Voltage drop to well pump

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wormy

Member
Im wireing a submersible well pump that is over 500ft deep. The pump at normal depth requires a 20amp 240v circuit. Because it is so deep the well contractor ran a #8 wire to the pump all the way to the pressure tank in the basement to allow for voltage drop. Does this #8 wire need to go all the way back to the panel box or can I run #12 to pressure tank. And do I still use a 20 amp breaker? Or should I use a 40A.
 

wormy

Member
12 and 20 is fine.

Why? I guess Im thinking of this circuit as a water pipe. The water being the voltage. Will the piece of #12 from the panel to the pressure tank where it changes to #8 not bottle neck the circuit somehow and defeat the purpose of the larger wire for voltage drop?
 

SAC

Senior Member
Location
Massachusetts
Im wireing a submersible well pump that is over 500ft deep. The pump at normal depth requires a 20amp 240v circuit. Because it is so deep the well contractor ran a #8 wire to the pump all the way to the pressure tank in the basement to allow for voltage drop. Does this #8 wire need to go all the way back to the panel box or can I run #12 to pressure tank. And do I still use a 20 amp breaker? Or should I use a 40A.

The breaker should be 20 amp. The wire gauge should be calculated such that the total voltage drop at the pump is acceptable. The voltage drop for 500' of #8 at 240v@20a is about 5.2%. Assuming you have 60' or so of additional wiring to the panel, #12 would add about 1.6% voltage drop (total drop of ~6.8%). Continuing to the panel with #8 would add about 0.6% voltage drop (total drop of ~5.8%). I don't know the minimum acceptable voltage for the pump, but my guess it is that it would be fine with ~6.8% total drop (nominally ~224v at the pump, and since the nominal load is probably less than 20a, the actual nominal drop would be less).
 

Electric-Light

Senior Member
You're driving a centrigural pump with very little momentum and minimal starting torque, so you don't have to worry much about voltage drop during startup.

If this was a hoist or a positive displacement compressor, it likely won't even start.
 
12 and 20 is fine.

I am curious how can you tell without knowing:
  1. What the actual current is,
  2. What is the distance betwwen the tank and the panel,
I presume that the cable is plain, unshielded cable from the pump to the tank. If the cable runs in a steel pipe the induction can affect the voltage drop. These type motors do have a lousy power factor and a very high inrush current due to the slender rotor and stator construction, both which could effect voltage drop issues at starting and running. ( Just some of the things that may help to understand when standard voltage drop calculations does not seem to work.)
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I am curious how can you tell without knowing:
  1. What the actual current is,
  2. What is the distance betwwen the tank and the panel,
From the OP's statements this appears to be a residence and the holding tank is in the basement. It can't be that far, IMO, that a #12 wouldn't work. Assumptions are made, of course. This would have to be a very large pump to not work with #12 to the pressure siwtch.

It is a PITA to terminate that #8 on the pressure switch.
 

iwire

Moderator
Staff member
Location
Massachusetts
I am curious how can you tell without knowing:
  1. What the actual current is,
  2. What is the distance betwwen the tank and the panel,



  1. I am that good.




    The pump at normal depth requires a 20amp 240v circuit.

    This tells me he has wired the same pump before with 12 and 20 so I am assuming he has been doing it right so far. And I bet he has.


    My experience with basements and panels tells me it is rarely going to be a long run to the panel.



    That said you are right, I am shooting from the hip, I will leave it to others to take a simple question and turn it into rocket science. :)
 
I am that good.






This tells me he has wired the same pump before with 12 and 20 so I am assuming he has been doing it right so far. And I bet he has.


My experience with basements and panels tells me it is rarely going to be a long run to the panel.



That said you are right, I am shooting from the hip, I will leave it to others to take a simple question and turn it into rocket science. :)

Assumptions are fine for some, not for others.

Isn't it better to work with data, than scartch head later WTH happened?

Isn't this forum a place to learn?
 

Electric-Light

Senior Member
[/LIST]From the OP's statements this appears to be a residence and the holding tank is in the basement. It can't be that far, IMO, that a #12 wouldn't work. Assumptions are made, of course. This would have to be a very large pump to not work with #12 to the pressure siwtch.

It is a PITA to terminate that #8 on the pressure switch.

It wouldn't be a huge deal if he pig-tailed it with #12 if the reason for running #8 was to reduce voltage drop over the long cable and the current level remains acceptable for #12 pig-tail.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
It wouldn't be a huge deal if he pig-tailed it with #12 if the reason for running #8 was to reduce voltage drop over the long cable and the current level remains acceptable for #12 pig-tail.
Those pressure switches are pretty small to make a pigtail. I think we have used stakons on the end of the #8
 

kwired

Electron manager
Location
NE Nebraska
Those pressure switches are pretty small to make a pigtail. I think we have used stakons on the end of the #8

Junction box with incoming line, outgoing load, flexible conduit to pressure switch that contains 12 AWG line and load conductors.

I do this often even if the load conductors are only 12 AWG - usually has a neater appearance. BTW if the breaker supplying this is not within sight then it is easy to nipple to a disconnect from the j box, disconnect required within sight of a motor controller which is what the pressure switch is if it opens the motor circuit conductors.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Junction box with incoming line, outgoing load, flexible conduit to pressure switch that contains 12 AWG line and load conductors.

I do this often even if the load conductors are only 12 AWG - usually has a neater appearance. BTW if the breaker supplying this is not within sight then it is easy to nipple to a disconnect from the j box, disconnect required within sight of a motor controller which is what the pressure switch is if it opens the motor circuit conductors.

Exactly. Can't get much better. We use AC disconnects.
 

mull982

Senior Member
It wouldn't be a huge deal if he pig-tailed it with #12 if the reason for running #8 was to reduce voltage drop over the long cable and the current level remains acceptable for #12 pig-tail.

Is there anything in the code preventing the splicing of two different size conductors as long as smaller conductor is rated for OCPD?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Junction box with incoming line, outgoing load, flexible conduit to pressure switch that contains 12 AWG line and load conductors.

I do this often even if the load conductors are only 12 AWG - usually has a neater appearance. BTW if the breaker supplying this is not within sight then it is easy to nipple to a disconnect from the j box, disconnect required within sight of a motor controller which is what the pressure switch is if it opens the motor circuit conductors.

I didn't say it couldn't be done, I said it was a PITA. There are those extra steps involved that are a bit of a hassle. Generally, the pump guys here are licensed to wire from the pressure switch to the pump so I don't worry about it.

I have had them make me run a #8 from the panel. I won't do that anymore unless they prove I need it.
 
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