Need Help with Solar

Hi All,
I am new to the forum, 2nd year trailer owner and still learning solar. Something unusual happened this weekend. Let me tell you about my set up first. I have (4) 100Watt solar panels connected in parallel. They are connected to just a cheap PMW charge controller (30amp) wich then goes to 8 SLA batteries. I have a 3000W pure sine wave inverter connected to the batteries (connected using 4AWG wire and that run is only about 2' long. Between the inverter and the batteries I have a 300amp fuse. From there I plug in my 30 amp "shore power" plug into my inverter which in turn powers everything in the trailer. In order to avoid issues, I unplug my charger/ converter so the batteries don't try to charge the batteries...
So I have been using this set up for a few months, no issues as long as I remember to unplug the charger/ converter. The first time I powered up the inverter I was unaware about the charger/ converter and blew to fuses in my converter. Luckily, that was the only damage.
The reason I am running a 3000W inverter is to have enough power to run a 900W coffee maker.
The other morning I powered up the inverter and turned on the coffee pot. I few minutes later I could smell something burning. I immediately turned off the inverter and realized I forgot to unplug the charger/converter. I unplugged it and I didn't blow any fuses on the charger /converter. I checked the inverter and the wires going from the inverter (4AWG) to the batteries were extremely hot and the line going to the 300amp fuse was melting and melted part of the plastic casing of the fuse but did not blow it.
So my questions are these:
- Why did the wires get so hot? I have been using this set up without issue until now. Maybe with the coffee pot (900w) and the charger/converter it was drawing too much power? Doesn't make sense to me
- is my wire size too small? again - never had an issue before
- After this weekend I am thing a 300amp fuse is too big if it will allow the wires to burn up but I used a calculation that people use for sizing (3000w / 10 = 300amps)

From your description, the wire connections at the 300A fuse were of relatively poor quality  (high resistance) and so it melted before the amp load reached 300 and blew the fuse. Even 30A is plenty to melt things - electric welders run on that (or less).

Leaving the converter/charger active creates a progressive power loop, i.e. the amp load keeps increasing (and quickly!). So yes, the combination of the charger and the coffee maker would produce a very high amp load almost instantly.

The fuse needs to be sized to protect the lowest capacity link in the circuit.  You sized the fuse to allow the inverter to produce its 3000W max, but apparently didn't size it to handle the max amps of the wire and any intervening gadgets, e.g. the fuse holder.
A single conductor copper wire of 4 AWG falls well short of 300A. Depending on the type of insulation and stranded vs solid, it could be as little as 70A.  Even 4/0 gauge wire falls into the 190-230A range.

The link below  is a white paper from Trojan battery on battery wiring and connections.
https://www.trojanbattery.com/pdf/WP_BatteryCableGuide_0512.pdf

As an aside, your inverter is way oversized for a 900W appliance - 1500W would have been plenty. Besides, you don't have anywhere near enough battery capacity to operate a 3000W inverter at full load.  I'd replace that melted fuse with a 150A model.

Gary RV_Wizard said:

From your description, the wire connections at the 300A fuse were of relatively poor quality  (high resistance) and so it melted before the amp load reached 300 and blew the fuse. Even 30A is plenty to melt things - electric welders run on that (or less).

Leaving the converter/charger active creates a progressive power loop, i.e. the amp load keeps increasing (and quickly!). So yes, the combination of the charger and the coffee maker would produce a very high amp load almost instantly.

The fuse needs to be sized to protect the lowest capacity link in the circuit.  You sized the fuse to allow the inverter to produce its 3000W max, but apparently didn't size it to handle the max amps of the wire and any intervening gadgets, e.g. the fuse holder.
A single conductor copper wire of 4 AWG falls well short of 300A. Depending on the type of insulation and stranded vs solid, it could be as little as 70A.  Even 4/0 gauge wire falls into the 190-230A range.

The link below  is a white paper from Trojan battery on battery wiring and connections.
https://www.trojanbattery.com/pdf/WP_BatteryCableGuide_0512.pdf

As an aside, your inverter is way oversized for a 900W appliance - 1500W would have been plenty. Besides, you don't have anywhere near enough battery capacity to operate a 3000W inverter at full load.  I'd replace that melted fuse with a 150A model.

Click to expand...

Thanks for the reply and information Gary.
I thought 300 amps was high as well but was going by a chart. I will swap it out for a 100amp. The wire is stranded and is very flexible (not stiff) which I thought was a high end wire. The ones that came with the inverter are probably 8 guage so I have upgraded to 4 guage. Also, The coffee machine will be the highest load so since I am not maxing out the 3000W, do I need to upgrade the wiring?
In terms of the charger converter, I am currently researching how to tie in a relay to the charger/converter that would kill power to it as soon as the inverter becomes energized. This would eliminate the human mistake factor...

I have to ask what brand of 3000 watt inverter comes with such tiny hookup wire?    You really need something in the 4/0 to 2/0 range minimum for a 3,000 watt inverter.

