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Testing limits, cooked 175a fuse

AlanGriff

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Jun 25, 2020
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Below photos is my system: 200ah lifepo4 battery pack, 4 100w panels, Epever MPPT controller, with Giandel 2200w inverter.

Normally I run my 1425w coffee maker in the morning with no problems for the past 2 months. Today I tried my 1500w microwave for a 2 minute runtime during noontime sun! Just over a minute, silence! 175a fuse fried!

So, what am I missing? Microwave was pulling the juice! My red positive 4gauge wire were nice and WARM!
Do I need a bigger fuse? Or, is the microwave to big, need a smaller one?
Frankly, I was surprised. But I’m trying to learn what the limits are...

Plus, was trying to get over 300w from my panels. Just curious if I could test it to those limits...

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Is the microwave 1500W of RF or is that the total consumption with its cooking power being more like 900-1000W?

If it's 1500W of cooking power it will be drawing about 1900W-2100W, or a bit more depending on how the magnetron has aged, from the AC. If it's a hefty transformer design microwave too and has poor power factor correction that will increase the current the inverter has to draw from the DC side as well. You may find that you are only just getting away with it before the inverter itself goes into overload if it is 1500W of RF.

A DC clamp meter (example only, not a recommendation) would soon tell you if you have higher than you expect current.

*edit to add more*
 
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Is the microwave 1500W of RF or is that the total consumption with its cooking power being more like 900-1000W?

If it's 1500W of cooking power it will be drawing about 1900W-2100W, or a bit more depending on how the magnetron has aged, from the AC. If it's a hefty transformer design microwave too and has poor power factor correction that will increase the current the inverter has to draw from the DC side as well. You may find that you are only just getting away with it before the inverter itself goes into overload if it is 1500W of RF.
The microwave is 1000w on the door, back side says 1500w.
 
My 1000W microwave (cooking power) runs fine on a 2000W inverter. Draws 150 amps from the battery. 2/0 cable with 250A fuse.
I can recommend this clamp multi meter. https://shop.marinehowto.com/products/sterling-power-dc-ac-amp-clamp-volt-meter
So, you’re saying 1) to put on larger cables, and 2) use a 250A fuse.
I won’t be buying the clamp do to cost and not usefully but one time.

Should I installed a circuit breaker on the inverter side (like I have on the controller side)? If so, what size, 200A?

thanks!
 
Putting the oven aside for the moment and just from a fusing point of view... your Giandel inverter is spec'd at 2200W. 2200W from a 12V battery is 183A but the inverter isn't 100% efficient so you could be seeing more current than that, plus you have to consider what happens as the battery voltage goes down during discharge. At 11.5V you are now at 191A, and we still aren't including inverter inefficiency. You fuse is too small for that capacity inverter. At the barest minimum you'd want 220A but the recommendation is 270A, 25% over rated to account for surges etc.
 
Ok, sounds good.
So I put in a 270A fuse. How about also a circuit breaker between fuse and inverter? Tripping a breaker would be better than a fried fuse. Circuit breaker at 270A or 250A? Or, would that be counter-productive ?
 
You'll find it hard to get a breaker that genuinely has those ratings that won't leave a pretty good hole in your pocket. A fuse is a better option IMO. You can still have an isolation switch of some type of course but the fuse is the protection.
 
Please size fuses to cable ampacity. Fuses protect wires. Only you can size your system and cable sizes. So I was only telling you what I have and my results. Not telling you what size cable to use.
 
2200 watts * 1.15 conversion factor / 12 volts = 211 amps
211 amps * 1.5 fuse headroom = 315 amps.
You want 2/0 awg with a 315 amp fuse.
4 awg is way under spec.
Out of curiosity, how did you end up with 4 awg?
 
BTW pulling 300 amps from a 200 ah lifepo4 battery is typically stressful for the battery.
 
In my experience microwaves and toaster ovens pull every bit of their rated wattage.
 
Toaster ovens will because they are simple themostatic heaters. Heavy transformer microwaves will too, because they can only be on or off, there is no reducing the power output of a magnetron.

'Inverter' microwaves can draw less than their rated power since they turn the magnetron on and off rapidly and the DC power supply in the oven averages that high frequency switching to be fairly smooth but lower draw on the AC side. Obviously an inverter microwave at 100% power will draw the full wattage on the specs plate.
 
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