Did I Design This Correctly? 24v/10.2KWh/1200w RV system. (Diagram attached)

[Jon22896]

Hello!

This is my first build. I was hoping the fine members of this forum might be willing to glance at my diagram and let me know if I'm missing anything. I'm most concerned about wire size and fuse calculations.

 

[John Frum]

I'm guessing the pigtails on that little dc2dc converter could melt or catch fire before a 100 amp fuse would blow.

 

[eXodus]

You are missing fuses between the panels and the combiner and charge controller.

What voltage are the panel running? Without that you can't tell if the wire to the mppt is correct.

Why do you got another ground from the inverter to the mppt?

Good start. Just a few details to flush out.

 

[rmaddy]

FYI - It's "American Wire Gauge" - AWG.

Post the full specs of your solar panels. How long will the wire be from the combiner box to the SCC? This will help determine if 8AWG is appropriate.

What is the amp rating on your 24V->12V DC-DC converter?

You have a 100/50 SCC. That means the output current is 50A. For that you only need 6AWG wire, not 2AWG. It's like the 100/50 won't even accept 2AWG. Yep, just checked - max of 6AWG. And the 100A fuse you plan is much bigger than needed. A 65A - 70A fuse is plenty for a 50A load.

Your 3000W 24V inverter can pull up to 140A. 4/0AWG wire is much bigger than needed. You'd be fine with 0AWG. You only need a 175A fuse, not 300A. All of your 4/0AWG wires can be 1/0AWG.

 

[rmaddy]

You are missing fuses between the panels and the combiner and charge controller.

Wouldn't the fuses be inside the combiner box?

 

[Jon22896]

You are missing fuses between the panels and the combiner and charge controller.

I was under the impression that if I'm running 2p I don't need to fuse the panels, but anything more than 2p fuses would be required. Is that incorrect? If I need fuses, what type size would you recommend?

What voltage are the panel running? Without that you can't tell if the wire to the mppt is correct.

Why do you got another ground from the inverter to the mppt?

This is something that was recommended at explorist.life. I'm not sure why, but I think it's simply because there are not enough connection points on the Lynx for the charge controller ground, so they recommended running it to the inverter then to the lynx. Do you recommend something different?

Good start. Just a few details to flush out.

Thanks so much for your comments!

 

[OzSolar]

Charge Controller wire could be stepped down to #6. Even though you'll rarely if ever see the max solar panel wattage we will still use it for the calculations.
1200 watts/24v = 50 amps which is also the most your CC will put out anyway. Then replace that fuse in the LYNX with 60.

A class T fuse on the output of each battery is the typical recommendation but some will say that the BMS in the batteries make that unnecessary.

In lieu of a Class T fuse you could install an MRCB which does give you the ability to troubleshoot and isolate a bad battery thus keep your system working while you're on the road trying to enjoy yourself.

 

[Jon22896]

FYI - It's "American Wire Gauge" - AWG.

Thanks for catching that typo! And all the great points.

Post the full specs of your solar panels. How long will the wire be from the combiner box to the SCC? This will help determine if 8AWG is appropriate.

Just posted the full specs in my last reply. See above.

What is the amp rating on your 24V->12V DC-DC converter?

The output is rated at 40 amp @ 12 v. This is the one I'm looking at: uxcell New BIG-Size Voltage Converter Regulator DC/DC DC 24V to DC 12V 40A 480W Buck Transformer Waterproof

You have a 100/50 SCC. That means the output current is 50A. For that you only need 6AWG wire, not 2AWG. It's like the 100/50 won't even accept 2AWG. Yep, just checked - max of 6AWG. And the 100A fuse you plan is much bigger than needed. A 65A - 70A fuse is plenty for a 50A load.

Thank you! That makes total sense.

Your 3000W 24V inverter can pull up to 140A. 4/0AWG wire is much bigger than needed. You'd be fine with 0AWG. You only need a 175A fuse, not 300A. All of your 4/0AWG wires can be 1/0AWG.

