24v to 12v wiring

[OldJimbo]

I want to use my 24v fuse box as the start of my 12 volt system. It goes 24v 40a fuse to 40a 24 to 12v converter to 12v fuse box. Is this OK? Not too late to send stuff back to Amazon, but I want to be sure to do this properly. I bought a Victron 100/20 solar charger, but need to be frugal for awhile.
I know that I should post a drawing of all the wiring, but don’t know how to do that on an iPhone.

 

[rmaddy]

Why not connect each "component" to the bus bars? The charge controller is connected to the bus bars. The 24V fuse panel is connected to the bus bars. The 24V-12V DC-DC converter is connected to the bus bars. The inverter is connected to the bus bars. Anything else is connected to the bus bars.

Each of those components gets its own fuse.

That's how I did mine. What you propose is doable but it means your 24V fuse box and its wiring is not only handling all of the 24V loads but also all of the 12V loads and their current.

Also keep in mind that your 40A 24V-12V DC-DC converter means the 12V output is 40A or 480W. So the input will be 480W plus some extra to account for conversion losses. Probably about 510W. At 24V that means the input is about 22A.

 

[OldJimbo]

OK. That would sure simplify things.
So it would go battery + to 100a fuse to + bus bar to 25a fuse to 24-12v converter to 40a fuse to 12v fuse box.
Battery- to shunt to - bus bar, etc ?

 

[OldJimbo]

But if I connect the solar charger directly to the + bus bar, doesn’t that risk getting a rush of 24v current back into the Victron 100/20 ?

 

[rmaddy]

So it would go battery + to 100a fuse to + bus bar to 25a fuse to 24-12v converter to 40a fuse to 12v fuse box.
Battery- to shunt to - bus bar, etc ?

That seems right except I would use a 50A fuse between the converter and 12V fuse box. Since the converter can output 40A it is likely to cause a 40A fuse to pop unnecessarily.

But if I connect the solar charger directly to the + bus bar, doesn’t that risk getting a rush of 24v current back into the Victron 100/20 ?

No. Loads pull only what they need. This is why you can have huge wires from the battery going to the bus bar and also a tiny little 20AWG wire for some 0.5A device connected to the same bus bar.

 

[HRTKD]

I have two Victron MPPT 100/50 directly attached to my common bus bars with a circuit breaker in between. That's a common way to connect it. I've seen no issues.

 

[Zwy]

I want to use my 24v fuse box as the start of my 12 volt system. It goes 24v 40a fuse to 40a 24 to 12v converter to 12v fuse box. Is this OK? Not too late to send stuff back to Amazon, but I want to be sure to do this properly. I bought a Victron 100/20 solar charger, but need to be frugal for awhile.
I know that I should post a drawing of all the wiring, but don’t know how to do that on an iPhone.

I did mine similar but you need to add a fuse between the converter and the 12V fuse panel that is either the max amperage of the converter or the max amp rating of the fuse panel.

What size is your 24V to 12V converter?

Mine is 720 watts. I have a 40a breaker from 24V busbar feeding #6 cable that runs some distance to the converter. After the converter, I have another breaker, 60 amp.

 

[OldJimbo]

My converter was 24/12 30 amp but then I moved it to 24/12 40 amp and now I’m thinking of changing again to 24/13.8 40 amp for $40.
The changes are driving me crazy - but once started, it’s easy to slip into the crevasse. Why not get rid of the whole 24 volt system and stay with 12v? Thanks to Amazon’s return policy, I can flounder around until January 31st. Can an mppt solar charger handle 4 each 12v 100 watt panels in parallel? How about 2 each 12v 200ah batteries in parallel? The only things I have which can’t be returned are the solar panels.

 

[rmaddy]

If you are still referring to the Victron 100/20 SCC then it can easily handle 4 100W panels in 4P, 4S, or 2S2P. That controller is good for about 500W of panels at 12V.

A SCC doesn't really care how many batteries you have as long as the charge current isn't too much for the batteries. 20A isn't even close to being too much for 2 batteries in parallel.

Your biggest issue here is that 2 12V 200Ah batteries is 2 x 12.8V x 200Ah = 5120Wh. 400W of solar would take 5120Wh / 400W = 12.8 hours to charge the batteries from 0% to 100%. Of course you would never need to put that much back into the batteries but you'll also rarely get a full 400W from the panels either. Adjust those calculations for how much of the battery you will actually use in a day and how many solar hours you can get in a day. Most likely you have too little solar for that much battery.

 

[Rednecktek]

Why not get rid of the whole 24 volt system and stay with 12v?

That was my first thought. The only difference between a 24v fuse block and a 12v fuse block is the voltage going to it from your batteries. Is there something on your system that is running 24v? Compressor? Fridge? Heater? If you don't have any 24v stuff, then why worry about a 24v fuse block?

I don't see if you're running a 12v or 24v system for your batteries, but if it's 12v then just skip the 24v. If it's a 24v battery system, then it would go Battery -> 50aFuse -> 24/12 Buck -> Fuse Block -> 12v Things

 

[12VoltInstalls]

Why not get rid of the whole 24 volt system and stay with 12v

That was my first thought.

I didn’t see what you are trying power. If your inverter - if you have one- isn’t over, say, 1500W at 120V output there isn’t the typical need for a 24V battery supply.

Is there something on your system that is running 24v? Compressor? Fridge? Heater? If you don't have any 24v stuff, then why worry about a 24v fuse block

That’s the actual question that needs to be answered: “why 24V?”

400W = 12.8 hours to charge the batteries

Is this a cabin? Backup system at home? RV trailer? Motor home?
With LFP it’s less critical but lead acid wants to be fully recharged each day. At 200Ah each I’m believing you have LFP.
If you have the room more panels make sense.

 

[Zwy]

My converter was 24/12 30 amp but then I moved it to 24/12 40 amp and now I’m thinking of changing again to 24/13.8 40 amp for $40.

Why didn't you just get a big one to start out with?

The changes are driving me crazy - but once started, it’s easy to slip into the crevasse. Why not get rid of the whole 24 volt system and stay with 12v?

The main reasons for 24V are running a higher wattage inverter, less voltage drop on those large loads, ability to run smaller cable for those tight spots, less amperage on a wire for the same wattage, the list is actually quite long.

I'd go completely 24V if I was building ground up on a custom RV or camper build. 24V fridge, lights, and other accessories. The only reason I needed 12V was due to the wiring already in place and the items are 12V on those circuits.