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.