be used when the chainsaw and leaf blower are also in use. Too much to deal with here IMHO

Personally I’d use the 32a and either put in a ring with 2.5mm or a radial with 4mm. That’ll give you a decent rating on all the sockets. Just round a garage the voltage drop wont be be a concern. If you do want to start adding heavy duty kit then you can add some 16a sockets on one of the other breakers. No need to bugger about with this one. Once in it’ll all need testing even if you aren’t getting it properly signed off

40A is nothing, was looking at an 8KA breaker today (750V D.C.).

I’ll raise you ‘Flash ‘ Testing today @32KV……

Will 4mm go in std sockets ? Seems a bit beefy.

Yes physically as on a radial its only one whereas on a ring with spurs you can have three 2.5 cables

“…on a radial its only one…”

Assuming that you have several socket outlets on a radial circuit then you’ll have a conductor in and a conductor out, i.e. two. A radial is effectively half of a ring, which is how you can overload one leg of a ring final

I’m not up on current regs, but you used to be able to branch off a radial socket as well

You will get 2 x 4mm cables in a socket – not the easiest to work with though.

Trying to explain to a graduate design engineer that whilst a 6mm ring main works perfectly well on a calculation you can’t physically get them into a socket is always a good one…

On a ring main the furthest point is the middle of the ring.  Without doing any calcs I’d say a 32A ring in  2.5mm T&E clipped direct would suffice. If you have a piece of equipment that draws a lot of current, has a big motor etc then maybe provide a radial to that on a separate mcb, gives some discrimination and means the lights won’t go out whilst you’re cutting/grinding etc!    Also, consider RCD or Rcbo protection seeing as there will be outdoor sockets on the ring…

Also worth noting, addition of a new circuit from the cu would be ‘notifiable work’ and you would either use a electrician who can test and sign it off or notify your local building control of the works you intend to carry out. Just sayin

Just get a proper electrician in for crying out loud!

Just get a proper electrician in for crying out loud!

Why?

So far we’ve had a handful of people that actually read the question and answered it. And a load of hysterical shouting about stuff that wasn’t related to it.

The CU is brand new, and fully populated with RCBO’s, and I’ve no intention of opening it. Just moving some sockets arround the garage and asking about whether an upgrade to the existing ring main is sensible in order to give a degree of future proofing as most of the wire will need replacing to move the sockets anyway.

And the answer turned out to be no, 2.5mm is fine (which I assumed was correct, and was hoping for as TJ said, 4mm is a PITA to get into the back of sockets.

Now if you’ll excuse me I’ve got work to do. I need to find some rubber gloves and wellies so I can cut the wires without getting all tingly.