twin and earth cabling size for commercial buildings

[marxman99]

Hi,
The current carrying capacity tables in the BS 7671 for twin and earth cables are only for buildings that have thermal insulation. I'm working on a commercial building, the inner walls do not have thermal insulation, what can i use for the current carrying capacity of the cables. The tables that i've seen (for T&E in BS7671) derates the cables too much. I hope to hear from you soon.
Rgs

 

[davesparks]

The tables in BS7671 are not only for buildings with thermal insulation, they contain the applicable basic installation methods too.

Cable size calculations are the same regardless of type of installation.

The current carrying capacity tables should always be used as part of the proper calculations and not as a go-to reference for current carrying capacity. The actual current capacity of a cable can be more or less than that shown in the tables depending on the applicable rating factors

 

[marxman99]

The tables in BS7671 are not only for buildings with thermal insulation, they contain the applicable basic installation methods too.

Cable size calculations are the same regardless of type of installation.

The current carrying capacity tables should always be used as part of the proper calculations and not as a go-to reference for current carrying capacity. The actual current capacity of a cable can be more or less than that shown in the tables depending on the applicable rating factors

Thanks for replying so fast davesparks. I am referring to Table 4D5, it's the only table in the regs for twin and earth. You'll see that apart from if you have the cables clipped direct, all the other columns refer to thermal insulated walls or ceiling. My cables will be run behind the walls.

 

[JBW175]

Thanks for replying so fast davesparks. I am referring to Table 4D5, it's the only table in the regs for twin and earth. You'll see that apart from if you have the cables clipped direct, all the other columns refer to thermal insulated walls or ceiling. My cables will be run behind the walls.

Will they run behind the wall in trunking? Clipped direct? On perforated cable tray? Etc…

I think you can see where I’m going with this.

 

[davesparks]

Thanks for replying so fast davesparks. I am referring to Table 4D5, it's the only table in the regs for twin and earth. You'll see that apart from if you have the cables clipped direct, all the other columns refer to thermal insulated walls or ceiling. My cables will be run behind the walls.

Which referenece method from table 4A2 are they going to be closest to?

 

[marxman99]

Will they run behind the wall in trunking? Clipped direct? On perforated cable tray? Etc…

I think you can see where I’m going with this.

I imagine they'll run in the cavity and not in trunking or conduit or tray, probably clipped when i think about it. Can't quite remember how i've seen this done on site. Back in the day it was conduit in the walls probably to costly for that these days. They are circuits for wall sockets. I'm actually a electrical design consultant.

 

[marxman99]

Will they run behind the wall in trunking? Clipped direct? On perforated cable tray? Etc…

I think you can see where I’m going with this.

I got it, so use reference method C. Can't believe i couldn't work that out before, thank you all for reigniting my brain cells.
Going to bed now. Night

 

[JBW175]

I got it, so use reference method C. Can't believe i couldn't work that out before, thank you all for reigniting my brain cells.
Going to bed now. Night

No worries, glad you got there in the end. Sometimes we can’t see the wood for the trees, and need a little inspiration.

 

[Vortigern]

I wonder why you would be using T&E in a commercial installation. Usually I would be thinking singles in conduit surface mounted all the way through. Alternatively, dado trunking. Personally I hate T&E in commercial, very untidy and inelegant. With singles and conduit/dado trunking you can chop and change to re-fit sockets and lighting at will without going into the fabric of the building. Any fault finding is inestimably easier as is repair and additions. And then of course with T&E you get those situations of spaghetti in the ceilings flopped all over the place and additions add to that over the years. Not a very well designed method imho.

 

[marxman99]

I wonder why you would be using T&E in a commercial installation. Usually I would be thinking singles in conduit surface mounted all the way through. Alternatively, dado trunking. Personally I hate T&E in commercial, very untidy and inelegant. With singles and conduit/dado trunking you can chop and change to re-fit sockets and lighting at will without going into the fabric of the building. Any fault finding is inestimably easier as is repair and additions. And then of course with T&E you get those situations of spaghetti in the ceilings flopped all over the place and additions add to that over the years. Not a very well designed method imho.

Thanks for sharing your thoughts. For lower cost which includes getting away with not using trunking for singles, i think these days a few of us are going over to T&E from time to time.

