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Why isn't twin and earth cables used in commercial?
Question

I was always taught that twin and earth cables should be used in domestic only and singles/multicore in commercial? 

Why is that ? I mean there isn't anything wrong with using 1.5mm2 twin and earth with 1.0mm2 CPC for lighting circuits.

Regards,

J

13 Replies
mapj1
4166 Posts

Plenty of light commercial is wired in T and E round here, and it seems to do no harm. Often in basket trays in the ceiling. It can be done well, and makes certain other things easier, but it can also be an excuse for thrown-in spaghetti hoops of the worst kind.

It was never forbidden, but it is not really a good choice for exposed wiring in a heavy industrial setting where SWA or conduit is a better choice, and less likely to go badly wrong when it meets a forklift.

Some organizations have ‘house rules’ that dictate what types of wiring are to be used where, and there may be insurance agreements, but not the wiring regs per-se.

Mike.

broadgage
878 Posts

Agree, not in general prohibited by regulations, but often prohibited by client preferences or by consultants specifications.

And twin with earth IS widely used for the cheaper end of the market, cafes, pubs, small shops and the like.

Chris Pearson
3072 Posts

It depends what you mean by “commercial”. As opposed to industrial, I would include shops and small offices, which might even have been converted from housing. Then there are official residences like embassies or palaces.

What is, IMHO, inappropriate is T&E in conduit (galvanised or PVC).

Blencathra
123 Posts

Chris Pearson: 
 

It depends what you mean by “commercial”. As opposed to industrial, I would include shops and small offices, which might even have been converted from housing. Then there are official residences like embassies or palaces.

What is, IMHO, inappropriate is T&E in conduit (galvanised or PVC).

Very common in schools, T&E on basket then galv drops to sockets

 

gkenyon
1792 Posts

Chris Pearson: 
 

What is, IMHO, inappropriate is T&E in conduit (galvanised or PVC).

BS 7671 Table 4D5 Ref Method A would tend to disagree.

Chris Pearson
3072 Posts

Chris Pearson: 
What is, IMHO, inappropriate is T&E in conduit (galvanised or PVC).

gkenyon:

BS 7671 Table 4D5 Ref Method A would tend to disagree.

Graham, you know fine well that that was not what I had in mind.

AJJewsbury
2958 Posts

There is a potential issue about the prospective fault currents and reduced c.p.c. (especially where MCBs are used as the energy-let though tends to increases with increasing fault current) - as some larger commercial and industrial premises tend to have beefier supplies and hence higher fault currents.

Using generic data (e.g. from BS EN 60898) you'd have a hard time proving that a 1.0mm2 c.p.c. was good above 3kA. 

   - Andy.

gkenyon
1792 Posts

AJJewsbury: 
 

There is a potential issue about the prospective fault currents and reduced c.p.c. (especially where MCBs are used as the energy-let though tends to increases with increasing fault current) - as some larger commercial and industrial premises tend to have beefier supplies and hence higher fault currents.

Using generic data (e.g. from BS EN 60898) you'd have a hard time proving that a 1.0mm2 c.p.c. was good above 3kA. 

   - Andy.

Using generic data from BS EN 60898, you'd have a job anyway, because only the maximum at the rated short-circuit capacity is provided, i.e. if you have a 6 kA breakers (as in most domestic CUs), the lowest let-through energy quoted is 35000 ampere-squared seconds.

This doesn't even permit 1.5 sq mm copper conductors (of live conductors, or cpc) for fault (that is, short-circuit) protection!

bb0d2aa36e3d05cd0fc01bd25346d2c9-origina

 

gkenyon
1792 Posts

However, back in the real world, if you're meeting disconnection times, or with the circuit lengths shown in Table 7.1(i) of OSG, we know the actual let-through energy will be much lower …

geoffsd
226 Posts

johnwicklover: 
 

I was always taught that twin and earth cables should be used in domestic only and singles/multicore in commercial? 

Why is that ? 

 

Because you were taught wrongly ?

 

ebee
1336 Posts

A lot of us were brought up on a lot of Urban Myths !

Not just in Electrics/Electronics - “Put as much salt as you want in the cooking but very little on the meal before you eat it. It`s bad for you sprinkled on the meal!”

Chris Pearson
3072 Posts

What about 3-core & earth? Even 1 sqmm could supply a 9 hp motor, but I doubt that it gets used for that very often.

Johno12345
144 Posts

Chris Pearson: 
 

What about 3-core & earth? Even 1 sqmm could supply a 9 hp motor, but I doubt that it gets used for that very often.

we have some in a lab wired just like that, it does feel a bit wrong, but its not

 

it will be exciting if it ever gets snipped live by mistake!

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