Supplying power to outside sheds - couple of questions

[Hallsy]

Cheers Spark123, the bit that was confusing me was in the regs it states that the volt drop must not exceed 4% between point of supply and next socket outlet or distribution, this made me wonder if it meant I would need 4mm to the first socket, then 2.5 would be fine thereafter, BUT looking at the table in OSG it looks at the radial in total length and suggest 4mm.

I'll be doing the lot in 4mm, (as long as they sort out some 4mm SWA) and will fuse the spurs at 3 or 5A and run 1.5 from these.

 

[ban-all-sheds]

Just realised that you don;t get single phase plus earth colours in SWA,

You never did - the pre-harmonised colours were red, blue & yellow. One of these matched the phase colour of single-phase fixed wiring, and one of them coincided with the neutral colour for the other single-phase colour scheme. The same colours were used in 3C+E cable, and people seemed to get on with not assuming that the blue was neutral without worrying about it.

The harmonised colours are brown, black and grey. One of these matches the phase colour of single-phase fixed wiring, and one of them coincides with the neutral colour for the other single-phase colour scheme. The same colours are used in 3C+E cable...

There is a certain amount of symmetry:

Like you, I was surprised that you couldn't get SWA in "single-phase + earth colours" (either the old red/black/g-y or the new brown/blue/g-y). Not being a pro sparky, let alone one with experience of 3-phase installations, I'm not sure, overall, what percentage of 3-core SWA is used in 3-phase applications with no neutral and how much is used in single-phase. As it seems it is manufactured, but is rare and expensive, then maybe the requirement is genuinely small. Or maybe it's the classic feedback loop of a product being expensive because not many people use it because it is expensive.

 

[Hallsy]

Yeah, but at least with old colours everyone knew red would be live, flex wiring had neutral blue so nothing extra-ordinary there, and yellow as earth is not far off either, but new wiring using grey as neutral and black as earth just seems wrong to me.

You're right though, loads more is used in industry, we do probably do mostly 3 ph plus neutral drops as you will nearly always need the neutral for your single phase supplies within the m/c, but we 3 core where 3 phase only is needed. Saying that, we use a lot of SY now as well, good mech properties and very flexible.

 

[Spark123]

this made me wonder if it meant I would need 4mm to the first socket, then 2.5 would be fine thereafter, BUT looking at the table in OSG it looks at the radial in total length and suggest 4mm.

Regulation refers to the point of origin of supply to the point of utilisation.
You can use smaller cables (current carrying capacity dependant) as long as you add up the volt drops and they do not exceed 240Vx4%=9.6v. To do this properly you need to do use volt drop tables 4D4A in the IEE regs to work out the overall volt drop as opposed to the OSG.

Example:

25m X 4mm² @20A = 11mV/A/M = (11x20x25)/1000 = 5.5v
10m X 2.5mm² @20A = 18mV/A/M = (18x20x10)/1000 = 3.6v
5.5v+3.6v=9.1v < 9.6v
So is OK for volt drop.

 

[Spark123]

As far as I remember, I have only seen 3c SWA used for 3phase mainly in industrial applications for the likes of motors, welders, lighting or power supply to applications where the apparatus is a bulk of 415v motors, controls are transformed locally from 415v-240v or 415v-110v and running a neutral in isn't worth the extra cost. On a production line where almost every motor can be 3phase, there can be a vast amount of 3phase (no-neutral) cables.
For single phase applications it is OK to use a 2c SWA cable and the armour for the CPC as long as either the adibatic equation or the conditions of table 54G can be met. For a 4mm² 90deg XPLE SWA cable to comply with table 54G, the required csa of the cpc is 4mm*(k1/k2), k1=143, k2=46, required csa of armour is 12.43mm². CSA of armour from manufacturer is 19mm² so requirement of 54G is met. Generally the conditions for table 54G are met for 1.5mm² to 6mm² 2c SWA. Above this in 2core, only 35mm² is satisfied for 54G. Apart from this either the adiabatic equation should be used or a suitable additional cpc should be run in (or 3core cable doubling the armour and earth core which is now the norm.)

 

[ban-all-sheds]

Green = OK

Red = too small

 

[Spark123]

Superb, can you put this in the Sticky section ban?
Thanks,

 

[Steve]

What the hell is 400mm² SWA used for???

 

[Hallsy]

this made me wonder if it meant I would need 4mm to the first socket, then 2.5 would be fine thereafter, BUT looking at the table in OSG it looks at the radial in total length and suggest 4mm.

Regulation refers to the point of origin of supply to the point of utilisation.
You can use smaller cables (current carrying capacity dependant) as long as you add up the volt drops and they do not exceed 240Vx4%=9.6v. To do this properly you need to do use volt drop tables 4D4A in the IEE regs to work out the overall volt drop as opposed to the OSG.

