Fuses are tripping, don't know why

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.
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.
Comments
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?
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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)
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
If you have a multimeter you could watch voltage sag to see how low it gets.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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.
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
The battery is likely 100ah at C/20 rate. It will behave like a much smaller battery being discharged at more like C/1.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
@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.
http://members.sti.net/offgridsolar/
E-mail [email protected]
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
@Estragon I"m going to get one of those. I do like my coffee!
Thank you all for helping me with this.
http://members.sti.net/offgridsolar/
E-mail [email protected]
19 SW285 panels
SE5000 inverter
grid tied
The panel mount breakers sold by NAWS from midnite or outback have posts with lots of room for big lugs.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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.
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
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.
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 ([email protected] load) before the sun hits the panels.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
- 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
-BillI am available for custom hardware/firmware development
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
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.
2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.