Parallel cables on SunSynk batteries and inverter (UK)

[improper3100]

I currently have an 8kW SunSynk that I've hooked up and am switching to a 16kW SunSynk soon. I have 11kW of Solar panels and 6 x 5.12kWh CATL SunSynk batteries.

The cables the batteries came with are 25mm2 copper, and I can't seem to find any bigger than this.

So right now my inverter is set to take 100A from and to the batteries, which at 48v is only 4800W.

I saw in the SunSynk manual that you can double up on the cables and get 200A out, but I don't really get the maths on this. I know when you do a radial circuit in the UK with 2.5mm2 cable, you can use a 20A breaker, but when you double up and make a ring, it doesn't go to 40A, you can only go to 32A... so how is 2 x 25mm2 cable = 200A?

Can someone help with the math on this? One site I looked at said you'd need 85mm2 conductors. Appreciate your help!

 

[SeaGal]

The cables the batteries came with are 25mm2 copper, and I can't seem to find any bigger than this.

Plenty of suppliers in UK with larger core cables, 35, 40 or 50mm+.
like bimble solar for example

35mm² Premade Cables

www.bimblesolar.com

50mm² Premade Cables

www.bimblesolar.com

I saw in the SunSynk manual that you can double up on the cables and get 200A out, but I don't really get the maths on this.

Yes, but better to use bigger core cable IMHO. Then no issue with a) contact resistances of lugs on lugs and b) you can't have the situation where one cable got disconnected and all load is goes through the remaining cable.

You will need suitable fuses to protect the cables you choose.

I know when you do a radial circuit in the UK with 2.5mm2 cable, you can use a 20A breaker, but when you double up and make a ring, it doesn't go to 40A, you can only go to 32A... so how is 2 x 25mm2 cable = 200A?

Can someone help with the math on this?

2 x 25mm2 is equivalent to 1 x 50mm2, assuming cable of same material / resistance.

I'm not a sparky, but the key maths to your question is that a ring circuit is not the same as having 2 parallel wires of equal distance. The current flow from a socket on a ring mains will not evenly use both sides of the circuit unless it is exactly half-way round the ring. There are other considerations as well, with regard to loading on house wiring, such as whether the cable is surface mounted, embedded in plaster or buried under insulation. Hence the conservative common usage of a 32A MCB on a 2.5mm2 T&E ring mains circuit.

Here's some more reading for you, with regard to your battery cable wiring - may be of interest...

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

 

[improper3100]

That's a really fantastic response. Clearly explained, and very helpful - thank you so much!

Plenty of suppliers in UK with larger core cables, 35, 40 or 50mm+.
like bimble solar for example

35mm² Premade Cables

www.bimblesolar.com

Yes, but better to use bigger core cable IMHO. Then no issue with contact resistances and if one cable got disconnected, all load is going through half the diameter cable.


2 x 25mm2 is equivalent to 1 x 50mm2, assuming cable of same resistance.

A ring circuit is not the same as 2 parallel wires of same distance. The load on a socket on a ring mains is not evenly distributed unless it is exactly half-way round the ring.

Here's some reading for you...

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

 

[SeaGal]

btw... I just edited / added to it a bit