6mm^2 T&E ratings

[Stephen]

Hello,

Looking at table 6D1 in the OSG (17th) am I right to read that 6mm^2 T&E clipped direct is rated at 47 Amps but enclosed in conduit the rating drops to 41 Amps?

Is the length of conduit relevant? I thought I read elsewhere in the guide that derating where the cable runs through insulation was dependent on how much cable ran through the insulation.

I would like to run 6mm^2 T&E to the garage. I could clip direct but does T&E agree with sunlight? I though that conduit might protect it from UV and impact?

The cable would be protected by a 40A RCBO in the house and terminate in a CU in the garage. I figure that 32A for sockets plus 6A for lights = 38A, which is below the 41A limit. But for a couple of sockets is it worth using a ring; I could use a 16A MCB and have them as spurs, meaning the maximum load would be 16+6=22A.

Subject to other considerations: earth impedance, voltage drop, etc. does this sound ok so far? (No burying or overhead cables involved).

Thanks.

[PeterC]

Had 2.5mm outside for ~40 years and it's still flexible (to some extent) and not cracked. It gets sun on it for about 4 -h/day in the 'Summer'.

This proves that T&E was OK back then.

[ARWadsworth]

What sort of load do you want to run in the garage? If you want to use arc welders or 110V transformers then you will need a 32 MCB as they often trip

16A MCBs.

Adam

[Stephen]

I don't have anything that clever (yet) ;)

There might be a freezer if I can't make room for it in the kitchen. I know it ought to be on a separate circuit but with 40A limit that won't be possible. That would be the only always-on load. Other than that it would be the occasional use of tools such as lawn mower, circular saw, etc.

[ARWadsworth]

The limit is not a total sum of the MCBs in the CU. If it was then most houses would blow the main fuse if you added up all the MCB ratings! There is no point putting your freezer on its own supply if the incoming supply is protected by an RCBO.

Even if you somehow maxed out your garage ring to 32A and installed 6A worth of lighting you are still below the 41 amps maximum load that you are allowed.

As I am a lazy sod I often wire garage sockets using a 4mm radial setup with a 32A MCB instead of a ring.

Other options are to use 10mm T&E for the supply or use armoured cable if you really want a bigger supply.

Adam

[NT]

Its fine, there's loads of exterior T&E in service. But I dont think its permitted any more.

either's good, but rings have safety and reliability advantages.

Only thing to note is if you have a freezer in there I personally would give it either its own RCD or no RCD, so put your RCD in the garage CU, leaving the exterior cable not RCD protected. Why? Food poisoning kills people, RCDless electric freezers dont to any significant extent. Plus an unnoticed trip is just an unnecessary waste of food worth money.

NT

[Stephen]

You are quite right, I would be unlikely to be pulling 32A through the sockets and 6A through the lights at the same time, so I do not need to worry about the MCBs added together: diversity and all that. I guess if someone fitted too many 500W PIRs the 6A MCB could trip but I don't use 500W PIRs myself so that won't be a problem for us.

Oops. I forgot I was in the garage, but on the house CU it is a good idea to have a separate freezer ring.

If it was a workshop with dozens of sockets I would use a ring but it isn't and I may just have two sockets. I wonder about using a radial. I was going to use 2.5mm T&E and a 16A MCB, though I note your previous comments about certain appliances wanting a 32A MCB.

I don't think armoured is necessary as it's not overhead or underground. I agree 10mm would give more capacity but I think I already have some 6mm, hence I was going to use that. I am sure 40A is more than I could need.

Thanks again.

[ARWadsworth]

I do believe that the 500W lamps are now part of our history.

Adam

[Stephen]

Sorry for the late reply, I've been painting (see other thread). Why is it not permitted any more and what has replaced it?

[Stephen]

Sorry for the late reply. Do you mean they are no longer sold? The problem always was that 500w lights were available for £4.99 from almost everywhere whereas the more appropriate (IMHO) 150w ones were less readily available and cost more to buy; I don't think people thought about running costs.

[Andrew Gabriel]

They aren't banned yet, but are due to be at a future date by the minimum efficiency rules imposed by the EU. The GE 375W retrofit will survive longer, being more efficient by using an infra-red reflective coating inside the tube. (There's a 225W replacement for 300W version too.)

500W lamps have been 3 for £1 in B&Q for some time. More recently, they have been 3 for 10p. I don't suppose they're top quality, but I haven't tried them.

B&Q also sell the GE 375W and 225W versions, which can be hard to find. They are expensive, until you take account of the electricity cost, in which case they're cheaper than even a free 500W lamp.