help with wiring for 12v system

DIY
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Hello everyone! This is my first post and I signed up to clarify a doubt. I’m a newbie to this and I need to install a small solar system for a cabin that is on a family plot where I will spend 2 weeks a month for a period of several months intermittently. The system will be for a 45w fan, two 6w LED bulbs and a 55w mini freezer. The mini freezer is an inverter and is off most of the time. I have used it before. It is a 12v system and here are the components.

4 x 100w solar panels
BlueSolar MPPT 150/70
Phoenix 12/1200
200ah lifepo4 battery

Most likely I will connect two 200w series and then connect them in parallel so I would be getting 40v and 10a approximately. I would use a 16a breaker. I can also connect all 4 panels in series to get about 80v and 5a and would use a 10a breaker (these values ​​are not real but it is an approximation of what I could generate with good sun)

I would use 10awg cable for a distance of 25 feet from the panels to the controller.

Now, here is where I have the serious problem. Due to the structure of the cabin the distance between the controller and the battery is approximately 18 feet. I read the manual and the controller should not be connected more than 5-6 feet away from the battery. I cannot raise the system to 24v because everything is already purchased and cannot be returned. And if I wanted to put the battery 5 feet away I have to break a wall in the room and the work and expense in construction materials would be very high.

I saw that through the Victron Connect App I can set the charging values ​​that the manufacturer requires and I can also limit the amps that comes out of the controller. I was thinking of limiting the load to 35a. I bought the 70a controller because I really want to use half of its capacity the same way I bought a 1200 inverter to use less than half of its capacity. I don’t know what cable to use because I don’t understand the voltage drop very well. My biggest concern is not getting the terminals hot and avoiding a fire. What would you recommend? Can I use 8-6awg cable for 18 feet of distance limiting the amps to 35 or should I use 4awg cable? Any other information would be great. I’m still reading and educating myself but getting help would be great to get my project up and running as quickly as possible. Excuse the vocabulary but I hate gas emissions and I’m using a 1200w inverter generator and the smell of gasoline is already bothering me.

Thanks in advance.

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For lead acid batteries the controller should be close to the batteries so they are at the same temperature as the controller assumes they are for temperature compensation. For lithium temperature compensation is not required so they can be spaced further apart. You can download an app called the Victron Toolkit and this will calculate voltage drop or look on line. You ideally need at max current 0.3V drop. For a distance of 18ft between battery and controller use 4awg wire for 0.3V drop at 35A.

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Thanks for your quick reply. So 4awg cable is the best option for my situation. I wanted to ask you two questions:

  1. Is there a formula to know which breaker I’m going to use? I’m going to use double pole breakers for all the system. I was thinking that if I do a series/parallel panel configuration (~40v 10a) I should use a 16a breaker and if I’m going to use the series configuration (~80v 5a) I should use a 10a breaker. Likewise, if I’m going to use 4awg cable from MPPT to battery the breaker would be 40-50a because I would be limiting the controller to 35a or the breaker must be 100a which is what the 4awg cable would carry?

  2. Since this is the first time I use Victron, is the load limitation via Victron Connect App safe? Will it always be at 35a even if there is a software update, or, for example, every time I turn it off and on for maintenance, do I have to configure everything again?

I know that 400w will never generate 70a even if I connect everything in series to have approximately 80v with good sun but I’m very worried about the overheating of the terminals and cables.

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Further help can be found in the Victron guide, “Wiring Unlimited”, which can be found at https://www.victronenergy.com/blog/2019/09/03/wiring-unlimited/

Typically the breaker needs to be rated at a minimum of 125% of continuous current to stop it getting too warm. For the controller to the battery the breaker could be anywhere from 45 to 80A. The cable will not get hot as it is large for the given load, being based on voltage drop.

My controller retains settings during firmware updates but they should always be checked afterwards.

1 Like
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Thanks you so much!