Which MCB for this cable?

[officeguy]

Thanks for the reply guys!!

So a 10A MCB it is!

Wow, I didn't know that the 50% was only for cable wrapped in insulation. Didn't think people really do that (I though covering with insulation was bad enough!).

Just another question. The 13A cable max. How does this value compare to the overload? Let's say an MCB allows 15A for an hour (I just picked these values out of the air), would that be bad for the cable, or reasonable?

I guess what I'm asking, is that the calculated figure of 13A, is that cold turkey max before you're a fire risk, or is that value for normal operation (with additional headroom for limited-time overload)?

Thanks

 

[JohnW2]

Just another question. The 13A cable max. How does this value compare to the overload? Let's say an MCB allows 15A for an hour (I just picked these values out of the air), would that be bad for the cable, or reasonable? ... I guess what I'm asking, is that the calculated figure of 13A, is that cold turkey max before you're a fire risk, or is that value for normal operation (with additional headroom for limited-time

Your 'out of the air' figure is very close. A 10A MCB will allow about 14.5A to flow for about an hour.

The 'current-carrying-capacities' (CCCs) we work with (with or without adjustment for insulation etc.) take into account the characteristics of the MCB or fuse which (as you say) will allow appreciably higher currents (than those 'CCC's) to flow for appreciable, but not very long, periods of time. Over and above that, there is also undoubtedly appreciably 'headroom' built into those CCCs we work with.

In other wotds, it all errs on the side of safety.

Kind Regards, John

 

[officeguy]

Excellent, that sums everything up nicely!

How do you know what is an acceptable limited-time overload current for a given CCC? Let's take the 13A in my cable's case. What maths did you do to realise that 14.5A for 1 hour is safe?

Sorry for all the questions. I'm not a spark, just very interested!

Thanks

 

[JohnW2]

Excellent, that sums everything up nicely! ... How do you know what is an acceptable limited-time overload current for a given CCC? Let's take the 13A in my cable's case. What maths did you do to realise that 14.5A for 1 hour is safe?

You're welcome. The specification of a Type B (the common domestic type) of MCB requires that it will allow 1.13 times it's rated current (e.g. 11.3A for a 10A MCB) to flow indefinitely, and that it will trip in no longer than one hour at a current of 1.45 times it's rated current (e.g. 14.5A for a 10A MCB). The tables of cable CCCs therefore take into account the possibility of these 'overcurrents' flowing for up to an hour.

Hope thathelps.

Kind Regards, John

 

[officeguy]

Excellent, that sums everything up nicely! ... How do you know what is an acceptable limited-time overload current for a given CCC? Let's take the 13A in my cable's case. What maths did you do to realise that 14.5A for 1 hour is safe?

You're welcome. The specification of a Type B (the common domestic type) of MCB requires that it will allow 1.13 times it's rated current (e.g. 11.3A for a 10A MCB) to flow indefinitely, and that it will trip in no longer than one hour at a current of 1.45 times it's rated current (e.g. 14.5A for a 10A MCB). The tables of cable CCCs therefore take into account the possibility of these 'overcurrents' flowing for up to an hour.

Hope thathelps.

Kind Regards, John

Ok great! But let's say I have a hypothetical 10A cable (I know my real cable is 13A). You say that a 10A MCB would allow 11.3A indefinitely. Is that still acceptable for this hypothetical cable?

 

[JohnW2]

Ok great! But let's say I have a hypothetical 10A cable (I know my real cable is 13A). You say that a 10A MCB would allow 11.3A indefinitely. Is that still acceptable for this hypothetical cable?

Exactly. When we (the Tables of CCCs we use) say that it is "a 10A cable" (i.e. has a CCC of 10A), that means that it is deemed to be 'safe' with 11.3A indefinitely or 14.5A for up to an hour.

To talk about a "10A cable" (or a "13A cable") is obviously not correct - since, as you know, the CCC of a particular cable will differ according to how it is installed (e.g. in relation to insulation). The effective CCC will also be different (lower) if it is protected by certain types of fuse (rather than an MCB) which can allow up 2 double (rather than '1.45 times') the 'rated' current to flow for up to an hour.

