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In my 2015 Subaru Forester, the 12V power outlet in the center console says that it is designed to supply loads that draw a max of 120W. To me, this would mean that there should be a max of 10 amps flowing through the outlet (120W/12V=10A). However, in the fuse box (and confirmed in the owner's manual) the fuse for the accessory power outlet is 20A, which would mean to me it wouldn't trip below 240W (12V*20A=240W).

I would like to buy a power inverter such as this one for use in my car, and I would like to understand what really is the maximum load I can use this inverter for given the constraints explained above.

My question essentially boils down to why is the 120W rating given for the outlet different than the fuse actually connected to it (240W)?

Edit - Here's a link to an online version of my owners manual I found. The power outlet info starts on page 330 of the PDF (part 6-9 of the manual) and the fuse info is on page 505 of the PDF (12-10 of the manual).

I will try to get a circuit schematic on here, not quite sure where to find that yet though

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    Does that fuse serve any other items than that outlet? Maybe a second outlet in the trunk? – Hobbes Dec 11 '16 at 16:54
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    From page 505 of the owners manual I linked above it seems that it's just the center console but I guess I can't be sure until I find a circuit schematic. – scuba Dec 11 '16 at 18:35
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The purpose of the fuse is to limit the current in the wire to prevent it from overheating. That could cause the insulation to melt or even start a fire. The other components in the circuit will (should) also selected to handle the same load. So, for example, the accessory outlet should also be able to handle a 20A load. The bottom line is that nothing in the circuit should fail before the fuse.

Since fuses are designed so that they will tolerate a small overload for a long time (hours or even days at around 5% or 10%) and to blow more and more quickly as the overload increases, the other components in the circuit should also have been selected with that in mind. The graph at the bottom shows the relationship between overload and time to blow (trip) for Littlefuse's automotive blade fuses.

Without seeing a schematic for the circuit serving the accessory outlet it's hard to say if the rating is based on an abundance of caution or other loads on the same circuit, or maybe even a bureaucratic limitation. If you can find a schematic for the circuit, check to see if there are other loads. If the outlet is the only load on the circuit, or if the other loads are predictable/controllable (perhaps it serves a front and rear accessory outlet), then you can probably get away with it. Some of the thinking that goes into the rating of the outlet will be avoiding "nuisance trips" – having the fuse blow due to a condition which "shouldn't" blow the fuse. An example of this would be the current inrush that happens when a computer power supply is plugged in.

One possible explanation for the rating might be that Subaru is being conservative – that's not a bad stance for them to take since inverters are typically rated for the amount of power they can deliver, not the amount that they consume. Since an inverter can't be 100% efficient it will need more than 240 Watts of input to produce 240 Watts of output. The current necessary to run the inverter will go up as the input voltage goes down, so derating the outlet by 50% provides some margin there too.

Another factor that could lead to derating the outlet is the desire to allow "headroom" for short term peak loads like motor starting and the current inrush that happens with power supplies.

I think you could safely try out the inverter, the worst that is likely to happen is that you'd blow the fuse. With luck the fuse doesn't also serve the ABS (actually I'd be shocked if it did).

Time-Current Characteristic Curves for Littlefuse Blade Fuses

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The fuse is for the entire circuit. There may be other things on that circuit, sharing that 20A fuse.

Also, the gauge of wires used in a circuit is part of the "safe" current limits (maybe why your 12V outlet has a 10A limit), while the fuse is there more for current "spikes" rather than steady loads.

I'd stick to the manufacturer's recommendation of a 120W steady load.

