Voltage drop calculator

simagic

So I'm looking at .... http://www.solar-guppy.com/forum/dow...calculator.zip to understand my voltage drop in the MC4 cable but am a bit confused. On line 30, which is #9, it speaks of figuring the length of the run. As i am not ......"exactly sure" .....( and there would be a big big difference if I didn't get this right), my question is.......If my panel is 35 feet away from the controller, do I enter 35 feet in line 7 or double that and put 70 feet . Also where it says "PANEL AMPS",do I enter IMP (max amps) or ISC ( short circuit amps). And where it says "PANEL VOLTS", do I enter VMP (max volts) or VOC (open circuit volts). And lastly "BATTERY VOLTAGE". Line 29 ( number eight) says just enter 12 volts ( so, I got that).. The biggest issue, I would think, would be the 35 vs 70 feet issue.. Thanks. I didn't see this question asked anywhere else. Could I be the only one who this isn't clear to?????.. thanks

Cariboocoot

Re: Voltage drop calculator

The idea here is to see what is the maximum stress the wire will be under. Since peak panel output is Imp @ Vmp, that's the current and Voltage you want to use. A panel at Voc produces basically no current, and a panel at Isc produces basically no Voltage.

The wire length is the full distance, to and from, for the complete length of the circuit.

niel

Re: Voltage drop calculator

you have 2 wires going 35 feet so add them together for 70 feet total.
actually that voltage was nominal. even the vmp may not be correct to enter as the voltage could be dropped further with mppt controllers. remember that a higher voltage will show a lower v drop so if i were to have a pv with a vmp at say 18v that i would want to enter 12v rather than 18. it will not be maintaining the rated 18v all of the time to use that figure and using that 18v figure would give a better v drop % than may be actually voltage experienced. i know it is somewhat confusing, but solar is a bit variable and to charge 12v batteries we have 12v pvs that actually have general vmps in the area of 17-19v. the voltage gets pulled downward for many reasons and is why the extra voltage is needed. 24v pvs would typically be seen as 34v to 38v on their vmp. of course with an mppt controller the pvs don't need to be in those ranges set for good operation with pwm ccs as it could go higher, but it would still be good to use the nominal pv voltage by asking yourself if say a 26.8v vmp on a pv would be good to charge a 24v battery with. it is not, so i would round it downward to a 12v nominal pv with just extra voltage headroom.

it was said elsewhere, but i don't mind saying it again as there is quite a bit of reading there.