 

[Risteard]

I wonder why you would be using T&E in a commercial installation. Usually I would be thinking singles in conduit surface mounted all the way through. Alternatively, dado trunking. Personally I hate T&E in commercial, very untidy and inelegant. With singles and conduit/dado trunking you can chop and change to re-fit sockets and lighting at will without going into the fabric of the building. Any fault finding is inestimably easier as is repair and additions. And then of course with T&E you get those situations of spaghetti in the ceilings flopped all over the place and additions add to that over the years. Not a very well designed method imho.

Very well said. T&E is a domestic cable and really isn't generally the right choice in a non-domestic setting.

 

[davesparks]

Thanks for sharing your thoughts. For lower cost which includes getting away with not using trunking for singles, i think these days a few of us are going over to T&E from time to time.

Is that the best approach to design? 'lower costs' and 'getting away with it'?

 

[davesparks]

Very well said. T&E is a domestic cable and really isn't generally the right choice in a non-domestic setting.

T&E is a general purpose cable, there is no rule or regulation limiting its use to domestic installations.

 

[davesparks]

I wonder why you would be using T&E in a commercial installation.

Why not? If T&E is suitable for the job why not use it?

Personally I hate T&E in commercial, very untidy and inelegant.

Surely that is a result of bad workmanship, not a problem caused by the cable itself?

I'm pretty sure a lot of us can produce a much neater and more elegant installation with T&E than some people manage to achieve with other wiring systems.

 

[Risteard]

T&E is a general purpose cable, there is no rule or regulation limiting its use to domestic installations.

I was careful not to state that it was generally prohibited, but the generally much higher fault currents on a reduced csa cpc and issues of adequate mechanical protection need to be considered. Really it is intended for domestic type installations and much superior choices exist for non-domestic installations. Let's be honest and agree that generally T&E is simply selected for low cost (comparatively) to proper containment systems etc.

 

[marxman99]

I was careful not to state that it was generally prohibited, but the generally much higher fault currents on a reduced csa cpc and issues of adequate mechanical protection need to be considered. Really it is intended for domestic type installations and much superior choices exist for non-domestic installations. Let's be honest and agree that generally T&E is simply selected for low cost (comparatively) to proper containment systems etc.

Hi, from Amtech cable calculations, the cpc is usually less than that you would usually use in an installation. For example, if you use singles then you would use the same size cpc as the live and neutrals, but on calculations it usually can be less. Not every case but usually

 

[default]

I imagine they'll run in the cavity and not in trunking or conduit or tray, probably clipped when i think about it. Can't quite remember how i've seen this done on site. Back in the day it was conduit in the walls probably to costly for that these days. They are circuits for wall sockets. I'm actually a electrical design consultant.

Clipped in a cavity, doesn't sound right. Think the installation method needs looking at a bit more closely.

 

[default]

I was careful not to state that it was generally prohibited, but the generally much higher fault currents on a reduced csa cpc and issues of adequate mechanical protection need to be considered. Really it is intended for domestic type installations and much superior choices exist for non-domestic installations. Let's be honest and agree that generally T&E is simply selected for low cost (comparatively) to proper

I was careful not to state that it was generally prohibited, but the generally much higher fault currents on a reduced csa cpc and issues of adequate mechanical protection need to be considered. Really it is intended for domestic type installations and much superior choices exist for non-domestic installations. Let's be honest and agree that generally T&E is simply selected for low cost (comparatively) to proper containment systems etc.

I was careful not to state that it was generally prohibited, but the generally much higher fault currents on a reduced csa cpc and issues of adequate mechanical protection need to be considered. Really it is intended for domestic type installations and much superior choices exist for non-domestic installations. Let's be honest and agree that generally T&E is simply selected for low cost (comparatively) to proper containment systems etc.

You'd be surprised at the amount of commercial buildings that are wired in T&E, even when they're using trunking.

 

[default]

I was careful not to state that it was generally prohibited, but the generally much higher fault currents on a reduced csa cpc and issues of adequate mechanical protection need to be considered. Really it is intended for domestic type installations and much superior choices exist for non-domestic installations. Let's be honest and agree that generally T&E is simply selected for low cost (comparatively) to proper containment systems etc.

