Fire Hazard? 2.5mm 20A Cable Changing To 4mm 25A Cable

[ban-all-sheds]

What about this:

?



Cross-sectional area.

 

[JohnW2]

Wow, I've missed all the excitement , I've got some reading up to do. Well, this is what I think but please correct me if I'm wrong (I'm sure I probably am anyway): ...

I really don't think you should concern yourself about what are 'spurs' and what are 'branches' - as you will have seen, the terminolgy is totally confused. As I said before, the important thing for you to understand is that a single cable supplying just one socket can be 2.5mm² - even when the circuit to which it is connected is wired in 4mm² cable and has a 32A MCB.**

And what does CSA mean?

Cross-sectional-area (i.e.'size') of cable conductors - like 2.5mm² or 4mm².

[** question for the electricians - given that a 2.5mm² unfused spur to one socket off a 4mm²/32A radial is permitted, that would almost seem to imply that a piece of 2.5mm² cable running straight from CU to one socket could be protected by a 32A OPD - any comments/thoughts??!]

Kind Regards, John.

 

[Electrifying]

[** question for the electricians - given that a 2.5mm² unfused spur to one socket off a 4mm²/32A radial is permitted, that would almost seem to imply that a piece of 2.5mm² cable running straight from CU to one socket could be protected by a 32A OPD - any comments/thoughts??!]

Kind Regards, John.

Yes, it can - but don't tell the DIYers, it'll only confuse them or get someone hurt........20A protective device , please

 

[EFLImpudence]

[** question for the electricians - given that a 2.5mm² unfused spur to one socket off a 4mm²/32A radial is permitted, that would almost seem to imply that a piece of 2.5mm² cable running straight from CU to one socket could be protected by a 32A OPD - any comments/thoughts??!]

As stated in appendix 15 (in the 'unfused spur' box).

It seems illogical but there is no difference in it being connected at the origin of the circuit or anywhere else on the circuit.
The situation is the same on a 2.5mm² 32A ring.

It will be protected by the one or two 13A fuses in the plugs used and governed by the regulations covering over current protection when a reduction in csa occurs.
As DIYers will not know about this, as I said, I think it unwise advising it on a forum

I suspect (know) you knew all that so what is it, exactly, you are asking.

The situation 'looks' even worse should the main circuit be removed.

 

[JohnW2]

[** question for the electricians - given that a 2.5mm² unfused spur to one socket off a 4mm²/32A radial is permitted, that would almost seem to imply that a piece of 2.5mm² cable running straight from CU to one socket could be protected by a 32A OPD - any comments/thoughts??!]

Yes, it can - but don't tell the DIYers, it'll only confuse them or get someone hurt........20A protective device , please

I'm not sure that I would like to tell a lot of electricians, let alone DIYers Is this the BAS-theory that one can rely on the downstream OPD (fuses) to protect the cable - such that even a 50A MCB would be acceptable?

Kind Regards, John.

 

[JohnW2]

I suspect (know) you knew all that so what is it, exactly, you are asking. The situation 'looks' even worse should the main circuit be removed.

That's really what I was asking. Is it your belief that it is technically compliant if one does remove the main circuit?

Kind Regards, John.

 

[EFLImpudence]

I suspect (know) you knew all that so what is it, exactly, you are asking. The situation 'looks' even worse should the main circuit be removed.

That's really what I was asking. Is it your belief that it is technically compliant if one does remove the main circuit?

I suppose it must be 'technically' compliant (because there it is)but pointless and no mention is made in appendix 15 of the restrictions.

Of course, it has to meet the fault current (and other requirements of 433) which it may not with a 50A opd. (I haven't worked it out).

 

[JohnW2]

That's really what I was asking. Is it your belief that it is technically compliant if one does remove the main circuit?

I suppose it must be 'technically' compliant (because there it is)but pointless and no mention is made in appendix 15 of the restrictions. Of course, it has to meet the fault current (and other requirements of 433) which it may not with a 50A opd. (I haven't worked it out).

