Small AC motors and Inverters

Chris Bryant wrote:
Because this thread isn't quite long enough, I'll point out that ampacity also hinges on the temperature rating of the wire- a #12 wire with 90 degree insulation is rated for 30 amps, standard building wire (75 degree) #12 is 20 amps. At the extreme, a nicrome heating element is rated way above these figures.

Yes, I believe the Blue Sea wire gauge charts assume 105C which seems pretty standard for battery cable. At least that's what my 6ga is.

If you use their wizard you can change that and see what happens.

Wanted to toss out there... if your teetering with a low voltage alarm at prime battery voltage... what happens when the batteries are at 80%?

You are gonna be in a constant state of low voltage (at the inverter) even though your batteries are still very useable.

LScamper wrote:
BFL wrote:
“I don't understand ampacity amounts for different gauge wires since they don't say (or I missed that) how long the wire is. AFAIK if the wire is shorter it can do more amps at the same gauge. “

“Thanks. I still don't understand when they say #4 is good for 100 amps if that means 3 feet of #4 or 30 feet of #4. “

I'll try to confuse you even more!

Ampacity is a rating to do with heat (power). A current flowing in a wire will heat it up. The amount of heat is determined by the current squared times the resistance of the wire per unit length. If the wire is 1 foot long and has 1 Amp flowing it will reach a set temperature. (Temperature will vary depending if it is in free air or in an enclosure like conduit.) If the same size wire is 10 feet long and has the same 1 Amp flowing in it the wire will reach the same temperature along the length of the wire as the 1 foot long wire. The amount of power dissipated (heat) is the same for every unit length of wire so the length of the wire has nothing to do with how hot it gets. The total power dissipated in 10 feet will be 10 time that of 1 foot but the power per unit length is the same. The ampacity it a rating of how hot the wire is allowed to get.

This is completely different than voltage drop across the wire which is a function of length and current.

Well that is not confusing at all! Now I understand why the length of the wire does not matter for ampacity. Good explanation, thanks.

The RV does not have a power source that can keep the amps up the same on the longer wire, so that must come into play for fusing a long wire vs the short wire of the same AWG?

I know when I reduced the R of my converter to battery path, more amps got to the battery and I needed a bigger fuse, but I was only looking at the amps the fuse had to let past, not the wire gauge. Hmmm.

Because this thread isn't quite long enough, I'll point out that ampacity also hinges on the temperature rating of the wire- a #12 wire with 90 degree insulation is rated for 30 amps, standard building wire (75 degree) #12 is 20 amps. At the extreme, a nicrome heating element is rated way above these figures.

To clarify, the inverter runs up to 1000w continuous according to its spec sheet, unlike the similar Prowatt 1000w PSW which ISTR is 900.

Mine only fried when I ran it at 1050w, with its overload light on, too many times ("too many" is defined as the number of times run when it fries a circuit board part)

The #6 and 60a should be safe, but I agree it could lead to low voltage alarms and shut downs sometimes. Just have to try it out.

To stay above 11v loaded, it is best to have fat wires and a big battery bank. With just two batteries, you have little margin to run the loads when the batteries are near 50% before you start the inverter. Having fat wires with just two batteries gives you more margin.

It was shown on here a while back that AGMs can take a big load and stay above 11v much better than Wets can. If you have a small rig that cannot carry more than two batteries, and you want to run big inverter loads, having AGMs would be the way to go.

This all matters to an early riser making his morning toast with his batteries low--to be recharged later in the day once "generator hours" open.

The Power jack had four terminals and called for 4 awg but I went with 2 awg anyways. The Trip lite only has two terminals and I didn't want to buy more wire until I knew what I needed for install so I doubled up the cables from the Power jack and all seems to be ok.

As far as running things on the Tripp lite. Aside from the air conditioner, heater and leaf blower, I have tried a blender, hair dryer, two drills, and a saws all. I really couldn't tell a difference between plugging them into the wall socket vs the inverter.

BFL wrote:
“I don't understand ampacity amounts for different gauge wires since they don't say (or I missed that) how long the wire is. AFAIK if the wire is shorter it can do more amps at the same gauge. “

“Thanks. I still don't understand when they say #4 is good for 100 amps if that means 3 feet of #4 or 30 feet of #4. “

I'll try to confuse you even more!

