Balcony Solar Cabling
Wire gauge and routing
With the right size determined planning the circuitry makes sense. Cabling must be calculated. All components must be connected safely and cleanly to each other.
The first part Balcony Solar Dimensioning was about system parameters. The next step is organizing the cables. Determining length is a simple task: measurement. With length and current losses can be calculated. Cabling can be split into 3 major parts:
- From panels to charger
- charger to batteries
- batteries to consumers
A total power of 400W and a system voltage of 36V a maximum current of 11A has to be transferred. Solar cabling comes in 4mm² or 6mm². With a path of 4m which needs to be doubled because of two cables it gives 32mΩ with 4mm² or 20mΩ for 6. It does not make a real difference so I chose 4mm². Which means close to 1.5W more loss.
Since the charger is very close to the batteries I assume roughly half a meter of cable in total. I also had a rest of 10mm². Charging power is not more than 15A (24V system voltage) and resistance is a single digit mΩ value. Easy to deal with.
The inverter's manufacturer recommends at least 10mm². I go up one step and use 16mm². Cable length of a meter is also very short. This yields a resistance of nearly one mΩ. But mind the current which is the highest of the entire system. The specification states a peak load of 1kW. I also add some head room or reserve of 20%. Discharged batteries mean a minimum voltage of 18V. Therefore maximum current could reach 50A which causes losses of 2-3W.
My panels came ready to be plugged with MC-4 connectors. They are rated up to 20A. They are easy to crimp to the solar cables. Going inside is a self made two layer copper foil conductor. I bought a sheet of 0.5mm copper and cut strips of appropriate length and width to reach 6mm². I rolled their ends to a tube and soldered the solar cabling into the tubes. Finally I stacked the two foils between plastic wrap to isolate them from each other and protect them from air and rain. The result is a two layer conductor 1.5mm thick and 7cm wide. No need to drill a hole in a wall or through the door frame.
The battery stack has a lot of connections. On the one hand batteries must be connected in series to reach 24V. on the other different capacities are paralleled to reach the high output current while increasing life time. As mentioned before I use 10mm² for the battery cabling. The middle connection takes 2x3 cables which requires a solid busbar. First I planned to use solid copper bars, drill holes, press or solder cable lugs and screw them onto the bars. A maximum short circuit current of 300A at 24V would weld any connection perfectly, too.
There are busbars easily avaible to be used in distribution boxes. Normally cutouts or circuit breakers are snapped onto them. They already come in stacks of 3 for all phases of a 230V system. It would cost more than 80€ and I'd have to drill a lot of holes and attach many lugs. So I used the ones for grounding which consist of a thick metal bar with many 25mm² holes and a single connection for a 15mm diameter grounding rod. Three of them are necessary to collect all cables for +, - and the middle collector. As a side effect three of these can be mounted according to their potential. The topmost is 24V, middle has 12V and the lowest is 0V/ GND. One costs less than 10€ and cables are simply pushed in and screwed clamps hold them tightly.
The upper and lower busbar with their 15mm diameter connection are the connecting point for the inverter cables.
During regular operation of the inverter delivering 200W I measured a voltage drop of 10mV for a single cable. Voltage difference from charger to battery is barely 1mV at 8A charging current. Losses along the solar cables cause a voltage drop of 3mV with 8A from panels to the charger. Take a certain lack of precision into consideration since I use two meters with a standard error of +/-1mV.
The last piece of cabling is a grounding cable. Follwing the recommendations from the manufacturer's manuals it is best to only ground the inverter. So all grounding happens at the potential of the inverter. I didn't ground the panels frames explicitely. They are screwed onto metal bars which are in contact with the metal rain gutter.