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Extending water pump wiring

  1. Mar 2, 2017 #1
    Good afternoon Wizards, (there are some really smart people here!)

    I have wired up my 1 hp water pump with #12-2 wire w/ground on 220 volts right at 400 feet. It registers the correct voltage and amperage at the pump. According to the wiring chart that came with the pump, I think I'm at the maximum length of this size wire to have the correct volts/amps. My question is I want to extend the wire another 200 feet to hook up to my other house's electric panel. Can I add 200 feet of #10 or #8 wire and expect to get 220v and 8 amps to operate the pump? I don't want to discard the entire #12 wire (already buried) unless I absolutely have to.

    Thanks in advance for your advice!


    Don L
  2. jcsd
  3. Mar 2, 2017 #2


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    Since this is an electrical improvement to your property, it must be taking place under a Building Permit from your local government, right? What do the people at the Building Permit office say? What does the pump manufacturer say?
  4. Mar 2, 2017 #3
    If you are at the limit with just the motor, then you can't add additional load and expect to still be OK. Required wire size is determined by 2 main factors:
    - 1) The size (like #12) has to be able to handle the full load rating of the pump motor.
    - 2) The voltage drop can't be excessive. This is the more stringent requirement when you're dealing with long lengths of wire. I normally use 5% as a maximum voltage drop for this calculation.

    If the #12 wire is at the limit based on the 400 ft distance and the full load amps of the motor, then adding an additional length of wire - even if it is a larger size like #8 or #10 - will still be adding additional amps to the wire run that feeds the motor. So it looks like you will have to replace the buried #12 wire in order to meet the normal requirements of the NEC or applicable governing standard.
  5. Mar 2, 2017 #4


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    A 1 HP pump on #12 at 400 feet is well over the limit in my opinion even on 240 volts. This is a well pump I assume? Upon startup your pump motor will draw considerably more current than 8 amps and at the distance you have now there will be considerable voltage drop.
  6. Mar 2, 2017 #5
    Thank you so much Mr. Magoo. Now that you explained it, makes perfect sense that the #12 would need to be capable of handling the full load.

    Can't thank you enough for your sage advice.


  7. Mar 2, 2017 #6
    I appreciate your input Nova. I realize, now, that I was foolish to stretch the limits of the #12. DUHHH on me. Looks like I'll be burying another wire. Ugh!

    Thanks so much,

  8. Mar 2, 2017 #7
    A point of clarification. To be precise, voltage drop as it applies to circuits in general is not actually handled in the NEC. It is included in the volume of NFPA 70 as an informational note; once called fine print notes, the term was changed to further convey that these helpful and aptly placed notes in the codebook are nevertheless not a part of the written standard which is the NEC and are not enforceable as code.

    In practice nobody worries about getting "red-tagged" based on a voltage drop issue. From an enforcement standpoint that's not unreasonable. A large motor or discharge lamp, for example, that fails to start for the end user is typically an unacceptable outcome that gets taken care of outside the sphere of the building inspector or AHJ's realm, so they don't worry about it. They won't prevent someone from burning up their well pump any more than they will prevent a homeowner from running three or four 100ft extension cords from the back porch outlet and burning up their circular saw.

    Five percent is a good rule of thumb but also as a general rule most motors will be willing to start at +/- 10V of their nameplate rating. They may not operate efficiently, or last all too long, but they are more or less guaranteed to start. Now keep in mind that 8A on run is not the issue here, its the 40-50+A of startup current that will really start to peel away your circuit voltage over a distance.

    If one were entirely dead set on not digging up the old wire, I dunno maybe you built an olympic tennis court and a mahogany gazebo over it, the good news is you may not absolutely have to. The math of voltage drop does not prohibit simply extending the circuit with a large enough conductor, maybe some 500kcmil CU URD cable? Hire an electrician, I'd bet you would not have a hard time finding one willing to do the work!
  9. Mar 3, 2017 #8
    Good morning Krater,

    I absolutely do NOT want to dig up the existing #12 wire. I have measured the voltage and amperage of the well pump while it was running and they check out to spec....8 amps and 230 volts as required. However, I never thought of measuring at start up. I will do that today.

