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DC ~ Watts/Volts/Amps

  1. Nov 30, 2009 #1
    Basic question here: I have a simple circuit consisting of a 12 vdc battery, a level switch and a water pump. The switch is rated for 1/4 HP @ 32 vdc, and the pump is 12 vdc rated at 15.5A.

    If 1/4 HP = 185 watts, I assume that equals 5.8A @ 32 vdc, and is not capable of handling the motor load of 15.5A. Is this right?

    I keep wondering if I am looking at this wrong because I see this exact motor/switch combination in use in various places...with the switch used to make/break the NEGATIVE line. What is the advantage of placing the switching mechanism on the negative side of a DC circuit?

    Any insight would very appreciated!
  2. jcsd
  3. Nov 30, 2009 #2


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    Staff: Mentor

    Your analysis seems correct to me: Typically, switches are rated for a certain amperage. It is odd to see a switch rated at 1/4 hp @ 32 vdc (strange voltage too!). But 185/32=5.8A

    Location of the switch in the circuit really doesn't matter much in a dc circuit.
  4. Dec 1, 2009 #3
    Is this switch is electromechanical or purely electronic? In purely electronic switches, negative side aka "low-side" switches can be simpler to implement usually. One reason might be that they don't need any voltage to be in the "off-state" but they need a positive control voltage to turn on. That is usually a desirable characteristic. I agree that rating a switch in terms of hp is not intuitive.
  5. Dec 1, 2009 #4
    I am working with a mechanical switch.

    Some further thinking about these ratings are confusing me even more. The switch is rated 1/4 HP (185 watts) @ 32vdc which equals 5.8A. Does this mean at 12vdc and 185 watts it is good to 15.4A?

    Another way to look at it is the pump is rated 15.5A @12vdc which I believe equals 186 watts. In this analysis it looks as if the switch is ok to use with this pump.

    I am just not sure what the important part of the spec is...1/4 hp @ 32vdc. Is it the calculated watts (185) or the amperage (5.8 @ 32vdc) that should be used as a limit??
  6. Dec 1, 2009 #5
  7. Dec 2, 2009 #6
    No 12vdc rating listed in spec sheet. I wish I could get the manufacturer to give me a spec...I guess I dont present enough buying potential to get them to do that though...

    Thanks for all the comments and help!
  8. Dec 2, 2009 #7
    The points being made are that common relays are governed nominally by two independent parameters: current rating and maximum voltage across the points. You normally shouldn't exceed either.
  9. Dec 3, 2009 #8
    You should draft shortly 12vdc battery rating.
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