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 DIY Chatroom Home Improvement Forum DC ~ Watts/Volts/Amps

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11-30-2009, 10:12 PM   #1

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## DC ~ Watts/Volts/Amps

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!

 11-30-2009, 11:25 PM #2 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,832 Rewards Points: 5,246 A 12V pump rated for 15.5 amps draws 15.5 * 12 = 186 watts. Your switch is rated for 1/4HP, which is 186 watts, so the switch is rated for the power of the pump. The fact that the switch is rated up to 32 volts means it is rated to handle 12V, which is what you are operating at.

 12-01-2009, 10:08 AM #3 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 For some reason I can't pull up table 430.248 from the NEC with my bookmark. It shows that small motors are worse than 50% efficient. Your 12v pump is probably 1/8 hp or less and a 1/4 hp motor-load-rated switch can probably handle 12A @ 32v. In this application your switch life may be less than the rated value of 1 million cycles, or 10 million cycles, or whatever. Since this is DC, buying or making [\$2 in parts and a half hour labor] an arc suppression network for your switch contacts will give you a longer service lifetime. If you want to see the arc that you are suppressing, connect the motor to the battery with jumper cables, and then disconnect it. But only do it once. Supposedly this little fireball gives off a large amount of UV light. Last edited by Yoyizit; 12-01-2009 at 10:21 AM.

12-01-2009, 04:49 PM   #4
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Quote:
 Originally Posted by arimike I 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?!
Coming from the battery, negative does not have to be neutral or ground. In fact some cars have positive as the ground and negative as the hot.
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