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35mm or 50mm cable?
#1
Trying to make a decision on the battery cable I buy.

Max current for the foreseeable future will be 125amp. (48v)

35mm looks small, but I know a lot use it.

Anyone using 50mm ?

Lugs for both sizes seem readily available, thou not sure how 50mm will go into shunt trip. (Which is another decision to make)
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#2
It's partly a question of distance.
If the run from the battery bus bar is longer, then go up a size to keep losses down.
Lower resistance path means the inverter's input capacitors have a little less stress.
Overload situations, eg too much load needs to be considered too.

Also consider having a pre-charge circuit between the batteries & the gear. eg a large 4.7ohm resistor & a separate smaller breaker around the main breaker.
Running off solar, DIY & electronics fan :-)
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#3
Round trip distance wouldn't be any more than 4 m.
Define 'Large' resistor :-P
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#4
(02-25-2020, 11:10 PM)slimf Wrote: Round trip distance wouldn't be any more than 4 m.
Define 'Large' resistor :-P
In the chart I use for my own reference.....   https://www.powerstream.com/Wire_Size.htm
- 33.6mm squared cross-section = 2AWG  with 181a chassis limit and 94a transmission limit.    This is on the small side.- 125a will make it 'hot'.
53.5mm squared cross-section = 1/0 AWG with   245a chassis and 150a transmission limit.   This will be OK in my experience and I think even a 1 AWG would work.  


I base this on my own experience with
- 4AWG wire (thick strands) of about 6feet range with 60-80a continuous and it gets nice and warm but this side of OK.  (Controllers -> Battery control box/bus)
- 2/0 (welding wire - hundred fine strands) in 4 foot range with 125-150a continuous and its get 'barely warm'.       (Battery -> Battery control box/bus -> Inverter)

Of course, larger is usually better Smile
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#5
I always refer to the NEC table for wire ampacities. It's a great reference even if you do not live in the United States.
https://www.usawire-cable.com/pdfs/NEC%20AMPACITIES.pdf
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#6
Peter for example, uses 35mm sq welding cable.. and he has a 5kw inverter.. my guess is 35mm sq is fine. My inverter will be Victron Multiplus II 48V 5000 and it wont hit 125A.

I suppose what im asking is - what is everyone else using? 35 or 50 (2 guage or 0 guage)
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#7
(02-26-2020, 01:22 AM)slimf Wrote: Peter for example, uses 35mm sq welding cable.. and he has a 5kw inverter.. my guess is 35mm sq is fine. My inverter will be Victron Multiplus II 48V 5000 and it wont hit 125A.

I suppose what im asking is - what is everyone else using? 35 or 50  (2 guage or 0 guage)
>My inverter will be Victron Multiplus II 48V 5000 and it wont hit 125A.
I presume "5000" is the max wattage output of the inverter?   5000w/48v = 105a (not 125a).   Using @mike's chart + mine (60% of chassis) then 0 guage.

Look - I'm don't have the ability to professionally go down a checklist, consult the charts / codes, and say 'do this'. Rather, I'm DIY and have operational experience running my own system... so I'm advising you from the DIY perspective and you should take it as such.

2 will likely be OK.   2 is close enough it won't 'melt' as you turn on the inverter and you can assure yourself by measuring temps undef full/continous load. On the other hand, if its no big deal to you due to cost and connectors and you have room - then larger is never 'wrong' so 0 will definitely do 105a.
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#8
You can also take surge ampacity into account. The 2 awg wire can handle the 105A (mentioned above) for short periods. So if there is a heavy load on the line and the inverter kicks up to 200% for a second or two, then that'll be fine. However, you will have to also take into consideration the voltage drop, though it should be small for the short period (make sure to not drop to low on SoC to mitigate this)
Another thing to consider is the distance total (Pos length + Neg length) into the equation. If there are 10's of feet, or meters, of distance, then you want to lean towards the heavier wire to keep voltage drop low.
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#9
For the 48/5000/70 model Victron actually suggest 70mm2, see the manual page 9
"Recommended cross section (mm2)
per + and - connection terminal
cable distance : size
0 – 5 m : 70 mm2"
Running off solar, DIY & electronics fan :-)
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#10
Thanks for the responses.

I think I will go for 50mm sq. It’s not crazy expensive (thou the lugs are stupid expensive!)

Now, an Aussie available shunt trip... anyone? Something that will accept lugs or at least 50mm :-)
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