Fuses are tripping, don't know why

wayneworkman2012 · March 2017

Good day,

I've been slowly building on a solar system at home - I'm just piecing it together. It's a 12v system.

I have a 3000 watt continuous pure sign wave inverter, this accepts 12 volts.
I have a DC 300A fuse between the inverter's positive pole and the battery bank's positive line.

And, I have a coffee pot that operates on 110v grid power.

I have a watt measuring device that I can plug into an AC outlet, and plug the coffee pot into - it says the coffee pot uses about 850 watts.

By my math, the coffee pot uses about 7 amps when it's using 110v, and the inverter might be pulling about 70 amps from the batteries when powering the coffee pot.

But when I try to run this coffee pot off of the inverter, the 300A fuse trips after about two minutes. The bottom of the fuse gets a little warm, and the cable from the fuse to the inverter is a little warm. The cables on the positive side are 2AWG with 5/16 wire lugs professionally fitted (I bought them premade). The line length between the fuse and inverter is 6 inches.

I don't understand why this fuse is tripping. Can someone shed some light? I'm just trying to learn this stuff before I invest in a big system.

Estragon · March 2017

What is supplying the power to the inverter (ie battery type/size/#)?

Batteries will sag in voltage as their rate of discharge increases. As the voltage sags, the current in amps required to power the load in watts goes up. As the amps go up, resistance in wire and batteries increases further, and so on. Warming of fuse and cable suggests excess current. I'm surprised the fuse is blowing before low voltage cut-off on the inverter shuts it down though, assuming voltage sag is ehat's happening. Do you know what the inverter low voltage battery cut-off is?

Photowhit · March 2017

First is it a fuse or a breaker? breakers trip, fuses melt.

A 3000 watt inverter on 12 volt should be connected by at least 4/0 wire, read through your owners manual.

Is anything else running? Do you know the efficiency of your inverter? 12 volt 3000 watt, I would guess best would be 85% so your 850 watt load has become 1000 watt load. 1000/12 = 83 amps.

Now how far is the inverter from the battery bank? You may have addition current running from the voltage drop (one of the reasons for the heavy wire required for low DC voltage inverters)

wayneworkman2012 · March 2017

@Estragon The inverter's low voltage cutoff is 10.6v I believe. I have two 100ah 12v batteries wired in parallel. They are sealed lead acid, the model number on the side is UB121000.

Estragon · March 2017

Maybe the LBCO needs to be <10.6v for several minutes to trigger? Are battery interconnects also #2 wire? Do wires from battery +/- to inverter come off the same battery?

If you have a multimeter you could watch voltage sag to see how low it gets.

wayneworkman2012 · March 2017

@Photowhit ; It's a breaker. Just the coffee pot was running. I've misplaced the manual, it'll turn up eventually - I know I held onto it.

There is an 18" 2AWG cable coming from the positive of one battery to the fuse, and a 6" 2AWG going from the fuse to the inverter. The negative cable is two 4AWG 24" cables put together from the negative pole on the inverter to the negative pole on the battery.

@Estragon ; The battery interconnects are advertised as 2AWG 1' cables - although the girth of the cable is less. Yes, the inverter is hooked to the same batteries poles.

Photowhit · March 2017

Is the breaker rated for DC?

Estragon · March 2017

With both +&- hooked up to the same battery, that battery will be taking more of the load, which is a big load for that size bank to begin with. It may help to take the + off one and the - off the other. My guess is you're pulling well over 100a (actually 300a if breaker is working properly) through wire that really should be much heavier (pretty sure 300a breaker is too big for 2ga wire). IIRC my interconnects are 4/0, which is about thumb size thick.

The battery is likely 100ah at C/20 rate. It will behave like a much smaller battery being discharged at more like C/1.

wayneworkman2012 · March 2017

@Photowhit ; I believe so, this is the one I have: https://www.amazon.com/KUMEED-Circuit-Breaker-Trolling-Inverter/dp/B01G3KGVYO/ref=sr_1_cc_3?s=aps&ie=UTF8&qid=1490458834&sr=1-3-catcorr&keywords=300A+breaker+dc

@Estragon ; so 4/0 cable and more batteries. I can add two more of the same battery type & get some cables ordered.

Any recommendations for other breakers or did I pick out a good one? The size of wire lug that this breaker accepts is limited due to the distance between the poles & the plastic box between them.

Dave Angelini · March 2017

The one thing you will learn when you invest in a "big" system is that it is pretty foolish to think about running electric coffee makers on a small system like this. Just a prediction

Estragon · March 2017

French press also makes better coffee! I did finally break down and get an electric toaster though

wayneworkman2012 · March 2017

@Dave Angelini I figured as much. My goal currently is to get my refrigerator running on solar. I'm not terribly interested in ROI, but the thought was - if there is a power outtage that lasts for a couple of days and I save 300 bucks worth of food from spoiling, I'll get my ROI much faster. I feel you can't put a price on energy independence really. I was just running the coffee pot to stress things (purposely).

