Power Inverter on the 12V Installation

Stranded wire is typical for automotive, because things vibrate.

For heavier gauges, many people use welder wire; it is made of extra-fine strands and is easier to work with than ordinary stranded wire.

Chances are, the independent auto electrical shop nearest you already has nice spools of red and black heavy-gauge welder wire and can roll off and supply whatever lengths you need.

I typically walk into the one in my neighborhood and just say I'd like so many feet of this color and so many feet of this, in this gauge, with these terminals crimped on the end. It takes him about two minutes, the terminals are crimped on with a much heavier tool than I'd ever go buy from Amazon, it's all nicely finished up with heat-shrink, and I get off cheap compared to buying the tools to do it myself.

As for the gauge, figure out where you're going to want your inverter to sit, and the wire lengths you will need to get back to the battery + and to good body ground. Look up the resistance per foot of different wire gauges, for example here:

Copper Wire - Electrical Resistance vs. Gauge

Look at the specs for your inverter and decide what voltage drop you will tolerate. For example, if you'll be using 2 feet of red, and 2 feet of black for the return, that's 4 feet total. Look for example at 4-gauge wire, which is 0.253 Ω per thousand feet (or, 0.253 mΩ per foot). So at 4 feet you'll have about 1 mΩ. When drawing near full load at 80 amps, the wire will be dropping 0.08 V. You'll be using 6.4 watts of power heating the wire up. These are very small losses.

With 8-gauge wire you'd be seeing a 0.21 V drop at full load, amounting to not quite twenty watts lost in your wires.

Make sure everything you do is well (solidly) crimped/made and well insulated and protected from chafing on edges of anything. Put the inverter where its cooling isn't obstructed. Be familiar with what safety features are designed into it.

I recommend attaching your + cable at the car side, not the battery side, of the fusible link that is in the battery + clamp. If you look at this photo (sorry, it was for a post about something else, there's a lot of stuff in it), the fuse link is in the part of the red clamp that hangs down the side of the battery:



There's a nut and stud on its bottom end there, a perfect place to attach your cable using a ring terminal.

Located there, your tap is protected by that clamp fuse between it and the battery, and by the front fusebox fusible link between it and the converter under the hood.

Also, you will pretty much always have the car in READY when drawing much power from your inverter (won't you?), which means the power you're using is coming from the front of the car. By locating your tap between the two fusible links, there is only one fuse voltage drop between you and where your power is coming from. If you attached right at the battery post, there would be two.

 

Maybe rethink your idea. Ignoring the AC/DC power requirements, remember P=VI (Power = Volts x Amps). At 12V, 1000 Watts means you could be drawing 83 Amps. That's a lot. It takes #4 wire to handle that (google wire ampacity). The wire size depends on the amps that are being run through them.

This is a fairly common problem with 12V inverters. A normal house plug provides 15A to 20A at 120V. That's 1800W-2400W. Every Amp you draw at 120V is 10 Amps at 12V. People go wanting an outlet or two, to run a fairly low-power 120 V appliance, but it turns out that it requires hooking up some major wires directly to the 12V battery.

The real problem begins with the fact that the 12V system in a Prius isn't made for that much load. The high voltage battery is used for a lot of what's normally powered by electricity in a car. Drawing that kind of power from that little battery will flatten it in fairly short order. Since it's small, I would also expect that it Toyota didn't provide the 12V charging system with that large of a charging system either.

The 12V battery has a capacity of 45AH (45 Amps times Hours. So 45 amps for 1 hour, or 45 hours at 1 amp). That 83 amps won't last but 1/2 hour (all things being ideal).

Even though the battery is less than half the size of a regular car battery, it costs about 3 times as much. Your mileage may vary, but I avoid asking too much from that battery.

 

The size of inverter the OP is asking about installing in a gen 3 is the exact size I have installed in my gen 3.

The key thing to remember is not to expect to draw that kind of power when the car is off (for the it's-a-dinky-battery reasons mentioned in #3). Plan on having the car READY for that. When the car is READY, you are getting your power from the DC/DC converter under the hood, not from the battery.