Isaac-1 said:

I have to ask what brand of 3000 watt inverter comes with such tiny hookup wire?    You really need something in the 4/0 to 2/0 range minimum for a 3,000 watt inverter.

Click to expand...

It's a cheap one... But do I need that wire size if my biggest load is a 900w coffee maker and maybe a couple of cell phones charging?

The wire needs to be sized for the max load you will require it to carry, and the fuse should be sized for the actual wire size.  If that is 1500W rather than 3000W, then OK. The risk is that you may accidentally try to draw more, e.g. 2000-2500 and the inverter will go along with it. Despite your best intentions, somebody will plug in a hair dryer or a few extra gizmos and exceed your self-imposed limit.  That's why you must use a fuse or breaker sized for the actual wire.

JimBigger said:

Hi All,
I am new to the forum, 2nd year trailer owner and still learning solar. Something unusual happened this weekend. Let me tell you about my set up first. I have (4) 100Watt solar panels connected in parallel. They are connected to just a cheap PMW charge controller (30amp) wich then goes to 8 SLA batteries. I have a 3000W pure sine wave inverter connected to the batteries (connected using 4AWG wire and that run is only about 2' long. Between the inverter and the batteries I have a 300amp fuse. From there I plug in my 30 amp "shore power" plug into my inverter which in turn powers everything in the trailer. In order to avoid issues, I unplug my charger/ converter so the batteries don't try to charge the batteries...
So I have been using this set up for a few months, no issues as long as I remember to unplug the charger/ converter. The first time I powered up the inverter I was unaware about the charger/ converter and blew to fuses in my converter. Luckily, that was the only damage.
The reason I am running a 3000W inverter is to have enough power to run a 900W coffee maker.
The other morning I powered up the inverter and turned on the coffee pot. I few minutes later I could smell something burning. I immediately turned off the inverter and realized I forgot to unplug the charger/converter. I unplugged it and I didn't blow any fuses on the charger /converter. I checked the inverter and the wires going from the inverter (4AWG) to the batteries were extremely hot and the line going to the 300amp fuse was melting and melted part of the plastic casing of the fuse but did not blow it.
So my questions are these:
- Why did the wires get so hot? I have been using this set up without issue until now. Maybe with the coffee pot (900w) and the charger/converter it was drawing too much power? Doesn't make sense to me
- is my wire size too small? again - never had an issue before
- After this weekend I am thing a 300amp fuse is too big if it will allow the wires to burn up but I used a calculation that people use for sizing (3000w / 10 = 300amps)

Click to expand...

Welcome to the forum, you will find plenty of good advice here, just ask the right questions..!


Your statement of "still learning solar" is right on, sadly you have made a number of novice mistakes and been burnt in the process ( literally ! )
you have found out the hard way how not to do things with an over sized inverter and under sized wiring.

Gary is quite correct, fuses are there to protect the wiring and should be sized accordingly. it's surprising how many
people think that the fuse it there to protect the appliance.

1. from my point of view you really have no business using a 3000 Watt inverter on a 12 Volt system, the currents are much too
high and the chance of fire is very high, you witnessed this first hand..
using a 3000 W device to power a 900 W coffee machine is like using a nascar vehicle to go shopping in.
I would suggest you need no more than 1500 W to run any device in an RV.

2. 4 AWG is way undersize for even a 1500 W inveter ! take a look at http://assets.bluesea.com/files/resources/reference/20010.pdf
for wire and fuse sizing. I personally use the Handbook of Electronic Tables and Formulas for American Wire Gauge.
AWG for 3000 W inverter at 12 V with cable length of 4 ft giving 3% or less voltage drop would be 2/0 AWG or better

3. 8 SLA batteries, i my business this is taboo, in the RV world this seems to be the norm, just be aware that multiple lead acid batteries in parallel
is a good formula for reduced lifespan, capacity and sulfation due to uneven charge and discharge currents. there are ways to reduce these effects, but
sadly it's intrinsic to the battery chemistry.

4. " calculation that people use for sizing (3000w / 10 = 300amps) " never heard of those people..
but I do see this as a "rule of thumb" that someone contrived.
if we took the inverter efficiency to be at 90% then your 3000 W inverter would draw
3000 W / 12 Volts * 1.1 = 275 Amps, 300 A would suggest 80% efficiency, 80% is low BTW.

5. 4 * 100 W panels.. this brings me to ask how much battery capacity do you have ?
panel sizing is related to capacity with minimums that need to be met.
a lead acid battery typically needs a minimum of C/10 charging current.
with 400 W panels you have an ideal max current of 33 Amps, in practice you will only
get 75% to 80% of that, so 400 * 0.8 / 12 = 26 Amps. for C/10 your capacity would be 260 Ahr
you can get away with C/20 but that's not a good place to be.. so you should not exceed 500 Ahr in my opinion.


for reference here is a link to some design info that may help you
in designing a solar system. http://www.rvforum.net/SMF_forum/index.php/topic,113514.0.html