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Inverter question

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Initially Posted - 10/28/2006 : 10:28:34

I just bought a Xantrex Pocket Power 175 Inverter.

OutputWaveform: Modified Sine Wave & DC
Output Type: Three-prong AC outlet and 5V DC 0.5A USB power port
DC Input Type: Airplane power plug with cigarette lighter adapter
Automatic Functions: Low voltage shutdown, overload protection, short circuit protection
Status Indicators: Power/fault light

Here is my question; Energy can be neither created nor destroyed, therefore the output of this unit cannot exceed the potential output of the socket into which it is plugged, however 12 volt sockets are seldom wired with larger than 14-16 gauge wire. I would never use that gauge for 110 volt purposes, should I rewire my 12 volt sockets with #10 wire to provide adequate infrastructure for this inverter? I will run a big Laptop and CPAP machine on the hook, (not together

)

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 10/28/2006 : 11:04:20

"should I rewire my 12 volt sockets with #10 wire to provide adequate infrastructure "

That's an excellent idea... if for no other reason than to reduce your losses due to resistance.

Edited by - on

Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 10/28/2006 : 11:30:02

OK, my knowledge of inverters is not much but it seems to me that the conversion happens in the inverter and that all you are drawing to the inverter is 12 volts and your existing 12 volt wiring will be fine. I cannot imagine that the manufacturers assume that we'll all upgrade to 110V wiring just to plug in an inverter.

Edited by - on

existentialsailor
Master Marine Consultant

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USA
1180 Posts

Response Posted - 10/28/2006 : 18:37:06

I would. Most boat wiring is lacking by today's standards. This inverter is intended for automotive or airplane usage and the probability is those newer sockets are wired accordingly.
You could try it as is and see how hot your wiring gets, but I'd be hesitant to take the risk myself.
Xantrex is good stuff, nice choice.

Edited by - on

britinusa
Web Editor

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USA
5404 Posts

Response Posted - 10/28/2006 : 19:33:47

According to the Xantrex site, this inverter has a max continuous rating of <blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">AC output power (continuous) 140 W <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Normally one would ensure the cables were up to the max power draw.

So 140 W = 12*i , so i(amps) = 140/12 = 11.7amps !!!!!

According to Don Casey (Pg 48 of Sailboat Electrics Simplified) you would need to have 10AWG if the length from battery to supply point and back is 20', 8AWG above that.

However, you might figure out the power consumption of the devices to be plugged in and then calculate the cable size.

If the laptop used 75watts, then
75/12=6.5amps
and that would require 14AWG - 12AWG (20-30feet)

Next I researched laptop power consumption... it not so bad!
I found one site where using a wattmeter, the power consumption of an iBook was 11Watts, and if charging the battery, the consumption was 26Watts.
26w/12v = 2.2amps
So in that case you would not need to recable unless the battery was on the end of a longer than 20' cable run (assuming you have 14AWG)

Anyone confirm my numbers?

Paul.

Edited by - britinusa on 10/28/2006 19:35:24

aeckhart
Master Marine Consultant

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USA
1709 Posts

Response Posted - 10/28/2006 : 20:32:15

Paul,

Care to take a short vacation to northern Michigan and check out my boat wiring? I'm not shocked

, but in awe.

Edited by - on

Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 10/28/2006 : 22:26:44

I have that exact inverter. You won't need to rewire a thing. Your cig light will run just fine on 12 guage wire. It may be able to convert 11.7 amps, but there is a fuse built into the thing in both the cig plug and one in the box at I believe both are 10 amps. Keep in mind that wire is still only carrying 12 volts DC and not more than 10 amps - the fuse on the unit would have blown. Paul's numbers are correct . However....
I did try watching a movie with my daughter on the way to Ellicottville,NY and my wife was charging her cell phone at the same time. The draw was about 8.5 amps (checked it later on the boat with the meter) and everything was a bit warm. I wouldn't recommend plugging in anything more than the PC with this particular unit.

I'd have to pull out the manual (who knows where it is) but if your Cpap machine pulls a lot of amperage, this unit might suck your battery dry rather quickly. They recommend you start the car every couple of hours in the manual and you might have to do the same with the motor. I would guess that getting up every 2 hours to start the outboard to charge the battery so that you can use a CPAP machine has just a touch of irony.

Edited by - on

britinusa
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5404 Posts

Response Posted - 10/29/2006 : 06:08:14

Thanks Al, I have a brother in-law somewhere up there.

paul.

Edited by - on

Jefffriday
Navigator

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USA
198 Posts

Response Posted - 10/30/2006 : 11:03:37

It's V x A = Watts, your inverter is rated at 140 watts Output... 140/110 = 1.27 amps... don't worry about the rewire...

Edited by - on

ClamBeach
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3072 Posts

Response Posted - 10/30/2006 : 11:17:57

>It's V x A = Watts, your inverter is rated at 140 watts Output... 140/110 = 1.27 amps... don't worry about the rewire...

It's the input current that's the concern here. The inverter has an input of 12VDC and is converting it to 120VAC. Power In >= Power Out so the inverter consumes [at least] 140 watts on the 12V side.

The input amperage required is roughly (140/12)... or about 12 amps @12VDC. Toss in 20% for inverter effeciency losses and you're needing about 15 amps DC to service the peak output of the inverter.

All that said, the inverter isn't rated to supply 140 continuous watts and his laptop only consumes 70 some-odd watts so his wiring should be ok. Still not a bad idea to rewire... especially if the wiring is original un-tinned copper.

Edited by - ClamBeach on 10/30/2006 11:18:37

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 10/30/2006 : 13:54:22

What kind of wiring is on a cigarette lighter, which this unit is designed to utilize?

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Champipple
Master Marine Consultant

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USA
6855 Posts

Response Posted - 10/30/2006 : 14:32:23

14 or 16 guage I would suspect.

Edited by - on

Jefffriday
Navigator

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USA
198 Posts

Response Posted - 10/30/2006 : 21:29:52

Might be worth checking what your laptops input requirements are, It is not very efficient to convert 12v DC to 110 AC, plug in your laptops power supply that then converts the 110 AC back to DC (mine likes 13V DC)... Just a thought

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