Can we use the armour of an SWA as the CPC???

[Engineer54]

can i just add, that they do not comply with table 54G, but may comply by calculation, using the adiabatic. I seem to remember that the only size that doesnt comply with the adiabatic is 400mm2 2c. This will need you to verify this though, dont take my word for it.

Using the table i re-posted above (post 155) (originally posted by Spinlondon) and i can assure you, ...you won't go far wrong. There is no need for any further calculation to the above complying cables...

 

[Tonkatoy27]

I've been dropped in at the deep end really. Story is: 2x tech workshops burnt out due to unknown cause a couple of months ago, whilst the workshops are refurbished the classes need to go ahead and they've decided to have 2 portacabin type classrooms delivered to a hard standing about 100 meters away as the crow flies. They can't tell me the exact load they will be using, only that each cabin will have 2x 3phase machine circuits. (lathes or pillar drill I expect) a couple of pc work stations, sockets, lights and more than likely heating as they will be up until at least xmas. The sockets will be used by classes using basic tools inc a rack of heat guns, vac pac machines etc. Quite a load if 5 or 6 kids all have heat guns on the go.
I'm going to use the 60amp MCB originally used to supply one of the burnt out classrooms which originally had 5-8 3phase machines in. (con unit well away from fire thankfully)
In used as they can't tell me at the moment exactly what Ib is and I suspect it will creep up and up as they find more machines to runs and it gets colder.

We do all the maintenance and smaller works at the school but installing an IT suite or installing a new 16amp welder socket is a far cry from this.

Thinking some more, it may be more cost effective to come from the existing 60amp mcb (can't change it for a different MCB as its about a 50 years old and no longer availible) to a standalone 32amp switch fuse, then use a much smaller cable? I'd rather do that as I can't believe 150m of 50mm 4core will be very easy to work with
Thanks for your help.

 

[malcolmsanford]

So are you going to give each cabin 30amps of 3 phase supply ( You said your main breaker was 60amp I assume 3 phase 60amp)

So if the cable works out at 50mm and is the most expensive part of the install, then why don't you fit 2 x 32amp 3 phase breakers under the existing 60amp one, and run 2 X 3 phase 32amp supplies out, you most likely would get away with 16mm and possibly 10mm if you had a better idea of your loads

 

[Tonkatoy27]

So are you going to give each cabin 30amps of 3 phase supply ( You said your main breaker was 60amp I assume 3 phase 60amp)

So if the cable works out at 50mm and is the most expensive part of the install, then why don't you fit 2 x 32amp 3 phase breakers under the existing 60amp one, and run 2 X 3 phase 32amp supplies out, you most likely would get away with 16mm and possibly 10mm if you had a better idea of your loads

Ok, that sounds like a winner, I'll get my head back in the books and work out sizes based on 2 32amp instead of 1 60amp. It'll also have the advantage of the rennovation sparkies having the option to supply the 2x32a board from a different place if they need to rip out the existing con unit as part of their work.

I'm going to take a guess at the proposed load per cabin.
2x 3phase lathes or pillar drills
6x 5' flurescents
1x pc workstation
various low powered hand tools. ie, soldering irons.
5x heat guns that may concievably be used at the same time.
2x 2kw convectors when it gets colder.
1x 13amp plastic former

Cable run is through ceiling void to outside, no applicable grouping, thermals or insulation. Clipped direct on wall, 3m duct under steps, clipped direct to fence, 1m duct under gateway, grouped with 32amp single phase swa along fence to cabins.
What size(s) would you go for?

 

[malcolmsanford]

Well mate you have a 3 phase supply so you put the 2 heaters on a phase each so there is 8/9 amps there you have a couple of smallish 3 phase lathes/ drilling machines so what would you say 9/12 amps spread over your 3 phases that make 3/4 per phase so your worse case will be 15amps per phase.

The rest of the load will go on the other phase lights, smallish hand tools etc, former so you may have a load of 15/17 amps per phase.

You could get away with 10mm especially if your being generous and the run is less that the 150 you think it is.

 

[Tonkatoy27]

Well mate you have a 3 phase supply so you put the 2 heaters on a phase each so there is 8/9 amps there you have a couple of smallish 3 phase lathes/ drilling machines so what would you say 9/12 amps spread over your 3 phases that make 3/4 per phase so your worse case will be 15amps per phase.

