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05-01-2005, 12:29 AM
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#1
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4 Rivet Member
1997 30' Excella
Waddell
, Arizona
Join Date: Apr 2003
Posts: 313
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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,
__________________
Steve Heywood
Waddell, AZ
1999 19' Bambi (SOLD)
1997 30' Excella (SOLD)
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05-01-2005, 12:58 AM
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#2
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a.k.a. Ambassador Tim
1960 28' Ambassador
Northern
, California
Join Date: Jun 2003
Posts: 1,921
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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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05-01-2005, 07:32 AM
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#3
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Moderator Emeritus
1972 31' Sovereign
High Springs
, Florida
Join Date: May 2002
Posts: 2,311
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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.
__________________
ARS WA8ZYT
2003 GMC 2500HD 4X4 D/A Ext. Cab
Propane Powered Honda EU2000i
Lots of Hot Sauce!
Air # 283
WBCCI 1350
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05-01-2005, 07:40 AM
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#4
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Rivet Master
1978 31' Sovereign
Texas Airstream Harbor
, Zavalla, in the Deep East Texas Piney Woods on Lake Sam Rayburn
Join Date: Jan 2003
Posts: 2,435
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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.....
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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.
__________________
Dennis
"Suck it up, spend the bucks, do it right the first time."
WBCCI # 1113
AirForums #1737
Trailer '78 31' Sovereign
Living Large at an Airstream Park on the Largest Lake Totally Contained in Texas
Texas Airstream Harbor, Inc.
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05-01-2005, 07:51 AM
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#5
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Old Paint, rolling again.
1973 Argosy 20
Lorain County
, Ohio
Join Date: Nov 2004
Posts: 556
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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
__________________
Have you never questioned those who travel? Have you paid no regard to their accounts- Job 21:29
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05-01-2005, 10:13 AM
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#6
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4 Rivet Member
1997 30' Excella
Waddell
, Arizona
Join Date: Apr 2003
Posts: 313
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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!
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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.
__________________
Steve Heywood
Waddell, AZ
1999 19' Bambi (SOLD)
1997 30' Excella (SOLD)
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05-01-2005, 10:17 AM
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#7
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4 Rivet Member
1997 30' Excella
Waddell
, Arizona
Join Date: Apr 2003
Posts: 313
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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
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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.
__________________
Steve Heywood
Waddell, AZ
1999 19' Bambi (SOLD)
1997 30' Excella (SOLD)
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05-01-2005, 02:03 PM
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#8
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4 Rivet Member
1997 30' Excella
Waddell
, Arizona
Join Date: Apr 2003
Posts: 313
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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.
__________________
Steve Heywood
Waddell, AZ
1999 19' Bambi (SOLD)
1997 30' Excella (SOLD)
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05-01-2005, 04:55 PM
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#9
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Old Paint, rolling again.
1973 Argosy 20
Lorain County
, Ohio
Join Date: Nov 2004
Posts: 556
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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
__________________
Have you never questioned those who travel? Have you paid no regard to their accounts- Job 21:29
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