35mm or 50mm cable?

slimf

slimf

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Sep 26, 2017
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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)
 
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.
 
Round trip distance wouldn't be any more than 4 m.
Define 'Large' resistor :p
 
slimf said:
Round trip distance wouldn't be any more than 4 m.
Define 'Large' resistor :p
Click to expand...
In the chart I use for my own reference..... https://www.powerstream.com/Wire_Size.htm
- 33.6mm squared cross-section = 2AWG with181a chassis limit and 94a transmission limit. This is on the small side.- 125a will make it 'hot'.
-[size=small]53.5mm squared cross-section = 1/0 AWG with245a chassis and 150a transmission limit. This will be OK in my experience and I think even a1 AWG would work.[/size]

I base this on my own experience with
- 4AWG wire (thick strands) of about 6feetrange 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 :)
 
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)
 
slimf said:
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)
Click to expand...
>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 itwon't 'melt' as you turn on the inverter andyou 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.
 
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.
 
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"
 
Thanks for the responses.

I think I will go for 50mm sq. Its 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 :)
 
for that Victron inverter, the manual says 70mm2 for 0-5m cable run, see manual page 9
It's so it performs under surge overloads, etc.

re the pre-charge, I've got 2x 10W 4.7 ohms resistors in parallel - via a 10A DC breaker they feed:
1x Victron Multiplus 1 48V/5000kVA inverter (like yours)
3x Victron 150/60 MPPT charge controllers
I have other in-rush devices for the electronics (each 48V connected device has one):
eg Batrium, 48 to 12V 300W PSU, 48 to 20 V 50W PSU (for laptop)
https://au.element14.com/epcos/b57237s0229m000/thermistor-ntc/dp/9754148?CMP=i-55c5-00001622
 
if you ask you most likely have the money and then you really dont have to ask. Go for 50mm2 and you save energy long term and sleep better :D
 
Hmm.. the manual does recommend 70mm. Thanks for pointing that out.
 
A 5kW unit can pull 8-10Kw and thats what you should design for.
Same goes to fusing. Follow recommended. And THEN you build the battery pack to support that current needed in max conditions.
 
If the 50 sq mm lugs are very expensive, You could always go for 2 times 35 sq. 35 sq mm lugs are reasonable, and so is 35 sq wire.

ChrisD
 
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