Twin and Earth CPC

[Cookie]

The chances are pretty slim when all new underground pipes to a house will be plastic these days.

What is CATV?

Coaxial cable, or cable television.

Also phone lines in the US have a shield which are required to be bonded to the service. This provides parellel paths which lower earth Z substantially.

 

[Cookie]

A bit late but if curious here is how US homes are served outside major metropolitan areas:

1-7 homes on a single transformer. Primary neutral ties in with the secondary neutral. Any phone or CATV is also tied to the secondary neutral at the pole and again at the home.

 

[LastManOnline]

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

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.

This would appear to be the logic behind it.I can appreciate both sides of the argument. Reducing the size of the CPC and not using Insulation for it has obvious economic benefits. On the other side of the equation, a full sized CPC lowers the Fli and allows us to extend the length of the supply cable for cicuits. A full sized CPC will also lower the "touch voltage" appreciably. From an installation point of view the adjustment to full size CPC in T&e cables has been a PAINas as it has knock on effects on the number of cables we can now fit into the standard wavin pipes

 

[Cookie]

What about larger circuits in the UK? Circuits over 32 amps? 63amps? 100 amps? 225 amps?

 

[pc1966]

What about larger circuits in the UK? Circuits over 32 amps? 63amps? 100 amps? 225 amps?

Regulation 543.1.3 suggests the use of the adiabatic equation to compute the minimum size.

But if that is not easy to apply (for example, if the I2t of the source OCPD is unknown) then regulation 543.1.4 has a table which basically has the same earth conductor as the live conductors to 16mm, 16mm for live conductors to 35mm, and then half the live size for conductors above 35mm (rounded up to the next standard size). To illustrate by example:

• 32A supply on 6mm cable would have a 6mm CPC (larger than UK style T&E which is based on adiabatic limits for standard fuse/MCB, but the norm for 3-core round cables)
• 100A supply on 25mm or 35mm cable would have 16mm CPC
• 225A supply on 95mm cable would have 50mm CPC

 

[Mark Wright]

id be more interested (because I hear uk stuff everyday) what the requirement for America are?

You call your CPC - Equipment Grounding Conductors (EGC) ? is that right?
what are the NEC? requirements for the gauge of EGC? your allowed smaller CPC in the USA ? etc

 

[Cookie]

id be more interested (because I hear uk stuff everyday) what the requirement for America are?

You call your CPC - Equipment Grounding Conductors (EGC) ? is that right?
what are the NEC? requirements for the gauge of EGC? your allowed smaller CPC in the USA ? etc

Yup- CPC is called "Equipment Grounding Conductor" or "EGC"

Up to 30 amps the EGC is the same size, beyond that it is allowed to be smaller. Table 250.122 determines the size:

https://imgur.com/MCuYZm0

View: https://Upload the image directly to the thread.com/MCuYZm0

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There is an exception for motors- a circuit breaker can be upsized by 250%. So 2.08mm2 wire can be placed on a 40 amp breaker.

 

[7029 dave]

Which is fine if using any other cable than twin and cpc cable.
No ones going to wire domestic installations in conduit and singles tho are they?

They used to back in the day with slip tube conduit, it was pure art eh.

 

[pc1966]

Up to 30 amps the EGC is the same size, beyond that it is allowed to be smaller. Table 250.122 determines the size:

Looking at an example point in that table, 8 AWG is apparently 8.37mm^2 of copper, for a 100A circuit I suspect you are going to hit the adiabatic limit for disconnection times under 5s. So it would need care in the breaker/fuse choice up front and/or Zs at end to make sure nothing serious happens.

 

[Cookie]

Looking at an example point in that table, 8 AWG is apparently 8.37mm^2 of copper, for a 100A circuit I suspect you are going to hit the adiabatic limit for disconnection times under 5s. So it would need care in the breaker/fuse choice up front and/or Zs at end to make sure nothing serious happens.

No Zs requirements in the NEC.

Yes, 8.367mm2- here is our AWG to mm2 table:

https://imgur.com/qsc6YNh

View: https://Upload the image directly to the thread.com/qsc6YNh

 

[LastManOnline]

Because some countries like to waste the rapidly dwindling natural resources of this planet.
Or maybe they just don't trust the people installing the cables to understand the relatively simple calculations required.