Surely the cable, containment and installation method you choose is dependent on the job your doing no matter if its domestic, commercial or industrial. Otherwise you just over or under engineering the job

 

[Electro-tech]

Probably Building Regs may apply if the cable is run in the cavity. Then get some polystyrene pumped in to help migration.

 

[Lister1987]

Being commercial, will it be open to public? If it is then isn't thr consensus to use FP or similar?

 

[Risteard]

Hi, from Amtech cable calculations, the cpc is usually less than that you would usually use in an installation. For example, if you use singles then you would use the same size cpc as the live and neutrals, but on calculations it usually can be less. Not every case but usually

Yes I agree that it often can be depending on fault current and disconnection time, but it remains a poor cable choice.

 

[default]

Being commercial, will it be open to public? If it is then isn't thr consensus to use FP or similar?

No

 

[default]

Probably Building Regs may apply if the cable is run in the cavity. Then get some polystyrene pumped in to help migration.

What

 

[Dustydazzler]

When I first started out it was a lot of over night shop fitting stuff and cut and shut office re-furbs. We used A LOT of LSF twin and earth and 3c or earth and lots of white and red FP cable to modift circuits.

Only when time and budget allowed did we put up galv conduit Or galv trunking work and wire in singles, which wasn't that often sadly

 

[Vortigern]

Why not? If T&E is suitable for the job why not use it?

As stated in my post. I am not saying dont/cant use it just expressing what I consider good design and good looking installation that is robust and easily changed, shows damage immediately, is easier to fault find on etc. Putting T&E in the fabric of the building without trunking (as the OP states) clipped (?) just seems to me poor design imho that is all I am saying. As a matter of fact I am just doing a commercial in T&E as the situation necessitates it, but not my first choice by any means. The other thing about is the whole first fix, second fix is not a big thing fitting in between framers, chippies, plasterers and plumbers etc. That whole process is a lot less complex. And don't get me started with the boarders/plasterers rather than pull the light switch wire into the hall by the door they stuffed it through the other wall in the far corner of the bedroom and other things!

 

[nicebutdim]

When I first started out it was a lot of over night shop fitting stuff and cut and shut office re-furbs. We used A LOT of LSF twin and earth and 3c or earth and lots of white and red FP cable to modift circuits.

Only when time and budget allowed did we put up galv conduit Or galv trunking work and wire in singles, which wasn't that often sadly

OP is a design consultant and, as such, it is up to them to specify as they see fit.

While T&E might be an option, it's one that neither customer nor installer will thank them for when it comes to any future additions. I guess there are times when customers will be intransigent where price of materials are concerned, but I'd much rather see designers lead customers toward sensible solutions for their premises and not simply specify on the basis of price.

Additions can often be problematic, but it's not a great feeling when faced with a job made unnecessarily difficult at the design stage.

 

[Dustydazzler]

This is where the Yanks have one over on us, in commercial sites over there you aren't allowed to use soft skin cable up in the voids or under the floors. It has to be in galv flexi or galv Emt. It is not acceptable to lash in twin and earth over there, twin and earth ( romex ) is just for domestic

 

[Vortigern]

I'm pretty sure a lot of us can produce a much neater and more elegant installation with T&E

I am sure we could, funny thing is I have never seen it where T&E is concerned. Just loadsa cables thrown over suspended ceilings, lights and mixed with data cables all over the show. But then I have worked on conduit, galv and pvc looks immaculate virtually everytime. And I have looked above a lot of suspended ceilings.

 

[Dustydazzler]

I am sure we could, funny thing is I have never seen it where T&E is concerned. Just loadsa cables thrown over suspended ceilings, lights and mixed with data cables all over the show. But then I have worked on conduit, galv and pvc looks immaculate virtually everytime. And I have looked above a lot of suspended ceilings.

Things is usually the glav conduit installed neatly saddled up to the concrete ceiling is installed before the drop ceiling grid went up. The t&e and data wires all lashed in is thrown above the ceiling at a later date by every tom dick and harry who has worked on the site

 

[davesparks]

Being commercial, will it be open to public? If it is then isn't thr consensus to use FP or similar?

