Correct size MPPT charge controller and How I determine correct gauge wiring?

wilyvero

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I bought 4 used 250W solar panels from San Tan Solar in Arizona. I want to put them on my RV. I initially purchased a 40A EPEVERS MPPT charge controller, but I've been seeing videos where it says that it can only handle 520W of power. What size MPPT controller do I need to buy? Also, how do I determine the correct gauge of wiring to use from the battery to the controller and the inverter ?
 

12VoltInstalls

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it can only handle 520W of power.
At 12V. And that’s not a ‘hard limit’ whereby factory smoke is released, the wording in the descriptions is weird (to me) and confusing. You can overpanel (watts) just not go over voltage. Either way it is output limited to 40A. Go over max volts, however, and output amps will go to zero. Forever.

What battery voltage do you plan to use? It will do more (double) watts at 24V and dbl again at 48.
how do I determine the correct gauge of wiring to use from the battery to the controller and the inverter ?
With a 40A controller you need at least 8ga to battery bank.
Without knowing the inverter output rating you can’t know what cable to use.
 

rickst29

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Which of the various EpEver MPPT controllers do you have? The "BN" models might have a bit more headroom than the "AN" models, due to a superior heatsink. As 12voltinstalls just described, "520 Watts" divided by 40 Amps is around 13 volts (a slightly low charging voltage, even for the case of lead acid batteries). Higher charging voltages increase the maximum power in a linear way.

When batteries don't need charging, or will accept only a small amount of current, the EpEver Solar Charge Controller (SCC) will leave a lot of "available" power up in the panels, unused. If your 4 panels create less than 30 Amps of current at rated output, most of the EpEver SCCs will handle them fine. If you can configure them series+parallel 2x2, you should have much less maximum current than that, while still having disconnected voltage well below the EpEver limit. It depends on the characteristics of your panels.

On less-than-perfect days, "excessive" panels are good. I have about 600 Watts of rated panels on my own EpEver 30 Amp "Tracer BN", and it never even gets warm. The user manual indicates that only '390 watts' are usable (at "12V"), but that's manifestly understated. I actually push upwards of 450 watts through the SCC in good sun conditions. I would personally feel very comfortable with your configuration on a 40 Amp Tracer "BN", but obviously can't guaranty it for you.

The wire size between the battery(s) and the Inverter depends on the Inverter, of course.
 

wilyvero

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I have the "BN" series charge controller and I plan on wiring the 4 solar panels in series/parallel which I estimated would give me approx 60 volts at 16 amps.
 

rickst29

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I have the "BN" series charge controller and I plan on wiring the 4 solar panels in series/parallel which I estimated would give me approx 60 volts at 16 amps.
Thanks! The "BN" can handle much higher Voltages (up to about 130 Volts Open Circuit), and your Amperage is also quite low. That should work great.
 

wilyvero

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So the fact that I have about 1000W of solar panels is ok for that charge controller ?
 

MichaelK

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Keep in mind that under the best conditions (ie:noon perfect blue ski), a panel really can't be expect to put out more than 85-90% of it's rated output. Secondly, if these panels are laying flat on the roof and not perpendicular to the sun, they won't get anywhere close to 85%, but maybe more like 60%.

Plug those parameters into you solar equation, and you get {(250W X 4 panels)/13W charging } X 0.6 efficiency = 46A. So, realistically a 40A controller will work fine about 90% of the time. On those rare occasions it bumps over 40A, those extra amps just get wasted.

Another fact to keep in mind is that charging will start in the morning when the sun is even lower in the sky, and won't even get anywhere near 60% for a couple of hours. As the controller starts getting the few meager amps in morning, and starts to charge, by the time output reaches the 60% level, the controller might already be throttling back amperage as it progresses through the bulk phase.

If you really want to be sure about this, just connect two of the four panels and monitor it's output for a couple of days. There will no need for concern if you never see any numbers past 20A.
 

wilyvero

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Which of the various EpEver MPPT controllers do you have? The "BN" models might have a bit more headroom than the "AN" models, due to a superior heatsink. As 12voltinstalls just described, "520 Watts" divided by 40 Amps is around 13 volts (a slightly low charging voltage, even for the case of lead acid batteries). Higher charging voltages increase the maximum power in a linear way.

When batteries don't need charging, or will accept only a small amount of current, the EpEver Solar Charge Controller (SCC) will leave a lot of "available" power up in the panels, unused. If your 4 panels create less than 30 Amps of current at rated output, most of the EpEver SCCs will handle them fine. If you can configure them series+parallel 2x2, you should have much less maximum current than that, while still having disconnected voltage well below the EpEver limit. It depends on the characteristics of your panels.

On less-than-perfect days, "excessive" panels are good. I have about 600 Watts of rated panels on my own EpEver 30 Amp "Tracer BN", and it never even gets warm. The user manual indicates that only '390 watts' are usable (at "12V"), but that's manifestly understated. I actually push upwards of 450 watts through the SCC in good sun conditions. I would personally feel very comfortable with your configuration on a 40 Amp Tracer "BN", but obviously can't guaranty it for you.

The wire size between the battery(s) and the Inverter depends on the Inverter, of course.
How do you calculate the size wire or cable ?
 

MichaelK

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How do you calculate the size wire or cable ?
Here's wire gauge table. Looking at the chart, it shows that 8 gauge wire can only handle a total of 40A without heating past 60degreesC. The next larger gauge is 6, which can handle 55A. Finally, the next bigger gauge is 4, which can handle 70A. So, I'd suggest selecting at least 6 gauge from the controller to the batteries, though 4 gauge would allow for future expansion.
 

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dehv

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So the fact that I have about 1000W of solar panels is ok for that charge controller ?
I have the 4515bn and can confirm abuse testing way more than 40a potential input, the unit will just ramp up to 40a current to battery and stay there. Good for cloudy days to have more panels than 40a.
 

12VoltInstalls

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I'd suggest selecting at least 6 gauge from the controller to the batteries, though 4 gauge would allow for future expansion.
I like overcabling. And the epever 40 will take 6ga which “is” oversized for 40A.

However, it’s doubtful you will see 40A of output on one hand, and on the other hand if you fuse at 40A with 8ga there isn’t a safety concern, really.
I have 8ga because it was way overkill when I initially installed it with 200W of panels. Now, two upgrades later 8ga is just “adequate” for my setup. I have an 8ga ATO fuseholder at the battery with a 40A fuse; so far the most I’ve seen is 32-34A with 600W split to 300W series in two directions. You could see max output regularly depending on your region/location.

If you are buying charge wire now 6ga makes the most sense.
 

rickst29

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I have the 4515bn and can confirm abuse testing way more than 40a potential input, the unit will just ramp up to 40a current to battery and stay there. Good for cloudy days to have more panels than 40a.
I can confirm the same for my smaller BN3215 (30 Amp), which is also over-paneled. When Solar Power exceeds the maximum charging power of my Tracer [(Solar Watts * 95% MPPT efficiency) / Charging Voltage would be greater than 30A], it happily limits output current to very slightly more than 30A and stays there, leaving unwanted extra Solar Power in the panels and "in the sky".

This is a programmable parameter - I have also "tuned" that 30A limit, using the MT-50, to be slightly less (28A). And my Tracer 'BN' obeys the newer lower limit in the same way, putting out a maximum of about 28.05A (measured by a separate coulomb counter).
 

dehv

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Beware of CCA copper coated aluminum wire if shopping online.
 
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