    My aim is to extend the circuit 200 feet to the house I will be living in so I can have control over the water supply when power is lost on occasion. I like your idea of extending the circuit with a large enough conductor. That was my thinking about using a #10 or 8 wire to do the extending. Perhaps they wouldn't be large enough?

    My only criticism of your reply was that I used Bolivian Rosewood for the gazebo! lol (and yeah...I've used a few 100 ft extension cords in my day lol)

    I will contact an electrician about the cable you suggested about the extension.

    Thanks so much for your idea and advice! You daMAN

  10. Mar 3, 2017 #9

    jim hardy

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    Just to elaborate a teeny bit on Krater's post

    What gets you in trouble with induction motors is "starting current".
    It's typically five to ten times running current and flows until the motor gets up to its running speed. Current then drops to normal
    If the voltage drop along your wires is enough that the motor can't make it to full speed then current won't drop off. That'll burn up the motor unless a circuit breaker or fuse opens to protect it.

    So you need a voltage&current reading during start.
    You'll hear the motor accelerate. If it's such a fast snappy start that your meter doesn't have time to settle on the reading then you're probably okay.
    Of course, be sure you test it in the well. Centrifugal pump is too easy to start when it's not pumping anything.

    If you find you have to run a bigger wire,,,, hmmm - if the old one has three conductors maybe repurpose it for power to the gazebo? With a 20 amp outlet you could run a cofeepot and toaster out there.
  11. Mar 3, 2017 #10
    Good morning Jim,

    Ive had the shallow well pump running nicely on the #12 wire for about a year...just wasn't smart enough to test it at start up. I'll make sure it's up to snuff.

    Great idea about a cup a joe. This bunch is tooooo funny! (Could be I could hook up a few 100 ft extension cords too)

    Much thanks

  12. Mar 6, 2017 #11

    jim hardy

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    I recently bought , at my scarp metal yard, two hundred feet of #10 in a coil about three feet diameter. There were three conductors twisted together, it is marked TWU suitable for wet location and direct burial. I'm sure it was somebody's well pump feeder they'd bought and found out they didn't need . I buried it in conduit out to a remote carport so i can run a compressor for tools and air up tires out there.

    Number 12 has resistance of 1.588 ohms per thousand feet. see https://en.wikipedia.org/wiki/American_wire_gauge
    So your 400 foot run has round trip resistance of 0.8 X 1.588 = 1.270 4 ohms.
    At 8 amps that'd drop 10,1 volts . That's running current.

    Adding 200 more feet of #10 , which is 1 ohm per thousand feet, will add 0.4 ohms , dropping 3.2 more volts when running.

    If you're going to have trouble it'll be at starting.
    See if your pump motor has on its nameplate an entry LRA (for Locked Rotor Amps) or KVA CODE (a clue to starting current)

    LRA is what the motor draws when starting .

    KVA code is how many KVA it draws when starting, they give it in KVA per hp
    example: your 2hp motor, if it's a code F would draw about 5.3 X 2 = 10.6 kva when starting.
    Amps = KVA/KV, so at 230volts(0.23KV) that'd be 10.6 / 0.23 = 48 amps.

    You don't have to meet that 5% voltage criterion during start because the pump is easy to turn until it begins lifting water.
    Instructions i've read say if you make 5% running at full load you'll be okay .
    That's why you need to listen to it - does it accelerate smoothly up to full speed ? If it labors in that last little bit of startup , well you're approaching the limit.

    That's the logic behind your testing.

    Good Luck !
  13. Mar 7, 2017 #12
    Hmm...re-reading post #8 got me thinking... You said the purpose of this wiring extension was to, "have control over the water supply when power is lost on occasion." Can you illustrate this intent a little further? Because as I understand it so far, you have a well pump that is fed from another structure with its own electrical supply. If by "have control over" you mean to say that you can supply standby power from one structure but not the other, then what you're proposing will be needed. However if this is simply some sort of load-shedding application and you just want to be able to remotely shut off the pump even when you're not in the building it's supplied from, well then you only need conductors capable of carrying a signal, not the full current of the pump. Contactors in the size you're after are inexpensive, and one might be able to get by with a few hundred feet of #14 run from the primary residence to the adjacent structure with the overcurrent device feeding the pump. Tell me if i'm completely off the mark here.
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