Fire danger from using existing 14 -16 gauge wiring with 300 watts solar???

[markopolo]

Originally Posted by mikeehlert

........... I know mine limits current..........

What solar controller do you have?

Originally Posted by mikeehlert

BTW, I just reviewed the Amazon listings for 30 Amp charge controllers from $13 to $130. Most explicitly say they limit current in their descriptions. I did not review install manuals.

Definitely provide links to solar controllers that limit output current. I think there's something in those descriptions that you're misinterpreting. They have a output current rating based on maximum input watts but that's is not the same as limits output current.

All solar controllers have output ratings in amps. They also have an input rating in watts. The Samlex 8A mentioned earlier has a 130W input limit so connecting 300W of solar is not a good idea. Likely result is smoke.

[jakegw2]

For reasons that baffle me, solar panels are not actually rated by the power you can get from them. Power(W)=Volts*Amps, so at 300W and 12 volts in theory you could get 25A. You won't. In fact, based on the values I have seen you can expect a maximum output of 16.5A from this setup. My only guess as to why is perhaps panels generate 18V, not 12V? You will only see this current on cloudless days when the sun is directly overhead. Typically it will be much lower.

In any case, for 16.5A of current @ 14.4V for 20' with a 2.3% voltage drop you will need a 10 AWG wire. Typical allowed voltage drops are 2% or 3%, and if you want a 2% max drop you will need a 9AWG wire.

You can calculate these values yourself using this site:

https://www.engineeringtoolbox.com/1...rt-d_1689.html

A 16AWG wire can only support that voltage / current for 6.5', 14AWG can go for 8' safely. Both of these assume a 3% voltage drop, which is less ideal than a 2% drop.

Bottom line: You need to run upgraded wiring from your solar charge controller to your battery bank.

[booster]

Originally Posted by jakegw2

For reasons that baffle me, solar panels are not actually rated by the power you can get from them. Power(W)=Volts*Amps, so at 300W and 12 volts in theory you could get 25A. You won't. In fact, based on the values I have seen you can expect a maximum output of 16.5A from this setup. My only guess as to why is perhaps panels generate 18V, not 12V? You will only see this current on cloudless days when the sun is directly overhead. Typically it will be much lower.

In any case, for 16.5A of current @ 14.4V for 20' with a 2.3% voltage drop you will need a 10 AWG wire. Typical allowed voltage drops are 2% or 3%, and if you want a 2% max drop you will need a 9AWG wire.

You can calculate these values yourself using this site:

https://www.engineeringtoolbox.com/1...rt-d_1689.html

A 16AWG wire can only support that voltage / current for 6.5', 14AWG can go for 8' safely. Both of these assume a 3% voltage drop, which is less ideal than a 2% drop.

Bottom line: You need to run upgraded wiring from your solar charge controller to your battery bank.

The thing is that they are rated in watts, not amps or volts, for maximum output and they have a "maximum power point", or MPPT, where they get the that maximum watts. That point is usually in 17.5v area for modern panels which would give a bit under 6 amps at that MPPT point. When you look at 12v coming out of the panel, you could theoretically get 8.33 amps if that was the MPPT point, but it is not so you get less watts at that voltage because the panel is running at lower voltage the where it will generate maximum power.


That is the reasoning behind MPPT charge controllers being needed. They entice the panel to run at whatever voltage will give the maximum watts to the batteries based on how much power the panel is capable of producing in the particular conditions. If the sun is good enough to run the panel at the maximum power point you should see the input to the MPPT controller at that 17.5v or so voltage and input amps will be at the 6ish amps. The controller takes those watts and switches it to battery charging voltage at lower voltage but higher amps at the same watts minus a bit of inefficiency in the controller.


A PWM controller does convert the extra power available above the battery voltage that is coming from the panel and can be less efficient because of that, but other conditions can change that a bit.