431.2.1 again

[Jeffmo65]

Commercial scenario :
lets say 54m , mmm ok 16amp OCPD, mmm ok at 52m I think I will put a 20amp D/P isolator , humour me I will go on . Actually oh dear I've ran out of 4mm T/E , tell you what I will just use 2.5mm instead . Anybody see an issue , cable clear run on open spacious cable tray no influences just straight clear run .
I just know that someone will say it's fine cos 2.5mm will take 25 odd amps .
Conversations requested guys

 

[Leesparkykent]

Mmmmmm

 

[Bobster]

Under new rules I'm sure trolling is worth a 3month ban.

 

[ruston]

Under new rules I'm sure trolling is worth a 3month ban.

Or 2-5 months

 

[Bobster]

Or 2-5 months

I see what you did there! Haha

 

[Jeffmo65]

Sorry guys not sure really what you mean . If I've broken some rules then I apologise . Oh and the cable size is after the isolator NOT A FUSED SPUR by the way

 

[ruston]

Lol , don't worry about it , some start posts like this as a wind up , but it has fallen out of favour in the last few days.

 

[Plonker 3]

So do VD,Zs and CCC comply with BS7671?

 

[Jeffmo65]

Oh good ,! Thought I was for the high jump . Anyway any views on my scenario ?.....

 

[Jeffmo65]

Oh don't worry about VD it's irrelevent ............ Isn't it . Let's see 2.5mm ooh about 40m max . 4mm what about 60 odd . Oh B type MCB BS60898 Schneider acti 9 10ka . Oh bugger lets just put the last few meters in 2.5mm

 

[ruston]

There again.......................

 

[Plonker 3]

Have you carried any calcs out to prove it doesnt comply?

 

[Jeffmo65]

Hi Dillb . Well this is purely a scenario but I would have thought that the length if you were to carry on from isolator with 4mm then great but this wasn't the question . I've seen lots of installations where other sparks have ran circuit in X cable to an isolator then for some unbeknown reason dropped to cable Y . My question is why not run the whole lot in cable Y if it's ok to use this on the last few meters . I think not . What's your thoughts ?.....??????

 

[Plonker 3]

Have you got some reg numbers to back up your thoughts?

 

[Jeffmo65]

431.2.1 ????

 

[Leesparkykent]

431.2.1 ????

What about it?

 

[Plonker 3]

What???

As long as the neutral conductor can carry the current without overloading then there is no problem

 

[Richard Burns]

Too slow

 

[Jeffmo65]

So voltage drop has nothing to do with it then ????

 

[Plonker 3]

So voltage drop has nothing to do with it then ????

What you going on about?

 

[Leesparkykent]

I don't really understand what your point is? Your OP is about 4mm then isolator then 2.5 and now your on about 431.2.1 which is about having a neutral that's CSA is smaller than the line conductor.

 

[Jeffmo65]

Ok let's rephrase . If calculations show that cable x should be used then how is it ok to use cable Y for the last few metres of this run . I'm not arguing I'm just posing a question that's all . If cable Y could be used for last few metres of said circuit then why are we using cable x for the first bit ?

 

[Jeffmo65]

sorry this is BRG NOT BGB

 

[Leesparkykent]

Ok let's rephrase . If calculations show that cable x should be used then how is it ok to use cable Y for the last few metres of this run . I'm not arguing I'm just posing a question that's all . If cable Y could be used for last few metres of said circuit then why are we using cable x for the first bit ?

Probably to meet max Zs figures and permitted volt drop

 

[Plonker 3]

YOu work out your cable calcs on the length and CSA of the conductors used, so if the first 50 meters is 4mm then the final 4 meters is in 2.5mm then you calculate it accordingly.

 

[Jeffmo65]

Now your confusing me how can voltage drop be better on a smaller cable at the end of a circuit . Maybe understand if other way round

 

[Plonker 3]

Its not, but there will less VD on a short run of cable

 

[Richard Burns]

OK ignoring the 431.2.1 reference which appears irrelevant.

If you have a circuit that start in a large csa and then drops to a smaller csa then it is likely that the designer was ensuring that the volt drop limits for the circuit were not exceeded, however you have said that volt drop is immaterial in an earlier post.

Your description of a 54m circuit in 2.5/1.5 would exceed the 11.5 V limit, however if half the circuit were run in 4/1.5 then you may just make the limit.

 

[Jeffmo65]

totally agree that you calculate on length and current carrying capacity but if total circuit length equates to using 4mm where and how do you calculate using 2.5 without fusing down at point of isolation .

 

[Leesparkykent]

totally agree that you calculate on length and current carrying capacity but if total circuit length equates to using 4mm where and how do you calculate using 2.5 without fusing down at point of isolation .

Then that's what you would use or a cable of a bigger CSA

 

[Jeffmo65]

I said VD WAS irrelevent being pedantic of course it matters . .ok let's say the circuit is designed on the limit in 4mm , putting 2.5 at end would not be correct . Yes?

 

[Richard Burns]

The whole circuit is suitable for 2.5/1.5 from a CCC consideration, so it is also suitable for 4/1.5 in terms of CCC.
therefore there is no need to fuse down.
The decreased resistance provided by the 4/1.5 serves to reduce the volt drop only.

 

[Jeffmo65]

Then that's what you would use or a cable of a bigger CSA

Hurar finally . Thank you Leesparks

 

[Jeffmo65]

The whole circuit is suitable for 2.5/1.5 from a CCC consideration, so it is also suitable for 4/1.5 in terms of CCC.
therefore there is no need to fuse down.
The decreased resistance provided by the 4/1.5 serves to reduce the volt drop only.

what do you abbreviate to CCC MATE

 

[Richard Burns]

what do you abbreviate to CCC MATE

Current carrying capacity

 

[Jeffmo65]

Ok so circuit regarding voltage drop is on limit at end of total circuit we can use 2.5mm for the last few meters ??

 

[ipf]

what do you abbreviate to CCC MATE

300....in roman numerals

 

[Plonker 3]

Ok so circuit regarding voltage drop is on limit at end of total circuit we can use 2.5mm for the last few meters ??

Strewth this is hard work. Only if by dropping the CCC the Zs and VD values comply with the BGB or BYB

 

[Jeffmo65]

Good one and to think I did Latin at school . In fact they were probably still speaking in Latin when I went to school .

 

[Jeffmo65]

Streuth your not wrong . Ok the CCC is fine , Zs is fine but OBVIOUSLY VD WOULD NOT BE BECAUSE OF THE 2.5 add on