Example:

25m X 4mm² @20A = 11mV/A/M = (11x20x25)/1000 = 5.5v
10m X 2.5mm² @20A = 18mV/A/M = (18x20x10)/1000 = 3.6v
5.5v+3.6v=9.1v < 9.6v
So is OK for volt drop.

Bugger, I'd gone out and bought a reel of 4mm T+E specially as well Thing is they were estimates on length, it may well be 30M of SWA or could be 20M (I only paced it out), prob only looking at about 5M of T+E in sheds as I'll be going between them with more SWA. Also I;d have to take into account the volt drop sustained down the 1.5mm from the Spurs to the lights.

It may well have just been inside the 4% if I used 2.5, but I suppose if I use 4mm throughout I'll have no worries at all if things work out to be a little longer than first thought.

Good work on the armouring table there as well BAS.

 

[2scoops0406]

Sorry, this is slight hijack of this post, but as this topic has been done nearly to death, I didn't really want to start a new thread.

What is the best way to get the cable from the outside trench into, in my case, the garage?

Drill through the walls, below the DPC, then dig a hole in the garage floor? This will break the garage DPM though I would guess. What I want to try to avoid is having SWA running up walls, and generally looking a mess. Also, Another consideration is that I have 16 sq mm SWA, so its bend radius is quite large.

Apologies in advance for the Hijack.

 

[Hallsy]

Sorry, can't help with your question Eddie although most I've seen do come up through garage floor as you say, but not sure what you do about damp course membrane.

Back to the topic in hand, this is turning into a joke. People wanting something for nothing can really drag you down. I left it with them trying to get some 4mm SWA off site (why, oh why just not buy some!!??), but I get a phone call today saying they can't get hold of any and as the 2.5mm SWA they have is long rnough, could I double up? I haven't bothered to work out the volt drop, etc but is this even permissbale? Wouldn't it then have to be wired like a ring? I'm just about fed up with this, all has came out of my pocket so far for materials, and now they are scrimping on the SWA!!

 

[Adam_151]

Surely ban you can't just say this armour is too small, this one is ok, it depends on things like if it'll meet required disconnect times if you use it, if the temperature rise under fault conditions is acceptable, which depends on PSCC (and is calculated using the adidiabitc equation)

Not totally sure myself, maybe someone like lectrician can enlighten us?

 

[Spark123]

For single phase applications it is OK to use a 2c SWA cable and the armour for the CPC as long as either the adibatic equation or the conditions of table 54G can be met. For a 4mm² 90deg XPLE SWA cable to comply with table 54G, the required csa of the cpc is 4mm*(k1/k2), k1=143, k2=46, required csa of armour is 12.43mm². CSA of armour from manufacturer is 19mm² so requirement of 54G is met. Generally the conditions for table 54G are met for 1.5mm² to 6mm² 2c SWA. Above this in 2core, only 35mm² is satisfied for 54G. Apart from this either the adiabatic equation should be used or a suitable additional cpc should be run in (or 3core cable doubling the armour and earth core which is now the norm.)

This is the post Ban was replying to though...

 

[Hallsy]

Anyway, back to the question I asked again, I wasn't thinking about it at the time as I was too ****ed off, but basically doubling up a cable will only give you increased current carrying capacity as the current will be split. The volt drop will still be the same, they have decided they only want one shed wired now so circuit is reduced BUT, even so

25m (approx but better too long than too short!!) x2.5@20A = (18mVx20x25)/1000 = 9V
Approx 3m internal wiring x 2.5@20A = (18mVx20x3)/1000 = 1.08V
Approx 1m to light x 1.5@5A(FCU) = (29mVx5x1)/1000 = 0.145V

Total = 10.225V which is greater than 9.6V so no go.

With doubled up 2.5 you have two runs at 10A each, so 2x((18x10x25)/1000) = 9V, so no different.

Even if I used 4mm T+E internal you would have (11mVx20x3)/1000 = 0.66V, so new total would be 9.805V so still too high.

Even though it is borderline I'm still sticking to what I said, that they need 4mm SWA for the 25m run to the shed, doubling up 2.5mm SWA will make no difference.

I am right in what I'm saying aren't I? Wouldn't mind some clarifcation just to make sure I'm right!!

Unless I dropped the mcb at the house to 16A, this would bring the volt drop down on the cable run and so 2.5mm could be used, but would this be enough current for a single shed? 2 double sockets and a fluorescent, to supply a freezer and various tools? Dropping from 20A to 16A doesn;t seem a lot and would make the isntall doable with what they have, but wouldn;t exactly be future prrof. Not sure what to do

 

[ban-all-sheds]

Superb, can you put this in the Sticky section ban?

Once I've corrected it.

I hadn't noticed until I added the automatic colour coding - look at the armour csa value for 25mm² 5-core. It can't be right.....