Twin and Earth CPC

[Cookie]

IEC 60364-5-54 states that phase conductors 16mm2 and smaller require a CPC of the same size, over 16 to 35mm2 a 16mm2 CPC and over 35mm2 a half size CPC is required.

My questions is why am I seeing catalogs with harmonized twin and earth with reduced size CPCs over 2.5mm2? And why doesn't the half size rule kick in when wire is over 4mm2 instead of 16mm2?

 

[Megawatt]

The adiabatic equation is scary to be honest... I thought 2.08mm2 on a 40 amp breaker wad bad The IEC seems to insinuate its better for TT earthing. And I could understand that, especially since TT earthing tends to be used in developing countries where cost is critical.

I'm trying to find the k factor for AWG wire sizes just so I can compare our CPCs.

Cookie I’ll say this up to 30 amps the neutral and ground has to be the same size, after that you can derate the ground only. As far as the neutral goes after 200 amps the you can derate the neutral 70% of the ungrounded conductors. I just can’t understand why we don’t keep the neutral the same size as the ungrounded conductors which would go a long way as far as harmonics are considered. I myself do not derate neutrals because of the manufacturing plants that I work in have all kinds of problems with VFD’s, electronic ballasts, etc.

 

[Cookie]

Cookie I’ll say this up to 30 amps the neutral and ground has to be the same size, after that you can derate the ground only. As far as the neutral goes after 200 amps the you can derate the neutral 70% of the ungrounded conductors. I just can’t understand why we don’t keep the neutral the same size as the ungrounded conductors which would go a long way as far as harmonics are considered. I myself do not derate neutrals because of the manufacturing plants that I work in have all kinds of problems with VFD’s, electronic ballasts, etc.

Yup and yup.

Though if you have to upsize the phase conductors for voltage drop you must also do so with the EGC.

 

[Cookie]

Why do you think Zs testing is a waste of time?
Whatever size CPC you install it is still an essential test to confirm the requirements of ADS have been met.

I like the way you think

 

[pc1966]

Waste time was a bit harsh but the money we save will eventually be lost to testing, where as if we used the same size CPC we would only need to test PEFC/Ze at origin like we do PSCC.

Testing is not just confirming the calculations you know...
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I'm trying to find the k factor for AWG wire sizes just so I can compare our CPCs.

The k-factor is about the material (copper, aluminium, steel) and acceptable temperature rise (so typically a few small variariation based on PVC, XLPE, etc). The units might vary, but so long as you work things out in one consistent standard (e.g. mm2) and then convert back in to equivalent AWG it is fine.

Actually that is a very good reason why SWG and AWG as cable sizing are pants - they have no direct usable value for calculation or comparison of size!
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As others have said, the size is purely down to regulations.

The CPC does not normally carry current so it only has to be big enough for fault clearing, but the historical and regional approach to sizing that may have varied massively depending on the type of circuit protection device and the assumed skill levels of those designing / installing the circuits.

In the UK at least you always had to size the OCPD for the thinnest of cables on the circuit for normal overload protection, so immediately under fault conditions (which a current to the CPC is) you would be looking at rapid disconnection and so the I2t ought to be low enough for a reduced-size CPC. But you also have to consider the mechanical aspect of CPC strength, termination and corrosion, etc, which is (I think) the reason for various lower limits on CPC size that you see in the BS regs.

In other regions they may have had different rules on circuit OCPD selection, or less rigorous guidance for installed length, etc, that resulted in a mandate for the same CPC size to manage the fault-clearing safely.

 

[Cookie]

Actually that is a very good reason why SWG and AWG as cable sizing are pants - they have no direct usable value for calculation or comparison of size!

I'm seeing that now. Can anyone give me a push?

 

[pc1966]

We in the UK used to use SWG, also wires would be given by strands/diameter (e.g. 3/0.029 for 3 strands of 0.029" diameter each, about 1.3mm2 CAS typically for light circuits, etc).

These days it is all the CSA that is specified, which helps a lot for sizing circuits!

 

[Julie.]

IEC 60364-5-54 states that phase conductors 16mm2 and smaller require a CPC of the same size, over 16 to 35mm2 a 16mm2 CPC and over 35mm2 a half size CPC is required.

My questions is why am I seeing catalogs with harmonized twin and earth with reduced size CPCs over 2.5mm2? And why doesn't the half size rule kick in when wire is over 4mm2 instead of 16mm2?

It doesn't - the cross sectional area is determined by calculation in accordance with 543.1.3, however if you don't want to calculate it then you can refer to 543.1.4 and table 54.7 in which case your summary is correct.

Did you just look at the table rather than reading the standard?

BTW IEC 60364-5-54 is chapter 54 in BS7671 other than the 9 clearly marked exceptions

 

[pc1966]

In reality few electricians have to use the adiabatic equation (though they should know how to) as the IET's On Site Guide has some useful tables that incorporate the information for circuit design. For example this table is for BS88 fuses and shows the maximum Zs values for different CPC sizes:

For example, if you have 10mm T&E cable with a 4mm CPC used as a sub-main feed so you could allow 5s disconnection, you might have a 63A fuse for short circuit protection only, and then the downstream DB can use a mix of MCBs up to 32A in order to provide overload protection with a reasonable chance of selectivity. Looking at the above table you see you max measured Zs is 0.49 ohms, so your final test at the nice new DB would be to confirm this is met.

Also you see the value is 0.62 ohms in all the larger CPC sizes - they are time-limited for the fuse action, where as at 4mm it is adiabatically limited (hence lower Zs for a shorter fault disconnection time).

 

[Gavin John Hyde]

slight variation on the theme, I had a roll of twin and earth recently and the cpc was insulated in green/yellow, made a change... I checked and turns out this is apparently standard in Ireland. suppliers had received wrong pallet and not realised. i think if it was put on sale over here it would sale. I would buy it as it saves sleeving.

 

[Mark Wright]

Some people always say non insulated CPC is safer/better because if there's a break in insulation or nail hammered in or something it will come into contact with the CPC easier, although I don't think this is in any official books you hear it alot.

 

[pc1966]

Some people always say non insulated CPC is safer/better because if there's a break in insulation or nail hammered in or something it will come into contact with the CPC easier, although I don't think this is in any official books you hear it alot.

That might be a factor, but I suspect it was originally driven by the desire to save cost of extra PVC and smaller overall size of cable to make installation easier.

 

[Mark Wright]

and what's the name of the IEC publication? is it free? or where can you buy it?

what country puts in the most input to the IEC regulations?

 

[Dustydazzler]

I was always told by older sparks that Rings and reduced cpcs were due to copper shortages during the war....

 

[Julie.]

and what's the name of the IEC publication? is it free? or where can you buy it?

what country puts in the most input to the IEC regulations?

and what's the name of the IEC publication? is it free? or where can you buy it?

what country puts in the most input to the IEC regulations?

IEC 60364-5-54:2011
Low-voltage electrical installations - Part 5-54: Selection and erection of electrical equipment - Earthing arrangements and protective conductors

Not free - Don't know the cost - I just buy the UK version which includes all sections and is around £76 - amazon, IET... loads of places.

Most of europe - UK is a big contributor

 

[Mark Wright]

IEC 60364-5-54:2011
Low-voltage electrical installations - Part 5-54: Selection and erection of electrical equipment - Earthing arrangements and protective conductors

Not free - Don't know the cost - I just buy the UK version which includes all sections and is around £76 - amazon, IET... loads of places.

Most of europe - UK is a big contributor

Thanks so the IEC is broken down into quite small publications where as BS7671 includes the whole lot in one publication.