I need help with sizing the solar fuses, wiring, and mppt controller.

[srponies]

I have been trying to figure out what I need for fuses for the solar panels on the roof, wire sizing between the roof and interior, interior shut offs, and charge controller size.

Is there a benefit for a roof top combiner box, or are MC4 inline fuses and branch connectors adequate?

How should I calculate the sizing of the wiring from the roof to inside the bus. 10ga, 8 ga, 4 ga?

Finally, the size of the MPPT controller. I want to wire the (3) panels in parallel. I'm looking at the Victron MPPT controllers. Our inverter (3000w) is a Victron unit.

The battery bank and DC system is a 12 volt system. We have (4) 155Ah AGM batteries.
We have (3) Solar panels on the roof.
Nom Power 310W
Optimum operating voltage (Pmax) 36.4V
Optimum operating current (Vmp) 8.52 A
Open circuit voltage (Voc) 44.9V
Short circuit current (Isc) 9.08A

I have tried to use several of the online calculators, but seem to get different results on each one.

Thank you for any help.
Steve

[Danjo]

The blue sea systems circuit wizard app is great to have for making these kind of calculations.

[john61ct]

one SC per panel will get the most out of the MPPT higher average output, Ah per day actually into the bank. Also one pair home run to each SC, no junction wiring

the 100/20 will let you capture every mAh each panel produces, but costs more, and really that extra will only be at relatively rare "peak output" times

75/15 a bit cheaper will be a only a slight compromise, how much depends on average insolation conditions.

One SC to control all three panels, go for the 150/60 or bigger for future expansion

[john61ct]

For wire gauge, download the BSS Circuit Wizard app

use round-trip distance

Round up to the next highest gauge available, rather than down (less losses from resistance, voltage drop)

post Qs here on other variables you don't grok

[john61ct]

I do not use any circuit protection between panels and SC

But you can use 120% of the max ampacity used for the wire calculation

[dzl_]

Originally Posted by srponies

I have been trying to figure out what I need for fuses for the solar panels on the roof, wire sizing between the roof and interior, interior shut offs, and charge controller size.

Is there a benefit for a roof top combiner box, or are MC4 inline fuses and branch connectors adequate?

How should I calculate the sizing of the wiring from the roof to inside the bus. 10ga, 8 ga, 4 ga?

Finally, the size of the MPPT controller. I want to wire the (3) panels in parallel. I'm looking at the Victron MPPT controllers. Our inverter (3000w) is a Victron unit.

The battery bank and DC system is a 12 volt system. We have (4) 155Ah AGM batteries.
We have (3) Solar panels on the roof.
Nom Power 310W
Optimum operating voltage (Pmax) 36.4V
Optimum operating current (Vmp) 8.52 A
Open circuit voltage (Voc) 44.9V
Short circuit current (Isc) 9.08A

I have tried to use several of the online calculators, but seem to get different results on each one.

Thank you for any help.
Steve

Different calculators come up with different numbers because they often make slightly different assumptions. One thing I've learned is its important to understand the assumptions they make so you can make sure they match you usage.


I second the recommendation of the blue sea circuit wizard, its a quality calculator, however its not well suited for solar wire sizing, since it doesn't allow you to specify voltage. Baymarinesupply.com has a good simple calculator that allows you to specify voltage.


As to your question about MC4 vs combiner box, this is usually determined by system size, MC4 is fine for smaller systems, but can only carry so much current (20A? 30A? can't remember).


Inline fuses are only necessary if you will have 3 or more panels in parallel, will this be the case for you? Breakers (rated for the proper DC current) have the advantage of also allowing you to easily disconnect the PV array from the rest of your system for maintenance etc, but there are other sometimes more convenient ways to accomplish this.


Edit: Its also important to be aware that any marine wire size calculator you use will assume 105*C rated wire unless you specify otherwise, 105*C exceeds the majority of household wiring so be conscious of this when choosing your wire or doing your calculations.

[srponies]

This is (3) panels - 310W, 36V. I was hoping to run them in parallel. I would also like to have only (2) wires coming through the roof. And use (1) Mppt Solar charge controller.

[john61ct]

Alan Ferber at Bay Marine is a fantastic vendor, right up there with Maine Sail.

The BSS app is set up for 12V nominal it's true.

Maybe talking propulsion use cases getting up to 60V+ or into hundreds of Volts, it may matter dunno.

But for solar storage, I don't think 24V is very different, it is really Amps that matter for heat risk, not so much Watts.

[dzl_]

Originally Posted by srponies

This is (3) panels - 310W, 36V. I was hoping to run them in parallel. I would also like to have only (2) wires coming through the roof. And use (1) Mppt Solar charge controller.

In this case, per panel fuses or breakers should be used. MC4 connectors and inline fuses, or a combiner box with breakers are both options. If you use MC4 you need to make sure the connectors are rated for at least 30A, and because you are close to this limit (Isc x 3 = 27.3A) you should buy from a reputable brand.



Per panel Fuses/breakers need to be greater than or equal to 10A and less than or equal to 15A


In terms of wire size, to help with that we need to know the approximate round trip distance from the panels to the charge controller.

[john61ct]

Originally Posted by srponies

This is (3) panels - 310W, 36V. I was hoping to run them in parallel. I would also like to have only (2) wires coming through the roof. And use (1) Mppt Solar charge controller.

Then the SmartSolar 150/60 is the one, and yes you'll need a junction box.

I would only use the MC4 connectors for pretty permanent joins

same with the built-in SC terminations.

If you might want to plug & unplug for safety / maintenance relatively regularly, Anderson is my standard.

[dzl_]

Originally Posted by john61ct

Then the SmartSolar 150/60 is the one, and yes you'll need a junction box.

