2 wire vs 3 wire

[Odysseus99]

I read this excellent thread from 2007 on this forum on this issue:

https://terrylove.com/forums/index.php?threads/2-wire-vs-3-wire-pumps.14991/

It really looks like 2 wire is best for me. I'll use a 1hp 7gpm with 300 ft static level and 350 ft total dynamic head. I need to string 750 feet of wire from pump to power, so the wire cost is significant.

Two questions-

Lightning- Where I am there is a fair amount of lightning. Well could be hit. Is a 2 wire pump MORE subject to lightning damage than a 3 wire? Not the control box, the pump. Do 3 wire pumps get damaged by lightning too?

Wire ampacity- I've read a few passing remarks that the wire on 2 wires needs to be larger than on 3 wire, but there is only one wire size chart, not two separate charts, one for 2 wire pumps and one for 3 wire. So what does this mean? With a 2 wire you should just be cautious to oversize if you are near the limit on a given wire size?

Here's one wire chart-

http://www.deanbennett.com/submersible-pump-wire.htm

Shows that for 1hp #10 wire should be used only up to 630 feet. Valveman thinks I should use #12wire down the casing, because Franklin says that a minimum wire size actually reduces start torque for 3wire pumps. That's interesting. But, if I use a 1hp 2 wire at 750 feet, by the chart I should use #8 wire.

Thoughts?

And THANKS for your input. Greatly appreciated.

 

[Valveman]

The 3rd wire in a three wire motor is just for starting. Once the start relay kicks out, the motor is running on 2 wires the same as a 2 wire motor. 2 wire motors have less starting torque. And I have been know to use a larger HP control box on a 3 wire motor to "jump start" when it is locked down. Sometimes it works, sometimes it doesn't, but at least I have the option.

The Kingsbury type thrust bearing is not likely "pitted", just stuck form sitting a long time without a film of water between the Stainless Steel Plate and the Graphite impregnated Teflon pads. Get it to turn and the film of water gives a hydroplane effect between the bearing surfaces and all is good.

So the wire size for 2 and 3 wire motors is the same. I suggested using larger wire underground to the well, like #8 copper or #6 Alu, then 400' of #12 down the hole for the reduced voltage start. If you run the full 750' with the same size wire, you will need #8 copper, and you won't get the reduced voltage soft start unless you use 990' of #8.

Lightning is going to get a 2 wire motor or a 3 wire motor the same. If the well is perfectly clean a 2 wire motor is fine. If you have sand or grit in the water as we sometimes do, the three wire is preferable.

 

[Sponsor]

 

[Reach4]

If the well is perfectly clean a 2 wire motor is fine. If you have sand or grit in the water as we sometimes do, the three wire is preferable.

You don't put value in this statement from the AIM manual?

When the motor is not free to turn, as with a sandlocked pump, the BIAC switch creates
a “reverse impact torque” in the motor in either direction. When the sand is dislodged,
the motor will start and operate in the correct direction.​

 

[Valveman]

No I don't put much value in that statement. Sounds like a good marketing gimmick. It has been my experience that if you sand lock a 2 wire, you will be pulling it up and manually hosing it out to get it to turn. I just don't have a sand lock problem with 3 wire motors because of the extra starting torque.

Also, I have been carrying around little demonstrators for 20+ years to show how a CSV works. These demonstrators have a ball valve I can turn to bypass the CSV, which causes the pump to cycle on and off rapidly. I let it cycle like this for a minute, then close the ball valve to make the CSV start working, and the cycling goes away instantly. I like to see peoples eyes get big when I do this. But the point is, I have to use 3 wire motors in my demonstrators to keep it from tripping the overload. The 3 wire motors will cycle like crazy for as long as I leave the CSV bypassed. But the 2 wire motors will trip the overload after 3 to 5 cycles. This alones tells me the 3 wires motors are much better for starting.

 

[Craigpump]

We only stock 2 wire pumps on our truck and a few 3 wire motors we can interchange. The only issues I've had are with 2 wire, 1 hp Grundfos motors. Put them in, 30+ amps, no start, pull them right out.....

That said, all 2 wire motors are more suseptible to overheating during short cycle conditions, but in reality, if your pump is short cycling the system is either failing or not designed correctly.

 

[Odysseus99]

Thanks a lot guys