Its not an issue of trust. The calculations speak for themselves. No issue. It's also not about being indifferent about resources. No issue. But speaking from a perspective where I have been using what I might term"British " T&E for most of my career and" Irish" T&E just recently. Obviously I am obliged to use it. But I see it as a trade off. A larger earth allows longer runs due to reduced FLI and volt drop. Touch voltages will be reduced. I have to say I, m also delighted to see the back of all those calculations we had to make for test reports due to the difference in the CSA of L and E in T&E
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Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.

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Its not an issue of trust. The calculations speak for themselves. No issue. It's also not about being indifferent about resources. No issue. But speaking from a perspective where I have been using what I might term"British " T&E for most of my career and" Irish" T&E just recently. Obviously I am obliged to use it. But I see it as a trade off. A larger earth allows longer runs due to reduced FLI and volt drop. Touch voltages will be reduced. I have to say I, m also delighted to see the back of all those calculations we had to make for test reports due to the difference in the CSA of L and E in T&E
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Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.
[/QUOT

Basically you are very unlikely to have an earth rod with an Ra below a couple of ohms, more likely tens of ohms, so under fault conditions the maximum current is in the tens to low hundred Ampere range and for a fraction of a second for the breaker to clear it that is OK. Also most TT installations have an incomer RCD that trips at levels of 100mA to 300mA usually.

So basically you won't see a large enough I2t to overload a 2.5mm conductor.

Having said that, personally I would not use anything below 4mm in that case for mechanical strength even for the protected cases.

I have seen the minimum size cable for an earth rod go from 2.5 to 4mm to 6mm to 10mm for domestic installations, over the years. Why the change? Soil resistance is still exactly the same as it was decades agbviously the requirement for greater protection through mechanical strength. the current flow under fault conditions remains determined by the rod res (on a good day a 100 ohms?). So under the worst fault conditions there will be max 2 to 3 amps flowing which of course means a 2.5mm is electrically fine (but mechanically not, according to the (our) regs).
I also think that it would unusual to find a, TT Supply system nowadays that does, nt incorporate rcd, s. Though they do exist.

 

[Cookie]

Regulation 543.1.3 suggests the use of the adiabatic equation to compute the minimum size.

But if that is not easy to apply (for example, if the I2t of the source OCPD is unknown) then regulation 543.1.4 has a table which basically has the same earth conductor as the live conductors to 16mm, 16mm for live conductors to 35mm, and then half the live size for conductors above 35mm (rounded up to the next standard size). To illustrate by example:

• 32A supply on 6mm cable would have a 6mm CPC (larger than UK style T&E which is based on adiabatic limits for standard fuse/MCB, but the norm for 3-core round cables)
• 100A supply on 25mm or 35mm cable would have 16mm CPC
• 225A supply on 95mm cable would have 50mm CPC

Question...

Why not have the half size option after 6mm2?

 

[Cookie]

For example- a table like this in BS7671:

Line Earth
-----------X--------------
1.5mm2=1.5mm2
2.5mm2=2.5mm2
4mm2=4mm2
6mm2=6mm2
10mm2=6mm2
16mm2=10mm2
25mm2=16mm2
35mm2=16mm2
50mm2=25mm2
70mm2=35mm2
95mm2=50mm2
120mm2=70mm2 (50mm2 could also make sense here)
150mm2=70mm2
185mm2=95mm2
240mm2=120mm2
300mm2=150mm2


Although to be frank these numbers would be inconceivable in the NEC- typically the EGC comes out to be about 10-12.5% of the phase conductors.

 

[pc1966]

Why not have the half size option after 6mm2?

You can, and that is basically the whole point about the UK-style of T&E cabling having a smaller CPC, simply by:

• (a) checking the adiabatic limits
• (b) ensuring it is protected against mechanical damage and corrosion.

Although you can get 16mm T&E in the UK it is rare, typically the biggest you will see in (sort of) common use is 10mm for the likes of large showers or big cookers in a domestic situation.

So the reality is most domestic final circuit wiring in the UK has a reduced CPC and that is fine due to the I2t from MCBs and fuses (when the design is ensuring a disconnection time below 0.4s) satisfying the adiabatic check.

