Voltage drop on a string with different wire lengths

[Zyb]

Hi,

just want to make sure that i got this right. in Voltage drop calculation V=2(D)(I)(R), the number 2 is constant to multiply D (one way length) in order to get the whole length of the circuit. can someone confirm if my calculation on the attached image is correct. Thank you!

 

[ggunn]

Hi,

just want to make sure that i got this right. in Voltage drop calculation V=2(D)(I)(R), the number 2 is constant to multiply D (one way length) in order to get the whole length of the circuit. can someone confirm if my calculation on the attached image is correct. Thank you!

No. Vd is calculated separately in the wire runs being measured; you have Vd in the 200' of #8 and a separate Vd in the total length of the other wiring, and you add them together. You do not add 200' to the Vd calculation in the smaller wire or the length of the smaller wire to the 200' of #8.

 

[wwhitney]

just want to make sure that i got this right. in Voltage drop calculation V=2(D)(I)(R), the number 2 is constant to multiply D (one way length) in order to get the whole length of the circuit.

Correct.

can someone confirm if my calculation on the attached image is correct. Thank you!

Yes, your calculation is correct. It's just resistors in series, so if you like you can add up the (length) * (resistance per length) for each wire segment to get the total resistance R, and then use V = I * R.

Cheers, Wayne

 

[ggunn]

Yes, your calculation is correct.

Maybe I misread his calcs, but wasn't he calculating Vd in the PV wire based on the entire length of all the wiring? It should be Vd in the #8 calculated on the length of #8, plus Vd in the PV wire based on the length of the PV wire, plus Vd in the trunk cable based on the length of the trunk cable.

 

[jaggedben]

Hi,

just want to make sure that i got this right. in Voltage drop calculation V=2(D)(I)(R), the number 2 is constant to multiply D (one way length) in order to get the whole length of the circuit. can someone confirm if my calculation on the attached image is correct. Thank you!

Looks right to me. Except do you really have almost 20ft of wire between each optimizer? Also depending on the optimizers (Solaredge) I wonder if it may be pointless to calculate the VD in between them, since as a group they may boost the voltage to whatever the inverter wants, anyway. But I'm not sure about that.

Maybe I misread his calcs, but wasn't he calculating Vd in the PV wire based on the entire length of all the wiring?

No, you misread.

 

[wwhitney]

Also depending on the optimizers (Solaredge) I wonder if it may be pointless to calculate the VD in between them, since as a group they may boost the voltage to whatever the inverter wants, anyway. But I'm not sure about that.

Perhaps pointless in terms of the inverter operating voltage window, but the VD still represents actual power loss in the DC wiring, if one cares about that.

Cheers, Wayne

 

[ggunn]

Looks right to me. Except do you really have almost 20ft of wire between each optimizer? Also depending on the optimizers (Solaredge) I wonder if it may be pointless to calculate the VD in between them, since as a group they may boost the voltage to whatever the inverter wants, anyway. But I'm not sure about that.


No, you misread.

For Vd in the PV wire he has (200' + 15')(15A)(0.00124 ohms per foot). The 200' is the length of the #8 and does not belong in there; how did I misread that?

 

[wwhitney]

For Vd in the PV wire he has (200' + 15')(15A)(0.00124 ohms per foot). The 200' does not belong in there; how did I misread that?

That 200' is the red line below the optimizers, and it belongs there as it is #10.

Cheers, Wayne

 

[jaggedben]

For Vd in the PV wire he has (200' + 15')(15A)(0.00124 ohms per foot). The 200' does not belong in there; how did I misread that?

Yes you are misreading, and it goes to the whole point of his question. He has 200+15 because his positive home run to the JB is 200ft and his negative home run to the JB is 15ft. As shown in his diagram. And that is correct, and also more correct than doubling one or both of them by using a formula for one-way distance. It's just a coincidence that the 8awg run is also 200ft.

Perhaps pointless in terms of the inverter operating voltage window, but the VD still represents actual power loss in the DC wiring, if one cares about that.

Cheers, Wayne

True. But when I was doing Solaredge we were pretty much always connecting the optimizers factory leads to each other, so I couldn't do anything to change that factor, so I didn't care. Not sure if there's something different here.

 

[Zyb]

Looks right to me. Except do you really have almost 20ft of wire between each optimizer? Also depending on the optimizers (Solaredge) I wonder if it may be pointless to calculate the VD in between them, since as a group they may boost the voltage to whatever the inverter wants, anyway. But I'm not sure about that.


No, you misread.

Thanks Jaggedben, i'm just putting number, it's not actually an actual job, i'm just trying to understand the voltage drop. thanks

 

[ggunn]

That 200' is the red line below the optimizers, and it belongs there as it is #10.

OK, I see that now. Emily Latella voice: Oh. That's very different, isn't it? Never mind.