Wianno,

For a non-pressure system, the item to me is a diverter. In essence, it is a box with two levels. When the tank is empty or partially full the water which comes in the lower section goes directly to the tank. When the water tank is full the water rises to the higher level and goes on out as an overflow. This also can be used for removing floating debris from the water stream. The overflow from the high level should be 2 x larger than the pipe into the lower level and into the tank to handle flushing debris out.

I get what you are saying, but water seeks its own height. I need the water to overflow when the tank is full which means I need to divert when the water in the tank is still below the water in the pipe. they should never be at the same height, the tank will always be about 6" below the feed pipe.

W,

Why do you need to divert the water before your cistern is full? When the cistern is full the water will back up in your inlet pipe up the down-spout enough to push the water over the higher level and any additional water because it can't go into the cistern will go out the diverter box. Aloha

Well, there is a reason. I need to put an electric junction in the tank. I plan to feed the power from the house through conduit into a junction box in the tank, then connect the pump power in the junction box with a low water level cutoff float switch. The pump power control will be in the house, about 40 feet away. If the tank fills to the top, the connections will become submerged. The tank is in the middle of the lawn and I do not want to see any part of it.

As far as I can see, pumps and float switches do not usually come with 50 ft or so of power leads. I suppose if it did, I could do all the connections in the house and then feed them to the tank.

Wianno,

If your land has the slope, you could run a large diameter relief pipe out to a daylight drain ditch from below the box. Mighty risky and plenty of digging. Ah, them critters love tunnel living though. My $0.04. Aloha