Armoured wire for 24V system

[MalcolmS]

Hi All,
I hope you are doing ok.
I have a question regarding the 24V system. I am going to build a pergola and I could install 4 panels on it. The inverter and batteries would be in a building that is about 3-4m away.
What armoured wire should I put in the ground in advance to connect these 4 panels? I have a trench ready.
Best regards

 

[Rockingit]

Does the panel / inverter manufacturer not have a recommended spec? You've a balancing act here of cost v resistance (equals less energy at the end).

 

[MalcolmS]

I come across this table. I reckon I have to work out.

 

[Rockingit]

I come across this table. I reckon I have to work out.

That looks to me like a chart for main loadings for North America, absolutely irrelevant (ish - the physics is the same) to this.

Off the top of my head I would say you're probably looking at 4mm SWA for the best bang:buck ratio but what wattage are the panels?

 

[MalcolmS]

That looks to me like a chart for main loadings for North America, absolutely irrelevant (ish - the physics is the same) to this.

Off the top of my head I would say you're probably looking at 4mm SWA for the best bang:buck ratio but what wattage are the panels?

Thanks for clarifying.
I don't have the panels yet but I home in on JA Solar 460W. In any case aim for panels of this size and wattage.

 

[davesparks]

At 24V it will need to be quite a lot bigger than it would be for 240V as the voltage drop is much more of a issue.

 

[MalcolmS]

At 24V it will need to be quite a lot bigger than it would be for 240V as the voltage drop is much more of a issue.

what do you mean by "bigger"?

 

[newfutile]

At 240 volts a 10v drop won't be much of an issue at 24v it's a big deal,you would have to work out the minimum accepted voltage at full current and use ohms law to work out the best size of cable

 

[Rockingit]

460 W @ 24V = 19A per panel
x 4 = 76A
1% VD

therefore .24v / 76A = 0.00315Ω /m max cable resistance
Closest standard matching cable would be 70mm² @ 0.0029Ω /m

allow 5m run by the time you've done a few bends and terminations, this is DC so applies both ways = 10m
= volt drop of .22V / 0.92%.

This is a very hefty cable - the workaround would be not to group all four panels togethers and take each one individually out to the inverter in maybe 2 x 4c SWA's instead - this would change the maths to 16mm² conductors which is far more pleasant on the pocket and the hands installing it!

 

[davesparks]

what do you mean by "bigger"?

I mean a much larger CSA than if it was at 240V to account for the voltage drop.

 

[MalcolmS]

Ok, thank you for your replies.
Now I understand why they install the equipment in the loft not to have to deal with the Voltage drop.
In this case wouldn't be better to just have two separate systems and merge them at 240V to be able to transfer electricity at longer distances.

It's just a hypothetical question and out of scope of the current discussion but currently I have a 10mm SWA in the ground and this is to bring the electricity to the garden but in the future could this be changed to bring the electricity to the house from PV system? Like a kind of evolution from grid to semi-of-grid.