Log in to the online community

Want to post a reply? You'll need to log in
Cable selection for three phase equipment. Does it have to be SWA?
eejit
2 Posts
Question
This is my first post and I'm currently reading my way through the regs so please go easy 🙏

In projects I've been involved in, in most cases, the electricians on the project have used SWA for supplies to three phase equipment, whether thats clipped along the wall, buried under ground, above a suspended or grid ceiling etc.

There's been one or two occasions I've seen SY cable being used although I'm not sure what the basis of that decision was.

Is there anything in the regs which makes it a requirement for SWA cable to be used? I would be grateful if anyone could point me to the most relevant sections.

More generally, what criteria exist which make an SWA cable a requirement, three phase or otherwise?

Thanks in advance :)
7 Replies
gkenyon
1378 Posts
SWA is not required for three-phase equipment. Other options exist. SWA is not always a good option for connecting to equipment subject to movement or vibration (such as commercial laundry equipment, where the manufacturer will often recommend a flexible cable type, such as H05RN-F or H07RN-F: H05VV-F may be suitable in locations where water is not usually present, but of course this example is laundry equipment).

Care is needed when selecting SY cable, because there is currently no standard for it (and BS 7671 requires equipment and cables to comply with standards, so a cable type complying with published standards is preferred to one that doesn't).

SWA is selected for mechanical protection, and the type of SWA might also be selected on other environmental reasons, such as water ingress (some SWA cable types such as PVC may not be suitable for installation in locations where there could be standing water, such as ducts where the drainage can't be guaranteed all the time, or underground below the water table).

Flexible cables are used to connect equipment where vibration is an issue. SY is not the only option - H05VV-F or H07RN-F, for example, might be acceptable, and this can be installed in metal flexible conduit where extra mechanical protection is needed.

If the equipment is fixed, not subject to vibration,, and suitable mechanical protection is available, there are various options for 3c+E and 4c+E solid core cable also.
mapj1
2851 Posts
In addition note that
You will see SY used as a shielded flex in places with an EMC problem (such as where robotics and automation bring delicate sensing electronics along side large motor currents).  The cores are multistrand. It is not wise to rely on the braid as a safety earth , as it is quite wispy compared to the armour on SWA and needs correctly terminating, so the braid is earthed, to get the proper shielding benefit.
To be honest there is nothing magic about 3 phase, you will see SWA and SY on single phase, and you can wire 3 phase as singles in conduit (rigid or flexible) if you wish to.
Mike
AJJewsbury
2163 Posts
More generally, what criteria exist which make an SWA cable a requirement, three phase or otherwise?
There are lots of factors that go into choosing a type of wiring system - they regs generally just specify what is to be achieved rather than mandating any particular type for any particular purpose (although they occasionally do specify specific types for very specific situations - e.g. for caravan hookup leads).

SWA had a number of features that make it attractive - the steel armour makes it reasonably resistant to knocks and bumps so suitable for rough environments - and as the armour is earthed it also fails safely if the damage gets to be dangerous (by triggering ADS) - so meeting the requirements for cables buried in the ground or omitting additional protection by 30mA RCD for cables concealed in walls etc. It's also reasonably cheap and quick and easy to install (compared with say singles in steel conduit) and less vulnerable to water ingress and corrosion. CY is less robust - but can still be suitable for some situations. For some reasons it seems very popular for air-con installations, especially above ceiling grids.

   - Andy.
eejit
2 Posts
So most of the projects I've been involved in are commercial kitchens and what I've often seen is the electrician take an SWA from consumer unit up into a grid ceiling and then back down into a rotary isolator and commando socket to which the tail of the three phase piece of equipment gets connected.

Usually the grid ceiling has enough space so that you don't have to worry about the cable being 50mm from a surface and being penetrated.

Looking at 521.9, the criteria I see is that the cable should be heavy duty and, if the equipment is intended to be moved, flexible. But if the equipment is not subject to vibration it can be non flexible.

I'm guessing the advantage of the SWA here is they do not need to install conduit where the cables come down from the grid ceiling but often that is such a small length of the total run of the cable it seems quite wasteful to me to use SWA for the entire run. I just feel like I'm missing something.

I would love to hear how others would select cables in the above scenario, Thanks for all the input so far
mapj1
2851 Posts
In that case, if you have a reel of SWA on the van, you may as well use it -  it is not much more expensive, partly as cable is rarely the dominant cost  on short hops compared to labour and the kit at each end, and as you say is easier to fit than conduit, and if it has to be routed in a stud wall or diagonally or something there are no concerns about safe zones and so forth. Your example of SY would not be 'safe' as if penetrated by a nail, the whispy braid may well fuse, leaving a live nail and an opening in the braid, rather than a reliable disconnection
I do not think you are missing anything other than the effect of material costs and labour rates is to make this the most cost effective, and it avoids needing a stock of too many types of cable and termination on the  off chance. and the last thing you want is a joint box and change of cable type mid-run.
M.
AJJewsbury
2163 Posts
Looking at 521.9, the criteria I see is that the cable should be heavy duty
521.9 is only talking about flex used for fixed wiring - so it has no bearing if you're using ordinary solid or stranded core fixed-wiring cables (be it SWA or singles in conduit or T&E). The requirement for heavy duty flex is also conditional - so ordinary duty flex is normally quite acceptable in benign environments (e.g. the final connection to fixed equipment in domestics and similar).

 
Usually the grid ceiling has enough space so that you don't have to worry about the cable being 50mm from a surface and being penetrated.
The general requirement for 50mm from the surface only applies to walls, not ceilings. In ceilings 50mm is only applicable where the cable passes though a joist or similar - which normally grid ceilings don't have (just the occasional steel wire - which you're not going to be able to pass a cable through).

   - Andy.
AJJewsbury
2163 Posts
I'm guessing the advantage of the SWA here is they do not need to install conduit where the cables come down from the grid ceiling but often that is such a small length of the total run of the cable it seems quite wasteful to me to use SWA for the entire run. I just feel like I'm missing something.
Turning it around, if you didn't use SWA then what would you use for the rest of the run? You can't just have singles without containment - and the labour for fitting conduit and then drawing singles through is much higher than just cleating a single cable. If it's 3-phase requiring a N you can't use T&E as there isn't a 4-core + c.p.c. version. You could use a unarmoured European style fixed wiring cable (e.g. NNY-J) but in these parts that's likely to be more expensive than SWA. Likewise any kind of flex is going to be more expensive (generally the finer the strands, the more there will have to be, so the more machine time is needed during manufacture - hence the price is higher). Foil screened cables (e.g. BS 8436) are often suggested as alternatives to SWA where the need for robustness isn't quite as great - but as they're still not common the availability and hence price counts against them.

   - Andy.

Share:

Log in

Want to post a reply? You'll need to log in