Volt drop

[Lectrician]

Anyone got an example of a volt drop calculation including the correction for ambient temp (6.1 page 258 in the red book)?

I need to calculate a cable to carry 63amp over 200 meter, and I will obviously be getting rediculous sizes of cable for this. I know I can reduce the cable size if I apply the correction factor, but it is my maths that is letting me down with this equation!

 

[ricicle]

Underground ?
SP or TP ?
Nature of load ?

 

[Lectrician]

I just want an example. I don't want it done for me! I am only interested in help with the volt drop.

But, hey.

It's a UG run of 90deg SWA, 3 core. It is a 200m run, with a 63amp MCB at the origin (possibly an MCCB, not sure yet). The load is to be assumed as fixed, with a 5% volt drop limit. Ze is 0.08.

I have had a quick go, but am not convinced.

I think my second to last line is calculated incorrectly. it should be 0.9 ish, which sounds better.

In the regs it says the equation can only be used if the device is other than a re-wireable, and the ambient temp is 30deg or greater which sounds odd to me.

Anyone had much experience using this equation?

 

[Lectrician]

I think I am actually on top of the equation now, but it still does not make the cable sizing that much smaller in terms of volt drop. You may be able to squeeze a cable size down, but not much else.

The idea of the equation is that if you have to use a larger cable for volt drop etc, then the cable is then unlikely to ever run at it's operating temperature as it is larger than required for the load. The mV/A/m figure therefor will be lower, and this equation should sort that for you.

 

[GaryMo]

Should you be selecting 90 degree SWA (XLPE)?

 

[NotHimAgain]

Leaving aside the conductor operating temperature connection for now - I would ask that you look at the following.

When I run this design through a rather antiquated copy of Amtech Single Cable I get a cable size of 70mm² - not the 16mm² you appear to be applying in your equation.

This is based on a Ze = 0.08 ohm, 63A single phase load, fed via a 200m cable run buried direct in the ground, and assuming a 5% voltage drop.

This is what I would expect. On closer examination I see that the software is selecting 70mm² because of the limit on impedance Zs. If we ignore that and just look at Table 4E4B we find that, given a 230 volt nominal supply voltage, we need a cable with a tabulated value for
(mV/A/M)r of

Vtab = ((230 * 0.05)) * 1000/(63 * 200) <= 0.91.

So we might be able to play and get to 50mm² with a bit of effort.

To use this we would have to protect the circuit with an RCD.

So the first thing you need to do is confirm the details of the load - will it really be 63A and if so for what period and for what sort of load(s).

 

[Spark123]

Lec, I think your sums went wrong as you should have worked out 0.521*60 and then subtracted this from 320.
I ended up with 0.9

 

[Lectrician]

This is feeding some hookup points on a caravan park, and will be looping through 3 posts with 4 sockets on each. We generally use a figure of 6amp for diversity per socket, but are looking at a 63amp circuit.

It is unfortunate that the run for these is so long. They are tightly grouped at the end of the run too, so you can't even take that into consideration.

I think we will be looking to price for a large cable terminated into a switch fuse of some kind, and then running smaller cables for the actual hookups. I need to have a better look tomorrow, I only measured up at lunchtime today, and have not really gone through the options yet.

I used 16mm just to see how the equation worked really. I am nieve enough to think it would be ok! I realised I made an error in my calculations, I think I posted that a bit further up.

 

[NotHimAgain]

Well with pitch supplies you will probably want to use a TT system anyway.

Winter loads are likely to be higher than summer so you may take some advantage of a generally lower ambient temperature in winter.

Are these holiday caravans ?

You may want to consider several runs of smaller cables rather than 200m of 50mm² or maybe 70mm². These cables would probably arrive on a big wooden drum and may take some handling .

 

[ColJack]

might be worth a small "service" room / stand being built in the middle of the pitches ( or to one end ) .. fit a CU in there and get the DNO to put a fresh 100A supply to it.. might be cheaper than 200m of 70mm² cable and your installation..