MPPT Cable Size Confusion

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kernowsolar asked Jul 20, '21

MPPT Cable Size Confusion

Hello, I have purchased a Victron Blue Solar 100/30 MPPT. In the manual it says to use 10mm2 cable to connect the solar panels to the controller and the controller to the battery bank.

I currently have a Tracer 40amp MPPT , on a 24 volt system, (5 panels x 250 watt) with 440ah lead acid batteries, connected to a Victron 800 watt Inverter

The panels are connected to the MPPT controller with 6mm2 cables , which are rated to 79amps

Why do I need 10mm2 cables, can I not just use my current 6mm2 cables and swap over the controller like for like?

I use 16mm2 cable to connect the inverter to the battery.

Any help would be greatly appreciated

MPPT Controllers

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michelg answered · Jul 20, 2021 at 05:43 PM

Hi Kernowsolar, My setup is using also 6mm2 and carrying 3KWp, but at 105VDC to the mppt, and from it I use 35mm2 cable. According to your setup, you should be fine with the current cabling. The recommendation is given to avoid maximum losses from the PVs to the mppt.

Michel.

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Answer 1 · 2021-07-21T16:52:46Z

kernowsolar answered · Jul 21, 2021 at 04:52 PM

Hello Michelg, that is reassuring, thank you for your reply

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Answer 2 · 2021-07-22T21:32:19Z

justmurph answered · Jul 22, 2021 at 09:32 PM

The short answer is that if it’s working fine now, then it will work much the same on the new controller.

The longer answer is that it entirely depends on how your panels are configured (series or parallel) and the maximum current they’re putting out. In series, you’re laughing, in parallel (depending on how it‘s wired) you could be seeing considerable losses on a 6mm cable. I certainly wouldn’t be putting 79A through one.

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Answer 3 · 2021-07-31T10:45:33Z

mort answered · Jul 31, 2021 at 10:45 AM

kernowsolar, you can look at a voltage drop calculator to answer that sort of question - https://www.jcalc.net/voltage-drop-calculator-as3008 is very good and meets the Australian standard for installations.

The current rating of 79A assigned to the cable is a maximum, and you have to remember that for each metre of length you will lose some voltage - it is that loss that you have to calculate for each installation. The more power (lost watts = voltage drop x amps) you lose in the cable, the less that is available to charge your batteries.

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