Not quite, I think you may be thinking of LSF/LSOH type cables. LSF T&E exists.

 

[davesparks]

Clipped in a cavity, doesn't sound right. Think the installation method needs looking at a bit more closely.

Why not? What's wrong with cables being clipped in a cavity?

 

[Electro-tech]

Why not? What's wrong with cables being clipped in a cavity?

Depends on the purpose of the cavity & what else will end up in there, other than a cable route. Especially an external cavity wall.

 

[nicebutdim]

Depends on the purpose of the cavity & what else will end up in there, other than a cable route. Especially an external cavity wall.

To be fair to the OP, their comments indicate partition walls and not external cavities in which it would be nigh on impossible to clip cables.

 

[default]

To be fair to the OP, their comments indicate partition walls and not external cavities in which it would be nigh on impossible to clip cables.

Can it still be classed as clip direct. Maybe reference method G?. And if they're stud walls off the outer block work there's Surely going to be insulation.

 

[nicebutdim]

Can it still be classed as clip direct. Maybe reference method G?. And if they're stud walls off the outer block work there's Surely going to be insulation.

OP stated internal walls with no thermal insulation.

 

[default]

OP stated internal walls with no thermal insulation.

Still can't be classed as clipped direct. It's in a wall.

 

[cireland]

There is no exact reference method for this but I'd say clipped direct is closest.

 

[default]

Still can't be classed as clipped direct. It's in a wall.

And

There is no exact reference method for this but I'd say clipped direct is closest.

I'd say 103. If there's an EICR done in the future they either won't have the original test to go by or they'll look at it and think who ever tested it put down the wrong classification. I do sometimes think the installation references could do with one or two more. C'est La Vie

 

[cireland]

103 is with thermal insulation which greatly de-rates the cable.

 

[default]

103 is with thermal insulation which greatly de-rates the cable.

Depending on length of run. Anyway this is all hearsay. That's why you would need more info. If someone said to design an installation & the installation method was " Behind the wall in the cavity " you'd want a bit more to go on. "Throw me a fricking bone". Clipped direct it is not.

 

[cireland]

It is still the closest to the actual installation. There is no insulation which rules out the 100's.
There is no tray, conduit or trunking. It sure as hell ain't method G.
The only difference between this and 'C' is there are no clips.

 

[default]

It is still the closest to the actual installation. There is no insulation which rules out the 100's.
There is no tray, conduit or trunking. It sure as hell ain't method G.
The only difference between this and 'C' is there are no clips.

And the fact it says clipped direct. I though that ment on the surface & visible, which it wouldn't be. Form what the OP is suggesting there is no classification. This is a new install with new walls going up. It's been surmised it's somehow behind an internal wall, in a cavity, clipped, with no insulation present. All I'm saying is more classification is required. You can't price for a job with best case scenario. It's worst case or having the correct information.

 

[JBW175]

Well he’s going have to clip it to something, or put it in containment of some sort.

We can’t have cables just hanging round unsupported throughout their runs.

 

[default]

Well he’s going have to clip it to something, or put it in containment of some sort.

We can’t have cables just hanging round unsupported throughout their runs.

Who said you could, that's why you'd go back to the main contractor for more information, and if it went in some sort of containment that would also change the outlook.

 

[JBW175]

Who said you could, that's why you'd go back to the main contractor for more information, and if it went in some sort of containment that would also change the outlook.

He’s the design consultant, he’s asking us for advice.

We’re giving him our opinions on how we interpret the regs and what to do in our experience.

What would you suggest?

 

[loz2754]

Table 4A2 number 40 gives ref method B, single or multiple cable in a building void.

 

[default]

He’s the design consultant, he’s asking us for advice.

We’re giving him our opinions on how we interpret the regs and what to do in our experience.

What would you suggest?

Yes. The advice is to get more information from the main contractor so you can make a more informed decision.

 

[davesparks]

And the fact it says clipped direct. I though that ment on the surface & visible,

Clipped direct means nothing about the visibility of the cable.
It means that the cable has been clipped directly to whatever it is installed on, no containment or anything else has been installed.

You may be confusing method C with clipped direct? Clipped direct is one of the methods which get grouped together as method C, cables embedded directly in plaster or in capping etc are also method C.