SWA earthing

[RF Lighting]

I have a supply to run in. It is a single phase 60A supply which is 135 meters long.

From my calculations, I'm going to need a 70mm² cable to keep volts drop within acceptable limits.

I'm trying to work out if it's going to be acceptable to use a 2 core cable, and rely on the armour only, or if a copper earth is required too.

I only need 14mm² of earth to satisfy the adiabatic equation, so I am well covered there, but I'm struggling to find the resistance per meter of steel to check my Zs is goint to be (a) low enough to provide ADS, and (b) low enough to supply final circuits and keep their Zs low enough.

 

[EFLImpudence]

Any good?

 

[Spark123]

XPLE or PVC?
For 70mm² 2 core PVC the max resistance for the steel armour is 1.9Ω/km and copper conductor 0.268Ω/km both at 20°C.
XPLE 2Ω/km armour and 0.268Ω/km copper BS5467 at 20°C.
Temp coefficient for armour is 0.0045, copper is 0.004

 

[ricicle]

70mm² is way oversized for VD !

 

[Monkeh]

70mm² is way oversized for VD !

Keeping it under 2% drop on the run out seems pretty sensible to me..

Damn expensive bit of cable, though, but I don't know what a pair of 15kVA+ transformers costs these days..

 

[ban-all-sheds]

I don't know what a pair of 15kVA+ transformers costs these days..

Do you have a ladder, cutting equipment and a complete lack of regard for your own safety?

 

[Monkeh]

I don't know what a pair of 15kVA+ transformers costs these days..

Do you have a ladder, cutting equipment and a complete lack of regard for your own safety?

Yes, yes, no. I can drive the equipment if you're willing to go play snippy, though.

 

[JohnW2]

70mm² is way oversized for VD !

Even if there is lighting at the end of it? The next size down (if you can find it) from 70mm² is 50mm² and, by my reckoning, 60A through 135m of 50mm² would result in a VD of about 7.53V (3.28%, aka >3%) from the SWA alone - i.e. before one considered any VD in thye final circuits and upstream of the SWA. Have I got my sums wrong?

Kind Regards, John.

 

[Lectrician]

Assuming a Ze or Zdb 0.02 (my softwares default), I get the below.

 

[JohnW2]

Assuming a Ze or Zdb 0.02 (my softwares default), I get the below. ....

Why does your software give a VD of 6.65V for 50mm², whereas using B7671 table figures gives 7.53V? Is your software perhaps assuming 20° conductor temperature, whereas BS7671 gives VD figures for 70° ?

Kind Regards, John

 

[ricicle]

Looking purely at VD, then 50mm² is satisfactory for a 3% max. drop running at 60A.
You would have to weigh up the likelihood of any expansion (which would involve uprated source protection anyway) against the price difference between 50mm² and 70mm² to see if it was worth bolt and braces oversized cable.
Also it may be that diversity is applicable in this case, therefore this can be used to give some room in calculation ie the load might never reach the full 60A capacity.

 

[JohnW2]

Looking purely at VD, then 50mm² is satisfactory for a 3% max. drop running at 60A.

As I've been saying, if one uses the BS7671 (70°C) figures, 60A through 135m of 50mm² appears to result in a 7.53V (3.28%) drop. However, as you say, it may be possible to work to a design current a little less than 60A in order to get that below 3% (although, as I've also said, one also needs to take into account the VD upstream and downstream of the SWA).

Kind Regards, John

 

[EFLImpudence]

Don't forget it is RF asking.

I am sure he has done the calculations correctly.

I am surprised he hasn't got the necessary information already.

 

[EFLImpudence]

Can he not get away wiith more than 3% if the light is not susceptable to such voltage drop?

 

[JohnW2]

Don't forget it is RF asking. I am sure he has done the calculations correctly.

Exactly - which is, I strongly suspect, why he concluded that 70mm² was required from the VD point of view, even though others are suggesting (perhaps because they are using VD figures at a different temperature) that 50mm² would be adequate in terms of VD.

Kind Regards, John