6.3.3??

[Hager]

HI

I've been reading the 16th Edition book, with 6.3.3 (sorry if i got this wrong- it's Radials on page 117) it doesn't really say what the minimum breaker size can be. I got the impression that it's 20A like a ring main but would like to check.

Thanks
Jake

 

[sparkticus]

The 16th edition has been replaced by the 17th edition (for several years now) with the first amendment to it already released and effective from Jan, 2012.

The maximum protective device rating for a radial wired in 2.5mm twin+earth is 20 amps. But there are several other factors to consider if you are installing a new radial circuit. By the way a ring final circuit is usually protected with a 30/32amp protective device not a 20amp device.

What is the maximum load?

What length is the cable run?

Does the cable pass through any thermal insulation?


Assuming a domestic environment, unless the new circuit is less than 50mm from the surface of any wall and not run in earthed conduit then it will need to be RCD protected (additional protection by RCD)

 

[ericmark]

I can't find that reference however the correction published on IET web site does give a summary of what is permitted.

As far as I am aware there is no minimum size only a maximum size. Where we have found faults it is common to down grade the MCB. For example on a radial where some one in error has used 2.5mm in part of it instead of 4mm reducing the MCB to 20A is a good method to stop overload until it can be rewired.

The same of course where one finds 1.5mm in a ring main. To protect at 16A would make it safe.

Although we would not design a circuit using 1.5mm we have to use some common sense when we find it and I can't see any reason why we should not down grade. Same applies with insulation in walls.

 

[EFLImpudence]

it doesn't really say what the minimum breaker size can be. I got the impression that it's 20A like a ring main but would like to check.

Did you really mean to say 'minimum'?
It seems an odd question to ask. Why would you want a lower rating than would be allowed?

Also, your numbers do not match my 16th edition.

You could have a 6A MCB on the circuit but it wouldn't be a lot of use.

There is another thread which has veered on to this subject at-
http://www.diynot.com/forums/viewtopic.php?t=286875&start=30

 

[ban-all-sheds]

I got the impression that it's 20A like a ring main

Err....

Why do you think that 20A is the minimum breaker size for a ring main (sic)?

 

[JohnW2]

You could have a 6A MCB on the circuit but it wouldn't be a lot of use.
There is another thread which has veered on to this subject at ....

There is an aspect of this which has not been considered, in either thread. Sure, from the point of view of the protection of cables, there's clearly no such concept as the 'minimum allowable' rating of a circuit's OPD. In that sense, a circuit with a 'surprisingly low' MCB rating would be 'safe'.

Compliance is perhaps a different matter. As I don't need to tell you, there is (not surprisingly) a requirement in BS7671 that the In of a protective device must be at least as high as the circuit's design current. Whilst one has to accept that 'determining' (if that's the word!) the design current for a sockets circuit is more a crystal ball job than any sort of science or engineering, I think it might be difficult to justify a design assumption that the 'design current' for a circuit supplying several 13A socket outlets was 6A!

Kind Regards, John.

 

[EFLImpudence]

I wasn't suggesting the circuit should have a 6A CPD; merely pointing out that it would not be of much use, in an attempt to discover the reason for the original question.

Indeed, the question only makes sense if maximum is substituted for minimum.

 

[westie101]

Consider the case of a sub-main to, say, a garage some distance from the house (there was a similar question some time ago for a shed 300 away)
Now you would possibly be looking at a design current of 45A, but using "normal" sized cables the volt drop would be too high!

As you are designing the circuit (not blindly following BS7671) there is nothing to stop you, in theory anyway, using a piece of, say, 95mm2 cable for volt drop purposes
You still could protect it to the design current.

It is something we do on a regular basis by using a 300mm2 for volt drop purposes, where for load a 95mm2 would do. We would also protect it with an appropriate fuse size to grade with other protection. This could see a fuse as small as 160A being used.

We also have a case, in a 185mm2 3 core cable, where two cores in parallel are the live and the other one and the normal N/E sheath is the neutral. This is fed from a 25kVA single phase transformer with a 100A fuse!
Suitable notices do appear at the transformer and on our cable records to this effect.

 

[JohnW2]

I wasn't suggesting the circuit should have a 6A CPD; merely pointing out that it would not be of much use, in an attempt to discover the reason for the original question.

