Length of DC cable run

[Morgan keith]

Hi guys. When using a DC volt drop calculator, when it comes to the length of DC cable run,does that refer to the distance of the panels from the inverter or the distance of the cable from the - terminal of the inverter to the positive. As in the length of the single core circuit from positive to negative??

Thanks in advance

 

[sedgy34]

what??????? just dont exceed 30 meters on 4mm

 

[moggy1968]

I take it as the distance from the last panel on the circuit (+&-)to the inverter

 

[Worcester]

We take it as the TOTAL length of the DC cable in a string, so 2 x 900mm per panel, plus the run out (-ve) plus the run back (+ve) there again we've always been conservative on all our calcs ...

 

[Morgan keith]

Thanks guys. It is a bit confusing. I was kind of thinking along the lines of Worcester but then thought than on an AC circuit you'd go with moggy1968's idea. Sedgy I agree, but I think the insulation will evaporate in a second on a 4mm from a 16 string combiner box haha

 

[moggy1968]

yep, you don't include the length of the neutral on an AC circuit so why would you consider the length of the -ve on the DC?

imagine the sort of runs we've considered elsewhere, 300m+. that would end up being very expensive if you count the return path!

 

[grantr37]

wouldn't worry about the voltage drop for such tiny distances that are involved with Solar etc...

 

[yorkshiremike]

For DC, you're meant to consider the total cable length = positive + negative, because the 'same' current has to flow through both. With AC, the 'neutral' is essentially at earth potential, so only consider the voltage drop in the 'active' live line.
But it depends what voltage drop calculator you're using. For example in Sunny design, you only specify the single cable length for both AC and DC, but if you look in the project summary, you'll see that the program has doubled up the DC cable length you specified, when it calculates the voltage drop, whereas the AC is left as the single length value (even for three phase)

 

[Worcester]

Our DC runs are often 150m - 200m plus round trip ... hence I've got a warehouse full of 6mm2 ( ) Our next job the runs are 45m out and back, total lengths therfore in the order of 150m or so per string, so yes I DO concern myself with the length and size of the cable.