Irrigation Pump Wiring

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  #1  
Old 08-13-07, 06:36 AM
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Irrigation Pump Wiring

Over the last few years my irrigation pumps have performed erratically (circuit bkr opening, pump stopping, overheating, etc.) and have had to be replaced. Now I am having problems getting the current one started (ckt bkr opens) Here is was I have for my 1/2 HP 115 volt motor/pump currently plugged into the outside GFI outlet:

...line in the house is 80 feet from the circuit box to the GFI outlet
...line outside the house is 120 ft from the outlet to the pump
...all lines are 12 guage, three wire lines
...pump requires 10 amps with 115 volts (which is what I have), but this
does not tell me what the surge voltage might be on startup
...all ground fault outlets in the bathrooms and outside and the burglar
alarm are on this 20A breaker. Only the burglar alarm would be running when I start the pump.
...my wiring handbook says:
there will be a 2% voltage drop with 10 amps for every 70 feet of 12 guage
wire

Hence with 115v, 10 amps and 200 ft to the pump I think I would be getting a voltage drop of 6% or about 6.5 to 7 volts.

Question: Is this enough of a drop to make the pump run poorly?
 
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  #2  
Old 08-13-07, 08:53 AM
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> but this does not tell me what the surge voltage might be on startup

A small motor will have start-up current of about 2x to 3x of running current; perhaps a little higher if the pump has head pressure when it starts.

> ...all ground fault outlets in the bathrooms...are on this 20A breaker.

This could be a significant problem. A hair dryer can easily draw 12A-15A.

> I think I would be getting a voltage drop of 6% or about 6.5 to 7 volts.

That's pretty close; I calculated 6.4% (7.7V) for running current of 10A. It's 12.8% for a 20A start-up current and 20% for a 30A start-up current.

> Is this enough of a drop to make the pump run poorly?

Based on your description of the circumstances, yes. You have all the symptoms of a voltage drop problem.

My recommended solution is to run a dedicated 20A circuit for the irrigation pump. To keep the voltage drop within 3% spec, you need to upsize to #8 copper. You could use #10 copper for 4% voltage drop which may or may not be acceptable depending on the particular motor.
 
  #3  
Old 08-13-07, 08:56 AM
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I can understand why the pump won't start, but I don't understand why the circuit breaker trips when it tries.
 
  #4  
Old 08-13-07, 01:16 PM
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Irrigation Pump Problem

Hi Ben,

That makes sense to me. When I had the first pump installed 20 years ago it ran much better and lasted longer (several years). I looked at this motor a couple of days ago and it was designed to draw about 8.6 amps whereas the current pump which lasted only two years is designed to draw 9.9 amps. So I am assuming that with a lower voltage at the motor, the 9.9 amp motor will run 'off design' more and as a result run hotter and cause a shorter lifetime.

Question: Do I have the right concept here?

Next item: I am the only one at home so there would be no other people runnig any of the circuits while I am runnning the pump. I am very carefull about that.

Thanks for the help,
Charlie
 
  #5  
Old 08-13-07, 06:03 PM
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"I can understand why the pump won't start, but I don't understand why the circuit breaker trips when it tries."



Voltage and current are inversely proportional, could it be that with the voltage drop, the motor is drawing much more than 20A for a time longer than the breaker can handle.
 
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Old 08-13-07, 06:20 PM
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Just curious____Is there any kind of obstruction ("anything" that might cause the motor to have a hard time starting). Haven't ever handled a irrigation pump. Does it have a Start Cap? Run Cap? I assume the pump and motor when bought is one assembly?
 
  #7  
Old 08-13-07, 06:52 PM
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Irrigation Pump

Hi Dwayne,
Yes, I think that is what is happening. I THINK when the pump is stopped and I hit it with the 115v, there is a current surge because there is no 'back emf' like there is when the motor is rotating. So there is a surge of current that trips the breaker. This motor is a 1/2 hp motor, same as I had years ago, but I didn't realize it was taking more amperage than the old pump until I compared the rating plates this weekend.
The second problem PROBABLY is that there is too much voltage line drop (higher amp rating with the new pump and small wire) so the motor even I get it started it doesn't like it and eventually heats up and trips.
Charlie
 
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Old 08-13-07, 07:02 PM
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Just to be sure, try hooking up this motor at a 120v source close to the panel and see what happens. I'm willing to bet that 20 amps should handle the start current of this motor, but not with the long run of #12 wire. I think I would run # 8 copper for this motor at 200' and I would like to see it as a dedicated circuit.
 
  #9  
Old 08-13-07, 08:23 PM
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The gradual speed up of the motor is what generates the CEMF to "hold back the current" Sounds like you already know this and have everything under control. Hope you figure it out soon.
 
  #10  
Old 08-14-07, 07:49 AM
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> Is there any kind of obstruction ("anything" that might cause the motor to
> have a hard time starting).

> try hooking up this motor at a 120v source close to the panel and see
> what happens.

These are both great suggestions. If the motor pump is obstructed, needs to be lubed, or something like that it will draw more start-up current.
 
  #11  
Old 08-18-07, 04:41 PM
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Irrigration Motor

Hi Dwayne and Rollie,
I agree with you guys. I got #10 guage wire layed underground today - I put it through plastic piping just in case I 'forget' about it later. I got the path from the circuit bkr box to the pump down to 110ft by drilling a hole thru the basement wall and going through a rock wall and through a gardened area. I just purchased a 20A breaker today and will now turn the project over to my electrician buddy to tie into the breaker box and make all the connections to new outlet out by the pump.
Yesterday the motor 'breathed' its last. It stopped after I got it running by plugging it in and them resetting the breaker after it tripped. After I got it running it ran for 2 hours real well. Temperature outside was about 80 degrees, the pump was in the shade with plenty of ventilation. I kept checking the motor for heat and after 2 hours it was getting warm, about 90F on the motor casing but before I could shut it off. It tripped the breaker for good. Attempts to restart the motor today were unsuccessful. It is 'dead'. It must be shorted.
I hope the shorter line and the #10 guage works better with a new motor. I will be looking for a new 1/2 horse motor/pump. Any ideas?
Charlie
 
  #12  
Old 08-18-07, 06:31 PM
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Good to hear you got this figured out.

"I will be looking for a new 1/2 horse motor/pump. Any ideas?"

I wouldnt presume to know enough about irrigation pump manufactures to say that any one brand or model was better than the other.

#10 awg copper should have a votage drop of about 2.2% when used for a 10A load @ 110' which means you will still have 117.8 volts at the load(if you assume a starting voltage of 120 V )


I suggest you try to find a motor that has a FLC of about 8 amps which will give you plenty of room to play.



P.S. Dwayne and Rollie are the same person.
 
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