Concern about '96 cig lighter wiring...

So heres the deal. For a while now I have had a small 350W rayovac brand inverter primarily used for small things like a cell phone charger or lcd screen. I think the most drain I have put on it thus far is with my laptop charger which is 60W. And thats where my concern comes in.

For a while I have been happily able to run my laptop off the inverter with no adverse effects or anything. But about a week or so ago the battery died because the headlights were left on and as soon as we managed to get the van running again, I stupidly tried to turn the inverter on again and it blew the fuse. in the owners manual it reads it as a 20A fuse. I didnt spot this right away and when we went to get a replacement from one of the local auto parts places they gave us a set of 15A fuses instead. Ever since I have not been able to run the inverter without the fuse immediatley blowing. But the thing that bothers me is that it doesnt look to be that much of a drain where it would be exceeding that 15A. Is this actually the case that the fuse is too small or is it probably something else?

350W / 12V (worst case battery) = 29.2 amps.
The point of this calculation is that the rated current draw of the inverter is above the rating of the fuse protecting the circuit that it is on.
What this means is that the fuse will blow before any protection on the inverter will.
Normal voltage would be about 14V..... 350 / 14 =25 amps, which is still above the fuse rating.
It is NOT a problem that the inverter protection is above the fuse rating.....
I am only stating why the fuse is blowing.
The inverter should NOT be blowing a 15 amp fuse with what you are doing.

If the inverter is blowing the fuse with no load, then the inverter is defective.
I would try the inverter with NO load....then try it with a very small load.
With no load, or a small load, the fuse should not be blowing.
Do NOT put a larger current fuse in than the correct rated fuse !!!!!!!!!!

I would also try another item in the same cig lighter.....like a cell phone, etc.
If you still blow the fuse, then give the socket a very good look.

You could try the inverter in another vehicle....but you better have the correct fuse for that vehicle handy.

well, the inverter seems to run fine with no load, but the second i plug my laptop charger in the fuse under the dash blows. the 35A fuse in the inverter of course is still good.

Does the inverter's manual indicate what current capacity circuit feed it requires? As Wiswind's calculations show 15 or 20 is too low for higher steady state loads. There is also an inrush current to consider. This is a short duration pulse when the inverter is first energized. Even a 60 Watt load can add the the peak value and even lengthen the pulse . This could easily blow a 15 amp fuse.

thing is though, as I already stated (or I may not have. I dunno. Im kind of out of it tonight) this configuration has worked fine before with no problems whatsoever. The only thing that has changed between the time it was working and now when its not is the fuse under the dash. the original fuse and what was listen in the owners manual is a 20A. the guy at the auto parts store gave us 15A instead and thats whats in there now. If i can get a confirmation that this fuse amperage mixup is whats causing the issue, I can simply go back and get new fuses rated for 20A.

If I understand there was no problem with the 20 amp fuse but the 15 amp fuse keeps blowing. Perhaps you are drawing 19 amps (for a time period enough to blow a fuse). It makes sense that the 20 was okay and the 15 is not. If the 20 worked for a while and then blew it could be something as simple as something else on the circuit too when it blew. Usually fuse supplies more than one accessory and the cigarrette lighter is most likely only one of the items fed by this fuse.

Remember though that the fuses are there to protect the wiring of the vehicle. Never replace a fuse with a higher rated one.

Remember though that the fuses are there to protect the wiring of the vehicle. Never replace a fuse with a higher rated one.
Yes. and I completely understand that. after being a long time member of mp3car.com and watching people trying to fry their car's, I kind of have some common sense when it comes to these things now.

But just as a clarification, this is the rear cig lighter I am talking about and the owners manual does rate the fuse at 20A. I even have the page here:
http://img469.imageshack.us/img469/8946/fusepage6je.th.gif (http://img469.imageshack.us/my.php?image=fusepage6je.gif)

Anyways though, I will do some testing later today with the 15A fuse its got now and see what goes. I know my cellphone charger is rated at 120V/0.1A so that should be a light enough load.

P.S. Oh, I almost forgot. the little power plug in the center console which is on the same fuse is currently being used by my mom's CB. It's nothing big at all because it fits fairly loosely in the thin cubby hole below the cupholder tray so I have doubts its pulling a whole lot of current, plus its not on alot anyhow. But I figured id put this out there JIC.

So there are two items being fed. On something like an accessory plug(s) the fuse really only needs to be large enough to supply the load. The 20 amp from the factory is there becasue the wiring was designed to support it.

Unless you have some sort of an intermitant short (even near the connector that only occurs when you plug something in) or someone has tapped into this circuit and forgotten the only load is your accessories. This is your CB and your power inverter. If you are blowing the 15 amp fuse then for whatever reason you are drawing more than 15 amps. I would not rule out the CB drawing more than you might think and certainly consider that there is a bit more current required during initial power up (albiet for a few milliseconds or so) but this may be enough to put you over the top. a current meter with a peak hold feature may shed some light. Why not replace with a 20 amp fuse since the wiring was designed to deliver 20 amps.

BTW I doubt that a CB should draw a lot of current but the size is not a factor. A cigarette lighter is small but probably draws 10 - 15 amps. A better clue for current draw is the amount of heat produced.

Well, that was the case. I just changed the fuse today and as I'm posting this, we are on the road going to see a baseball game and my laptop is happily charging through the inverter with no problems.