Cable sizes again!

[syvictoria]

Hi! I have an old Steca Solarix Gamma 12A Solar Charge Controller which I'm going to connect up to a Solar Trader 32W flexible panel to charge a pair of 6v Trojan T105s. The Steca's manual recommends 10mm2 / 8 AWG cable to both the panel (max 10m) and the battery (max 2m). My distances will be about half the maximum for both cables. 10mm2 seems quite large, and the factory fitted cable on the panel that I will be joining to is quite a bit smaller. Can I use a smaller cable, particularly for the panel connection? I'm not too worried about the battery to Steca cable as this will only be about 1m long, but for the Steca to panel cable, surely it's a little pointless to connect a larger cable to the smaller 30cm or so length at the panel end?

Also, my Trojans came with a low profile thread type terminal. Does anyone have any recommendations for the best type of nut to use on this?

Thank you, and sorry for posting yet more dumb questions!

 

[nigelmercier]

For this size of panel, you could use a smaller cable. But would you remember to upgrade it if you fitted a larger one into the same controller?

 

[syvictoria]

I will eventually be connecting two further 32W flexis...

 

[rob2]

Joining the three 32W panels to a larger feeder makes sense. The larger the conductor the lower the losses, which seems realistic for solar power in the UK! The other thing to consider is check whether the lengths quoted against cable size is for the distance between equipment or there and back - most measure both conductors, so a 5m twin core cable counts as 10m.

Rob.

 

[noelex]

Your three panels will put out a little more than 6a. The total round trip distance is 5+1 =6x2=12m.
The voltage loss for 10 swg. (6mm) @6A is 0.51v. This is a little high. So 10swg is minimium the next size up would be better.
8 swg (10mm) gives a loss of 0.29v for the same current and distance.
There is nothing inappropriate about connecting a thick wire to thin one. It's all about voltage drop which is small in a short section of thin wire, but not in a long run of thin wire.

It's worth measuring the distance carefully. The wire is often longer than looks by the time it has been fed around obstructions. The above calculations are for a distance of 6m between the battery and panel.

 

[lw395]

Voltage drop in a solar panel circuit is not as critical as in for instance an alternator circuit, because the solar panel has quite a high open circuit voltage.
You have to look at the v/I curve for the panel at a particular light level to see what the current would be into the load with the two different cable resistances.
I would expect the change from 0.51v drop to 0.29 to be less than 1% in current.
And of course, once the controller is limiting things it will make no difference at all.

Whereas an extra 0.2V drop in an alternator circuit would seriously reduce charging.

You should take the temperature rise of the cable into consideration if it is going to be well insulated thermally, but it's usually not an issue in 12V systems.

 

[syvictoria]

Thanks very much for the replies - much appreciated.