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Cable Size Difference for 12V/3KVA

I was looking at the recommended cable size in the manuals for the MultiPlus 12/3000, the Phoenix Inverter 12/3000 and the Phoenix Inverter Smart 12/3000 (230VAC versions). All of them have 3KVA inverters but the recommended cable sizes are different.


Multiplus manual recommends 2 x 50mm2

Phoenix Inverter recommends 1 x 90mm2

Phoenix Inverter Smart recommends 2 x 95mm2 (with a note saying one cable must be able to carry full fuse rated current without overheating)

All 3 recommend a 400A fuse


Is the recommendation for the Phoenix Smart a bit over kill or is there a good reason for the much larger cable requirement? The 95mm2 cable doesn't fit into the cut outs in the Phoenix Inverter Smart body, the small plastic cut out liners need to be removed in order to fit 95mm2 cable into the inverter. The 95mm2 cable not fitting the cut outs and being double the recommendation for the other two devices seems strange. Can anyone let me know why the Phoenix Inverter Smart requires the extra cable size?

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Could anyone from Victron confirm if this is correct or if it is a typo in the manual please? Are 2 x 95mm2 cables required for the Phoenix Inverter Smart 3KVA. 95mm2 is not the cheapest cable so I don't want to be using it if it is not required.

@Guy Stewart maybe you would know? Thanks

2 Answers
seb71 avatar image
seb71 answered ·

I think 2x95mm2 (for 3000VA/12V) it's a typo and should be 2x50mm2.


Edit:
Although, yeah, when using two wires in parallel and one fuse for both is problematic.

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Thanks Seb, I was also thinking that it might be a typo, maybe if someone from Victron is reading this could you please confirm if this is a typo? Thank you

Ulrich avatar image
Ulrich answered ·

Hello,

There are several cable cross-section calculators on the network.

An example:
A cable with a length of 2m, 150A load at 13.2V (1980VA ~ 1600W) has a loss of
10qmm: 8.2% = 12.13V
16qmm: 5.08% = 12.53V
25qmm : 3,26% = 12,77V
32qmm: 2.5% = 12.87V
50qmm: 1.59% = 12.99V
90qmm: 0.91% = 13.08V

The loss is converted into heat. If the cable was only 1qmm, it would burn at 150A.

My German source: https://campofant.com/kabelquerschnitt-leitungsquerschnitt-berechnen/

I hope you can now calculate everything yourself.

greetings

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Hi Ulrich,


Thank you for your reply. I understand that cables have certain current ratings and associated voltage drops, that is not really the issue that I am raising. I'm trying to understand why in the Multiplus manual 2 x 50mm2 was acceptable for a 3KVA Multiplus but for the Phoenix Inverter Smart 3KVA it now requires 2 x 95mm2 according to the manual.


Both units are rated at 3KVA which is about 250A in a 12V system. Peak power is 6KVA which is about 500A in a 12V system. The 50mm2 cable I use is rated for 345A so I understand that the 2 x 50mm2 cable can carry 690A which is fine for the 3KVA Multiplus.


The 95mm2 cable I use is rated for 500A. 2 x 95mm2 cable can carry 1000A. The 3KVA in normal or peak operation is not going to demand 1000A so I'm trying to figure out the reason that 2 x 95mm2 is recommended when 2 x 50mm2 is sufficient for the MultiPlus. (I'm talking about cable lengths less than 5m)

Don't forget the length makes a big difference.
Example:
2m length, 25qmm, 13.2V, 150A = 3.26% loss = ok
4m length, 25qmm, 13.2V, 150A = 6.44% loss = not ok
2m length, 25qmm, 13.2V, 300A = 6.44% loss = not ok

That is why there are so different statements.

The peak load is only very short-term. However, if this should really work, the voltage must not drop too much, otherwise the inverter switches off due to undervoltage.

How long is the cable between your inverter and the battery? How high is your load that you need over several minutes?

(Please note that when using two batteries in parallel, the battery cables to the bus bar must be exactly the same length! And the inverter is then also connected to the bus bar.)