Sub main query

[RF Lighting]

You can't use XPLE figures unless the devices it's terminated onto are rated for 90°C connections.

I see. In that case, what figures does one use? It would presumably be inappropriate to use 70°C thermoplastic figures (which would give 78A for buried 16mm² SWA), since they relate to a different material.

Kind Regards, John

I would think using the PVC figures would be a good place to start as they assume a maximum conductor operating temperature of 70°C, and I can't imagine the thermal insulating properties of PVC differs that much from XLPE.

I think you could probably use the adiabatic equation to work out the maximum CCC of XLPE conductors at 70°C if you really wanted to.

 

[JohnW2]

Quite apart from the (probably limiting) issue of voltage drop to which RF has referred, in design terms you appear to be approaching this issue somewhat 'back-to-front'. The 'design load/current' should be the actual required load (or, at least, an estimate thereof). The cable is then required to have a CCC at least as great as that design load, and, in turn, the device protecting that cable has to have an In no higher than that CCC.

This doesn't seem the right way round. You calculate the load and then the minimum size for the protective device. This is used to calculate the size of the cable.

Good to see you - an hour of so early this week! Since a further opportunity may not arise, let me take the opportunity to offer my Season's Greetings.

Anyway ... the crucial part of the point I was making is that one should work backwards from the design load, rather than what the OP appears to have been doing (and, despite countless requests from several of us, we still have no real idea of what sort of load we are talking about). However, yes, I agree with the rest of your point. Given the limited number of 'sizes' of OPD available, it will usually be necessary to use one which has a higher In than the design current of the circuit - in which case, as you suggest, a cable has to be selected which is adequately protected by that OPD, even if a smaller cable would have been adequate to carry the design current.

As for the thread, if the OP is an electrician, he is out of his depth.

I'm not sure that statement really need to be qualified by "if the OP is an electrician". He is certainly putting himself in the position of a designer and, whether an electrician or not, he appears to be out of his depth!

Kind Regards, John

 

[JonathanA]

If I knew it all, I wouldn't be seeking advice here.

Thanks to those who have provided constructive advice.

Regards,

J

FWIW I am actually a proctologist.

 

[JohnW2]

You can't use XPLE figures unless the devices it's terminated onto are rated for 90°C connections.

I see. In that case, what figures does one use? It would presumably be inappropriate to use 70°C thermoplastic figures (which would give 78A for buried 16mm² SWA), since they relate to a different material.

I would think using the PVC figures would be a good place to start as they assume a maximum conductor operating temperature of 70°C, and I can't imagine the thermal insulating properties of PVC differs that much from XLPE.

Yes, it's probably a reasonable 'starting point'. I also agree that the properties of the two materials do not differ appreciably, but the 78A figure for PVC at 70°C is so close to 80A that a very small difference could have made a difference to the OP.

I think you could probably use the adiabatic equation to work out the maximum CCC of XLPE conductors at 70°C if you really wanted to.

That approach would be inappropriate. CCC is all about a thermal steady state during continuous current flow - which is the antithesis of an adiabatic process. An adiabatic calculation would only tell you what (very high) current the conductor could tolerate for a very short period of time (usually taken as <5s) - and that current would be many times greater than the 'CCC' of the conductor.

Kind Regards, John

 

[ban-all-sheds]

The more I'm reading here the more I get the feeling the OP is well out of his depth with this job.

It was clear from the outset that he was never in his depth.

He's not an electrician, he can't even realistically pretend to be one, and yet for some unfathomable reason seems to think that he can act as one.

 

[ban-all-sheds]

As for the thread, if the OP is an electrician, he is out of his depth.

He's not.

And I wrote that, and my previous post, before I got to Page 4.

It beggars belief that anybody could ever have thought that he was.

 

[ban-all-sheds]

If I knew it all, I wouldn't be seeking advice here.

And if you had any common sense or decency you would not have decided that ignorance and incompetence should not be a bar to you blundering on anyway.

FWIW I am actually a proctologist.

Really. I wonder what you would think of someone who decided to have a bash at a bit of colorectal surgery even though they didn't know about it?

