CPC sizing in a conduit

[boltonsparky]

I was at the wholesalers today purchasing some 2.5mm singles to be installed in steel conduit and got to thinking.

You can use the conduit as the CPC I know,  but the way I was taught and the way I've always done it is to install a CPC with a CSA matching that of the size of the Line conductors. 

I was curious as to why we don't usually reduce the CPC size like that of a twin and earth cable and install the smaller sized CPC in singles, providing the adiabatic equation worked out.

Looking in the regs at 541.1.1. which takes a bit of unpacking, the minimum size for a CPC is 2.5mm unless it's an integral part of a cable, formed by a conduit, ducting or trunking or contained in an enclosure formed by a wiring system. 

But then comes the contradiction which states that the CSA shall not be less than 2.5mm if mechanical protection is provided or 4mm without. 

I've interpreted this as meaning that in a conduit we could install a 1.5mm CPC for a circuit with 2.5mm line conductors, providing that the adiabatic equation worked out ok, but is this over ruled by the sentence below it and a 2.5mm CPC should be installed, and if so why doesn't this over ruling apply to T&E?

Based on the above should a 1.5mm line conductor be protected with a 2.5mm CPC?

However if we don't do the adiabatic equation then we must use the same size CPC, as per table 54.7?

And another matter relating to this.

I've always thought that you can run one CPC for multiple circuits in conduit, which in theory would be the same as using the conduit itself as the CPC, but I can't find a reg to back this up, other than a mentioning of it in 543.1.2 which talks about if a common CPC is used, but doesn't state that a common CPC can be used just alludes to it. Can anyone point me in the direction of a reg stating that a common CPC between circuits is permitted? I can't find one.

 

[binky]

don't think I've ever seen anything about common CPC not being allowed, Seems a lot better than running multiple cpcs through the same conduit? Like wise pinching a cpc from another cct in a house is allowed eg you have a twin cable lighting cct and want to earth a metal light fitting - nothing to stop you tapping an earth into a socket cct.

 

[Phoenix]

I've interpreted this as meaning that in a conduit we could install a 1.5mm CPC for a circuit with 2.5mm line conductors, providing that the adiabatic equation worked out ok, but is this over ruled by the sentence below it and a 2.5mm CPC should be installed, and if so why doesn't this over ruling apply to T&E?

Thats correct, but generally its easier to reach for teh drum of 2.5m g/y than start doing calculations. If using T/E on commercial then really you should do the calaculations as well, its just that the on site guide has it a standard circuit which makes some assumptions that there generally true in domestic, but might not be elsewhere.

Generally if the PEFC at the board is less than about 3ka, the breaker is of a B type and the disconnection times are met then the adiabatic is going to pass for a 1.5mm cpc on 32A device, if you have higher PEFCs or C types then you might be wise to check, I'd avoid reduced size CPCs with D types.

Charts here of max I²t from 60898 https://www.proface.com/support/index?page=content&country=ES&lang=en&locale=en_US&id=FA22629&prd= giving I²t. The best way to check the adiabatic is to claauclte K²S² of the cable and compare, as long as the cable figure is bigger than the MCB value its fine.

1.5mm in a conduit or T/E would be 1.5*1.5*115*115 = 29,756. Modern MCBs are mostly all energy limiting class three (look for a number in a square), it refers to rated short circuit capacity, but thats an oddity, as they are tested at their max ka rating, so assume the column is refering to actual KA, rather than breaker max. So upto 3kA both B type (18,000) and c type (22000) are alright for the 1.5mm cpc, at 4.5ka both are not, but the B type is not over my much, so you could guess its probably topping out about 4ka, the C type much lower.

If your PFC is massive then BS88 fuses are kinder on cables, the values tend towards a maximum rather than increase constantly with PEFC as the I²t is proportional to energy disapated, and the energy requried to vapourise a fuse element is more or less a constant, whereas mechanical contacts, its the time that will be constant and in worse cases the I²t could therefore increase in proportion to the sqaure of teh fault current (and did for non limiting types) so could get big very quickly.

 

[Evans Electric]

Like wise pinching a cpc from another cct in a house is allowed eg you have a twin cable lighting cct and want to earth a metal light fitting - nothing to stop you tapping an earth into a socket cct.

I don't think you actually  can  Binky     although it  definitely happens .     

I remember the observation that  the earthed circuit could be removed by others  who wouldn't know there was a borrowed earth connection .  

 

[boltonsparky]

Thanks for the input and information everyone. 

Thats correct, but generally its easier to reach for teh drum of 2.5m g/y than start doing calculations. If using T/E on commercial then really you should do the calaculations as well, its just that the on site guide has it a standard circuit which makes some assumptions that there generally true in domestic, but might not be elsewhere.

Generally if the PEFC at the board is less than about 3ka, the breaker is of a B type and the disconnection times are met then the adiabatic is going to pass for a 1.5mm cpc on 32A device, if you have higher PEFCs or C types then you might be wise to check, I'd avoid reduced size CPCs with D types.