I realised that, but was pointing out that it would not only be of not much use, but wouldn't even be compliant with BS7671 unless you could justify a belief that 6A was an appropriate design current for the circuit (which I imagine would be difficult).

Kind Regards, John.

 

[westie101]

but wouldn't even be compliant with BS7671 unless you could justify a belief that 6A was an appropriate design current for the circuit (which I imagine would be difficult).

Though it would not be much practical use as a circuit, there is nothing to stop any designer specifying that a 6A protective device be installed.
It is no less safe!

As a designer I only have to show compliance with the safety requirements of any legislation. What I choose as design parameters do not need justifying!

 

[JohnW2]

Consider the case of a sub-main to, say, a garage some distance from the house (there was a similar question some time ago for a shed 300 away)
Now you would possibly be looking at a design current of 45A, but using "normal" sized cables the volt drop would be too high!
As you are designing the circuit (not blindly following BS7671) there is nothing to stop you, in theory anyway, using a piece of, say, 95mm2 cable for volt drop purposes You still could protect it to the design current.

That is not in dispute; I think you may have misunderstood the point that I was discussing.

There is absolutely nothing wrong with having a cable which is larger than the minimum required for the design current, nor with having an OPD with an In lower than the maximum that would be acceptable to protect the cable - and that is true whether those things are being done 'for the hell of it' or for some good reason such as you are suggesting (voltage drop).

What I was talking about was a situation in which the design current (which cannot be greater than the In of the OPD) is arguably too low for what one might realistically expect the demand on the circuit to be. I was suggesting that eyebrows could be raised at the suggestion that the design current was 6A for a circuilt supplying multiple 13A socket outlets. Of course, there are some situations in which one might be pretty confident that it was unlikley that the 6A total demand would ever be exceeded (despite the multiple 13A sockets), but I personally feel that it would be difficult to justify that in most 'normal' socket circuit situations; let's face it, a good few of the things which may be plugged into 13A sockets will individually take considerably more than 6A.

Kind Regards, John.

 

[EFLImpudence]

wouldn't even be compliant with BS7671 unless you could justify a belief that 6A was an appropriate design current for the circuit

Then it would.

Belief and appropriate are not the right words if the design current were 6A.

Would the main argument not be that too large a cable had been used?
Although, again this would be extra safe and not non-compliant.

This is even more hypothetical than usual as we, or at least I, have no idea what the OP has in mind.

 

[westie101]

John regarding the loading on a circuit I would tend to agree.
My main point around that is that whilst a designer has to justify his circuit design, I would be surprised if he had to justify picking a 6A design requirement and if in that position would strongly argue the point.
OK it's a subtle difference but I think important

 

[JohnW2]

Though it would not be much practical use as a circuit, there is nothing to stop any designer specifying that a 6A protective device be installed.
It is no less safe!
As a designer I only have to show compliance with the safety requirements of any legislation. What I choose as design parameters do not need justifying!

That may well be true will your rules, but not BS7671. We have all agreed that an MCB with a 'surprisingly low' MCB is 'no less safe'

However, BS7671 does impose requirements on the design process. Specifically, it requires than the In of the OPD shall not (Edit: delete 'not'!!) be greater than the 'design current' of the circuit - and that is defined (something like) the total current that the designer believes is likely to ever be demanded. If the designer cannot demonstrate that the design current is 'reasonable' (per that definition), and below In of the OPD, then that cuircuit will not be compliant with BS7671.

Kind Regards, John.

 

[westie101]

So I'm wiring my new house and have decided to have one RFC per room (wasteful, a bit odd but permitted)
One of these rooms will be for home entertainment and contain a large screen TV, DVD with surround sound to that end I am not fitting any sockets but feeding the equipment from suitably fused FCOs to prevent higher loaded appliances being used.
The total load being 3A

So as I do not, at this point in time, wish to use anything of a higher loading in this room I am fitting a 6A MCB supplying 2.5mm2 cable. It does mean though that if things change in the future I could, if I wished change the MCB to anything up to 32A

And you are saying I cannot as it does not accord with a non-statutory Code of Practice.

I can show it meets all requirements for safety, volt drop etc.