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Old 07-01-2013, 05:40 AM   #1
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cable size for 12VDC application


Hello
I am installing a backup sump pump (12VDC) in my basement
I was given (yes, given ...) 2 brand new deep cycle YUASA batteries (38Ah each)
I will interconnect them in parallel
The pump draws 20A peak, and ~10Amp continuous
I was thinking to use 10AWG? Is it OK, or should I go up to 8AWG?

Thanks
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Old 07-01-2013, 05:50 AM   #2
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No reason not to use #12
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Old 07-01-2013, 05:56 AM   #3
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AWG12 is enough?
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Old 07-01-2013, 07:17 AM   #4
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Quote:
Originally Posted by french_guy View Post
AWG12 is enough?


#12 is good for 20 amps at 120 volts or 12 volts. I assume the batteries will only be a short distance from the pump
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Old 07-01-2013, 08:07 AM   #5
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#12 stranded. They actually make harnesses that allow you to quick connect/disconnect, that you keep one end on the battery, and the other on the pump. Then you can swap out a set while the other set is charging on say your car or truck.

They also make Solar trickle chargers that can act as backup on house power.



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Old 07-01-2013, 03:40 PM   #6
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With a 10 amp load, you are going to have about 30 minutes of pumping time per battery, or about an hour with the pair. Depending on the scenario you are looking at, that may not take you through the power outage.
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Old 07-01-2013, 03:51 PM   #7
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Yes, but it's better than nothing.......
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Old 07-02-2013, 01:57 PM   #8
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Quote:
Originally Posted by Oso954 View Post
With a 10 amp load, you are going to have about 30 minutes of pumping time per battery, or about an hour with the pair. Depending on the scenario you are looking at, that may not take you through the power outage.
How did you come up to these values?
2 batteries in // will represent 76Ah, meaning the battery can deliver (theoritically) 76 Amps for 1 hour
So with a pump drawing 10 Amps, I should be fine for....7,5 hours of running time. How did you calculate 30 min or 1 hour?
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Old 07-02-2013, 05:04 PM   #9
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I didn’t calculate it, I pulled it off of a not so clear battery chart from a vendors site. But in doing so, I did make several mistakes, and for that I apologize. My biggest error was reading a 1Ca curve as a 10amp curve for the specific battery

So, you should get about 3 hours from one battery, and about double that with two, before making other adjustments that would reduce those times. To refine the numbers, we would need to know the voltage where the gpm drops off on the pump. I used the 10V mark on the chart. If the gpm starts dropping off seriously at 11V that would have to be adjusted for.

The numbers from the chart are also based on 77 degrees and should be temperature adjusted.

So, you are much better off than the numbers I originally posted. Again, I will apologize for that, and for not catching the error myself. But my intent was to flag that you cannot divide 76 by 10 and say I have 7.6 hours. You have 6 hours of pumping at best, and probably less.
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Old 07-02-2013, 05:14 PM   #10
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Isnt 10 amps a heck of a lot for a sump pump to pull? What is this pump like 1.5 HP? That seems very high for a sump pump, does it not?
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Old 07-02-2013, 05:31 PM   #11
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120 watts, at 12VDC.
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Old 07-02-2013, 06:46 PM   #12
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Quote:
Originally Posted by french_guy View Post
How did you come up to these values?
2 batteries in // will represent 76Ah, meaning the battery can deliver (theoritically) 76 Amps for 1 hour
So with a pump drawing 10 Amps, I should be fine for....7,5 hours of running time. How did you calculate 30 min or 1 hour?
Then you have the answers to your own question, ????
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Old 07-02-2013, 07:29 PM   #13
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10A x 12V = 120 watts, as previously noted. A 120 watt pump is about 1/6HP, which is a very small pump. If the pump is say 80% efficient, the actual brake horsepower is perhaps 1/8 HP, which isn't going to pump much water, or pull much head.
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Old 07-02-2013, 11:00 PM   #14
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Quote:
Originally Posted by jagans View Post
Isnt 10 amps a heck of a lot for a sump pump to pull? What is this pump like 1.5 HP? That seems very high for a sump pump, does it not?
Just remember that a device at a given wattage will draw 10 times more amperage at 12VDC than 120VAC.

120 Watts @ 120VAC = 1 Amp
120 Watts @ 12VDC = 10 Amps
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Old 07-03-2013, 02:01 AM   #15
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Given battery cables are available in rather limited gauges and are usually for automotive uses, I'd just go with 4awg which is going to be the easiest to find. It's going to be more than enough.

Also to get more capacity I'd go with a couple deep cycle marine batteries. They are usually 100ah and cost a bit over 100 bucks. You'll also want to monitor the voltage and hydrogen levels to ensure the charger is operating properly.
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