The converter's rated output is 120 amps at 13.5 to 15 volts (more details here). That's around 1600 watts, taking the lower voltage figure.



Of course those 1600 watts aren't all for you. The car has to use some. Toyota had no reason to oversize the converter, so you can be pretty sure when you have lots of power-hungry car equipment (lights, heater blower on high, defrosters, seat heaters, etc.) there is very little capacity left over.

On the other hand, the car can be using as little as 400 watts or so in READY when you've got most of that stuff turned off. So at such times you've got no problem drawing 1000 watts for your inverter. Just be aware of the limits if you start turning on a bunch of the other car stuff.

The fact that your power is really coming from the converter then, and not from the dinky battery, is one of the reasons for making the + tap on the car side, not the battery side, of the battery + clamp fusible link. That way, the path from the power source to you doesn't incur a voltage drop across that link.

Wire ampacities are more of a calculated thing for this kind of application. The lookup tables that might be more familiar are typically for the kind of wiring done in buildings.

When designing a 12 volt system with highish currents and shortish runs, it's common to go right to the calculations, just as #2 showed. Your goals are to see what the wiring voltage drop will be at full load, to make sure the inverter will stay happy, and to see how much power will be lost to heat in the wire, and avoid the wire getting too hot.

A handful of watts being dissipated in four feet of heavy copper is not likely to make it very hot.

 

I did not know that the charge system was capable of that many amps. My main point was it could overtax the 12V system, which for a Prius has less capacity as well as costs at least three times as much to repair as ICE cars.

I was also trying to make the point that 120 V inverters can easily overtax regular ICE cars 12 V systems. People install these things without realizing what that's trying to do. In this case, the inverter has 4 120V outlets - each one was meant for up to 15A of load. As long as what you plug in doesn't draw that much, you'll be fine - but at some point, we exceed a limit. The OP asked a valid question as to what precautions he should take.

Ampacity is the calculated thing that the NEC/NFPA uses when writing electrical codes. The best guidance is not what you can get away with, but what's used by electrical engineers when designing circuits. If you only need to run it for 4', that's great. You only need to buy 8' of it. Electrical codes also derate the wire's ampacity when the run exceeds 100', and also when the circuit is used for things like a heater.

 

Right. And those codes provide some precalculated tables based on the temperature ratings of different insulation types, and on the temperature and heat-carrying capacity of different surroundings. The National Electrical Code has an "Annex B" that goes further into how to do that work when it hasn't been done for you.

Most of the wires already in your car would seem noncompliant if you compared them to household wiring for the same currents. Differences in the insulation and in the conditions of use are factors. If you look in good ampacity tables, they have several columns for different insulation temperature ratings. Be sure to know the temp rating of the wire you will use. Ampacity significantly depends on that.

Because most people working from the NEC are working at higher voltages, 120 and up, they don't run as often into the practicalities that crop up working on 12 volt systems. There's more about those in references that are specific to cars and boats, like this one, which makes the point:

In practice the Voltage Drop Table usually take precedence over the Ampacity Tables for DC wires​

(they mean "low voltage" by "DC wires", in contrast to 120 V AC wires that might also be on a boat. It's not the DC/AC that makes the difference.) A run of wire that drops 5 volts under load would be acceptable in a 120 V system, but would be nearly half the voltage on a 12 V system.

So: don't ignore ampacity for your 12 V wiring. (If I sounded like I said to ignore that, I take it back.) Be sure to take the temperature rating into account. But also consider voltage drop; it's a bigger deal for 12 V systems than it is for houses.

Yes, there.

 

so... DO NOT USE THE WIRES THAT CAME WITH YOUR INVERTER... DONT USE THE ONES FROM HARBOR FREIGHT EITHER!

they all melted, I was running 1700 watts on an air fryer and all of those doubled wires failed... dont skimp on wiring!

here... look at the wires I used, I can run two 2000W inverters now one with an air fryer and the other with an induction stove without issues.

https://black.jmyntrn.com/2021/05/12/custom-battery-and-inverter-cables-for-the-win/?utm_source=PriusChat&utm_medium=weblinkt&utm_campaign=commentlinkJune