Twin and Earth American style

[Mark Wright]

Can anyone tell me more about Twin and Earth American style?

What's it's full name? NM?
Is it always flat?
Do you add a sheath to the ground/earth cable?
Thanks

 

[Mark Wright]

Thanks for the info.

NM or Romex is flat for 14/2, 12/2, or 10/2 or 3 conductor. Starting at # 8 or # 6 is normally round. I still can’t understand why we have naked grounds but it’s easy to purchase shrink tubing that can be installed on the naked wire. Be sure and mark it green.

Thanks, I assume it's naked like us for cost? but also the Earth/Ground is normally of a smaller size/diameter than the Live and Neutral.

But I would be interested in there is some other thinking behind the ground being bare. I mean I have known people in the UK say it's for safety incase a nail is driven into the cable.

 

[westward10]

Love it, naked ground.

 

[Megawatt]

Thanks for the info.

Thanks, I assume it's naked like us for cost? but also the Earth/Ground is normally of a smaller size/diameter than the Live and Neutral.

But I would be interested in there is some other thinking behind the ground being bare. I mean I have known people in the UK say it's for safety incase a nail is driven into the cable.

The ground is the same size as the conductors because you can’t derate the ground until you reach 40 amps or more.

 

[pc1966]

Thanks, I assume it's naked like us for cost? but also the Earth/Ground is normally of a smaller size/diameter than the Live and Neutral.

The ground size depends on the adiabatic limit and for that the USA has had less demanding specs than the UK for some time, so as @Megawatt says reduced CPC size is only for quite big cables. Similar to the EU in that respect.

The UK has long favoured fuses as OCPD and they are very good at limiting the I2t let-through, also our design requierments on Zs to guarantee the disconnection time also is a factor in keeping I2t down due to the short 't' value and they are rather more onerous than EU/USA.

But I would be interested in there is some other thinking behind the ground being bare. I mean I have known people in the UK say it's for safety incase a nail is driven into the cable.

I think it is partly cost (not just the PVC needed, but the resulting size and weight for shipping), and partly to make the cable smaller so easier to work with.

If you nail a cable it is pot-luck as to what you hit anyway. I think the bigger safety aspect is the E/CPC between the live conductors so less likely to get L-N fault and so more chance of RCD picking up damage due to rodent chewing, etc. But that is more speculation.

 

[Dustydazzler]

Like most things , I imagine the bare copper earth / cpc / ground is purely and simply a Cost thing

 

[Mark Wright]

earth / cpc / ground

All bases covered there

 

[Mark Wright]

In Ireland the CPC is now insulated and the same size - I believe.

 

[Dustydazzler]

In Ireland the CPC is now insulated and the same size - I believe.

And in Australia and New Zealand they have Twin and Earth with a fully insulated earth / cpc / ground

What is interesting in America / Canada they leave the copper ground wire bare , where as we over sleeve it

 

[Mark Wright]

I mean it is kind of worksite speculation but there is a little bit of truth that it adds safety. If the Live or Neutral insulation are damaged they automatically come into contact with the bare CPC. Metal enclosures are kind of automatically earthed.

But I agree and have always been taught it's because of cost.

 

[loz2754]

I mean it is kind of worksite speculation but there is a little bit of truth that it adds safety. If the Live or Neutral insulation are damaged they automatically come into contact with the bare CPC. Metal enclosures are kind of automatically earthed.

But I agree and have always been taught it's because of cost.

This kind of speculation has led some people to believe that it's a requirement for all cables to contain a CPC. (Even on this forum!)
As you say, the only real design advantage is the reduced cost.

 

[Mark Wright]

This kind of speculation has led some people to believe that it's a requirement for all cables to contain a CPC. (Even on this forum!)
As you say, the only real design advantage is the reduced cost.

All cables should contain a CPC shouldn't they?

 

[nicebutdim]

All cables should contain a CPC shouldn't they?

No.

Surely you've encountered, if not installed, numerous cables that didn't contain a protective conductor?

Edit: to expand upon this, common examples would be 4 core 3 phase cable or insulated and sheathed singles, such as meter tails.

 

[Lucien Nunes]

Amongst the countries that use a flat cable:

USA: Bare earth, same CSA as line conductor
AUS/NZ/IE/RU: Insulated earth, same CSA as line conductor
UK: Bare earth, reduced CSA
JP: Insulated earth, reduced CSA

Japan is about the only example I can find with a smaller, insulated earth. Granted, they don't often use it and it's strange in other ways (the green CPC is bonded down the outside of the other two so it can be zipped off at the terminations). But it sort-of completes the square of permutations.

Obviously there are countries / systems that normally use round cables, such as NYM-J in Europe, where the earth is usually insulated and same size.

 

[Mark Wright]

Amongst the countries that use a flat cable:

USA: Bare earth, same CSA as line conductor
AUS/NZ/IE/RU: Insulated earth, same CSA as line conductor
UK: Bare earth, reduced CSA
JP: Insulated earth, reduced CSA

Japan is about the only example I can find with a smaller, insulated earth. Granted, they don't often use it and it's strange in other ways (the green CPC is bonded down the outside of the other two so it can be zipped off at the terminations). But it sort-of completes the square of permutations.

Obviously there are countries / systems that normally use round cables, such as NYM-J in Europe, where the earth is usually insulated and same size.

Great bit of info

 

[Dustydazzler]

The interesting thing with Romex is the outer sheath / jacket is much harder than our PVC , it looks more like LSF cable.
I wonder if it has similar properties to LSF / LS0H cable

 

[Mark Wright]

I've said it before but the old White sheath stuff seems tougher to me.

There was some old White sheath - Black and Red stranded resting on a hot pipe for about 20years and it had hardly done anything to it - would probably be ok for another 20. I suspect the Grey wouldn't have lasted.

 

[Dustydazzler]

I've said it before but the old White sheath stuff seems tougher to me.

There was some old White sheath - Black and Red stranded resting on a hot pipe for about 20years and it had hardly done anything to it - would probably be ok for another 20. I suspect the Grey wouldn't have lasted.

Could be LSF cable which is much tougher than Pvc

 

[Mark Wright]

Was the old get anywhere(B&Q) White sheath cable LSF? think its just PVC but might be wrong.

 

[Dustydazzler]

Was the old get anywhere(B&Q) White sheath cable LSF? think its just PVC but might be wrong.

yes There used to be some god awful pvc in the big DIY sheds that was white and near impossible to strip. it was the worst cable every produced

 

[brianmoooore]

Ordinary white PVC sheathed T&E was around long before LSF was even invented (as was I).