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

[automerge]1594377687[/automerge]
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