I am slightly confused as to why it states 70mm2 cable connection when the charger is rated at 70amp. I am off grid and sourcing 70mm2 is nearly impossible. My battery bank 3x Polyontech us5000 states 25mm cables which I can get.
I will probably parallel another 48/5000 later so the cable sizes are important
Any comments welcome as I might have miss read the documents
It does seem a bit large, but Its all based on distance, the longer the run the bigger the diameter. I have been running mine on 35mmsq when the PV Ac coupled is pushing max watts and the Multi is pumping 65Amps to the battery the cable gets luke warm. Ambient's have been around the mid 20s and the length is around half a meter to the bus bar, where two battery banks connect.(Also 35mmsq)
I have just over night upgraded to a Lynx unit and rerun all the cables through 100mm duct, to tidy up the install. I have also upgraded to 50mmsq as the lengths were just under a meter each as I am parrallelng two units now. Our Ambient's do get to 45 mid summer, hence the second unit to cope with the load and a cable upgrade for same reason.
Hope this helps.
Your multi will draw much more than its charging current from the battery. Around 100A. Your batteries will take one third each. Cable length, voltage drop and heat are also important.
If you wire the Pylons with their standard layout (battery to battery to battery to multi) you are on the limit of those cables and they will get hot.
That said, I've used 50mm and with full AC charging and an MPPT running about 150A there's only moderate warming.
@Boatman i wouldnt be using pylontechs guidance on the connec to the multiplus. Victron states clearly 70mm2 or if like me you couldn’t get a good 70mm2 go for the 120mm2 which is also recommended by Victron you don’t want to undersize DC cable it can go wrong pretty fast
Also the pylon cables have a special insulation on them that allow them to run hot without fear of fire. Check their amp rating not diameter..
The same goes for all cables. 70mm fine strand has more current ability that coarse strand.
But heat means energy lost so all in all not desired.
Just to add my 2p worth - the Multiplus-II can run continually at just over 100A of battery current but in an off grid setup I believe it can peak at 9000w which is somewhere around 180A. It can only sustain 9000w for a very short time (15 seconds maybe?) but I think this is why the recommended fuse rating is 200A.
Also, since the inverter fuse is 200A, really you should be using wire that can handle a fault current at that level so 25mm2 isn't really sufficient, even if you used the same high temperature insulation that Pylontech use on their cables.
Lastly, as @Alexandra said, any heat in the cables is energy that you are losing so my view is to use the largest cable which is supported by the inverter. In this case that is 70mm2. I wouldn't recommend cable larger than 70mm2 because you'll have difficulty getting it onto the battery terminals on the inverter and you risk placing too much stress on those terminals and breaking them.
I bought my cable here in the UK from here ( https://www.12voltplanet.co.uk/extra-flexible-tinned-copper-pvc-battery-cable-70mm-485a.html ) and they say they can offer international delivery so if you are stuck then it might be an option. Note that it's important to use fine strand cable rather than solid core or multi-core in this application. HTH
Yes 70mm2 is for 0-5m and 120mm2 is for 5-10m but I would hope that most installations can manage to keep under the 5m length. In my own installation my 70mm2 cables from bus bar to inverter are probably around 400mm and yet they still warm up a bit at full load. I think most of this heat comes from the connections to the isolator switch though.
Keep to the letter of the victron manual then you can't go wrong i struggled to get 70mm2 so i said to the Victron suppler that i could get 90mm2 which would be better than the 70mm2 i was told nope only the two sized cables are approved 70mm2 and 120mm2 so i opted for the latter.
The latter is not without complication which can be fixed easy enough by shaving down the sides of the eyelets that connect the cable to the Multiplus2, @Craig Chamberlain is correct the cable is heavy but as long as a clamp is placed on the wire so it isn't hanging free weight of the entire cable then there is no issues using this cable size as its as spec by victron just a bit more work and a massive crimping tool needed lol.
looped then clamped to take the weight,
Good timing, I came here with the same question. The Quattro manual has the same specs. When I do a VDI calc using 200 amps, 10 ft, 50 volts, and 2% voltage drop, the result is to use 4 AWG wire, which has a CSA of 21.2 mm2. The VE manual seems very conservative. VDI calc from https://www.altestore.com/diy-solar-resources/wire-sizing-tool-for-12-24-and-48-volt-dc-systems/
Part of the reduction is the difference between 10 ft and 5 m (16.5 ft), but not that much.
The inverter type and design also affects wire gauge choice not always the amps for perceived 'loads'.
Look at it this way, if u are pulling 5000w(48/5000 will easily do this)and you use a cable with a 2% loss , you effectively have a 100w heating element in the form of your cable.
Personally that is not acceptable to me.
I have 48/5000 mp with 70mm approx 700mm in lenght and at full chat (@ 60v - 18S lipo4)the cables get slightly warm. Apart from full chat cables remain at virtually ambient temp.
Yes a smaller cable would more than likely survive, but would heat up considerably more - choice is up to the individual.
The 100 W heating element description is very good, it finally makes sense. Thank you.
Any % losses are generally in the form of heat, and heat generally is not to the good or longevity of electrical circuits.
Thanks for your answers, it is nice to have second opinions. I am installing a "full blown" off grid system for a 5 bed house and It sometimes gets complicated
It's not complicated when broken down into the individual parts, gets a bit more tricky integrating the individual parts together.
Grasp the basics before trying to run or a plethora of unneeded bits will be left, once you realise you have the wrong bits.
For instance I thought a ANL 200A fuse was a suitable fuse, but oh no should be a class T , oh well lesson learnt and ANL fuse sold on fleebay.
So for information
min 8 hours sun per day 365 days a year almost guaranteed
10 x 550 watt panels
all panels individually fused with 20amp in a distribution box.
Feeding 2 x 250/100 mppt tr chargers
4 x pythonteck us5000 batteries and.......
2 x 48/5000/70 inverters
Charger function on 48/5000 is turned off
And of course a GX colour control (ha ha)
Given you have three batteries in parallel and can source 25mm2 then the solution, imho, is simply to wire each battery to the MultiPlus with its own 25mm2 wires, preferably all of equal length. You may need a small bus bar at the MultiPlus end.
