Understanding long pv wire voltage drop

[Hedges]

A 5% voltage drop for AC (inverter drives wires to 5% higher than grid) could be enough to cause disconnect, because often grid is driven a bit high to make up for voltage sag due to loads.

A 5%, 25%, maybe even 50% voltage drop for PV doesn't stop MPPT from operating. If 150V max, 120V nominal Voc, 48V battery, then 50% voltage drop would be a bit too much for MPPT operation but it would keep working, just wouldn't draw as much current.

 

[Checkthisout]

Yeah, I’m off a bit in my math… duh… sorry.

Wow… big conductors for a pump. 1500’ is certainly a long run, and motor start loads are important to factor in loss… but wow, thats a lot for a pump feeder.

That's more than a quarter mile deep.

 

[Jennifer]

That's more than a quarter mile deep.

Yea wow never thought of it like that and I sure don't want to have to pull up and redrop 1,500' of galvanized pipe if the size isn't appropriate.

 

[dogfud]

The sheer weight of the drop pipe is hard to fathom, 1400' of 1-1/4" sch 40 plus #2/3 cable is in the 4500-5000 lb range. Pretty much like lowering a pickup truck down the hole.

Better hold on tight while lowering it, lol. And jacking it out to replace a pump is going to be a sizeable operation. Also I don't know if a safety cable is standard, but I don't see one on the bill of materials.

 

[Jennifer]

The sheer weight of the drop pipe is hard to fathom, 1400' of 1-1/4" sch 40 plus #2/3 cable is in the 4500-5000 lb range. Pretty much like lowering a pickup truck down the hole.

Better hold on tight while lowering it, lol. And jacking it out to replace a pump is going to be a sizeable operation. Also I don't know if a safety cable is standard, but I don't see one on the bill of materials.

The more you guys remind me of this type stuff the more I think what a good deal I'm getting.... LOL

 

[Danke]

Have you thought about taking up rain dancing classes instead?

 

[jfharper]

After initially sizing pv wire for ampacity, why don't we have to account for ampacity drop for long runs? For example, if my mini splt requires 10 amps or less, and I size the wire lets say to 10 or 12 awg, over 300' (600' round trip), if following NEC rules of 80% ampacity for wire AND 1.2 factor for each 100' distance for ampacity drop, my required awg would be really high...like 90 amp requirement...or something like that. Is it because the voltage of the unit is determined after the scc or inverter? So for ac long runs of wire are 120v each, so the voltage is fixed...whereas pv the voltage is not determined yet until it hits the scc or inverter, where the voltage is then set and amps then determined? Do I have this sort of correct? Just trying to learn.

 

[Hedges]

Voltage drop over long runs starves the load, especially motors which might remain stalled and not start running if they can't draw surge of about 5x operating current.

For PV, whatever percentage voltage drop is just that percentage power loss. So long as voltage remains high enough to charge battery (SCC requires some volts above to operate), it just works.

The voltage difference between Voc and Vmp of the PV panel is about 10%, but that is OK too. If IR drop pulls voltage down another 25% or so, the system continues to work. SCC MPPT adjusts current draw up and down to maintain highest V x A = W harvested.

We used to care about watts when PV panels cost us $5.00/W. Not so much now that we might spend $0.25 to $0.50/W.

 

[Jennifer]

Impressed a 3hp can do that. Is your run current really under 14.3A? My neighbor's Grundfos burned about 17-18A @ 230V to get ~7gpm @ 700'.

Shit... just holy shit... I probably would have cheaped out with #6/3 for a 6.9% voltage drop.

Why is copper so luscious?

I found the wire requirement document for the well pump motor I'm putting in..... at 1400' it's right between two sizes, shown by arrow.