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!

 

[shambock]

Cheers Voltz

Thanks for trying, i think a new day with a new head will help see through the dark mist!!

 

[scotsparky]

As its a ring have you devided total length by 4

 

[Geoffsd]

I'm not sure what you are trying to do exactly but you will have to change the derating factor by choosing another installation method.

Are the circuits all 78m run together? Move the db or have an additional one nearer.

 

[VoltzElectrical]

is that not for just checking expected values of R1 + R2 on a RFC test? or is there something else about design I missed?

 

[Sparky Ninja]

Ring circuit cable tabulated rating (It) formula

In BS7671 the rules are changed for the special case of the 30 and 32 A final ring circuit (Regulation 433.1.5), where overload protection requirements are amended allowing 20 A rated cables (Iz > or equal to 20 A) providing the load current in any part of the ring is unlikely to exceed 20 A.

Therefore Iz > or equal to 20 but Iz = It Ca Cg Ci Cc

Hence It > or equal to 20/(CaCgCiCc)


Example

A ring final circuit wired in thermoplastic (PVC) cable a sper Table 4D5 enclosed in conduit in an insulated wall and protected by a 32 A circuit breaker (In = 32 A); the cable rating must equal or exceed 20 A. The cable rating,

It > or equal to 20/(CaCgCiCc) if Ca = Cg = Ci = Cc = 1 then It > or equal to 20 A

From Table 4D5 (column 7) a 2.5mm cable is therefore adequate.

 

[Engineer54]

Are yours and all these other 16 circuits operating at full load currents?? Now, there's a little clue for you ...lol!!!

 

[VoltzElectrical]

Widdler. an excellent response. But how do we proceed given that the OP had specified that the circuit be run in singles, ref method B. does not table 4D5 not apply here as that's for twin and earth?

 

[VoltzElectrical]

'54, how do you apply diversity to this situation (i take it that is what you are implying?) OSG states that diversity to socket outlets 10 A then 50% in domestic.

It's a confusing scenario, with not enough detail, granted

How then do I find myself pulling many meters of many circuits into sometimes 6" trunking on runs in schools etc..?

 

[VoltzElectrical]

Surely we have to start with the protective device rating, then work our way up ( or down) from there?

 

[Sparky Ninja]

Widdler. an excellent response. But how do we proceed given that the OP had specified that the circuit be run in singles, ref method B. does not table 4D5 not apply here as that's for twin and earth?

i was just giving an example.

its not too hard to adjust the data to suit the OP's scenario.

 

[Deleted member 26818]

I take it that from your correction factor of 0.41, that you have 6 circuits in the trunking?
This is a RFC, which means that there are two conductors in parallel, the correction factor should be doubled.
Which would give you a conductor size of 6mm².

 

[pushrod]

I take it that from your correction factor of 0.41, that you have 6 circuits in the trunking?
.

6 or 16??

OP, TBH if there is this large number i can't see how this can be calculated without at least some indication of what the expected load is and whether it will be evenly distributed - is this for some commercial or industrial premises? Is installing submains not an option? Is it 3 phase or single phase?
Why so many circuits?
If it is domectic premises table 7.1 in the OSG gives guidance on ring main length with various PDs.

 

[shambock]

6 or 16??

OP, TBH if there is this large number i can't see how this can be calculated without at least some indication of what the expected load is and whether it will be evenly distributed - is this for some commercial or industrial premises? Is installing submains not an option? Is it 3 phase or single phase?
Why so many circuits?
If it is domectic premises table 7.1 in the OSG gives guidance on ring main length with various PDs.

I am taking the area of the trunking where most circuits will meet ie right above the DB in question, i can get it down to 12 circuits but am unable due to cost run seperate circuits in seperate tubes thus removing correction factors all together.

The lighting and fixed power outlets eg 4.2KW dishwasher have been done and compleated by me but the ring is proving difficult due to the fact i have no idea what will be plugged in.

 

[pushrod]

This might be of use to you - If your lighting circuits are operating at less than 30% of the capacity for their cables they can be ignored from the total number of circuits when working out the rating factor for grouping see note 9, P126 OSG or P268 of BS 7671.

Also if your cables are more than 2x their diameter, horizontally apart from others then their rating factor can be ignored. eg if you had 10 cables and you cable tie them in 2 groups to opposite sides of the trunking with sufficient space between them then they can be counted just as 5 circuits (see note 2)

 

[Geoffsd]

Also if your cables are more than 2x their diameter, horizontally apart from others then their rating factor can be ignored. eg if you had 10 cables and you cable tie them in 2 groups to opposite sides of the trunking with sufficient space between them then they can be counted just as 5 circuits (see note 2)

Surely, If there's room to do this, then then leaving them loose would not warrant the 0.41 derating factor in the first place?