How to find out Voltage drop ?

Voltage drop example question

You have a PV array in a field and an inverter at a house 250 feet away. You are

using 10AWG stranded copper wire. There is just one string and the voltage of

the string operates at Vmp of 200V and an Imp of 5A. What is the percentage

power loss due to dc voltage drop?

Answer

Voltage drop is calculated with Ohm’s Law, which states:

                                  Vdrop = I X R

We know that current is I = 5A

We will solve for resistance by multiplying:

ohm / kFT X kFT = ohm

The distance from the PV to the inverter is 250 feet, so there will be two wires

to complete the circuit, therefore the wire length is:

                        250 feet X 2 directions = 500 feet

                      500 feet / 1000 feet per kFT = 0.5kFT

                                  Distance = 0.5kFT

From looking up the properties of 10AWG stranded copper wire, we get

1.24 ohms/kFT, so:

                              ohm / kFT X kFT = ohm

                      1.24 ohms / kFT X 0.5 kFT = 0.62 ohm

                               Resistance = 0.62 ohm

Now back to Ohm’s Law:

                                     Vdrop = I X R

                             Vdrop = 5A X 0.62 ohm

                                 Vdrop = 3.1 volts

We now know that during peak sun conditions, we are losing 3.1 volts on the

wire and if we measured the voltage at the array, it would be 3.1 volts higher

than at the inverter.

To figure out our losses as a percentage we will divide voltage drop by system

voltage and then turn that into a percentage.

          (Vdrop / system voltage) X 100% = voltage drop percentage

                       (3.1V / 200V) X 100% = 1.55% voltage drop

Since voltage multiplied by current is power, then voltage drop percentage is the

same as power loss, so we are losing 1.55% of our power in this case.

           Answer to voltage drop question: 1.55% power loss at STC

                                          table use for ohm/KFT on wire.