Ring Final Circuit - Single core CPC size

[SgtTrojan]

Lads,

I was always under the impression that when a ring is wired in singles the CPC had to be the same size of the line and neutral conductors (2.5mm).

Appendix 15 (BS7671:2008 p.362) states that the sizes shown (2.5mm) are for flat twin and earth only but a reduced CPC size is permitted ie 1.5mm. Appendix 15 doesn't state whether the same is applied to single core in my eyes.

I cannot find the regulation relating to the above quandary.
Can anyone shed any light and provide Reg number/s please?

 

[Risteard]

You answered your own question re: T&E.

Use the adiabatic equation.

There is no requirement to select cpc size from the table.

 

[holmslaw]

..

 

[Risteard]

Table 54.7 on page 130, if conductor is smaller than 16mm cpc should be the same size.

Not if you apply the adiabatic equation. As I said, T&E does not comply with the table.

 

[SgtTrojan]

Ok.. worst case scenario.

Max Zs for B32 BSEN60898 = 1.15 ohms
@230v Ia calculates to 200A and disconnection time is 0.1 sec. K would be 143. Therefore:

S = 0.44mm but who would install a 0.5mm cpc for a ring main?

 

[Risteard]

Ok.. worst case scenario.

Max Zs for B32 BSEN60898 = 1.15 ohms
@230v Ia calculates to 200A and disconnection time is 0.1 sec. K would be 143. Therefore:

S = 0.44mm but who would install a 0.5mm cpc for a ring main?

For a given size of cpc that wouldn't tell you that 0.5mm2 is acceptable, it simply means that if the size of cpc you used for the equation is equal to or greater than it then it is acceptable.

 

[Spark123]

You need to use actual fault current values as opposed to the minimum required to operate the MCB in 0.1s

 

[Risteard]

You need to use actual fault current values as opposed to the minimum required to operate the MCB in 0.1s

Correct.

Ia = Uo/Zs

 

[SgtTrojan]

So there is no where that says you must use 2.5 for cpc when using single core?

 

[ban-all-sheds]

No.

Let's turn it around though.

Why do you not want to use the same size, and how much work are you prepared to do in order to not use the same size?

 

[holmslaw]

..

 

[Risteard]

"Doing the adiabatic" sounds clever, but base a design on it and you could end up with a severely bitten 4rse.

Not true, because during the Design stage it is your responsibility to ensure that the circuit will disconnect in time. The adiabatic may need to be done more than once.

It doesn't tell you what size of cpc to use - it tells you whether the selected size of cpc is adequate. You will have already confirmed that disconnection times will be met / volt drop acceptable etc.

 

[holmslaw]

..

 

[Risteard]

Wrong. I wasn't saying to use it to select cpc size, but to confirm adequacy of selected cpc size.

You need to ensure disconnection times are met. This does not mean that you can forego the adiabatic equation because it ensures that the cpc can handle the fault current without damage from heat before the circuit disconnects.

I think it is you who misunderstands the design process.

 

[Spark123]

I can't see why you can't use the adiabatic to calculate the minimum size of CPC required for a given circumstance.

You can work to table 54.7 if you wish which is what a lot of people do when pulling in singles, however I have no problems with people using 1.5mm for a CPC in a 2.5mm conduit run which is allowed using the adiabatic, the very reason a 1.5mm CPC is allowed in 2.5mm twin and earth.

One situation I am aware of which doesn't pass the adiabatic is having 2.5mm twin and earth with a 1mm CPC (which was available for a short period many moons ago) where the OCPD is a 30A BS3036.