SY cable current rating ??

[Buzz1980]

Hi All

got a question regarding SY cable. I have tried to find the current rating for 6mm SY cable and have found a data sheet the GEM cables which states 33 amps in conduit thermal wall and 54amps when in free air. Also seen a few other manufactures state these ratings but a lot stating 44amps for 6mm cable. What table is best to go with in BS7671 to suit this cable. Reason being have a site transformer with a pull of 40 amps have installed this on a 40amp type D MCB to suit the transformer. But didn’t take into account the difference in current rating from SWA , T&E and SY cables ? Hope that makes sense ? any info will be much appreciated

 

[Buzz1980]

Also have noticed that SY rated at 70 degrees op temp ?

 

[Strima]

SY isn't recognised by BS7671 as it isn't manufactured to any recognised British standard. You would need to consult manufacturers data.

Also it's use is classed as a departure from BS7671.

 

[Buzz1980]

Hi strima

Thanks for the reply. Ok will consult with manufacturer. Too be honest might just go with the general rating of a 6mm SY cable that I have been seeing rating it at 44amps. This install is in free air on its own and only 10metre run so I think the 6mm will be good to go on a 40amp MCB to protect the cable. Have seen other tables state that there SY cables are rated for 54amps in free air ?

 

[Buzz1980]

See loads of SY install feeding lights, motors etc in industrial plants all the time ?

 

[pc1966]

It is a perfectly good cable to use for many things, but it is not a recognised UK standard for "fixed wiring" and similar. So if used you should consult the manufacturer of the cable for precise data and note that in any documentation relating to the design and testing.

And yes, that means multiple manufacturers with possibly different specs if you have different brands in use. Sadly that is the consequence of it not being a formal standard.

 

[Buzz1980]

Yeah noticed that but seen it all over must have comments in the test sheet for using SY instead of SWA. I mean I can’t see the current rating of the SY being much less than a standard T&E ?

 

[pc1966]

It is very unlikely to be less as T&E assumes a max of 70C for PVC insulation. So if you start with that I don't see it as a risk.

 

[Buzz1980]

Yeah have looked through different manufactures. Some just state 44 amps but some state 33 amps in conduit and 54amps in free air for 6mm SY so even taking it from T&E spec then I will have 47amps free air which is more than enough. So can protect the 6mm SY with 40amp MCB. ? ?

 

[pc1966]

6mm in conduit is down to 32A MCB protection, but if your SY is in free air then 40A would seem perfectly reasonable (assuming it is not a hot environment near an oven, etc).

 

[Buzz1980]

Yeah 32amp protection for install in conduit ? yeah cable is only in a switch room 10-15 M of cable to the transformer from the dist board in free air. So all good for 40amp ? would have probably put it on 10mm if I installed the cable but transformer already had 6mm SY installed on it and rated at 40 amps?

 

[westward10]

Is it single or three phase.

 

[Buzz1980]

Hi westward Single phase it is ?

 

[westward10]

Should be okay then as correction factors need to be applied to five core cables.

 

[Buzz1980]

Yeah it is a 3 core cable 1 core being the CPC. Phase - brown Neutral - blue cpc - green / yellow earth down the braid also ?

 

[DefyG]

6mm in conduit is down to 32A MCB protection, but if your SY is in free air then 40A would seem perfectly reasonable (assuming it is not a hot environment near an oven, etc).

Or in sunlight as the sheath offers no UV protection I believe.

 

[Strima]

Why use SY when there’s perfectly acceptable cable out there that is compliant to use?

The braid should not be relied on for earthing purposes as it only adds some money nor mechanical protection.

Personally I would look at using SWA/Hi-Tuff rather than SY.

 

[pc1966]

Probably because they are already in place?

If you need a screened control cable that is far more flexible than SWA they are useful, but I would probably go for CY (copper braid) or for fine cores / lower voltages some of the DEF-STAN 61-12 sort of stuff.

But for power then as you say SWA/Hi-Tuff if likely to get some abuse, or H07RN-F (with ferrules) if it is flexing a lot and not too much at risk of mechanical damage.

 

[Darkwood]

SY isn't recognised by BS7671 as it isn't manufactured to any recognised British standard. You would need to consult manufacturers data.

Also it's use is classed as a departure from BS7671.

You are half correct here, the BS 7671 does not rule on cable standards but it may say cables should meet BS standards, I can't quite recall now, it is the 'BS EN 50525-2-11' that I think you may be referring for cable standards which has no provision for the braiding thus the cable isn't covered and by default is not compliant, however it does not stop you using it but you need to do a write up on it suitability and use and why you selected it instead of a compliant cable, this also means any premature failure, injury or death resulting from the cable been used is on the shoulders of those who selected it for use and it cannot be turned on the manufacturers as easy.

SY is not an alternative for flex.
SY screening should always be earthed but not relied on as a fault path or used as an earth for low voltage installations.
SY should not be used outdoors, it has very low UV tolerance which will quickly breakdown the sheath.

SY is a screened control cable and careful consideration must be used before you use it for anything else, its outer clear sheath is very soft and offers little protection for the braid, its braid will rust/corrode quickly if moisture gets in too, if fact this is by design as it found a use in the railway underground network where routine checks for cable damage were made easier as the sheath was see through and all you needed to look for in the damp conditions is rusty sections of braid.

I often despair at the amount of this stuff I have seen misused and in some cases created a lethal scenario, 3 times I have seen this used as extension cable with the braiding not earthed and live due to damage along the cable, I throw this stuff in the bin at every opportunity when I see it used in this way, for the OP - some HO7RN-F will be more than suitable and resistant to many of the oils and chemicals it will be exposed to unlike standard PVC.