The "half phase size above 16mm" rule is a very basic choice if you are not in a position to validate the I2t that a fault could have. Typically by that size you might be looking at up to 5s disconnection times for a sub-main or main feed cable, and so quite high I2t (even at short times if it is a MCCB that is providing the over-current protection from a source with a very high PFC).
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I have seen the minimum size cable for an earth rod go from 2.5 to 4mm to 6mm to 10mm for domestic installations, over the years. Why the change?

For a typical earth rod you are quite right, no way are you going to roast even 2.5mm on a fault on a TT system, so going to 4mm or more would only be for mechanical strength I guess.

Where the UK has a higher earth bond conductor requirement is for TN-C-S where it might be bonding to a shared water pipe or gas pipe, etc, and in a PME fault case could be seeing high tens (or possibly hundred-ish) of amps flowing as all of the unbalanced neutral currents in that section try to use whatever route the can get home by.

 

[Cookie]

You can, and that is basically the whole point about the UK-style of T&E cabling having a smaller CPC, simply by:

• (a) checking the adiabatic limits
• (b) ensuring it is protected against mechanical damage and corrosion.

Although you can get 16mm T&E in the UK it is rare, typically the biggest you will see in (sort of) common use is 10mm for the likes of large showers or big cookers in a domestic situation.

So the reality is most domestic final circuit wiring in the UK has a reduced CPC and that is fine due to the I2t from MCBs and fuses (when the design is ensuring a disconnection time below 0.4s) satisfying the adiabatic check.

The "half phase size above 16mm" rule is a very basic choice if you are not in a position to validate the I2t that a fault could have. Typically by that size you might be looking at up to 5s disconnection times for a sub-main or main feed cable, and so quite high I2t (even at short times if it is a MCCB that is providing the over-current protection from a source with a very high PFC).
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For a typical earth rod you are quite right, no way are you going to roast even 2.5mm on a fault on a TT system, so going to 4mm or more would only be for mechanical strength I guess.

Where the UK has a higher earth bond conductor requirement is for TN-C-S where it might be bonding to a shared water pipe or gas pipe, etc, and in a PME fault case could be seeing high tens (or possibly hundred-ish) of amps flowing as all of the unbalanced neutral currents in that section try to use whatever route the can get home by.

Alright- good info!

Regarding the last part- I think its time the UK goes back to TN-S.

 

[pc1966]

But speaking from a perspective where I have been using what I might term "British" T&E for most of my career and "Irish" T&E just recently. Obviously I am obliged to use it. But I see it as a trade off. A larger earth allows longer runs due to reduced FLI and volt drop.

The volt drop is unchanged by the larger CPC size, only the fault Zs is lower so you may be able to get a longer run in some case.

But the reality is few final circuits are limited in distance by the R2 value. A quick look over the example cases from the On-Site Guide Table 7.1(i) shows the distances limited in most cases by volt drop in the "RCD" column (where Zs is not significant) and when the FLI kicks in (in the no RCD cases).

If you are looking at B-curve MCBs and the TN-C-S upper example Ze values then practically none of the circuits distances are limited in any real sense by Zs (and thus R2 from the CPC size).

In the TN-S case you do see distance limitations come in as the Ze = 0.8 ohm assumption starts to eat in to the R1+R2 allowed to meet the OCPD device Zs, same as the use of C-curve MCB would cause problems on distance by lowering the Zs requirement.

 

[Cookie]

I thought you needed FLI, even with an RCD or TT supply?


Regarding a 133/230Y supply, would you use a TN-S or a TN-C-S Ze values?

 

[Mark Wright]

if we look at this touch voltage theory

Appendix: The Touch Voltage Concept - Wiley Online Library https://onlinelibrary.wiley.com › doi › pdf - Recherche Google

although not really used today a same size CPC does seem to reduce touch voltage better the a smaller CPC obviously - finding info on Earthing and touch voltages in nearly impossible, over shadowed by ADS there is also a good Irish paper on touch voltages.

Also on TT systems with high Ze same size CPC is beneficial.

 

[pc1966]

Also on TT systems with high Ze same size CPC is beneficial.

On a TT system CPC size is practically immaterial!

Earth rod Ra ranges typically from around 10 ohm to 200 ohm and is OK, so going from, say, 0.1 to 0.2 ohms for R2 ain't a factor!

 

[Mark Wright]

If! you can just meet Ze on a TT to provide ADS a CPC of the same size is beneficial instead of just relying on RCD protection.