The cabins are coming pre installed, I'm going to check with the supplier today but you can bet there is only one ring main. If 5 kids are all using heat guns at the time then thats 30 amps pulled through one phase staraight away. :0(
I'll get hold of the tech department and ask them for a list of machines that will be used at the same time to try and get an accurate Ib as unless I'm missing something, I'm still struggling to get my Voltdrop acceptable using 10 or even 16mm over 150m
Thanks for the help.e

The rest of the load will go on the other phase lights, smallish hand tools etc, former so you may have a load of 15/17 amps per phase.

You could get away with 10mm especially if your being generous and the run is less that the 150 you think it is.

The cabins are coming pre installed so I'll bet they only have 1 ring main. If 5 kids are using heat guns then that 30amps through one phase straight away. I'll call the cabin company today and find out.
I'll also get hold of technology and ask what will be used at the same time as if I calculate with 10 or even 16mm over 150m with 32a Ib I'm still struggling to get an acceptable vd. (unless I'm missing something)
Again, thanks for your help

 

[Tonkatoy27]

And the prat of the week award goes to......me. I was indeed missing something.

I was presuming my 3% vd for lighting was 3% of 230v as it was single phase equipment. With a bit of digging I am now wiser. I'm allowed 12v drop not 6.9v

'hangs head in shame'

 

[netblindpaul]

Tonkatoy,Don't forget that Technology classrooms must comply the CoP BS4163.This require further control considerations for the machinery.

 

[Skittles]

Well here is the table to tell you.

That chart is incorrect or at best misleading, for example it appears to state that the swa of a 1.5mm 2core swa cable is equivalent to a copper Cu conductor of 6.85mm ?

 

[johnboy6083]

That chart is incorrect or at best misleading, for example it appears to state that the swa of a 1.5mm 2core swa cable is equivalent to a copper Cu conductor of 6.85mm ?

Bear in mind that the table refers to the heating capacity of the material, and not it's current carrying capacity.So in other words will the material be able to take the fault current for 5 secs without rising to a temperature that would cause adverse effects to the cable.

 

[Skittles]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

 

[stef]

If you take a 10 mm swa two core and take the swa off the cable and bundle it together you'll find that it will make up more than 10 mm so it would be OK to use as cpc. However, as a good code of practice use a separate copper conductor half the size ( in this case 6mm ) additional to the swa as cpc.

 

[Skittles]

If you take a 2 core SWA pvc cable the swa is equivalent to a 4.7mm copper conducter
a 2 core SWA XLPE cable the swa is equivalent to a 3mm copper conducter

Now either we are discussing different matters , or that Chart is seriously flawed ?

 

[Engineer54]

If you take a 10 mm swa two core and take the swa off the cable and bundle it together you'll find that it will make up more than 10 mm so it would be OK to use as cpc. However, as a good code of practice use a separate copper conductor half the size ( in this case 6mm ) additional to the swa as cpc.

Where do you get the idea that running a separate CPC for a more than adequately protected SWA cable is a ''good code of practice'' ?? If you think about it a little more, you may find that it could be just the opposite!!!

 

[Engineer54]

If you take a 2 core SWA pvc cable the swa is equivalent to a 4.7mm copper conducter
a 2 core SWA XLPE cable the swa is equivalent to a 3mm copper conducter

Now either we are discussing different matters , or that Chart is seriously flawed ?

Are you talking about the chart at the beginning of this thread, or the cart i posted (post 155) Which is a far better table to follow...

Anyway we are talking about using the SWA as the CPC, ...NOT using it as a bonding conductor...

 

[stef]

Brass gland and swa in moist atmosphere means contact corrosion ( for some reason it builds up) A crimp lug provides a more secure contact than a brass gland used on a Glasgow isolator. Less mistakes can be made. So, yes, I still think it is a good practise to do so.

 

[Engineer54]

Brass gland and swa in moist atmosphere means contact corrosion ( for some reason it builds up)

In this instance it would be even better practice to replace the shroud with an adhesive lined heat shrink tube!!!

 

[Chr!s]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

Times by 8

 

[Engineer54]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

Both the tables posted in this thread are relating to the armour CSA complying with suitability as a CPC. As i stated previously, the tables do not, and are not intended to be used to size for main bonding purposes. If you require your SWA cable to carry a bonding requirement,t then you'll need to use one of the cores (if 10mm or over) or run a separate bonding cable of the correct size, with the SWA....

In essence, don't confuse CPC's with Bonding, ...they are completely different animals, with completely different compliance requirements...

 

[Guest123]

From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

You need to be careful when considering using the armour as a bonding conductor on a TN-C-S network due to the possibility of overheating of the live conductors under the (very rare admittedly) diverted neutral current situation arising from an open circuited neutral conductor on the supply system.