Kind Regards, John

 

[officeguy]

That's very simple actually. I wish that was well documented online as I've been scratching my head over this for ages! I was worried that any calculated rating value would have to include the MCB over-currents.

So basically, after taking into account the rating based on your installation method and the CCC tables, it is still safe for 1.13x indefinitely, or for 1.45x for 1 hour.

Cheers

 

[ban-all-sheds]

Note: I live in a part of the UK where there is no Part P.

Unless you think that because of that it's OK for any work you do not to be reasonably safe, I'm not sure why you think it's relevant.

 

[JohnW2]

That's very simple actually. I wish that was well documented online as I've been scratching my head over this for ages! I was worried that any calculated rating value would have to include the MCB over-currents. ... So basically, after taking into account the rating based on your installation method and the CCC tables, it is still safe for 1.13x indefinitely, or for 1.45x for 1 hour.

Indeed, but the whole point is that these Tables has been drawn up in such a way that you don't need to get involved in all this thinking about the 1.13 and 1.45 factors. If the Table says that the CCC is 10A (after adjustment for insulation etc., if necessary), then that cable will be safe with an MCB rated no higher than 10A - "Simples", as a well-know meerkat would say

Kind Regards, John

 

[officeguy]

Note: I live in a part of the UK where there is no Part P.

Unless you think that because of that it's OK for any work you do not to be reasonably safe, I'm not sure why you think it's relevant.

Now where did you get that from? Obviously, I am here asking questions to make sure that the installation is safe, and want to understand it.

I just didn't want any lectures on Part-P notifications etc as it doesn't apply to me.

Thanks

 

[JohnW2]

Note: I live in a part of the UK where there is no Part P.

Unless you think that because of that it's OK for any work you do not to be reasonably safe, I'm not sure why you think it's relevant.

I suspect he was hoping to (seemingly not entirely successfully!) pre-empt responses of a sort which he has not infrequently seen in this forum in the past.

It is apparent from the questions that the OP is asking me that he is very concerned about matters of safety, and wishes to have an understanding of the reason for what he's being told, all of which is clearly laudable.

Kind Regards, John

 

[officeguy]

That's very simple actually. I wish that was well documented online as I've been scratching my head over this for ages! I was worried that any calculated rating value would have to include the MCB over-currents. ... So basically, after taking into account the rating based on your installation method and the CCC tables, it is still safe for 1.13x indefinitely, or for 1.45x for 1 hour.

Indeed, but the whole point is that these Tables has been drawn up in such a way that you don't need to get involved in all this thinking about the 1.13 and 1.45 factors. If the Table says that the CCC is 10A (after adjustment for insulation etc., if necessary), then that cable will be safe with an MCB rated no higher than 10A - "Simples", as a well-know meerkat would say

Kind Regards, John

Incidentally, is the table available online somewhere where I can see the 13A figure? Also, when you talk about "some kinds of fuses", is that in reference to rewireable fuses in the CU (instead of MCB), or do my 5A fused switches (in line with each heater) mess the calculations up?

Thanks

 

[ban-all-sheds]

I just didn't want any lectures on Part-P notifications etc as it doesn't apply to me.

Notification is something completely separate from Part P.

 

[officeguy]

Note: I live in a part of the UK where there is no Part P.

Unless you think that because of that it's OK for any work you do not to be reasonably safe, I'm not sure why you think it's relevant.

It is apparent from the questions that the OP is asking me that he is very concerned about matters of safety, and wishes to have an understanding of the reason for what he's being told, all of which is clearly laudable.

Kind Regards, John

Thanks John

I'm not doing the work myself actually. I just want to be able to understand the work someone else is doing for me. He is registered with the NICEIC, but I still want my own understanding to be on the safe side Don't want anyone pulling the wool over my eyes.

 

[ban-all-sheds]

It is apparent from the questions that the OP is asking me that he is very concerned about matters of safety, and wishes to have an understanding of the reason for what he's being told, all of which is clearly laudable.

Indeed.

So why go to the trouble of saying that he isn't under any legal obligation to make reasonable provision for safety?