I'd rather not do this unless it is strictly necessary since it will invite all sorts of difficult questions!

I think it fair to say that I would be out of a job if I had to either uprate the cable or limit the load to 50A so neither are going to happen!

Just what kind of proctologist job will you lose when (not if) your electrical incompetence comes to light?









Does anyone have any idea what this Wikipedia paragraph means?

"Colorectal surgery is a field in medicine, dealing with disorders of the rectum, anus, and colon. The field is also known as proctology, but the latter term is now used infrequently within medicine, and is most often employed to identify practices relating to the anus and rectum in particular."

 

[scousespark]

As for the thread, if the OP is an electrician, he is out of his depth.

He's not.

And I wrote that, and my previous post, before I got to Page 4.

It beggars belief that anybody could ever have thought that he was.

The OP wrote the following in his first response.

I think it fair to say that I would be out of a job if I had to either uprate the cable or limit the load to 50A so neither are going to happen!

This is why I asked. It would not be the first time an inexperienced electrician had overstretched hemselves with a job.

 

[ban-all-sheds]

And nor would it be the first time that someone who had no claim to being any sort of electrician had decided that it was perfectly OK to pretend to some hapless client that he was one and to take money under false pretences.

Were it not for the "out of a job" comment he would have come across as an out-of-his-depth DIYer.

With the comment he came across as an out-of-his-depth builder or landscape gardener or handyman.

But not an electrician of any sort.

 

[scousespark]

In general, gardeners and handymen don't: quote supply characteristics; state they have considered switchfuse or mcb; give the maximum ccc for their chosen cable. In the world of web forums (not sure if this is the plural) and academia - everything is always perfect. This is not always the case in the real world.

Given the OP has said he's not an electrician, this is all moot.

 

[scousespark]

Good to see you - an hour of so early this week! Since a further opportunity may not arise, let me take the opportunity to offer my Season's Greetings.

Kind Regards, John

And Seaon's Greetings to you and yours John.

 

[deadshort]

If I knew it all, I wouldn't be seeking advice here.

And if you had any common sense or decency you would not have decided that ignorance and incompetence should not be a bar to you blundering on anyway.

FWIW I am actually a proctologist.

Really. I wonder what you would think of someone who decided to have a bash at a bit of colorectal surgery even though they didn't know about it?

I'd rather not do this unless it is strictly necessary since it will invite all sorts of difficult questions!

I think it fair to say that I would be out of a job if I had to either uprate the cable or limit the load to 50A so neither are going to happen!

Just what kind of proctologist job will you lose when (not if) your electrical incompetence comes to light?









Does anyone have any idea what this Wikipedia paragraph means?

"Colorectal surgery is a field in medicine, dealing with disorders of the rectum, anus, and colon. The field is also known as proctology, but the latter term is now used infrequently within medicine, and is most often employed to identify practices relating to the anus and rectum in particular."

He should take to electrics like a duck to water then Apart from the drop in status and salary that is.

DS

 

[EFLImpudence]

http://www.electriciansforums.co.uk...-electrical-forum/98065-sub-main-query-2.html

 

[ban-all-sheds]

Oh dear oh dear.

Even more telling is to read this:

http://www.electriciansforums.co.uk...al-electrical-forum/97044-sub-main-house.html

I wonder exactly when (as predicted on the other site which I imagine he has abandoned because the advice there wasn't what he'd already decided he should be getting) "I will not be doing this work myself" morphed into "I have run the cable through underground ducting and already backfilled the trench"?



JonA wrote: "I am not an electrician and am probably a bit out of my depth."

Indeed you are.

But you didn't let that stop you from going ahead and doing things outwith your competence and doing them wrongly, did it?

BTW - what do you now expect to happen about Building Regulations sign-off?

 

[Adam_151]

I think you could probably use the adiabatic equation to work out the maximum CCC of XLPE conductors at 70°C if you really wanted to.

You could probably work it out, but not with the adiabatic equation.

(The adiabatic assumes no heat is lost to the surroundings, which is an approximation which gives a reasonable result up to about 5 seconds)