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Old 04-22-2008, 12:19 PM   #1
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irrigation pump


Help! I'm a DIYer working with an irrigation contractor on a lake based pump. I'm providing the power from the house. The power source is approximately 500 feet from the pump location and I'll need 30 A of 240 Volt power at the water source. What wire should be used and is conduit required?
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Old 04-22-2008, 01:09 PM   #2
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I hope you want that irrigation pump real bad because your going to need some large wire to get the amperage and voltage that far. And your going to want to set a panel at the pump to connect those big wires to so you can down size wire to get the pump connected.

500 ft takes #2 copper to get 230 volts to the pump assuming your starting with 240 volts at the panel feeding the pump.

It takes 1/0 aluminum to do the same

You can use direct burial insulated wires or wire in conduit.

A 30 amp 240 volt breaker will not accept that big of wire so you will have some issues in that regard, either crimp connectors, barrel connectors. Or sub-feed lugs if the main panel is outside and the unlimited tap rule is allowed.

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Old 04-22-2008, 07:15 PM   #3
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What size (HP) is the motor? Is the pump the only thing out there? Where is the pump controlled from? It'd help us alot if we knew exactly what we're dealing with.

Rob
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Old 04-22-2008, 09:02 PM   #4
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I just paid $1.78 ft for #3. So I know #2 will be little more.

$1.78 ft @ 500 ft== $890.00 before taxes for #3.
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Old 04-22-2008, 10:29 PM   #5
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The pump is a 1.5 horse pump, and I would like to add a receptacle to use at the lake as well. However, if the receptacle adds significantly to the cost, this may have to be re-examined.
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Old 04-22-2008, 10:40 PM   #6
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Plug cost is small compared to 500ft copper
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Old 04-22-2008, 10:56 PM   #7
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Quote:
Originally Posted by mr500 View Post
Plug cost is small compared to 500ft copper
Actually, it will add quite a bit. He'll need to drag a neutral with those two "hots" and find a way to split the circuits up properly, most likely using a small panel.
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Old 04-23-2008, 11:45 PM   #8
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If it's just the pump, #10's will be ok. Here's the math; 500' out and 500' back=1000'. A 1 1/2HP single phase motor operating at 230 volts draws about 10 amps. The resistance of #10 copper is right around 1 ohm per thousand feet. 1 ohm X 10 amps = 10 volts drop. Therefore, if you start out with 240 volts at the panel, you'll have 230 volts at the motor.

The starting current of a typical induction motor is about 6 times its running current, in this case about 60 amps. This would result in 60 volts of drop, or 180 volts at the motor during starting. I realize this sounds like alot, but in reality for a close-coupled centrifugal pump it's actually reasonable. This is because the HP required by a centrifugal pump is proportional to the flow through it. There's very little at low speeds. The high flow starts at about 3/4 of rated speed. This also happens to be the point where a motor is capable of maximum torque at lowest voltage.

Sizing wire for motors, especially long runs, is more of an art than a science. While #10's will work ok in this case, if this same motor started fully loaded, I'd go with #6's. This would result in about 24 volts drop during starting, or about 216 volts at the motor. Full starting torque in not needed in this case, so the wire size can be reduced.

If a receptacle is to be installed, I'd use 2 #6's and 1 #8 for the neutral. Don't forget the ground wire, if you up-size the hots, the ground must be up-sized as well.

Rob
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Old 04-24-2008, 12:39 PM   #9
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I very much agree with Micromind. My load calculations and wire sizes were based on 30 amps to the pump location for motor and receptacle with 3% voltage drop. This was assuming that the motor will be running at the same time you operate your tools. If you use good load management at the receptacle or switch the motor off then I see no reason to do more than what micromind has suggested.
I apologize for possibly not considering other options but you said 30 amps so thats what I gave you.


Last edited by Stubbie; 04-24-2008 at 12:41 PM.
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