Power calculation

[Darren_]

What cable do I need if I want to power a 24VAC device from a power supply that is 200m away?

[Flurfl]

A 200m one ;-)

I can't be bothered with the calculation at the moment but the main factor is resistance of the wire, having said that I don't think the power loss over 200m is going to be enough to worry about.

[Tweed]

The thickness of the wire is determined by the current carrying capabilities of it, therefore you'd need to say what the wattage/resistance of the load is. But the voltage loss over that distance should be fairly negligible as Flurfl said already.

[Hippy]

Remember, power (Watts) is a function of Volts times Amps. For a given power, the higher the voltage the lower the current and vice versa. That is why the long distance wires taking electricity over the pylons in the countryside carry extremely high voltage. If they were to operate at 24 volts, then the current would be so high that the cables would have to be many metres thick to cope.

So, let's say your 24v device is rated at 100 Watts. The curent required to power this device will be 100 divided by 24 making 4.17 Amps. So if, in this case, you were to use 5 amp cable then you would be within acceptable limits. However, the cable has a finite resistance (Ohms) and for 5 amps flowing it only needs to offer 0.2 of an Ohm before you loose 1 Volt across it. Also remember that to complete the circuit you need TWO conductors therefore the effective length for calculating the resitance of the wire is 400 metres. In the above example (and it is only an example not an attempted description of the real situation), I would be inclined to go one step up from the minimum required to reduce the voltage drop and go for 10 Amp cable. But how much is that voltage drop? More than you might at first imagine!

Darren, your situation will depend entirely on the power that you wish to transmit over your 400 metres of wire, and that is the vital element missing from your question. I hope that all this will help you figure it out.

... continued next due to charcter limit.

[Hippy]

... continued

I have just measured a 25 metre single run of 13 Amp flexible mains cable at 5.3 Ohms. So that same cable running 400 metres would offer 5.3 multiplied by 16 equals 84.8 Ohms. So, our nominal 5 Amps flowing here would result in 424 Volts being  dropped across the cable (I*R=V 5*84.8=424). The supply end, would have to be 424 + 24 = 448 volts to deliver 24 Volts at the device end and allow 5 Amps to flow through the circuit. If the current demanded by the device reduced then the voltage at the device end would shoot up and could cause failure and danger. This arrangement is very bad and should not be attempted. Far better to have a supply of power much closer to the device, or a high tension transmission line and a transformer at the device end.

[Thunderchild]

what hippy said is true daren if you tell me the watts or the amps then we have a better idea of what we r dealing with. if the wattage is like 100 or over and we r talking about a permanent thing i do advise to use transformers either take 240 volts straight to the location or a higher voltage if avialable and then reduce it with a transformer or if you have 24 VAC direct use a 240/24 transformer reversed to up the voltage reducing the amperage and then reverse it with another transformer. in case you want to know the same transformer can be used to up or down as you wish just make sure u connect the right side up or it will go BANG.

good luck if you really still need to transmit 24 volts over 200 metre give me the amperage or wattage and i'll work out the cable section u need

[jake-the-peg]

If this is just running an electric drill or lawnmower outside etc then ignore the following.

However if you are trying to install mains voltage outside for lights, pumps power to a garage etc. then you are not permitted to do this without building regulations approval unless you are certified to do so.

Do not pass GO, do not save �200 and do not pick up a roll of armoured cable and carry on regardless ( which is what you'd need if you were running mains outside)