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I am currently installing an (off-grid, battery based) PV system for 5kWp at 48V (resulting in ~100A DC). In order to keep the system extensible in the future (don't want to reinstall cables because it involves damaging walls and floors/ceilings), I have already chosen the cables for 200A, which led me to a cross section of 50mm² according to DIN VDE 0298-4 (for installation in air, practically in a very broad duct in my house).

But my 5kW inverter's PV terminals are only capable of taking up ferrules for like 10mm². My first idea was to go with 50mm² into a 100A circuit breaker and from there go on with 10mm² into the inverter. But then again, I guess that the 100A circuit breaker will also just support 10mm². On the other hand, using a 200A circuit breaker (which could take up the thick cable) would be inappropriate for the 10mm² outgoing cable.

What would be the proper engineering way to reduce the diameter so that I can stick my 50mm² cables into 10mm² terminals?

Meanwhile, I have added a picture of the situation: In the foreground are the PV terminals, into one of which I have stuck a 10mm² ferrule, which just tightly fits. That is consistent with the 50 A the PV input is specified for: I have oversized my PV modules to 100A peak so they supply enough current also in spring and autumn. In the beackground there are the battery terminals, which offer much more space, and which are specified for 110A combined utility and solar charging current.

enter image description here

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  • \$\begingroup\$ Why not locate the inverter right next to the batteries? \$\endgroup\$
    – bobflux
    Commented Mar 12, 2022 at 9:36
  • \$\begingroup\$ It will be. But the problem are not the batteries but the solar cells. The batteries have screw terminals with lots of space around them, which I can access with M6 50mm2 ring cable shoes. While the PV terminals are for sticking in rather small ferrules (if it's not 10mm² then maybe max 25mm² but definitely not 50mm²). I don't know why they made them so small, but that's how it is. \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 9:41
  • \$\begingroup\$ An alternative would be to solder in bigger terminals, but I clearly want to avoid that for reasons of warranty. \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 9:42
  • \$\begingroup\$ My bad, I thought you used the solar panels in series. \$\endgroup\$
    – bobflux
    Commented Mar 12, 2022 at 9:43
  • \$\begingroup\$ In Series (partly) with each other, but not with that battery. The inverter is also a charger, so I connect PV and Inverter on one set of terminals, and battery and inverter on another set of terminals. \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 9:46

4 Answers 4

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Not sure if it is the best way to do it, but I think you could just use 2 ring cable shoes (one for 10mm², one for 50mm²) and push them together using bolt and nut.

Or if you say that 10mm² are enough for 100A (I haven't checked, it also depends a lot on the voltage drop you allow), and you want to prepare for 200A in future, then you could also just install a second 10mm² cable that is not used for now.

If in future you double your solar pannels, you can just use the second cable for the new ones, and connect both cables at the inverter.

Pros :

  • no longer a problem switching section
  • you can keep your 2 solar pannel assemblies separated if you which (for example injectig the second assembly directly into the main grid (ie reselling the energy) while keeping the first one with storage for self consumption
  • in case one cable is damaged, it's easy to switch cable (as long as you haven't doubled your solar pannels), or you keep at least half the production untils repaired (if you already doubled)

Cons :

  • you don't bennefit from the lower voltage drop (as you would with the 50mm² before doubling section)
  • it might be more expensive (to be checked : a 50mm² cable is 2.5 times more copper, so it might not be more expensive)
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  • \$\begingroup\$ The double cable shoe approach is pretty interesting, improvisation-wise. I wonder how I could take care of isolation. I am not sure if heat-shrink tubing is specified for such high currents (which are associated with a higher fire risk or probably higher temperature load). \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 11:36
  • \$\begingroup\$ As to the question if 10mm² is enough for 100A: it is not, according to the standards I have mentioned. The 10mm² on the PV input terminals of the inverter is due to the inverter being specified for only 50A of PV current. I have oversized my solar cell configuration to 100A, so that in spring and autumn I have still enough power on average to charge my batteries/supply my daily energy demand. The 200A target does, of course, require a different inverter in the future (the one I have chosen for now was pretty cheap, that is my way of doing rolling-wave planning :-) ) \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 11:41
  • \$\begingroup\$ As to the doubling the cable count: interesting idea, but I have already ordered the 50mm². \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 11:42
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    \$\begingroup\$ For insulation : to retract heat-shrink tube, you usually need to go above 100°C, so it withstands this temperature without problem. And if you get your wire above 100°C, it means you already have a big problem. As an additionnal security, you could just cut your 2 wires at different length (for example 20cm) : this way, the 2 connexions can't touch each other even if the insulation should fail. (for quick and dirty hobby projects, I use this trick without any insulation, but of course, for high current applications at home, adding insulation is required) \$\endgroup\$
    – Sandro
    Commented Mar 12, 2022 at 11:54
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For now you may reduce 50 mm² cable to 25 mm² using the following:

  1. 200 A bus bar terminals

    Image credit - westmarine.com

enter image description here

  1. 50 mm² - M8 and 25 mm² - M8 lugs

    Image credit - moglix.ae

enter image description here

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  • \$\begingroup\$ I have already ordered M6/50mm² lugs. But the 200 A bus bars aren't critical at the moment, since the 25mm² side is only capable for 100 A anyway. So I will look for 100 A M6 bus bars, which I will be replacing probably somewhere in 2027. \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 14:12
  • \$\begingroup\$ Hi Oliver, Thanks for the info. \$\endgroup\$
    – vu2nan
    Commented Mar 12, 2022 at 15:37
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Could you use a bus bar like this or this to step down from 50mm to 10mm?

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  • \$\begingroup\$ That is a good idea I guess. I admit that I have thought about it, but I was unsure if that would be considered a "standard" way of solving such an issue (or if my problem was kind of a "standard issue" at all). \$\endgroup\$
    – oliver
    Commented Mar 12, 2022 at 11:32
  • \$\begingroup\$ Thomas Burns - Hi, Link-only answers do not count as actual answers on Stack Exchange since if the links die in future, the answer then becomes useless :-( Therefore to avoid this answer being deleted (it is already being flagged by other site members) please can you edit it & add at least specific relevant keywords and images of the suggested solution, to help people find them if the links do die? (Please keep the source links for the webpages in your answer, to satisfy the site rule about referencing of images etc.) Thanks. \$\endgroup\$
    – SamGibson ♦
    Commented Mar 12, 2022 at 11:53
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My first idea was to go with 50mm² into a 100A circuit breaker and from there go on with 10mm² into the inverter. But then again, I guess that the 100A circuit breaker will also just support 10mm².

Breakers commonly support larger dimensions. The reason for this is simple: the incoming wire has to be rated for the upstream breaker. If we look at for instance this breaker from Schneider it supports up to 50mm^2 wire.

You can also find terminals (such as this wdu 50N which supports 10-70mm^2 wire. This means you'll have to replace a short section of wire when upgrading, but the installed cable can up to 70mm^2.

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