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I have two systems connected to 1500W power inverter. Per case as shown below, what size (4mm, 5mm, 6mm ....) cable should I buy?

Note: Based on my researches, the smaller ah output the thinner cable thus the system becomes more reliable, safer and cheaper etc. It is also suggested not to go above 83ah, if possible.

  • Case 1) One 12V 120ah battery. Output is 12V 120ah. 1500W/12V=125amp
  • Case 2) Two 12V 65ah battery (wired in series). Output is 24V 65ah. 1500W/24V=65amp
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    \$\begingroup\$ You're mixing up your units. 'Ah' is capacity. 'A' is output current. (Note capital A for ampere.) There are plenty of web resources on cable sizing that you should research before asking a question here. You have omitted any details on cable length and the voltage drop on a high current system will be more important than the cable heating, I suspect. \$\endgroup\$ – Transistor Jul 13 '17 at 16:33
  • \$\begingroup\$ I've gone thru many resources and finally got confused as some of them contradict each other hence reason I had to come here. Another thing is, it is easy to mix up small but important things like A, ah, amp etc. for people who doesn't really come from particular background. I know I could do better than that. Anyway, cable length will be about 2 meters. \$\endgroup\$ – BentCoder Jul 13 '17 at 16:39
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Ampere-hours [Ah] is a charge. It says nothing about power, so you cannot determine your cable sizes from that. Batteries have a "12 Voltage, 120 Ah capacity" value. Don't mix that up with amperes [A]. They are not the same.

Your 1500W inverter will draw up to 125A from 12V, or 62.5A from 24V. It is generally adviseable to have lower currents because of smaller losses and easier construction, as long as you don't leave the low voltage range. (I would even advise to get your hands on a 48V inverter and battery set to drop the losses even lower.)

A rule of thumb is at minimum 0.1mm² of cross section per ampere of current. The lower the system voltage and the longer the cables, the more cross section per ampere you want to reduce the voltage drop. I recommend at least 0.2mm²/A for the 24V system, and 0.3mm²/A for the 12V system. (Same: don't mix up cross section [mm²] with diameter [mm]. They are not the same.)

So, I recommend:

  • 4mm² for 31.25A@48V
  • 12mm² for 62.5A@24V
  • 35mm² for 125A@12V

Short cables may be much thinner.

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  • \$\begingroup\$ If what you have suggested is the minimum area, then if the length increases the area should as well since the losses are proportional the the square of the current multiplied by the resistance. \$\endgroup\$ – Solar Mike Jul 13 '17 at 16:40
  • \$\begingroup\$ It's a rule of thumb which is good for cables in the 10m range. One wouldn't normally place the inverter much further away from the batteries and the solar panels. \$\endgroup\$ – Janka Jul 13 '17 at 16:43
  • \$\begingroup\$ So looks like I should get 4mm cable for 24V system. Some useful info there though. Thanks. \$\endgroup\$ – BentCoder Jul 13 '17 at 16:44
  • \$\begingroup\$ Check if it's 4mm diameter or 4mm² cross section. Usually, cables are advertised with cross section. \$\endgroup\$ – Janka Jul 13 '17 at 16:45
  • \$\begingroup\$ @Janka 10m was fine for the panels on the house I built and the power comes down at 450vDC to keep the cable size and the losses down... But other systems needed cable runs of 60m or more so the length and losses were calculated very carefully... \$\endgroup\$ – Solar Mike Jul 13 '17 at 16:57

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