You did not specify DC voltage, so we will assume 12V nominal. 3000W/12V is 250 amps, even at 24V, you will see over 125 amps at 3kw load (because of inverter efficiency). Your cables are heating up because they are dropping too much voltage, the currents are too high for 4 gauge wire. Check wire ampacity charts online, you will see that you usually need to have 2/0 for most 3kw inverters, at 12V I'd go bigger. Also if you check the manual for any UL/ETL listed inverter, they will have a wire chart vs. loads.

Ah I just looked at that battery and see that it is 12V! So yeah for a 12V system you need bigger (or more parallel) cables.
Tim

Cerrowire Resources - Ampacity Charts

Ampacity is the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Cerrowire's ampacity chart helps calculate the load requirement for a circuit.

www.cerrowire.com

 

You did not specify DC voltage, so we will assume 12V nominal. 3000W/12V is 250 amps, even at 24V, you will see over 125 amps at 3kw load (because of inverter efficiency). Your cables are heating up because they are dropping too much voltage, the currents are too high for 4 gauge wire. Check wire ampacity charts online, you will see that you usually need to have 2/0 for most 3kw inverters, at 12V I'd go bigger. Also if you check the manual for any UL/ETL listed inverter, they will have a wire chart vs. loads.

Ah I just looked at that battery and see that it is 12V! So yeah for a 12V system you need bigger (or more parallel) cables.
Tim

Cerrowire Resources - Ampacity Charts

Ampacity is the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Cerrowire's ampacity chart helps calculate the load requirement for a circuit.

www.cerrowire.com

Interesting. I had actually used a couple of online calculators to check that. My understanding was that length was also a factor. With the short length (about 1.5ft) I thought I was ok.

 

Is it possible that the calculator indicated 4/0 (as in 0000 ) and you might have interpreted this as 4 awg?

The wire length is the complete round trip length - not the one way length.

I used the bluesea circuit wizard and it indicates at least 2/0.

Circuit Wizard - Blue Sea Systems

There are two steps needed to pick a wire size:
- Ampacity (for a given wire surface temperature) this is the absolute max rating that the wire can handle in free air and still pass electrical code.

It is used when weight is more important than power losses

- 1 or 3 % loss based design

This is used for most van applications - where efficiency and having wires that feel relatively "normal to the touch" matter.

_

As far as the batteries, you are using them at nearly the max continuous discharge current so not a lot of room to spare. They are going to be warm at the terminals. Might not seem important - but what this means is that they are designed to cool the terminals by dissipating the heat into the wires.

 

In low voltage applications it’s better to use the blue sea chart for your wire sizes. A tiny bit of extra resistance in a connection will generate lots of heat. They also have a wire and fuse calculator on their website. Skip the Chinese circuit breaker and fuses also, often they will melt before they open.

 

After several crummy high current breakers from Amazon I decided to purchase some breakers with good reputation on the forum and test them. I purchased optifuse which is Chinese and mechanical products (MP) which is US made. Both worked great. They match their spec sheets to the t.

MP breaker is larger, more sturdy and have a lower contact resistance than the optifuse. It's small difference but measurable.

Sample of one, YMMV

 

I'm also using 4awg on our 3000w inverter (though, we have 5x 100AH batteries) and I doubled up. Putting 1800w made the cable uncomfortably warm on a single cable. I've since done the max that it is breaker'd for (250A) and cables were just warm.

The run was super short too, also 1.5ft each way (if that).

 

The cable sizing charts are conservative. You can get by with less. BUT if you don't know what you are doing you risk fire (buried in flammable insulation, etc.), or simply wasted power (long runs). It also exposes you to liability if anything goes wrong.

I'm guessing the proximal cause of the original fault is a poor crimp or torque on the lug. In my experience connections are the primary source of faults (capacitors come next). I crimped my lugs with a hydraulic press, and they seem good. Some folks in the forum order cables with lugs professionally crimped and that might be worth the expense. Once a connection gets too hot and starts to corrode it is a fast slope to melting. (Thermal cycling will hasten the failure)

If your wires are in open air and you can grab them and hold on for a minute, they are not too hot. Otherwise, you need bigger wires. If you use the wire charts, they should not noticeably warm.

I use #1 for my 2kw inverter with a very short loop and minimal lugs and #4 for the battery interconnect. They do warm with high loads (1800wt) but do not get hot. I figure I lose ~50-80wt at 1800wt draw.

IIR four gauges is roughly doubling the area of a wire (e.g. #1 is double #4) so @Dekkars experience matches mine.

I might upgrade to a third battery and would upgrade the inverter loop cables to 2/0. I was just being cheap the first time around.

 

Question; how warm is too warm?

Our rule of thumb: If we can hold onto the wire/connection, it's not too warm.

 

FWIW, my 2000W inverter specs 2/0 for short distances, 4/0 for longer.

 

For about $50 you can buy the cables you need on Amazon. If you ever plan on doing this work in the future the crimp tools are very affordable now and you can get the tool, lugs with heat shrink, and cable for slihtly more than the pre-made cables and make them exactly the length you want. Remember to connect the pos and neg leads to the load on opposite ends of the string of batteries so they all share the load equally. Also make sure the wire is actual copper and not the copper coated aluminum junk they sell for car stereos.

 

That's some good feedback there. One thing you can do after installation, if you know someone with a FLIR or other IR camera, is...you can take some pictures operating at high power of the wire, battery and inverter. The wire AND terminals should all be the same color (temperature), within a couple degrees, on the camera. If the terminals are hotter you are a) not getting a good enough crimp or b) your bolts to the inverter or battery are not tight enough. Easy and fast to check.
Tim

 

Oh one more thing. Tangeli, after re-reading your post you said "got so hot they melted the negative connector on the inverter. I smelled it, and shut it down. "
So I recommend to send the inverter back for replacement or repair to the negative post (connector) on the inverter. That sounds like an unsafe condition.
Tim