What Size Wire for my Inverter?

Before you spend the money on an inverter that’s larger than 300W, it’s worth knowing that you can’t just plug it into your cigarette lighter.

I’m always reading other people’s van camping articles and I’ve noticed something.  A lot of people seem to think you can just buy a 1500W inverter, plug it in, and run a coffee pot. If you don’t already know why you can’t, read on.  I’ll give a bit of a detailed explanation, but sum it up under “All you really need to know” at the end of this post.  By the way, for the person that finds this article at the beginning of their research, an “inverter” converts the 12 Volts of your battery to 120 Volts (ac, but don’t worry about that) so you can use household appliances in your van camper.


Power.  What does that term mean?  Most people of a certain age are familiar with the 100W lightbulb (before your government banned them) or a 1500W hairdryer.  They probably also know that W or Watts, stands for “power”.  But what does that term actually mean?

I’m going to use the “hose analogy” that anyone who has worked with electricity is familiar with.  I’d apologize, but if you are tired of the analogy, you probably already know everything I’ll say in this post, so it’s not really for you.  Plus, it’s a great analogy.

Volts and Amps

When talking about electricity, a useful analogy is to think about voltage (12 Volts in this case, or 12V) as the water pressure in a hose.  The flow of water through the hose is analogous to the current (aka, the Amperes, Amps, or A) through a wire.  Now let’s imagine a bucket as a power demand (your coffee pot).  The total water delivered to the bucket is a product of the pressure and the flow.  That’s your “power” in this analogy and the power (Watts or W) is literally the product of the current and the voltage.  Remember that in math, the “product” means you are multiplying “one number times the other.  So power is Volts x Amps.   In an example I’ll use later, 12V x 20A=240W

Installing an inverter in my minivan

This inverter is wired to the battery of the Phase 1 Odyssey. It can provide 800W (1250W peak), which is WAY more power than I needed. I find I can do everything I want with 300W, like make coffee using my immersion heater.For this example, I’ll say that we have 12 Volts available (12v), because the starting battery in your van is 12V.  That’s our hose pressure.  But what do I have available for the wire (hose size)?  That’s going to vary by vehicle, but it’s easier to solve than you think.  That’s because the automaker has already installed the wires, so if I’m going to use my cigarette lighter to power the inverter, that part of the puzzle is a “given” and doesn’t really matter.  What does matter is the limit they’ve put on how much water can flow through the hose.  That limit is called a fuse and it shuts off the current (by melting and breaking the connection) when it exceeds a certain amount.  If you look in your owner’s manual, you’ll see that the lighter is fused at 15A (amps) or 20A (amps).  That’s also what the number on the fuse means.

So, I know I have 12V and my current is limited to no more than 20A (probably 15 in a newer minivan).  Doing the math (12V x 20A) gives me 240W.  That means that even if you buy a 3000W inverter, you cannot plug in more than 240W or your fuse will blow.

I need a thick wire to get the full 400W (peak) power out of “Bessie” my Bestek 300W inverter.  This is the best $50 inverter I’ve found and it will also charge your phone and iPad.  You can find Bessie on my Kit channel at: https://kit.com/odysseycamper

Back to the bucket for a moment.  You can have a tiny hole with lot’s of pressure, but it takes some time to fill the bucket.  You can also have a huge hose with very low pressure and again, it takes time to fill the bucket.  Ideally, you have enough pressure and flow to fill the bucket in a reasonable amount of time.  That means I have to have a hose (or wire) that’s big enough to flow enough water (or current) at the pressure (or voltage) that I have available.

You can’t just increase the value of the fuse to solve the problem because that will let too much current through the wire (hose) and it will melt or catch fire (burst, in the case of the hose).  So what do you do if you want to run an inverter bigger than 240 Watts?  Unfortunately, you’ve got a little bit of work to do. You’ll need to run a thicker wire (bigger hose) to the inverter.  That wire needs to be big enough to handle the current and then a fuse needs to be added.  You can also use a “circuit breaker” (which I recommend) which is just a resettable fuse.