That's obviously the crunch - but the fact is that the situation is no different with or without the 'main circuit' connected, and most people probably interpret the regs (particularly Appendix 15, albeit only informative) as indicating that 32A MCB is always adequate to protect a 2.5mm² cable against fault current, even though whether or not that is true surely depends on the Ze/PFC?

Kind Regards, John.

 

[Tozzy]

So just to clarify I presume when you say my 2nd diagram is correct (think I got a mind blank today and thought it was wrong), i assume the current would travel like this (in red):
2.5mm cable not affected by rads in use:

Untitled

• Tozzy
• 12 Aug 2012
• 1

And not like this:

Untitled

• Tozzy
• 12 Aug 2012
• 1

 

[ban-all-sheds]

Both drawings are the same and they are both wrong - how would the current be the same after the first load, and why would a 350W load draw 20A?

 

[JohnW2]

Both drawings are the same and they are both wrong - how would the current be the same after the first load, and why would a 350W load draw 20A?

The indicated currents are not the currents flowing but are what Tozzy believed to be the CCCs of the cables (thanks to a misleading DIYnot 'information' page).

Kind Regards, John

 

[Tozzy]

Both drawings are the same and they are both wrong - how would the current be the same after the first load, and why would a 350W load draw 20A?

No no they are not the same.
The first diagram shows the 2 rads drawing 5KW in total and the current is only flowing through the 4mm cable, not the 2.5mm cable hence why it's shown in black.
The second diagram shows the 2 rads drawing 5KW in total and the current is flowing through the 4mm cable, AND the 2.5mm one but as you can see, the telly is switched off!

Sorry if not clear, I can explain again, it's a bit of a silly question really but I'm just trying to eliminate all the niggling things I wonder about electrics.

I think diagram 1 is right and diagram 2 is wrong. All i need now is for someone to confirm.

 

[JohnW2]

So just to clarify I presume when you say my 2nd diagram is correct (think I got a mind blank today and thought it was wrong), i assume the current would travel like this (in red):
2.5mm cable not affected by rads in use:

Untitled

• Tozzy
• 12 Aug 2012
• 1

And not like this:

Untitled

• Tozzy
• 12 Aug 2012
• 1

That is correct (in terms of current to the rads) - the only current flowing beyond the second socket would be that going to the TV (or anything else plugged into that third socket.

Kind Regards, John

 

[securespark]

Those misleading CCC's in the wiki/sticky/whatever are very mixed up.

Some are values for flexibles, others are neither for flex or cable.

 

[ban-all-sheds]

No no they are not the same.
The first diagram shows the 2 rads drawing 5KW in total and the current is only flowing through the 4mm cable, not the 2.5mm cable hence why it's shown in black.

My apologies - I didn't spot the change in colour - I was too busy being distracted by the figures which I thought represented the currents flowing.

Which they couldn't have been doing anyway, given the loads....

The second diagram shows the 2 rads drawing 5KW in total and the current is flowing through the 4mm cable, AND the 2.5mm one but as you can see, the telly is switched off!

So if the TV is off what would be causing current to flow in the final segment?

I think diagram 1 is right and diagram 2 is wrong. All i need now is for someone to confirm.

I think it's great that you want to learn how things work, but if you are still at the stage where you don't understand the basics of electrical circuits well enough to know, without needing someone to confirm, that if there is no load then there will be no current then you really should do some studying which is properly structured, rather than asking random questions to try and eliminate the niggling things that you might think are all you need to clarify.

The problem with your approach is that you are only going to ask about the things you realise you don't know or don't understand. If there are things you don't know but don't even realise you don't know because you don't know they even exist then you won't ask about them. Ditto with things which you think you understand but are actually wrong.

There's not much that Donald Rumsfeld and I see eye-to-eye on, but he was spot on when he said:

There are known knowns; there are things we know that we know.
There are known unknowns; that is to say there are things that we now know we don't know.
But there are also unknown unknowns – there are things we do not know we don't know.

Asking questions here can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.

You won't ask about things which you don't realise you don't know, or don't realise you've got wrong.

• http://www.google.co.uk/search?q=electrical+circuits+101
  
  
• http://www.google.co.uk/search?q=multimeter+101
  
  
• http://kevinboone.net/electricity.html
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• //www.diynot.com/wiki/electrics