Long distance armoured cable - size?

tullula · 2 Jul 2006

We're trying to get a mains water connection at our farm - however because the house is too high we need to put a tank (and a pump) some distance away from the house, down a field.

Apparently the pump will need 6 amps, and will be between 550m and 600m away from the electric supply.

I have a horrible feeling that the voltage drop on this distance will make it an enormous armoured cable (wants to be 4 core).

Any ideas how I start working this out?

dingbat · 2 Jul 2006

Working on 230V and a max allowed volt drop of 4%, that's 9.2V, or 9200 milli-Volts.

Volt drop figures for cables are given in milliVolts-per-Ampere-per-metre.
So, given your values of a 6A load over 600 metres, you have:

9200 = VD x 6 x 600, so:

VD = 9200/3600 = 2.5 mV/A/m (or less)

Using three-core SWA you'd need to use 25 sq mm cable.

Ever thought of using a generator?

RF Lighting · 2 Jul 2006

You say it will need to be a 4 core cable.
Is it a 3 phase pump, or just a single phase pump?

dingbat · 2 Jul 2006

Good point. I just read that!

Spark123 · 2 Jul 2006

If it is 3phase, wouldn't a 3core be cheaper, unless there is some other reason?

RF Lighting · 2 Jul 2006

Also on a run of such a great distance the EFLI is going to be an issue. I can't find my resistance per meter table at the moment, but this will need to be considered too.

Lectrician · 2 Jul 2006

Surprisingly, a C10 would be ok - close, but ok.

My calc shows 16mm just in for v-drop.

600m of cable isn't going to be easy to handle - you will need cable jacks, bar, rollers and plenty of hands and man-power.

ryanunderwood · 2 Jul 2006

There are more pertinent points to consider including the safe isolation of the circuit under fault conditions. The Earth Fault Loop (EFL) impedance is critical especially if you are on a farm. I would assume that living on a farm will give you a higher external impedance thus decreasing the fault current which will then take longer to clear the fault. I would suggest that you seek advise from an engineer that can consider all the aspects into the design of a cable as it is not as easy as looking at the voltage drop. In general safety is paramount. In experience I would say that you are likely to need a cable not far off 50mm 4 core plus a seperate earth conductor to ensure compliancy with BS7671.

Adam_151 · 2 Jul 2006

Which way does the electricity come from? might be cheaper to have the DNO provide a separate supply if their cables are nearby

RF Lighting · 2 Jul 2006

How do you come up with 50mm² 4core?
Also a lot of farms these days are supplied with TN-C-S supplies, so Ze is becoming less of a concern, although there is actually no mention of a farm in the OP.

tullula · 2 Jul 2006

crickey!

1. It wants to be 4 core because we want to have a separate core running a frost heater.

2. 3 phase pump.

plugwash · 2 Jul 2006

3 phase pump and seperate frost heater, won't that mean you'll need 5 cores (unless you supply the frost heater on a step down from 415V)?

RF Lighting · 2 Jul 2006

You could use the SWA to feed a small sub mains, then that a TP supply to the pump and a SP&N supply to the heater.

SparkyMarky · 2 Jul 2006

I had a similar problem recently where we had to supply some lighting on a footbridge accessible by the public from our private substation (long story

). Total cable length and load were similar to your situation (I can't remember the specifics now). In the end the advice we got was to use transformers at each end of the cable to step up and down the transmission voltage so that we ended up using a reasonably sized cable. Obviously this is a pretty specialised job to ensure that it's safe. We ended up using the contracting arm of our DNO to carry out the work for us.

plugwash · 2 Jul 2006

i can't see why it would be that specialist, normal SWA is rated at 1000V line to line, if you step up to that voltage you've quatered your cable requirements over 415V.

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Long distance armoured cable - size?

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