Electrical Question

[grrr]

Disclaimer: I don't know too much about electricity.

Background: Our well and water pump are located as far away as possible from our little "baan suan" where the electricity is. As a temporary solution I run a homemade extension cord straight across our land (about 80 meters) to the pump when we need to pump water to the tank.

The Plan: My idea is to run an electric line along the perimeter of our land from the little house to the pump, which is 166 meters. I would put the cable inside 3/8" PVC and bury it along with the 1" PVC water pipe.

Questions: Is 166 meters too far for the cable? If not, what size/type of cable should I use?

The specs on the pump are: DAM single phase, 1 HP motor, 746 W, Amp. 5.4, 50 Hz.

Thanks for any information.

[lopburi3]

Normal electric cable in conduct does not last long barred in my experience in earlier years when door bell AC bell. 5 meters probably averaged a year or two between failures. I now use battery/radio door bells and electric in conduct above ground (along walls) for lighting - that has lasted much better. I suspect direct burial cable may be available these days but you would be talking big money for that amount.

You are talking a huge run and will leave it to those with there RPN calculators to provide a wire size but I would look at moving the pump closer to the house (unless it is a vertical lift that the pump can't handle).

[Crossy]

I agree with Lop ^^^ that is a long run.

To get some perspective, a quick and dirty calculation:-

You want to drop less than 5% of your supply (say 11V).

Motor draws about 3.5A on run 5.4A on start

Run is 166m

2.5mm2 cable with an out-and-back volt drop of 18 mV/A/m will drop 166*18*5.4/1000 = 16.1V on start 166*18*3.5/1000 = 10.4V on run (ok if your mains is really 220V but marginal on start if your supply is on the low end of the tolerance).

4mm2 cable with an out-and-back volt drop of 11 mV/A/m will drop 166*11*5.4/1000 = 9.8V on start 166*11*3.5/1000 = 6.3V on run

I would install 4mm2 for safety.

You need to minimise the run length (straight across the land?) or as Lop suggests move the pump nearer the house.

[grrr]

Thanks for the info. I'm not sure moving the pump is an option. It's pulling water up almost 12 meters out of the well, and pushing it 166 meters through 1" pipe to the tank. I'm pretty sure guy who sold us the pump said the two intake pipes need to be as close to the water as possible. It sounds like the better option would be to run the cable straight across the land (dodging fruit trees), which would be around 80 meters. Cable recommendations for that length?

[Crossy]

It sounds like the better option would be to run the cable straight across the land (dodging fruit trees), which would be around 80 meters. Cable recommendations for that length?

It's always best to minimise the run length (keeps the cost down too).

2.5mm2 cable with an out-and-back volt drop of 18 mV/A/m will drop 80*18*5.4/1000 = 7.7V on start 80*18*3.5/1000 = 5V on run.

2.5mm2 it is

You should use directly buriable cable, regular cable in underground conduit can lead to issues with pests and water ingress.

Make sure your pump is properly grounded, you can use a ground spike at the pump to avoid running a long (expensive) ground wire.

Since the pipe already runs around the edge, it may be sensible to run a bigger (4mm2) cable alongside it, to avoid future issues with someone digging it up

Edited March 26, 2008 by Crossy

[Crossy]

At the prompting of another respected board member (let's call it a 'peer review') I've reviewed my sums for the cable sizes (I thought they seemed a bit small when I originally posted).

I've made an error in the start current, this motor has a RUN current of 5.4A, start is going to be (briefly) 4x that say 22A (ouch).

If we assume a worst case 10% drop on start (22V) to maximise motor life then we need to use bigger cable

For the full 166m run.

10mm2 copper with an out and back drop of 4.4 mV/A/m this will give a drop on start of 16V (this is the size that the calculations based on Aussie regs gives)

6mm2 copper with an out and back drop of 7.3 mV/A/m this will give a drop on start of 26V (this is the minimum recommended size)

For the shorter 80m run.

6mm2 copper with an out and back drop of 7.3 mV/A/m this will give a drop on start of 12.8V

4mm2 copper with an out and back drop of 11 mV/A/m this will give a drop on start of 19V (this is the minimum recommended size)

I do apologise for the misinformation, we all suffer from brain fade occasionally, actually I'm a bit surprised that the other sparkies on the forum haven't popped up with their flames lit

Edited March 27, 2008 by Crossy