  • Thank you. The second paragraph does make sense. So then if I stick to the 120W load I have one followup question. The 120W relates to the power going into/coming out of the 12V outlet itself but due to the non 100% efficiency of the converters etc. involved in the inverter linked above the power at the input of the inverter (what really matters) may be higher than the output (what my load actually sees). Is there any rule of thumb I should use (perhaps only actually use a 100W laptop charger as the efficiency losses will actually pull 120W from the battery) regarding this? Thanks again. – scuba Dec 11 '16 at 2:59
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    Electrical power ratings are always nebulous. Sometimes, they add all the outputs together even if it's physically impossible to use all of them at the same time (they may share a same bus), sometimes they state the max load, sometimes they state the safe steady load. The one you linked to has a "rating" of 300W, a "max load" of 350W, and a "peak load" of 600W. It'll do 300W all day, it can push 350W for periods of time, you can probably spike it to 600W for brief periods of time. Trust the lowest number. – tlhIngan Dec 11 '16 at 3:12
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    Clumsy power ratings may also be why your 120W outlet is on a 240W circuit, just in case somebody hooks-up an inverter "rated" for 120W but can push 240W for brief periods of time. – tlhIngan Dec 11 '16 at 3:17
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    I think before we can make any kind of definitive statement it would be good to see the schematic for the circuit. – dlu Dec 11 '16 at 7:32
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On a properly designed circuit it should be safe to plug in a 20-amp device in the lighter port - the fuse is supposed to be the weakest link in the circuit so the port's wires should be able to handle at least 20A.

This doesn't mean it will work, as there may be other stuff on that circuit that would draw current as well so that the fuse may blow even if you didn't take the full 20A from the lighter port, but I would give it a try anyway, especially when in most cases you won't be drawing the full 20A from the inverter at all times.

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You're over-thinking this. Any reasonable inverter (and even lots of dirt cheap crappy ones you get that plug into the cigarette lighter socket) is going to contain its own over-load protection; most do it in the form of a self-resetting circuit breaker of some sort. The fuse in the fuse box is for protection against shorts in the wiring to the inverter or a catastrophic malfunction of the inverter, not to prevent you from plugging excessive loads into the inverter.

If you want to plug a larger inverter into the existing 20A circuit in your vehicle, then as long as the vehicle was wired correctly and the wiring was not modified, it's "safe" to do so, but it's possible that you might blow the fuse. You can either try and see, or try to get specs on the peak draw (input) of the inverter, which might be considerably higher than the 12V current equivalent of the output wattage, especially if the battery voltage droops below 12V while the inverter is in use. Most inverters produced with the intent of plugging them into an existing cigarette lighter circuit are rated low enough not to approach the limit so you don't blow a fuse.

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    Thanks for the reply. But if I get a 300W inverter and I plug in a 250W load, shouldn't the car fuse trip before the inverter overload protection using the numbers above(and assuming 100% efficiency which in reality is not the case)? – scuba Dec 11 '16 at 18:15
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    If you're going to get your own 300W inverter, you need to add wiring that's appropriate for it, and a fuse to match. The existing wiring in your vehicle is almost certainly not suitable for such a load. Maybe I'm missing something, but the only high-wattage automotive inverters I've seen come with very low gauge wire and clamps to connect to the battery terminals (or to rewire yourself in an appropriate manner), not cigarette lighter plugs. – R.. Dec 11 '16 at 18:18
  • I'll update my answer with more info. – R.. Dec 11 '16 at 18:22
  • @scuba, A 300 Watt inverter is capable of delivering 300 Watts, it won't do that unless the load asks for it. A 250 Watt load on a 20 Amp nominal 12 Volt circuit may never cause the fuse to blow (blade fuses are designed with about 50% margin). On top of that 20 A at an operating voltage of 14ish Volts is closer to 280 Watts. If this is the only load on the circuit and you understand the behavior you're likely to find that it works just fine. – dlu Dec 11 '16 at 19:40
  • @dlu: 20A at 14V is 280 watts, but 20A at 11V (the moment you switch your headlights on, for example) is only 220 watts, and if you've got a 250+ watt load on it, you very well might blow the fuse. – R.. Dec 12 '16 at 0:59
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Something I did not see mentioned here is that some if not most subaru power ports have an additional inline fuse on the back of the socket. therefore the fuse box fuse may not be the one to blow. i'm guessing the inline fuse is a 10 amp. personal experience

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