How big that wire needs to be is determined from an ampacity chart.  Here’s one from Pinterest, if you want to pin it for later.

wire size for van inverter









The wire will come from either your starter battery (the one under the hood) or your battery bank.  It doesn’t really matter which it comes from in this discussion, so I’ll leave that for another post.

Let’s say we want to run a coffee pot off of our inverter.  I’d rather use an immersion heater and heat a smaller quantity of water, but let’s go BIG for this discussion.  I just looked at the sticker on my coffee pot and it says 1250W.  I’ll need to feed it at least 1250 Watts to make coffee.  So, I’ll need an inverter that is at least that big, like this one.

That inverter is a little pricey because it is a “pure sine” inverter.  You don’t NEED to use an expensive inverter, but I prefer pure sine inverters because they are not that much more and can run any appliance.  If you don’t know what that means, check out my article about pure vs modified sine inverters.

OK, now I’ve got an inverter that is big enough for the appliance I want to put on it.  It makes sense to run a big enough wire to get the FULL 1500W out of the inverter, but you don’t need to.  One reason that you might not want to is if you are running the wire all the way to the starter battery.  That might be a LOT of wire and thick copper wire is pretty expensive.

Even a small 300W inverter pulls more current than a cigarette lighter can supply. Here I’ve connected my inverter to the starter battery. This is my backup system in case the solar battery bank runs dry. The red and black thingy that the wire goes through is the circuit breaker. It also lets you turn off the whole system if you need to work on it.

Now we’ll employ a little algebra to figure out how much current I (flow) I will need to get to the inverter.  Using the lighter load of 1250W I can divide 1250W by 12V to get 104 Amps.  1250/12=104.  Looking at the chart, I will need a 1 AWG wire.  That’s commonly called a “one gauge” wire and the “AWG” stands for “American Wire Gauge”.  I’ll also need a circuit breaker that size.  To see how this looks in real life, check out my post about installing an inverter in your minvan RV.

All You Really Need to Know

The simple version?  You can only run about 240W from your cigarette lighter (20A fuse).  Maybe only 180W (15A fuse).  To run more, you’ll need to run a wire to the battery.  To get the wire size, take the power draw of your largest appliance and divide it by 12 to get the current draw.  In our example 1250W/12V= 104A.  Now look to the “Maximum amps for power transmission” column in the chart and you’ll see that you’ll need a 1 gauge (AWG) wire to get enough power to your inverter.  That’s about as big around as your pinky finger.  It’s a big “hose” and why I prefer to downsize my power requirements before setting up the inverter and batteries.

Closing Details

For the sake of simplicity, I left out some technical details that would probably interest some other readers.  The big thing I left out concerns the voltage available from the battery.  A fully charged battery will give more than 12V as will a vehicle with the engine running because the alternator might charge at 13.8V or more.  If I put that voltage in my equation, I find that I would only pull 91 Amps.  However, I wanted to give a real-world example where the batteries were not charging and a heavy load was applied.

I’ve used a simplified ampacity chart and suggested the power transmission column, to err on the side of caution.  It is possible to run more current through a smaller wire, but they will get hot.  Given that we are running wires under carpets and such, I went with the safer numbers.

It’s also worth noting that if you ran this coffee pot off the starter battery, with the van engine shut off, you’d completely drain most batteries (40Ah in a minivan) in 24 min.  Since you don’t want to drain that battery more than 50%, you’d have about 12 min of run time available if you just used the battery.  You’ll want to start the van up when running this heavy of a load.

In my next DIY post, I’ll talk about whether you need solar panels, and/or a battery bank to run your inverter, of if you can save a lot of money by just using an inverter.

For a complete list of my DIY minivan camper conversion projects, click on the menu item at the top.

–Darren at OdysseyCamper

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