What Wire Size For a 400W Inverter?

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400 watt inverters usually have a cigarette light adapter so you can use it with your car. But you can only draw 250 watts at best. You need to hardwire the inverter to get maximum power, but what size should you use?

A 4 AWG gauge is ideal for a 400 watt inverter because it allows you to use longer cables without losing power. An inverter and battery connected by 4 AWG wire gauge can be 10 feet apart and still run at maximum capacity.

How to Calculate Inverter Wire Size

4 AWG wire gauge sizes are going to work with inverters up to 1500 watts. So 400, 500, 1000 to 1500 watt inverters will run on this wire without issues.

Inverters under than 500 watts can use 6 AWG. So you can use that for a 400 watt inverter, but we recommend 4 AWG because the longer the cable, the greater the voltage drop will be.

There are a lot of 4 AWG wires available, but we like the 4 Gauge Windynation Wire Set as it works great with 400 watt inverters and is easy to set up.

This table shows different inverters and the ideal AWG wire sizes. As you can see, the greater the distance between the inverter and battery, the large the wire.

Inverter Size 0-3 feet 3 feet to 6 feet6 feet to 10 feet

Calculate AWG Wire Size For inverter Amps

If you want to manually calculate the AWG wire size, use this formula:

Inverter watt load / battery voltage = amperage

Suppose you have something like the Ampeak 400W Power Inverter and a 12V battery. Divide the wattage by the voltage:

400 / 12 = 33

The inverter output is 33 amps. The AWG wire size will depend on how far the inverter is from the battery. We like 4 AWG because it gives you a lot of flexibility up to 10 feet or so.

Use 6 AWG if the inverter is 3 to 6 feet away from the battery. If the inverter is less than 3 feet away, 8 AWG is going to work.

These calculations also work with other inverter sizes. As long as the inverter is 1500 watts or less, 4 AWG will be fine.

For 2000 watt inverters we suggest 2/0 for distances between 3 to 6 feet. Use 250 AWG if the battery is further away. For 3000 watt inverters and higher, go with AWG 3/0 up to 500.

These figures assume you are going to run the inverter at full capacity. If you only load 250 watts on the system, use that number in the calculations. It is not the inverter size that matters, but the load.

The longer the cable the greater the resistance, and the thicker the gauge has to be. Thick cables cost more so it makes sense to get a short, thick cable.

Long cables are not practical for RVs and cars (which is where you will probably use this inverter). Bear in mind that wire sizes have voltage limitations too. But it is not an issue since 400 watt inverters are invariably 12V and so are the cables for them.

All you need to do is figure out your inverter size and how far it is from the battery. With that information you can determine what AWG size to get. Just like with solar panels and batteries, the shorter the distance between the inverter and battery the better.

What is the Right Distance between a 400W Inverter and Battery?

The maximum distance between a 400 watt inverter and battery is 10 feet. Anything longer than that and you are better off using AC.

In AWG wire sizing, the lower the number the thicker the wire. A thick, short wire can hold more current. A 4 AWG wire carries 90 amps up to a distance of 10 feet, while a 8 AWG can only carry 40 amps over the same distance.

In another post we discussed how many batteries you can connect to an inverter. Even if you have the right number of batteries, you have to make sure the wiring is correct.

Our calculations show that a 400 watt inverter is equal to 33 amps. You can use 8 or 6 AWG, but you will be limited by the distance. With 4 AWG you have more options in terms of cable.

For a 400 watt inverter, the cable size comes down to how many amps the cable has to carry and how far the inverter is from the battery.

Make sure the inverter and batteries have matching voltages. Again this should not be a problem as most 400 watt inverters are 12 volts, and you can easily find 12V batteries.

The bottom line: keep the distance between the inverter and battery at 10 feet or less. If the distance is greater than 10 feet, get a thicker wire gauge.

What Happens if the Inverter Wire is Too Small?

The biggest drawback with using smaller AWG wires is the voltage drop. The acceptable voltage drop for solar systems is 2%, and higher losses might lead to a system failure.

The resistance is determined by the wire length and gauge. If the wire is too small there won’t be enough room for current to pass through.

A bigger problem is heat. Current generates heat when it passes through wires, and if its walls are not thick enough the wire could melt. This could cause a fire and lead to serious damage.

And these are the reasons why it is always better to go with the right wire gauge. It is better to invest in quality wiring than risk a fire in your RV.

What Happens if the Inverter Wire is Too Big?

There are also issues if the wire is too big, though not as serious compared with undersized cables. These are weight, cost and installation.

Cost is probably the biggest issue. Thick cables are efficient but they are also expensive. Fortunately you don’t need a lot of thick cables for a 400 watt inverter so the price will be manageable.

This is another reason why the inverter and battery should be as close to each other as possible. Short distances mean shorter and more affordable cables.

The thicker the wire the heavier it gets. Again this won’t be an issue with a 400 watt inverter as you will be using small wires. But for larger inverters it could be.

Lastly, thick cables can be difficult to install. They are heavy and finding space for them can be a challenge. But these issues are minimal and insignificant compared to using an undersized inverter wire.

How to Choose an Inverter Cable

The following guidelines are applicable for 400 watt inverters, but they should also work for any inverter regardless of size.

  • Add the total amps you will load. With a 400 watt inverter that would be 33 amps. Watts / battery voltage equals amperage.
  • Refer to the table above for the AWG gauge. The size will depend on how far the inverter is from the battery. We recommend a distance of no more than 10 feet.
  • Buy short, thick cables.
  • Buy from reputable manufacturer.
  • Make sure the wire voltage capacity is compatible with your inverter and battery. In this case that would be 12 vols.
  • Some inverters are bundled with the wires so you don’t have to do any calculations. If you want to buy the wires separately, check its amp capacity.

The easiest way approach is to buy an inverter installation kit. Many of these are for 600 watt and higher inverters, but their cables should also work with 400W systems. These kits have all the cables, connectors and other hardware you need to set up an inverter.

Installing an inverter is not that difficult. But you should consult a professional installer if you have no experience handling electrical components. With a 400 watt inverter the process should be easier than a large system though.


For any 400 watt inverter to work, it must have the right wire connected to it. This is true for the batteries, the solar panels and charge controller as well. You can go with a smaller size as we explained here, but leaving room for allowance is always better.