Sizing Wires & Breakers/Fuses

[jsmit209]

I've finally gotten the components for my van solar power system, and now I'm working on getting everything connected. Here's what I have:

• Four 200ah 12v LifePO4 batteries
• Six 100w 12v solar panels
• 3000w 12v inverter
• Victron Blue Solar 100/50 MPPT solar charge controller
• Renogy 40a 12v DC-DC charger
• Victron smart shunt

Assuming that I plan to max out the inverter, what wire size/breaker will I need for the following connections?

• Battery Bank to Inverter - 4/0 AWG & 250a breaker?
• Battery to Battery - 1/0 AWG or 4/0 AWG? (Trying to understand if the amp draw splits between the 4 batteries)

Also, what size should I be using for the following:

• Solar array to Charge Controller - wired in 2s3p configuration - 12 AWG & 20a fuse?
• 50a Charge Controller to battery bank - 6 AWG & 50a breaker?
• 40a DC-DC charger & battery bank - 8 AWG & 40A breaker?

How does all of that sound? Any additional considerations? Any special way I need to wire in the smart shunt with specific wire sizes?

I hope that I've done my homework correctly, but I appreciate anyone who can tell me where I'm wrong!

 

[zulu]

Battery Bank to Inverter - 4/0 AWG & 250a breaker?

I believe you'll need a 300A fuse. If you're using a Victron MultiPlus II, I think Victron recommends a 400A fuse.

Solar array to Charge Controller - wired in 2s3p configuration - 12 AWG & 20a fuse?

You could also use a solar panel disconnect switch like this one.

Any special way I need to wire in the smart shunt with specific wire sizes?

Shunt should be as close to the batteries as possible using the 4/0 wire.

 

[OffGridForGood]

There is a decent wire size calculator with option for type of wire enclosure/free air Solarwiresizecalculator.com
Or use the Omnicalculator.com
There are sites that are metric also if needed.

 

[jsmit209]

I believe you'll need a 300A fuse. If you're using a Victron MultiPlus II, I think Victron recommends a 400A fuse.

Is this for when the battery voltage drops, raising the amperage? I'm using an inexpensive 3000a inverter at the moment, but will probably upgrade in time. Is 300-400a safe on 4/0 wire?

 

[jsmit209]

There is a decent wire size calculator with option for type of wire enclosure/free air Solarwiresizecalculator.com
Or use the Omnicalculator.com
There are sites that are metric also if needed.

Thanks! I'll check that out.

 

[OffGridForGood]

I believe you'll need a 300A fuse.

the fuse size needs to be set to protect the wire.
depending on wire length, open to air or raceway/conduit, I believe 4/0 will max out at 220-230A ie 200A T-class fuse, will depend on what temps your wire can handle, and if losses above 3% are tolerated.

 

[OffGridForGood]

300-400a safe on 4/0 wire?

in my research, no not for more than a short surge. maybe 220 A you may feel ok at 250A, if the wire is open air (for cooling)

 

[OffGridForGood]

Running the numbers, looks to me that 12vDC 240A is max for short run of 4/0 cable if you have 90-degree C rated cable.
this would be continuous load, so you could go higher for a surge (start up load).
At 70-degrees C, 4/0 cable will top out at 220A
Keep in mind, if you run cable inside confined areas they heat up.
Fuses protect wires, T-class to protect your 4/0 likely 200 or 250A.
you can carry higher load using twin runs of cable, say twin 3/0 for example to carry 400A if you need this much amperage. (you need to twin both pos and neg btw)

 

[jsmit209]

in my research, no not for more than a short surge. maybe 220 A you may feel ok at 250A, if the wire is open air (for cooling)

Hmm. Everything I see about 3000W inverters recommends 4/0 wire, but it's very tight on max amperage.

What about between the batteries? What gauge there?

