After last night's blackout, I went to harbor freight and picked up an inverter. 400 watts continuous, 800W peak. I would like to wire this into my truck under the passenger seat.

After the maths, this inverter can suck about 73 and some change worth of amps at it's maximum of 800 watts on the 120VAC side.

Obviously I don't want something that can pull that much current to be on all the time, so I want to wire it up so that it's live only when the key is on. That in of itself isn't hard to accomplish. use a fuse tap and take a wire off a hot-in-run only circuit (preferably in the mass distribution box) and tap that off onto a relay to control the inverter. There's a small quandary I'm running into, though. Do I use a relay or a continuous duty solenoid, like what would be used for an Efan install?

I want this relay/solenoid to withstand 100 amps for sake of a safety margin. I've seen standard looking 5 pin relays that supposedly can carry 100 amps across it, but I don't want to splay out 4 dollars for something that's gonna melt on me the first time I use it and make a nice fiery puddle of relay. At the same time though, I don't want to shell out for a solenoid when I don't need one. I've never wired something up so power hungry before. Of course, if there are any tips anybody has for this type of application, I'd love to hear them.

For sake of argument, here's an example of one of these china relays that can supposedly take 100 amps.

529 New Relay 5 Pin 100 Amp 12V Car Automotive Truck Alarm Bulb Set 2 Two Pcs | eBay

So; I'll turn it over to you guys. What are your thoughts?

 

The inverter doesn't draw power unless you have something(120v) plugged in and turned on, so no relay is needed.

You would just run a larger gauge wire from Alternator B+ to the inverter, with 100amp fuse in-line(mega fuse), and have inverter in the engine bay, a smaller inverter in the cab that plugs in to cigarette lighter for charging laptop is OK but that larger inverter shouldn't be in the cab.
Wire run is too long and overheating can occur.

Your issue will really be alternators power output, a 200amp alternator doesn't output 200amps at idle, same for 100amp, usually you need engine RPMs above 2,000 for full output.

Running a vehicle engine at those RPMs without airflow can cause issues, so make sure fan and cooling system is working well, and check engine temp often when it is being used for this.

Portable generators of those wattage's are under $500, so consider the possibility of even something minor happening to the vehicle so you can't drive it until repaired, which could be an issue if power is out for awhile.

Also the usual use for inverters of this type is shorter term , i.e. on a job site or camping for a few hours a day

 

True, the inverter doesn't take anything significant (around 15 milliamps on standby for this particular inverter) while nothing is plugged in, however the reason I want it switched by the ignition (albeit indirectly) is for this scenario, which I'm sure would happen sooner or later. Say, for example, someone takes their laptop and decides to use it outside for a while with it plugged into my inverter for whatever reason. My particular laptop, while plugged in, has a draw of around 3.3 amps on the DC side of the inverter. Over the course of a few hours, I could try to start the vehicle, unaware of the use and wind up with a dead battery. At least with it being switched by the ignition, I would have to deliberately kill the battery; assuming the engine isn't running.

I, personally, wouldn't use this inverter for something too demanding, myself. The most I would ever expect out of this, while installed under the seat at least, is powering a laptop, perhaps some sort of trouble light, I would say a radio but there's already one in the dash, etc. Nothing too demanding.

This inverter also wouldn't be put into full time service. Even though I'm not all that inverter savvy, I would have bought something a bit beefier for daily use with a higher wattage rating.

About this 4awg getting hot, according to this chart, 4awg is supposed to be able to withstand up to 100 amps (more than what this particular inverter draws at it's 800 watt peak).

 

A relay would be needed though if you had something plugged into your inverter, but it would only charge with the engine on. I believe this is what the OP is trying to accomplish.

 

Then I guess you could use a Golf Cart relay, they are made for continuous duty, vs a Starter relay.

 

One simple solution would be to use three 40 amp SPST Bosch mechanical relays in parallel. This would give you 120 amp DC control capacity.

 

Hmm. That's a thought. I'll have to give that a try. Would certainly be cheaper considering I get all my relays for free.