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Old 05-01-2005, 01:29 AM   #1
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Inverter Questions???

How many watts 12 volts DC does it take to produce 400 watts 120 volts AC through an inverter?

I ask because I bought and installed a 400 watt (800 watt peak) inverter to power the 120 watt air pump for my Select Comfort bed. Problem is the inverter will not start the air pump (the inverter shuts off). I strongly suspect my problem lies with the fact that my batteries are nearly 30 feet away.

Thanks,
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Old 05-01-2005, 01:58 AM   #2
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The newer electronic type of inverters can be around 80% efficent. So for a 400 watt output you will need 500 watts on the input. So the current is I=P/V or 500w/12v=41.6 amps. You will need some big wire really close to the inverter. Depending on distance, but I would shoot for #8 or bigger to get the full power out of it. But you only need 120w@120v so you will need 150w/12v=12.5amps which a #10 wire would be OK.

Having thourghly confused everyone including myself, if you didn't pay a couple of hundred for it, it's probably severly over-rated. Those cigarettle lighter jobbies are pretty cheap.

Here are the ones I like.

Good luck with your project!
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Old 05-01-2005, 08:32 AM   #3
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Best to mount the inverter as close to the battery as possible, then run the AC line to the location that the power will be used at.
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Old 05-01-2005, 08:40 AM   #4
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Quote:
Originally Posted by Safari Tim
The newer electronic type of inverters can be around 80% efficent. So for a 400 watt output you will need 500 watts on the input. So the current is I=P/V or 500w/12v=41.6 amps. You will need some big wire really close to the inverter. Depending on distance, but I would shoot for #8 or bigger to get the full power out of it.....
In order to properly size the wire for 120 AC, and 24 and 12 volt DC, follow this link:

http://www.windsun.com/Hardware/Wire_Table.htm

As seen in the 12 volt table, 40 amps at 30 feet requires a #2 wire - and that is at a 5% voltage loss - for a 2% loss you have to go with a 2-0 or a 3-0 wire - a properly sized wire is truely critical.

in analyzing the total wire footage requirement, don't forget to add the length required for horizintal turns and vertical rises and falls - a couple of feet here and a few feet there really add up.

Luck.
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Old 05-01-2005, 08:51 AM   #5
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The web site says that the pump only uses 60 watts. Perhaps you could piggy back your inverter on one of those portable battery packs. Or go with one that has the inverter built in like the Xantrex I found in The Northern Tool catalog: see attachment and

http://www.northerntool.com/

I had the same problem in my truck using the cig lighter. Then I hot wired the inverter to the battery and now get full rated wattage at the 120 volt outlet.

Good luck,

Steve
Attached Files
File Type: pdf powerpack.pdf (82.9 KB, 174 views)
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Old 05-01-2005, 11:13 AM   #6
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Quote:
Originally Posted by Safari Tim
The newer electronic type of inverters can be around 80% efficent. So for a 400 watt output you will need 500 watts on the input. So the current is I=P/V or 500w/12v=41.6 amps. You will need some big wire really close to the inverter. Depending on distance, but I would shoot for #8 or bigger to get the full power out of it. But you only need 120w@120v so you will need 150w/12v=12.5amps which a #10 wire would be OK.

Having thourghly confused everyone including myself, if you didn't pay a couple of hundred for it, it's probably severly over-rated. Those cigarettle lighter jobbies are pretty cheap.

Good luck with your project!
As I suspected, I bet my voltage at the inverter goes in the toilet whenever I put any kind of load on it. Running #8 wire from the front of my trailer to the bedroom (in the rear) isn't something I want to consider doing right now. I have an idea which involves a sealed motorcycle battery, if it works I'll report the details here.
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Old 05-01-2005, 11:17 AM   #7
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Quote:
Originally Posted by sfixx
The web site says that the pump only uses 60 watts. Perhaps you could piggy back your inverter on one of those portable battery packs. Or go with one that has the inverter built in like the Xantrex I found in The Northern Tool catalog: see attachment and

http://www.northerntool.com/

I had the same problem in my truck using the cig lighter. Then I hot wired the inverter to the battery and now get full rated wattage at the 120 volt outlet.

Good luck,

Steve
While the website says 60 watts, the airpump and paperwork both say 120. I suspect the 60 watt figure is what the airpump draws to power up and tell me how much air is in the bed. My problem is when I tray to increase air pressure. The inverter shuts down 120 volt power and starts screaming at me, probably a low voltage alarm.
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Old 05-01-2005, 03:03 PM   #8
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Follow Up, or how I figured out my mistake

As inverters go 400 watts is pretty small. That was my mistake, treating it like a low wattage 12 volt accessory. The fact that the manufacturer provided 2 different power leads (light guage that plugs into a lighter, heavier guage that hooks directly to a battery) and clearly states "For loads over 150 watts hook directly to the battery using the supplied leads" should have clued me in.

Since I happened to have a fairly large 12 volt motorcycle battery I tried hooking it directly to the inverter using the heavy guage wires and the inverter had no trouble running the airpump. Problem solved! Since my motorcycle battery is in good condition and sealed I am going to install it under the bed and run it as a "total loss" system. I will remove the battery between trips and recharge it. Not as clean as I had hoped, but a LOT LESS HASSLE than running 30' of heavy guage wire through my trailer.
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Old 05-01-2005, 05:55 PM   #9
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Good solution Steve, glad you worked it out

Or instead of letting the battery drain hook it up to a small dashtop solar charger...which is basically a trickle charger.

Steve
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