24V voltage drop on 250ft run

Discussion in 'The Projects Forum' started by Boyscout, Jul 14, 2010.

  1. Boyscout

    Thread Starter New Member

    Jun 12, 2010
    Hi all,
    I have a water tank at the top of a slope that is approximately 250ft away from the house. The system is gravity feed but at the bottom of the hill is a pump that will refill the tank when the water gets low.

    My issue is the float switch at the water tank will be controlling a relay/contactor which will control the pump itself. The power will be low voltage between the house and the tank (24V) so I am trying to work out what gauge wire to put in so I dont have voltage loss problems. Also, will I run into problems with a 24V relay when the pump it is controlling is 110V?

    Thank you in advance...I wasnt sure if this was the right place to post as its not actually electronics but electrical.
  2. SgtWookie


    Jul 17, 2007
    Is it 24V DC or AC?

    If it's AC, you might consider using a pair of 120v/240 to 24v step-down transformers, but one in reverse as a step-up transformer. The power transferred will be the same, but you can use much smaller gauge wire.

    You didn't mention what current or VA is required on the 24v circuit. Without knowing the current or VA requirement, it's not possible to tell you how much of a voltage drop you'll get across whatever gauge wire.
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    The voltage drop depends upon the relay holding current.

    You could save wiring by accepting a 12v drop in the wires and using a 12v relay, or you could send a pulse down the line to a latch at the pump end which fires a second relay there.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    as for the relay.. You need to look at the contact ratings to ensure it is suitable for your 120V load.. Relays have coil ratings and contact ratings. Typically you will find something like a 24VDC rated coil with contact ratings of like 3A @125VDC.. But you need to check the specifications of the specific relay you are using.
  5. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    You need to know the coil resistance of the relay/contactor you plan on using. The lower the resistance the higher the current and higher the voltage drop. I would sugest using a small relay to then operate a larger relay. The small one would draw less current and there for would require smaller guage wire. Remember Ohms law E=I*R.

    Eg. 16AWG has a resistance of 0.4 ohms per 100ft. So your 250ft run will have a resistance of 1ohm. If you relay draws 1 amp then the voltage drop over that distance will be 1 volt. 2 amp = 2 volts. and so on. You can also double the distance you are running the wire as it will need a return path. This means you would have 2 ohms.

    As most relays are very slack a 24V relay will probably pull in at 18V so as you can see there would be no problems. If you use 20 AWG the resistance is about 1 ohm per 100 ft so you would have 2.5 ohms and a 2.5V drop at 1A. I beleive 1A for a relay would be realistic if you were trying to switch the AC directly possibly even a bit high.

    In conclusion the answer to your question regarding wire size I think you would be safe using 16 or 18 AWG.

    I hope all those calcs are correct. Way too early in the morning for the brain to function, only had one coffee.:p
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    Alternatively, use a transistor to operate the relay, and control the transistor via the long wire. Currents will be in the milliamp range, so the voltage drop will be minimal even if the wire is small.

    But if this installation will be outdoors, maybe there's an issue of the mechanical strength of the wire. You might want thick wire just so it won't be easily damaged.