Ampacity is a rating to do with heat (power). A current flowing in a wire will heat it up. The amount of heat is determined by the current squared times the resistance of the wire per unit length. If the wire is 1 foot long and has 1 Amp flowing it will reach a set temperature. (Temperature will vary depending if it is in free air or in an enclosure like conduit.) If the same size wire is 10 feet long and has the same 1 Amp flowing in it the wire will reach the same temperature along the length of the wire as the 1 foot long wire. The amount of power dissipated (heat) is the same for every unit length of wire so the length of the wire has nothing to do with how hot it gets. The total power dissipated in 10 feet will be 10 time that of 1 foot but the power per unit length is the same. The ampacity it a rating of how hot the wire is allowed to get.

This is completely different than voltage drop across the wire which is a function of length and current.

SoundGuy wrote:

...
JMO, but I'd forget the hair dryer, forget the toaster, or anything else that draws anywhere near as much power, as I suspect you too would find this inverter won't perform to your expectations unless you do use the heaviest gauge cable practical to feed it and to keep those cables as short as possible.

Yep. Then I would go to 4ga wire and maybe an 80A fuse.
That low a fuse to protect the Inverter which sounds more like 900W continuous, or 72A at 12.5V. Could go higher, say 100W, just to protect the 4ga cable, according to the Blue Sea tables. But suspect that would blow out the Inverter ala BFL and you.

brulaz wrote:
And so, after sleeping on it, I've decided to go the simple and cheap route. It's only a coffee grinder and a 220Ah battery bank. The 1000W PSW inverter is overkill for that and only purchased because it was so cheap and readily available.

I already have 10' of 6ga wire, MRBF 60A fuses and holder, and lugs. The 60A fuse will protect the 6ga wire and should be good for ~700W before blowing, enough for the coffee grinder and other low watt stuff. It will also protect the 1000W inverter which I think BFL claims is really only good for 900W continuous. Unlike BFL, I don't intend to find out. :)

I've been following this thread as I thought there was something familiar about this Crappy Tire inverter and it's now finally come to me - I bought one myself in Mar of last year when it was on sale! :S Frankly, I had forgotten about it because after extensive testing I returned it, deciding it just wasn't going to help me avoid buying another EU2000i which among other tasks will easily power my wife's hair dryer each morning which this inverter would just not do. ;) It would run my toaster which my Kill-a-Watt meter indicated was pulling 672 watts ... well, sort of, because at times it would shut down anyway before toasting was completed ... I suspect, after following this thread, that the doubled up 8 gauge cable I was using to feed it was the reason.

Now it's another year later, I did buy that EU2000i in the meantime so recharging a depleted battery has become a non-issue, but I think I'm going to invest in this Crappy Tire inverter once again anyway so I can power our 120 vac fans overnight when dry camping. I've got an inexpensive 300 watt MSW inverter that runs them OK but the squeal from the motors drives me nuts and this sine wave inverter will solve that nicely. For this a 1000 watt PSW is really overkill but considering there's not really much out there, even used, that's significantly cheaper I might as well go with this puppy now that it's on sale. JMO, but I'd forget the hair dryer, forget the toaster, or anything else that draws anywhere near as much power, as I suspect you too would find this inverter won't perform to your expectations unless you do use the heaviest gauge cable practical to feed it and to keep those cables as short as possible.

In addition to the whiz-bang Circuit Wizard, Blue Sea has published two charts and general info for choosing wire gauge and fuse size in DC circuits. They can be found here:

Wire Size
Fuse Size

These pages have been useful to me and seem generally inline with the advice received here.

And so, after sleeping on it, I've decided to go the simple and cheap route. It's only a coffee grinder and a 220Ah battery bank. The 1000W PSW inverter is overkill for that and only purchased because it was so cheap and readily available.

I already have 10' of 6ga wire, MRBF 60A fuses and holder, and lugs. The 60A fuse will protect the 6ga wire and should be good for ~700W before blowing, enough for the coffee grinder and other low watt stuff. It will also protect the 1000W inverter which I think BFL claims is really only good for 900W continuous. Unlike BFL, I don't intend to find out. :)

We do plan to upgrade the battery bank but that will require a new truck and/or AGMs (tongue weight issues :(), so it won't be this year. Whenever that happens we may upgrade the Inverter to something that can run the Microwave or a coffee maker. When/if that happens ALL the wiring and fusing will get bigger.

Thanks for all the good advice.