Charts here of max I²t from 60898 https://www.proface.com/support/index?page=content&country=ES&lang=en&locale=en_US&id=FA22629&prd= giving I²t. The best way to check the adiabatic is to claauclte K²S² of the cable and compare, as long as the cable figure is bigger than the MCB value its fine.

1.5mm in a conduit or T/E would be 1.5*1.5*115*115 = 29,756. Modern MCBs are mostly all energy limiting class three (look for a number in a square), it refers to rated short circuit capacity, but thats an oddity, as they are tested at their max ka rating, so assume the column is refering to actual KA, rather than breaker max. So upto 3kA both B type (18,000) and c type (22000) are alright for the 1.5mm cpc, at 4.5ka both are not, but the B type is not over my much, so you could guess its probably topping out about 4ka, the C type much lower.

If your PFC is massive then BS88 fuses are kinder on cables, the values tend towards a maximum rather than increase constantly with PEFC as the I²t is proportional to energy disapated, and the energy requried to vapourise a fuse element is more or less a constant, whereas mechanical contacts, its the time that will be constant and in worse cases the I²t could therefore increase in proportion to the sqaure of teh fault current (and did for non limiting types) so could get big very quickly.

I'm guessing this is why the 2.5mm minimum is stated in 541.1.1. 

It'd be interesting to know how many installers calculate the suitability of T&E for use in settings other than domestic. 

I don't think you actually  can  Binky     although it  definitely happens .     

I remember the observation that  the earthed circuit could be removed by others  who wouldn't know there was a borrowed earth connection .  

In a domestic installation where you wouldn't expect it it sounds familiar to me too, isolate one circuit and you've lost the CPC for another. In conduits/trunking I'd be expecting to find shared CPCs so any disconnection/isolation would be carefully looked into.   

 

[SPECIAL LOCATION]

don't think I've ever seen anything about common CPC not being allowed, Seems a lot better than running multiple cpcs through the same conduit? Like wise pinching a cpc from another cct in a house is allowed eg you have a twin cable lighting cct and want to earth a metal light fitting - nothing to stop you tapping an earth into a socket cct.

I don't think you actually  can  Binky     although it  definitely happens .     

I remember the observation that  the earthed circuit could be removed by others  who wouldn't know there was a borrowed earth connection .  

543.6.1 Says where overcurrent protective devices are used for fault protection CPC must be part of the same wiring system, (Or in its immediate proximity)..

So as I understand it a conduit wiring system with multiple circuits inside it could share a CPC..

But individual PVC cables with integral CPC should be a CPC function dedicated to that circuit. 

Guinness

 

[binky]

543.6.1 Says where overcurrent protective devices are used for fault protection CPC must be part of the same wiring system, (Or in its immediate proximity)..

So as I understand it a conduit wiring system with multiple circuits inside it could share a CPC..

But individual PVC cables with integral CPC should be a CPC function dedicated to that circuit. 

Guinness

fair enough, I havn't read that section for years. Magic word there is 'should' which implies that whilst that is normal requirement, it doesn't rule out sharing a CP.  I did come across a house in severe need of rewiring that had 4mm stranded unsleeved earth wire running everywhere - took me a while to realise what it was! Must have been 12th edition of something - the owners didn't quite see the need to rewire...  :shakehead

 

[SPECIAL LOCATION]

fair enough, I havn't read that section for years. Magic word there is 'should' which implies that whilst that is normal requirement, it doesn't rule out sharing a CP.  I did come across a house in severe need of rewiring that had 4mm stranded unsleeved earth wire running everywhere - took me a while to realise what it was! Must have been 12th edition of something - the owners didn't quite see the need to rewire...  :shakehead

Or consider old lighting wired in single..  

Lives looped at switch....

Neutrals looped at lights...

Switch-wires joining the in-between bits...

So if you wanted to add an earth to a metal fitting..

Do you follow the Lives or Neutrals to be part of the same wiring system???

Think its just where you apply a bit of common sense..

If you know about it just add a note by or on the Consumer Unit..

:coffee

 

[phil d]

fair enough, I havn't read that section for years. Magic word there is 'should' which implies that whilst that is normal requirement, it doesn't rule out sharing a CP.  I did come across a house in severe need of rewiring that had 4mm stranded unsleeved earth wire running everywhere - took me a while to realise what it was! Must have been 12th edition of something - the owners didn't quite see the need to rewire...  :shakehead

Ha, I remember doing that on my mam and dads house when I was an apprentice, the wiring was sound but the lights were only twin, no earth. it was about that time that everone was going daft over metal light fittings, almost every house had a chandelier of some sort. I 'borrowed' a roll of 1.5 GY singles from work and looped it around all the lights, then back to the board, couldn't get down to the switches so it was no brass rope edged switches for her, lol.