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markre avatar image
markre asked

Help with wire & fuse sizing

Hello friends!


First time working with solar so please forgive my ignorance.


I'm trying to put my system together and just want a sanity check and guidance on a couple of items.


I have 2x350W panels wired in series.

VOC is 46.7V and ISC is 9.72A.


I have ran 10AWG wire from PV to a 2 pole 1000V 20A breaker and from there to a Victron 150/60 MPPT.

Q: Does this seem right so far?


And this is where I stopped at the moment.


I have 2 12V 100AH Lifepo4 batteries that I'm planning on wiring in parallel to have a 12V system.


I haven't decided about an inverter yet but it seems 3000W is somewhat standard and I think I will pursue that.


The question is mostly about wire & fuse sizes and I'm getting a bit confused there looking at these diagrams.


Can someone shed light and teach me, what would be safe wire and fuse for:

1. Interconnecting batteries (consensus seems to be go as large as possible so 2/0 maybe?)

2. MPPT to battery (since max output of the controller is 60A seems like 6AWG wire with 70AMP fuse would be good?)

3. Battery to inverter? (Have no idea about this one yet but 200ah combined 2/0 again with 220amp fuse?)


Any other recommendations are welcome.


Thanks!

solar sizing
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2 Answers
Justin Cook avatar image
Justin Cook answered ·

Have a look at Victron's free book "Wiring Unlimited", it will answer those questions and many more that you don't even know you have yet!

HTML5 version: https://www.victronenergy.com/media/pg/The_Wiring_Unlimited_book/en/index-en.html

PDF DL version: https://www.victronenergy.com/upload/documents/The_Wiring_Unlimited_book/43562-Wiring_Unlimited-pdf-en.pdf


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markre avatar image markre commented ·
Thank you, I will certainly give that a read.
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Alexandra avatar image
Alexandra answered ·

@markre

Use a bus bar. (At least a 5/6 pole bus bar with min 250A ampacity)

Fuse and wire each battery to the bus bar.

Fuse and wire the 150/60 mppt to the bus bar. Fuse rating here. I would use at least 16 mm² cable there.

Fuse and wire the inverter to the bus bar. 3000va is 250A. So 50mm² at least from each battery to the bus bar. 125A fuses each min. (Can your batteries do that?).

The 3000va 12v uses 400A max fuse and 2 x 50mm² cables. The product manuals on their pages have recommended sizing.

(Sorry don't work in awg.) Make a drawing even boxes with lines will help clear it up.

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markre avatar image markre commented ·
Hi and thank you for your reply.


I currently have 3/8 bus bar so you think I should upgrade to 5/6?


Also, help me understand this, would you run both batteries to bus bar or you would tie batteries together in paralel and then have one run to the bus bar?


For the run from mppt to the bus bar/battery I mentioned 6awg and 70 amp fuse which correspondence to what your link chart shows and your metric size, 6awg = 16mm.


However, battery and inverter cables i underestimated. You said 50mm which is equal to 1/0.


Thanks!


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dc-marine avatar image dc-marine markre commented ·

Hi Markre.

A 3/8" busbar will be fine, Alexandra was referring to the numbers of studs when she said 5/6 poles.


Wiring between batteries vs to a busbar is somewhat dependant on a few criterias, usually if there's room for stacking big cables on top of the batteries or not. Second one would be if the power being draw is too great to trust the battery terminal to make good connections between the stacks of lugs.


In your case, I would use 4/0 awg cable between batteries and not worry about bussing or fusing the batteries individually. Typical setup would be 4/0awg jumpers, then 4/0awg from positive end of the bank to a 400A Class T fuse, then 4/0 to a minimum 350a continous rated battery switch such as the Blue Sea E-Series 9003e. Then 4/0 to a minimum 250a continous busbar. From there (still all in 4/0awg) I would go to a 400a ANL fuse then to another battery switch (can be a M-Series 6006 300a continous) the to my 3kva Multi.


Negative would also be all 4/0 from batteries to a negative bus (same or bigger than positive bus) and to the Multi. A minimum of 3/0awg should be used to ground the chassis of the Multi back to either the negative bus (on a boat) or vehicle chassis.


The point of the ANL vs Class T is the trip curve of ANL will allow it to blow before the Class T, helping to reduce the chance of blowing your main fuse rather than the branch fuse if there's a fault.


You'll want at least 6awg and a 80A breaker to wire your MPPT to the positive busbar to match the MPPT rating (in case you do add more PV power). Always wire your equipment according to max output/draw it is designed for.


Is it a common practice outside North America to run smaller cable in parallel vs one big cable to reduce voltage drop. However if you do so, you must fuse each cable individually to their respective ampacity, such as 2x 200a fuses vs a single 400a fuse. Unless you have a very long cable run (over 20ft-30ft each way), the single 4/0 will do and be more cost effective. I have recently completed an install of twin 3kva multi on 12v using 4/0awg and voltage drop wasn't an issue at close to 20ft of cable lenght each way with Lithium batteries.


One last thing, you may want to check what is the maximum continous discharge of your batteries is, as the multi can draw up to 250a continous and peak to near twice as much. Most 100ah batteries only output 100a continous with a short peak output. You may need a third battery to match the specs of your Multi or go to a 2kva Multi. If you go with a 2kva, I would still wire up the batteries up to the busbars with 4/0, but use 2/0 to the inverter with 300a fuse.

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