Great! I had them all at 1/0 AWG, but then I saw this chart and upped everything to 4/0. I'll size that back down to 1/0.

 

[Jon22896]

I'm guessing the pigtails on that little dc2dc converter could melt or catch fire before a 100 amp fuse would blow.

I was a bit stuck on this one. the converter output is rated for 40 amps @ 12v. Does that mean the input is approx 20 amps @ 24v? In that case, should I fuse that with, say, a 25 or 30 amp fuse?

 

[rmaddy]

was under the impression that if I'm running 2p I don't need to fuse the panels, but anything more than 2p fuses would be required. Is that incorrect?

Your diagram indicates your 6 panels will be in 6P so all 6 need to be fused. If you actually meant your panels to be wired in 3S2P (which is supported by the 100/50) then you do not need any fuses or a combiner box. But it is common to have a DPST disconnect switch near the SCC. Make sure the switch/breaker is rated for the voltage of your array and that it has more amps than your array.

One you clarify your actual panel arrangement then we can verify the details.

The output is rated at 40 amp @ 12 v.

OK. The 40A is the 12V output. The 24V input will only be half of that (plus a tiny bit more due to inefficiencies). So assume 25A input. You only need 10AWG wire from the Lynx to the converter. And you only need a 30A fuse. 6AWG is way too big. The converter will only have 10AWG leads at best, maybe even 12AWG. For the 12V output you should use 8AWG but again, the leads won't be that big.

I had them all at 1/0 AWG, but then I saw this chart and upped everything to 4/0. I'll size that back down to 1/0.

That chart seems aggressive. It claims the max inverter input amps for a 24V 3000W inverter is 200A. But 3000W / 24V / 90% = 140A.

 

[Jon22896]

Your diagram indicates your 6 panels will be in 6P so all 6 need to be fused. If you actually meant your panels to be wired in 3S2P (which is supported by the 100/50) then you do not need any fuses or a combiner box. But it is common to have a DPST disconnect switch near the SCC. Make sure the switch/breaker is rated for the voltage of your array and that it has more amps than your array.

One you clarify your actual panel arrangement then we can verify the details.

Yes, that was unclear. The panel arrangement will be 3s2p. I was hoping to do all of the series and parallel connecting inside the junction box so that it is easy to rearrange. I'd like to add a second SCC and a few more panels to the system down the road.

OK. The 40A is the 12V output. The 24V input will only be half of that (plus a tiny bit more due to inefficiencies). So assume 25A input. You only need 10AWG wire from the Lynx to the converter. And you only need a 30A fuse. 6AWG is way too big. The converter will only have 10AWG leads at best, maybe even 12AWG. For the 12V output you should use 8AWG but again, the leads won't be that big.

That makes sense to me. I'll make those changes now.

That chart seems aggressive. It claims the max inverter input amps for a 24V 3000W inverter is 200A. But 3000W / 24V / 90% = 140A.

That's what I thought too. Thank you for confirming.

 

[Jon22896]

Charge Controller wire could be stepped down to #6. Even though you'll rarely if ever see the max solar panel wattage we will still use it for the calculations.
1200 watts/24v = 50 amps which is also the most your CC will put out anyway. Then replace that fuse in the LYNX with 60.

Thank you! This seems to be the consensus. I'll make those changes.

A class T fuse on the output of each battery is the typical recommendation but some will say that the BMS in the batteries make that unnecessary.

The EG4 batteries each have their own 125 amp breaker built-in. Do you think an additional fuse is necessary?

In lieu of a Class T fuse you could install an MRCB which does give you the ability to troubleshoot and isolate a bad battery thus keep your system working while you're on the road trying to enjoy yourself.

I think the built-in 125 amp breaker on each EG4 battery will also accomplish this. Right?

 

[John Frum]

I was a bit stuck on this one. the converter output is rated for 40 amps @ 12v. Does that mean the input is approx 20 amps @ 24v? In that case, should I fuse that with, say, a 25 or 30 amp fuse?