@Estragon I"m going to get one of those. I do like my coffee!

Thank you all for helping me with this.

Dave Angelini · March 2017

Toaster ovens !

Bake bread and pizza.

dennis461 · March 2017

I see where feedback on Amazon reads the breaker trips too soon and gets hot. I would replace it with a better quality breaker.

Estragon · March 2017

@wayneworkman2012

The panel mount breakers sold by NAWS from midnite or outback have posts with lots of room for big lugs.

Photowhit · March 2017

Estragon said:

The panel mount breakers sold by NAWS from midnite or outback have posts with lots of room for big lugs.

Great advice. Please note, these usually top out at 250 amps. Again you might want to check your manual. 3000watts/12volts=250amps. Breakers will not trip instantly.

Also you should size your breaker to the wire. 2awg should be somewhere around 100 amps. but you should have 4/0 minimum for a 3000 watt inverter.

wayneworkman2012 · September 2017

@Photowhit @Estragon @dennis461 @Dave Angelini I came back here to tell you all about the outcome of this.

I've increased the battery bank size - it's 600AH now, which is just 6x 100AH 12v batteries in parallel, it's the exact same battery model I was using before, so there is a total of 6 now.

I've replaced all of my wires between the batteries and inverter, they are all 4/0 now. I have 12" jumpers between the batteries, a 18" between the front-most positive pole and the previous 300A fuse, and a 12" between the fuse and inverter. The negative line from the inverter to the far-most negative pole is about 4' long.

I tried the same coffee pot again this morning, which draws between 850 and 900 watts. My starting voltage before trying was 13.1. When I turned on the coffee pot, voltage sagged down to 12.5 within about a half minute and then the breaker tripped (again).

The wires didn't get hot whatsoever, and initially the breaker didn't either - but after some moments passed after it tripping, I guess the innards of the thing were hot enough to heat up the bottom of the breaker - because it only got a little warm moments after it tripped.

So I guess this breaker just sucks. I need a replacement that can power a 3000 watt continuous pure sign-wave inverter.

Estragon · September 2017

I think with 6 batteries in parallel you should really have a fuse on the positive wire to each battery, sized for the wire from each battery (2ga?) to the common positive connection point. The reason for this is in the event a battery fails short, you could have the combined current from the remaining 5 going through the shorted one.

The voltage sag to 12.5 doesn't seem too bad for a ~70a load, so you're probably right that the breaker on the inverter feed should be replaced. My ~300ah 48v bank sags a volt or two making toast (~20a@48v load) before the sun hits the panels.

wayneworkman2012 · September 2017

@Estragon I will do that, I guess I'd use a bus bar to tie each battery's positive to. I'm using 4/0 cable.

BB. · September 2017

Double check the fuse rating... Worst case, a 1,000 watts on 12 volt system should draw:

1,000 Watts * 1/0.85 inverter eff * 1/10.5 volts battery cutoff voltage = 112 Amps worst case

And, if this was a long term load (every day, for a length of time of more than a few minutes), I would suggest the 80% derating from NEC:

112 Amps * 1/0.80 NEC derating = 140 Amp rated fuse/breaker/wiring

-Bill

jonr · September 2017

Looks like that breaker is about 1/3 to 1/6 the price of other designs.

Vic · September 2017

Those type breakers are Thermal. They are cheap, and inexpensive, as well as compact.

It is often said, that thermal breakers often trip at lower, and lower current levels on each trip.

Personally, would never use any thermal breaker for DC applications, particularly when used with an over-sized inverter on 12 V.

Magnetic-Hydraulic breakers usually have lower internal voltage drop, are reliable and repeatable.

The breaker pictured on Amazon appears to be a knock-off of a Bussman/Cooper breaker, did not find specs for your breaker on Amazon (they may be there).

Normally you would want to use a breaker that is connected to a battery, that has an AIC Current Rating of at least 3,000 A -- 5,000 A would be better, IMO.

If you need a high quality 250 A breaker (which is commonly used with 4/0 cable) it will be a large Frame size (F-frame), it will be large, not inexpensive, and should be mounted in a metal enclosure:
https://www.solar-electric.com/mnedc250.html

Just opinions, Good Luck, Vic

wayneworkman2012 · October 2017

@Vic Just purchased that DC breaker, finally.

Vic · November 2017

OK Wayne, good move. It should be very reliable, and do the job well. Thanks for the update, Vic

sunnyman · November 2017

Wayne. I had that same circuit breaker and removed it as it was doing the same thing to me.
First I had a 150A then I got the 300A and it still tripped well below its rated limit.
It is all over ebay. It really sucks.

littleharbor2 · November 2017

If you hit that with a full 300 amps I bet it would either melt or go up in smoke. High current protection is not an area where people should try to save a few bucks. The consequences can be disastrous, or worse.

Fuses are tripping, don't know why

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