I would only use the MC4 connectors for pretty permanent joins

same with the built-in SC terminations.

If you might want to plug & unplug for safety / maintenance relatively regularly, Anderson is my standard.

150/60 seems a bit undersized doesn't it? Good enough to capture most of the energy in most real world conditions, but only a maximum of 77% of the arrays max power at STC. I think a 70A or 80A controller would be ideally suited, particularly considering that the 70A version of the Victron Smartsolar is less than 10% more $ than the 60A version.

[john61ct]

Sure if that's the case.

Just that if this is a "mostly solar" setup planning / sizing should really be focused on the worst case conditions, and maximizing **average** Ah per day actually getting captured into storage.

When absolute peak conditions get up to that over 90% of theoretical rated output (IME very rare)

it is usually not being captured by the bank anyway.

Hitting anything over 300Ah per day would be a great result in the real world - assuming the bank is depleted enough during the day to absorb that much after concurrent loads have been fed.

Also, depends on your charge setpoint, 14.85V quite different from what I would set for GEL or LFP.

[srponies]

I have a question about using the baymarinesupply wiring calculator. Do I use the operating current (Imp)x3 or the short circuit current (Isc)x3 for the AMPS box? And, operating voltage or Open circuit voltage for the VOLTS box?

If I input 25.5 amps, 144 length, and 36.4 volts, it looks like I can use 4GA wire. I would run this between a combiner box or quality Mc4 branch connectors, and the solar charge controller.

Can I use a "standard" red knobbed battery disconnect between the roof and the charge controller? Or is a DIN railed circuit breaker/shutoff a better option.

Thank you for all of the information.

[srponies]

Can you please tell me how you are figuring out the recommended charge controller size. I don't understand how you are getting the 60-80 amp recommendation. If each panel is putting out 8 to 9 amps, that is 27 amps for the 3 panels.

Thanks,
Steve

[TruculentTurnip]

My understanding is that charge controllers are normally rated by the amperage to the battery (and/or load). Thus you have to do a bit of math to find whether your panels exceed it's rating.

36.4v * 8.52A * 3 panels = 930 watts

If your battery bank is a 12v bank then;

930 watts / 12v = 77.5A peak charging current

The MPPT controllers I've looked at claim to safely only draw up to their rated current even if up to 1.5x more is available. (Those nice MPPT curves would just get a flat top on them, when the controller tops out.)

So in this case, the theoretical capacity of your panels could put 77 amps into your battery, but you could probably get away with a 60A charge controller since most of the time the panels will never hit their rated maximum power output.

[john61ct]

Maximum Watts (see the SC specs in the datasheet & manual)

divided by the charge Voltage you will use

is IMO at least 10-15% higher Amps than you need to use.

The first voltage rating number (150/x) is an **input** max, you need to stay 10-15% lower with Voc to avoid damaging the controller. In your case only an issue if you serial connect.

But the second current rating number (y/60) is **output** maximum amps, and Victron has built their SCs to withstand a much higher input current

aka to allow for the overpanelling I discuss above.

[john61ct]

Originally Posted by john61ct

Also, depends on your charge setpoint, 14.85V quite different from what I would set for GEL or LFP.

930 ÷ 14.85V

vs

939 ÷ (say) 13.8V

Originally Posted by TruculentTurnip

If your battery bank is a 12v bank then;

930 watts / 12v = 77.5A peak charging current

nominal 12V is ~50% SoC number, nothing to do with actual charge voltage

[dzl_]

Originally Posted by srponies

Can you please tell me how you are figuring out the recommended charge controller size. I don't understand how you are getting the 60-80 amp recommendation. If each panel is putting out 8 to 9 amps, that is 27 amps for the 3 panels.

Thanks,
Steve

So basically there are 3 numbers on the charge controller side to pay attention to.


1. Max Input Voltage (this is very important and a safety consideration)
2. Max Input Current
3. Max output Current (not a safety consideration on most controllers, but it is a bottleneck if it is exceeded).


Your arrangement is safe on points 1 and 2 (Your 3 panels in parallel have a max voltage of 45V x 1.2 safety factor for low temperatures, and a max current of 27.3A on the input side.


However, the output side is where your system is bottlenecked. The controller takes the power being delivered by your PV array at the array voltage, and converts it down to a level that is safe for your batteries (~12v), so to supply that same amount of power (930W) at 12v, the amps have to be much higher on the output side. There is some flexibility here because with a good controller, exceeding the output current rating is not unsafe, it just means in peak conditions you will be leaving a bit of power on the table.


The math to figure this out is [PV Array Watts] / [Battery Bank Voltage] = [Max amps on the battery side of the controller]


So for instance 930W / 12V = 77.5A


What specific voltage you use will affect the result you get. And the actual voltage differs depending on how full or empty your batteries are. I don't know lead acid well, but with lifepo4 I would use 12v if I wanted to see what current would be at a low state of charge, and 13.6v if I wanted to know the current at a high state of charge.


edit: and John is correct about the 'on paper' figure being more than what you will see in practice the majority of the time. This is why many people 'overpanel' a little bit to save money.

[srponies]

Thank you so much for all of the information here.

On the output side of the controller - I used the wire size calculator.. 78amps, 12 volts, and 96" of cable.. Looks like 4/0 cable is just out of the Excellent category. I can't get the charge controller any closer than 4-5 ft from my batteries. Should I run a longer cable from the roof, to get the output cable as short as possible? I had planned to cable from the charge controller, through a circuit breaker/fuse, to my buss bar. And the cable from my buss bar to the batteries is 4/0.

[srponies]

This is my electrical setup. All located under my dinette bench. The solar controller will mount outside of this area and mid-height on the wall in front of the bench. My batteries are located in the box under the bench.