This is a more complicated question than you've considered. To size a submersible pump, you need to know the total discharge head and the required flow rate. Part of the discharge head is calculated based on friction loss, which is based on the pipe diameter and length.
In irrigation systems, the flow rate is dependent on the type and number of sprinkler heads.
If you have any concerns that your well can't produce the required flow, you should test it. This basically means putting a very large pump down the well (which has a flow rate much greater than you need), and trying to pump the well dry. If it can't go dry, you know the rate is at least what the test pump is rated for. If it goes dry, you let it recover for a measured amount of time, then measure how much water you get out before it goes dry again. Basic math will give you the yield (in eg, gallons per minute).
If your well can't keep up, you either need lower-flow heads, or to install a low-yield system which basically consists of two parts: a well pump that continuously fills a big tank, and a second pump that draws from the tank. Once again, sizing of everything is dependent on the yield, required flow, elevation differences, etc.
You size the actual pump based on the performance curve of the pump, once you know other values.
In short, you effectively need to design the entire system up front, so you know it will all work. For example, if you run too small a pipe, your friction loss will be too high and you won't get enough flow. If you install too large a pump, it will be operating with too little head and not be as efficient as it should be, and probably dramatically shorten the life of the pump.
This is a very common setup, and basically the situation in my house. What you need is called a shallow well jet pump, and I believe this is what you are seeing in the store. Your suspicion is correct - that local mechanic is wrong. Some notes:
- Size the pump 1/2 or 3/4 hp based on the flow rates and demands you expect. There's insufficient details to provide an accurate recommendation here.
- You will need a pressure tank ("bladder") connected to the line after the pump in order to even out line pressure and prevent the pump from turning on all the time. This will very greatly extend the life of the pump.
- To keep the pump primed, you need a foot valve or check valve on the end of the line that is submersed in water.
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As far as I know, all the switching for these is done at the control box. Rather than a centrifugal switch, I believe the relay is simply a fixed time-delay contact.
But I consider them archaic these days - For single-phase supply, I buy 2-wire pumps, and would replace any 3 wire pump with a 2-wire pump when the 3-wire pump stopped working. The arguments in favor of 3-wire have become much like arguments in favor of a buggy-whip to make your automobile go faster...
If 3-phase supply is available that's a whole different animal, but it's uncommon for residential applications where I live.