What is the wire gauge and insulation rating on the in and out wires?

 

[OzSolar]

I think the built-in 125 amp breaker on each EG4 battery will also accomplish this. Right?

It totally will!

I have not had my hands on those specific batteries so my brain was stuck on ones that don't have built in breakers which is super nice.

 

[rmaddy]

What is the wire gauge and insulation rating on the in and out wires?

There is a picture on the Amazon link the OP provided that shows 12AWG on the input and 10AWG on the output. The 10AWG should be OK as long as it's kept short.

 

[rmaddy]

The panel arrangement will be 3s2p. I was hoping to do all of the series and parallel connecting inside the junction box so that it is easy to rearrange. I'd like to add a second SCC and a few more panels to the system down the road.

Connecting the panels in the desired arrangement inside the junction box should work. With 3S2P you don't need any fuses. You will have about 75V at 20A on the output. The proper size wire depends on how long the wire will need to be from the panels/junction box to the SCC.

 

[Jon22896]

Connecting the panels in the desired arrangement inside the junction box should work. With 3S2P you don't need any fuses. You will have about 75V at 20A on the output. The proper size wire depends on how long the wire will need to be from the panels/junction box to the SCC.

The wire might be as short as 10 ft if I drill a new hole in roof or 25 ft if I use existing refrigerator vent. I'm leaning towards 25 ft.

 

[Jon22896]

Connecting the panels in the desired arrangement inside the junction box should work. With 3S2P you don't need any fuses. You will have about 75V at 20A on the output. The proper size wire depends on how long the wire will need to be from the panels/junction box to the SCC.

The wire might be as short as 10 ft if I drill a new hole in roof or 25 ft if I use existing refrigerator vent. I'm leaning towards 25 ft

 

[rmaddy]

At 75V and 20A, 10AWG would be plenty for 25'.

 

[Jon22896]

Thank you all for the feedback, especially @rmaddy! I think I got all of your feedback into this updated diagram. Let me know If I missed anyting.

 

[John Frum]

Thank you all for the feedback, especially @rmaddy! I think I got all of your feedback into this updated diagram. Let me know If I missed anyting.
View attachment 91521

Did your diagram get hosed?

 

[rmaddy]

Did your diagram get hosed?

Looks good to me.

 

[eXodus]

I was under the impression that if I'm running 2p I don't need to fuse the panels, but anything more than 2p fuses would be required

With 2 strings you can sometimes get away with not fusing between them. The scenario is - one of the panels shorts out - then one string feeds into the other string - since they are parallel and heats that up.
This scenario is very rare - and nothing catastrophic usually happens - panels get warm and die over a few weeks.
You panel is rated with 9.5. A but only has a fuse rating of 15A - so that's less then 2x There are panels with 10A but have a fuse rating of 30A.
So recommended to have fuse between strings -not required.

I personally don't like combiner boxes on the roof - the RV roof is already cluttered enough, further they really get a beating up in the weather.
Your 3s 2P you can easy just hook up directly. 3 panels in series with just wires and then a Y-Splitter.

This is something that was recommended at explorist.life. I'm not sure why, but I think it's simply because there are not enough connection points on the Lynx for the charge controller ground, so they recommended running it to the inverter then to the lynx. Do you recommend something different?

It depends where you mount your equipment? If you got an RV with Fiberglass walls it can be difficult to find something to ground to, then I can see that configuration you are doing. Usually I try to ground to the closest metal structure of the vehicle. My charge controller for instance is grounded to the screw of a seatbelt on the floor. My batteries are directly grounded to Chassis frame of my Class A.
It depends what people define as their "reference ground"

With a 3S 2P config you will be running 60V at around 20A so a 10AWG (max 30A) between panels and MPPT should be fine. 8AWG gives you more expansion capacity.

I've built a similar system from the 120V side - just FYI you can use the existing 120V -12V converter charger in the powercenter and don't do the 24V-12V conversion if you 12V battery is still fine. It's only slightly less efficient (depends on the 12V charger) and adds a layer of backup.