300 amp inverter fuse blew. Expected?

[steve&renee]

So I've got a tiny little Aims 1500 watt inverter/charger. We're out camping/boon-docking today, and it was "our" turn to make breakfast for the group. Renee pulled out her electric fry cooker and was busy cooking away using a circuit on the inverter supplied sub-panel. Working great. I stumbled out of bed, and decided I'd like a cup of coffee. We use a Keurig-style coffee maker. Well I know that the fry cooker pulls a ton of amps, and the coffee maker also pulls a ton of amps, and I know you should only use one high current-draw device at a time, but I did it anyway. I loaded the coffee maker and pushed the button.

Fry cooker was cooking. Soon as I hit the coffee maker go-button, the 110 volt sub-panel died. Dead as a door-nail.

Well I went outside to investigate. In the compartment where the inverter connects to the battery bank, we had a new "something burned" smell. I touch the 300 amp fuse that sits between the battery and the inverter, and it's warm. Looks kinda "rode hard" as well.

So I do carry a spare fuse. It's a 300 amp ANL bolt-on fuse. Lots of choices on Amazon, but not so much in auto parts or big box stores. I'm really glad I had a spare, because this is what happened: when I added the coffee maker load to the fry cooker load, I blew that 300 amp fuse.

So here's my technical analysis, and I'm looking for feedback about whether I'm thinking about it correctly. The fry cooker draws 1500 watts. Same for the coffee maker. The system can handle those individual loads just fine. 1500 watts / 12 volts = 125 amps of current on the 300 amp inverter fuse. But add the two loads together, plus whatever else was running off the inverter at the time. Hmm. So I checked my inverter manual. The little Aims can deliver 4500 watts surge power for 20 seconds. 4500 / 12 = 375 amps.

So I'm thinking that when I pushed the coffee maker go-button, the sum of the fry cooker, the coffee maker, and the other loads resulted in the inverter supplying surge power greater ~ 12 * 300 = 3600 watts, and that was enough to blow the fuse.

If that analysis is correct, I still have a question. Should I size the fuse greater than 300 amps. One of the manufacturers says size the fuse 1.3 time the devices max load rating. If the surge load rating for the inverter is 4500 watts, 4500 / 12 = 375 amps * 1.3 = 487.5 amps.

A quick web-search suggests the nearest size fuse would be 450 amps.

My gut says, be safe and stick with a 300 amp fuse and REMEMBER to only run 1 high current device off the inverter at a time.

Advice?

-Steve

[TucsonJim]

Hi Steve. Does your inverter's owner's manual state the size of fuse that should be used with your unit?

There's one other item you're not considering. With everything powered up, your running voltage probably dropped below 12V. Let's say it was 11.6VDC which is pretty typical with a heavy load. Now, your 4500 watts/11.6V = 388 amps being drawn. The lower the voltage goes, the more current needed to meet the power requirements of your devices.

If if were me... First, I'd check out the owner's manual and use the fuse size they specify. Second. If the manual does not recommend a fuse size, I'd leave it at 300 amps, and manage my loads to make sure it's not exceeding the fuse rating. If you keep going higher with the fuse, you run the real risk of the wires becoming the fuse and overheating if there is excessive current flow.

Jim

[Cate&Rob]

steve&reneeTucsonJim

My initial thought is similar to Jim’s . . . don’t over-fuse the capacity of the 12V wiring. You would need 4/0 or perhaps multiple wires (depending on distance) for 450 amps.

Rob

[ncitro]

steve&reneeTucsonJim

My initial thought is similar to Jim’s . . . don’t over-fuse the capacity of the 12V wiring. You would need 4/0 or perhaps multiple wires (depending on distance) for 450 amps.

Rob

This was my thought too, if you want to go to 400 amps you’ll need the wiring to support it. Your fuse may also not be rated that high. I ran a 300 amp ANL on my Magnum inverter, but the Victron called for 400 amps (with 4/0 wire), and a class T fuse. Took me a minute to find a holder rated that high, some were not.

[steve&renee]

Hi Steve. Does your inverter's owner's manual state the size of fuse that should be used with your unit?

Manual doesn't call out fuse size, just says you need one. But it does recommend 1/0 AWG wiring and less than 3 meter runs. I'm running 2/0 for about a foot. Blue Sea chart says max ANL fuse-size for 2/0 wire is 350 amps. I just ordered a couple more spares and I stuck with the 300 amp size. Better safe than dead -- that's what I always say. . .

Thanks for the advice.

-Steve