 

[OffGridForGood]

Battery to Battery - 1/0 AWG or 4/0 AWG? (Trying to understand if the amp draw splits between the 4 batteries)

With 4 batteries, you can connect them parallel then your main pos from battery-1 to inverter, and main neg from battery -4 to inverter to force load sharing.
The other (my preference) option, run cables from each battery-Pos to a large copper Pos-bus bar, (ditto from Neg to neg bus bar) then from the bus bars to the inverter, keep each battery to bus wire the same length, so they all have exactly the same resistance. (each battery pos wire one length, each battery Neg can be different than pos length, just make all the neg wires the same, so the overall path is same resistance) hope this makes sense.
PV panels to charge controller - not enough information: VOC (volts open circuit) ISC (current short circuit) and how the panels connect, maybe all in parallel pos to pos to pos, and neg to neg to neg? This holds the voltage same as VOC of one panel, but adds up the currents, better confirm so we can help pick the best wire size for here.
Good luck with your project, hope it goes great.

 

[John Frum]

Everything I see about 3000W inverters recommends 4/0 wire, but it's very tight on max amperage.

3000 ac watts / .85 conversion factor / 10 volts low cutoff = 352.941176471 dc amps
352.941176471 dc amps / .8 fuse headroom = 441.176470588 fault amps.
For pure copper wire with insulation rated for 90C or better you need 250MCM and a 450 amp fuse.

https://www.uglys.net/docs/default-source/uglys-documents/errata/UglysResidentialWiring2017_Errata_Page2.pdf

May need to go even higher when distance based voltage drop is considered.

3000 watts is in my opinion too much wattage for a 12 volt system.

 

[OffGridForGood]

What about between the batteries? What gauge there?

If you connect one battery to the next, and then main pos to inverter Battery 1 and main neg to inverter battery-4. - use 4/0
If you used bus bars, you could expect your 400A load to be equally shared with each battery providing about 100A to the bus and you could use 3AWG wire to the bus from each battery. maybe 2AWG in case you remove a battery one day for service.

 

[OffGridForGood]

3000 watts is in my opinion too much wattage for a 12 volt system.

John is correct typically above 2000W we go to 24vDC, above 3000W 48vDC

 

[jsmit209]

PV panels to charge controller - not enough information: VOC (volts open circuit) ISC (current short circuit) and how the panels connect, maybe all in parallel pos to pos to pos, and neg to neg to neg? This holds the voltage same as VOC of one panel, but adds up the currents, better confirm so we can help pick the best wire size for here.

I don't have the panels in hand. They arrive tomorrow. They're nominal 12v, so whatever that means for VOC... 16ish? I'm planning to wire in 2s3p to achieve 300W at 24v (32ish). Should be roughly 12.5a or less, if my math is correct.

 

[OffGridForGood]

90C or better you need 250MCM and a 450 amp fuse.

Or twin 4/0 wires?

 

[jsmit209]

3000 watts is in my opinion too much wattage for a 12 volt system.

I'm starting to fear that you're correct. I could wire my batteries 2s2p for a 24v system. I'd just have to return the Renogy 12v 40a DC-DC charger and find another solution for that. I'd also need to get a converter to step down the voltage for my 12v appliances and fixtures.

 

[OffGridForGood]

They're nominal 12v, so whatever that means for VOC... 16ish? I'm planning to wire in 2s3p to achieve 300W at 24v (32ish). Should be roughly 12.5a or less, if my math is correct.

Lets say the ISC is 10.5A x3 you need wire to carry the 31.5A allowing for low temps 8AWG min. check with the calculator after you see the ISC and VOC and knwo the wire lengths

 

[jsmit209]

John is correct typically above 2000W we go to 24vDC, above 3000W 48vDC

Could I wire two 12v 40a DC-DC chargers in series to charge a 24v battery bank? Assuming that wouldn't melt my alternator.

 

[John Frum]

Or twin 4/0 wires?

2x 1/0 awg each with a 250 amp fuse would be cheaper and sufficient.

 

[jsmit209]

2x 1/0 awg each with a 250 amp fuse would be cheaper and sufficient.

If I double up 1/0 wire between the battery and inverter, with a 250a fuse on both connections, that would be sufficient to carry 400+ amps?