Capacity Of An Armoured Cable.

[nomisg]

Hello,

I am not a qualified electrician, although I worked on domestic and industrial installations many years ago. Regulations are so different that I would be grateful for advice before anything. I am running a cable from a 3-phase board to a location approximately 30 meters away. This is to enable lighting and power to be available in a stable building. The system will be single phase. I WILL NOT BE MAKING ANY CONNECTIONS, ONLY RUNNING THE CABLE!

Since there appear to be different ways of calculating the current capacity of cables, I hope someone can tell me the capacity of the cable I have ready to install. The cable is as follows: 3 core armoured described on the invoice as

XLPE3X16.  3 X 16 XLPE, PVC, SWA, PVC 1KV BS5467 CORES BRN BLACK GREY BASEC.

Looking at the bare end of the roll, it is 20mm. diameter overall, black outer, steel armour, white interior with three cores of copper, each measuring 6mm in diameter, and each formed of 7 strands. It will be clipped to the interior wall of the building in "free" air at high level. No other cables nearby. I realise there are potential losses for distance etc., but would like to know the current that it can handle in order to work out what size of mounting board I should be thinking of making a suitable place for in the stable. It is quite awkward to handle, so should I be allowing space at the "board" for some form of junction box to terminate the armoured, since it seems fairly meaty to go directly into a consumer unit. Hope someone can give some advice. Many thanks

 

[Andy™]

there is a lot more to cable design than simply x size cable can carry y amount of current. youre also doing it wrong, you start with your design current then go for a suitable cable size given current & length (& other factors). simply putting a cable in then finding something to match is capacity is not the way to go, and can easily end up with a non-compliant install and potentially dangerous one

also the mounting board will need to be the same size or larger than the DB thats going to be installed. Ask your sparky for the sizes

 

[Lurch]

The board will be the size it needs to be to accommodate the final circuits. The supply cable size will have little to no bearing on it.

As always, if you are planning on having someone coming in later to connect up then ask them as it is no good asking us what your unknown electrician may or may not be happy with.

 

[ProDave]

And shall we discuss the earthing for your stable?

 

[Lurch]

And shall we discuss the earthing for your stable?

No point, he's already said he isn't doing any of the electrics so why confuse the issue?

 

[Essex1]

The guy is only asking for the CCC of a SWA cable. He is not asking anything more. Someone tell him the answer without bringing up things the op has not asked for.

 

[ProDave]

To answer the question we need to understand it.

but would like to know the current that it can handle in order to work out what size of mounting board I should be thinking of making a suitable place for in the stable. It is quite awkward to handle, so should I be allowing space at the "board" for some form of junction box to terminate the armoured, since it seems fairly meaty to go directly into a consumer unit. Hope someone can give some advice. Many thanks

I don't understand what you mean by "mounting board"  Are you talking about the bit of wood to fix the distribution board to?

Anyway, you are asking a completely irelevant question.

It will terminate into a 3 phase distribution board. 99% of 3 phase boards are metal, so there will be no problem terminating a SWA cable directly into them.  Virtually all of them will be rated for 125A or more, I doubt you will find one rated at less than 100A.  So it's not as though you can buy a "small" one.

So it boils down to how many circuits you expect to have as to what size board you have.

The cable will be protected at it's origin by a suitable mcb for the size of cable.

What is it you actually want to know?

 

[Essex1]

I read it is a mounting board to secure the SWA to along the route he wants to wire it along. He wants to know the CCC of a 16mm SWA, single phase, clipped direct. He is not asking for a full design here. If I had my book I would tell him but I do not.

 

[Lurch]

It will terminate into a 3 phase distribution board.

Try again.

The system will be single phase.

 

[ProDave]

Try again.

Okay I missed that, thank you.

but the CU will still be rated at least for 100A so the size is just how many circuits you want.

 

[kerching]

Dave,,,,it is a. Single phase board!...I had to read it twice

 

[Essex1]

Out on a limb here - about 115 amps.

 

[Lurch]

but the CU will still be rated at least for 100A so the size is just how many circuits you want.

Exactly what I said at the start, the only person who can provide that information is the electrician that will be doing the install.

 

[davetheglitz]

Have a look on the TLC website technical section volt drop calculator. Should give you what you want.

https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html?cable=SWA_3_CoreXLPE&application=clipped_direct&max_perct_volt_drop=3&ambient_temp=25&no_circuits=1&circuit_layout=bunched&power=20&power_units=1000&voltage=230&length=30&submit=Calculate+Min+Cable+Size

 

[SLIPSHOD & SLAPDASH]

The only person who should/could  answer correctly is the person doing the installation , connections & testing.

 

[nomisg]

Thank you for your replies. I fully appreciate your points about asking an electrician. I already have the cable mentioned in my post and the opportunity to clip it up using a professional platform which is available for the next couple of days - quite useful in view of the weight! The electricians I have contacted should be available to come and have a look within a week or so, depending on their work load, at which point I will no doubt get an answer to everything. At a chargeout rate of £220 + VAT per day, I am quite keen to do a little simple work in advance!

Apologies for the confusion about the "board". Basically the access into the stable room is a bit fiddly, so my question about a "junction" box before a consumer unit was simply to be sure that I leave the cable end in a position where a wooden mounting board of sufficient size can be fitted. Answer: leave plenty of space. It is definitely a single phase installation - sorry!

Working up from a single strand of the copper wire and using the usual maths, it is clear that the piece of paper on the drum is correct. Each core is 16mm^2 and it is 3 core. Various professional sites indicate that this size of cable is capable of handling 85amps. On this basis I will take the risk of fixing it up; it will hopefully be adequate for a 63amp garage unit even allowing for voltage drop. If I am completely wrong tell me please! As I said, regulations are totally changed, and I am therefore trying to be cautious. Working at Dungeness power station in the 1960's, I recall the electrician sticking the earth in with the neutral to get a single phase supply from a 3-phase terminal! Thanks to all.

 

[SLIPSHOD & SLAPDASH]

A '63A garage unit' would be the maximum current draw.  If you are referring to an assembled consumer unit with a main switch Rcd & 2 circuit breakers which are normally 16 & 6A (even 1x 32A)  then   it appears your cable will be ok. If it is not, the electrician can calculate what size OPD should be fitted to suit your cable.

 

[Andy™]

, I recall the electrician sticking the earth in with the neutral to get a single phase supply from a 3-phase terminal!

which is not allowed, unless you get home office permission

 

[nomisg]

Hello Andy, It was the 1960's and we were "only" working on an atomic power station - at 37p (7s 3d) per hour! Thank you S&S for the point about OPD's. With this as a "backstop" to be determined by the electrician I feel I can go ahead and lay the cable with some confidence that it will not be a wasted effort. A few fluorescents on the lighting side and some 13amp plugs to enable the use of "domestic" tools for the restoration of a 1920's car are the requirements if all goes well. Many thanks.

 

[Lurch]

which is not allowed, unless you get home office permission

Depends on why he did it, and which way you look at it (as in earth in with neutral or neutral in with earth). Power stations are not bound by BS7671 anyway so moot point.