4mm radial on a 32A?

[HappyHippyDad]

Bit bored, boys in bed, beers in tummy and I thought I'd ask everyone why a 4mm radial is allowed to have a 32A MCB protecting it when its current carrying capacity is only above 32A when it is clipped direct or in free air?

I'm thinking diversity, but what if its your only circuit for sockets, then no diversity applies and I kind of assumed diversity was for max demand of household anyway?

Cheers :thinking:

ps... may be a little sporadic replying as just starting Star Trek!!

 

[ipf]

Bit bored, boys in bed, beers in tummy and I thought I'd ask everyone why a 4mm radial is allowed to have a 32A MCB protecting it when its current carrying capacity is only above 32A when it is clipped direct or in free air?

I'm thinking diversity, but what if its your only circuit for sockets, then no diversity applies and I kind of assumed diversity was for max demand of household anyway?

Cheers :thinking:

ps... may be a little sporadic replying as just starting Star Trek!!

To boredly go where no man has gone before, eh?

 

[HappyHippyDad]

Any takers? Otherwise I'm going to be thinking about this for the next few years? :thinking:

 

[telectrix]

IMO if the 4mm is to be derated due to insulation, whereby it's CCC is reduced below 32A, then a lower rated MCB should be used. generally, 4mm radial would be clipped direct and maybe partly buried in masonry, so a 32A would then be suitable.

 

[Silly Sausage]

Because it says so in Appendix 15. So it must be right?!

 

[telectrix]

it says "persil washes whiter" on buses, but they don't take in washing.

 

[HappyHippyDad]

Because it says so in Appendix 15. So it must be right?!

I think you're probably saying that a bit tongue in cheek. Appendix 15 does say that, so I thought I must be missing something as BS7671 is pretty thorough and precise. I guess this is where I'm reminded that BS7671 is not infallable and common sense and knowledge should be used as well.

 

[Silly Sausage]

Aye, a hint of sarcasm!


I go with what Tel says.

 

[Dohnut]

We have been using 4mm 32 amp radials for about 3 years after it was recommended by our Elecsa inspector, we use them for Kitchens circuits and use 2.5mm 16 amp radials for the rest of the property.

 

[kingeri]

It's true 4mm radials are just teetering on the edge when it comes to CCC on a 32a breaker. It's one of their limitations, and probably why the ring is still so popular! My kitchen radial is on a 25a breaker due to installation method. I also find 4mm T&E a bit limiting on occasion when it comes to max circuit length....would be so much more useful if it had a 2.5mm CPC.

 

[Dohnut]

It's true 4mm radials are just teetering on the edge when it comes to CCC on a 32a breaker. It's one of their limitations, and probably why the ring is still so popular! My kitchen radial is on a 25a breaker due to installation method. I also find 4mm T&E a bit limiting on occasion when it comes to max circuit length....would be so much more useful if it had a 2.5mm CPC.

Agreed a 2.5mm CPC would be very useful.

 

[jumpin jax]

Agreed a 2.5mm CPC would be very useful.

Only for fault protection to bring Zs down, but if covered by RCD then no problem, if it has 4.5mm conductors even better! Tin hat on.

 

[Dohnut]

Only for fault protection to bring Zs down

Exactly what i wanted it for.

 

[telectrix]

IMO, BS7671 has got the figures wrong. 2.5mm is rated at 27A (method C). 4mm is almost double the csa of 2.5mm) and is also stranded cable, so should be rated accordingly. a rating of 40A ( method C) would be more appropriate. tin hat on.

 

[kingeri]

Only for fault protection to bring Zs down, but if covered by RCD then no problem, if it has 4.5mm conductors even better! Tin hat on.

That was the whole point. Why else would I mention having a bigger CPC? To make it easier to strip the cable???

And are you suggesting that we should ignore the disconnection times for the fuse or breaker if an RCD is present? So if we use an RCD then we can have circuits as long as we want them?

 

[jumpin jax]

My tin hat is bigger than your tin hat

 

[Guest55]

.... but if covered by RCD then no problem,

so max Zs values are out the window so long as some cheap chinese rcd is fitted ?
you'll need more than a tin hat justifing that.

 

[kingeri]

IMO, BS7671 has got the figures wrong. 2.5mm is rated at 27A (method C). 4mm is almost double the csa of 2.5mm) and is also stranded cable, so should be rated accordingly. a rating of 40A ( method C) would be more appropriate. tin hat on.

I know what you mean Tel, I often gaze at 4mm T&E, imagining what that beefy stuff can really do. I'm not being sarcastic either.

 

[jumpin jax]

so max Zs values are out the window so long as some cheap chinese rcd is fitted ?
you'll need more than a tin hat justifing that.

Does this not happen in a TT arrangement?

 

[telectrix]

So if we use an RCD then we can have circuits as long as we want them?


you should have a circuit for as long as you want to keep it. they're not rented. :army:

​

 

[telectrix]

and if we delve deeply into the physics , we will find that electrons travel easier on the surface of a conductorthan towards it's centre , thus making stranded cable a better conductor than it's solid equivalent, so size for size, stranded cable will have a greater CCC than a solid one.

 

[jumpin jax]

And if they didnt twist the strands the electrons would get there quicker

 

[Guest55]

Does this not happen in a TT arrangement?

it does , which was bs7671's intended use for this guidance as 99% of rod installs are above 10 ohms giving you no chance of getting an acceptable Zs.
however many seem to think it can be applied to any situation like some "get out of jail" card as so often posted on this forum.

 

[jumpin jax]

I agree but it does happen. The guide does also allow for an RCD to be used for fault protection in a TN system as per 411.4.4, Why do they put 1.5mm cpc in a 4mm anyway?

 

[Dan]

Bump