2391-20 cable calcs

[shambock]

Hi

Anyone help with some cable calcs for my designe project, i must be missing something here.

Im trying to work out and show my calculations for cable sizing of a 32A ring

Length is 78 M

Number of 13A double sockets is 7

Ib is 32A
In is 32A RCBO
It should be In / Correction factors of wich i only have a grouping factor of 0.41
This makes it 32/0.41 = 78A

The above formula say that i should be using a cable capable of carrying 78A wich brings it upto 25mm ????????????? (ref meth B,singles) table 4D1A.

I know this is wrong but where am i mucking up.

Please help

 

[Risteard]

Personally I would use Ib of 26A for a ring final circuit.

As for sizing cable, BS 7671 stipulates that it must have a current carrying capacity of at least 20 amperes.

 

[VoltzElectrical]

YOUR NUMBERS sound ok, just checked the BRB. Perhaps I am wrong! Is this just a question to check for your understanding of the regs etc.. with maybe the next part 'ok, so this is obviously Unpractical, so how would you design the installation differently'?

 

[VoltzElectrical]

As a second thought, perhaps you should be applying diversity.

 

[Geoffsd]

Where did you get 0.41.

The only 0.41 in the table is for 16 cables 'bunched'. i,e. tied together.

 

[shambock]

Table 4C1 says buched,embedded or enclosed (ie trunking or conduit)

 

[Geoffsd]

Didn't know if you were given the figure in an example or had worked it out - possibly incorrectly.

Are there no variables?

 

[shambock]

YOUR NUMBERS sound ok, just checked the BRB. Perhaps I am wrong! Is this just a question to check for your understanding of the regs etc.. with maybe the next part 'ok, so this is obviously Unpractical, so how would you design the installation differently'?

No this is my deign from scratch

 

[shambock]

Didn't know if you were given the figure in an example or had worked it out - possibly incorrectly.

Are there no variables?

could try to get rid of correction factor by tubing each circuit but proves unpractical and expensive

 

[VoltzElectrical]

Where is your design current from?

 

[VoltzElectrical]

Sorry, daft question!

 

[shambock]

Where is your design current from?

The design current i am using is based on the fact that i have no known current using equpment.

am i wrong in assuming that the length of run would require a ring with most rings put on a 32A cb

 

[VoltzElectrical]

i'm gonna get the books out! Please provide all details of the proposed circuit.

 

[shambock]

OK Voltz hope you have better luck than me.

Length is 78 M run in trunking and conduit - ref meth B (probably singles)

Number of 13A double sockets is 7

Ib is 32A

In is 32A RCBO

It - should be In / Correction factors of wich i only have a grouping factor of 0.41 for 16 circuits in trunking

This makes it 32/0.41 = 78A

if i get rid of correction factor i can get away with a 4mm cable

Good luck

 

[VoltzElectrical]

Luck ran out. I await words from the wise ones! I have personally run many, many time, circuits in 4"x4" trunking (ok maybe not up to 16, but maybe) in schools, hospitals etc..

Standard lighting and power in 2.5mm, 1.5mm, singles, and it has all been designed, inspected, and passed. There is something definitely amiss here. How on earth do you propose to terminate said conductors at accessories?

I too have followed the cable calculations protocol and come out with the same results. What do they design large capacity trunking for?

Lets hope we find out the answer. I feel a naked lady coming on!