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Very long PV wire

Rocketman

Solar Wizard
Joined
Sep 27, 2020
Messages
2,235
Hi everyone,

I am considering purchasing a property that I would have a very long distance between the solar panels and the mppt.

I don’t know exactly details yet… here is my best guess… of what I would have - I am not sure how to figure voltage/amp drops over these distances. I have just picked items to get close enough to ask questions with real numbers.

Distance - 1700 feet
Mppt - Victron 450/100
Array 1. Eight 405-ish panels in series (Voc 45.6 Vmp 37.9v Isc 10.69a -
So Voc 369v Vmp 303v Isc 10.69 - Total watts 3240w
Array2 - exactly the same.
Cable - Aluminum Direct bury 4-4-4-4 - yes way overkill for 10.69 amps, but at $1.55 ft it’s either the cheapest or same price for much larger thicker wire.
It can get to -20 degrees C in the area.

What are the voltage and amperage losses going to look like? And how to I figure them out for a run like this?

Thanks
 
That is a long run for a 300VDC nominal run.

For this setup I'd probably compute the loss to do microinverters and step-up the 240VAC to something like 3kV and then back down at the other end.

High voltage stuff requires a lot of special handling, so there's a lot of extra complexity if you go that route.

Looks like at 3kV you can use 18AWG at 2amps no problem... These folks have rated wire... https://www.daburn.com/2725---Extra-Flexible-PVC-Hook-Up-Wire-UL-10093-3-000V-AC-105-C-VW-1.aspx

Residential code on 3kV systems would be interesting...
 
This is not strictly correct. It depends on the setup.

Since I'm DC coupled, I bond my "chassis" at the array and only run DC wires in my conduit back to the MPPTs.

AC coupled is different. I'd have to double check some code stuff... but I doubt you'd run a ground wire all that way.

You need a "chassis" or equipment ground at the ground-mount array.
 
Voltage drop isn't super important for DC PV circuits.
The cost of larger conductors usually outweigh the benefits.
This is the way... I probably wouldn't worry too much about it. If anything just add another panel in the series loop. It will probably be fine, most of the time.
 
Ok this is where my knowledge (RV stuff) ends.

Ok I knew that I needed to tie all the ground mount panels together- but I was under the impression that the ground would go to a ground rod driven next to the array, not all the way back to the mppt. Sounds like my impression is incorrect…

Why does the ground go all the way back to the mppt (and then to the earth) and not just to the earth? If it makes any difference- this is completely off-grid.
 
Ok this is where my knowledge (RV stuff) ends.

Ok I knew that I needed to tie all the ground mount panels together- but I was under the impression that the ground would go to a ground rod driven next to the array, not all the way back to the mppt. Sounds like my impression is incorrect…

Why does the ground go all the way back to the mppt (and then to the earth) and not just to the earth? If it makes any difference- this is completely off-grid.
You're not going back to the other earth connection.
You are going back to the existig grounding system. To protect against a shock hazzard to anyone who can touch the array.
The earth can't provide this protection.
 
Shielded cat 6 in the same conduit as AC wires? Never ever?
You can find 600 V rated cat6 cable, has a thicker outer insulator. So that could legally be put into the same conduit as AC wires.

It isn't a great idea, though. Chance to pick up noise, and if something compromises the conduit it can short the various wires together.

What are you sending via cat6? Maybe a wireless link would be a better choice?

Mike C.
 
For this setup I'd probably compute the loss to do microinverters and step-up the 240VAC to something like 3kV and then back down at the other end.
The losses of two AC transformers will exceed the losses of the DC PV wire, and cost a lot more.

Simplest is to run heavy gauge wire and/or string the panels to higher voltages (if the inverter allows).

Mike C.
 
You can find 600 V rated cat6 cable, has a thicker outer insulator. So that could legally be put into the same conduit as AC wires.

It isn't a great idea, though. Chance to pick up noise, and if something compromises the conduit it can short the various wires together.

What are you sending via cat6? Maybe a wireless link would be a better choice?

Mike C.
Gridboss to Flexboss comms.
 
Shielded cat 6 in the same conduit as AC wires? Never ever?
If the insulation is rated for the same voltage as the other conductors in the conduit, it's fine. But connections must be isolated from each other.
 
Always
Absolutely incorrect.
Everything needs to have an EGC ran back to the grounding system for personal protection.
Connecting to the local earth provides zero protection for anyone who could touch the array

The rules are different for roof mounted vs ground mounted (separate structures).

Roof mounted 100% has to tie into the existing grounding system -- i.e. your MPPT chassis, panel ground lug, etc.

There's a reason the PoCo doesn't run a bonding conductor from the transformer to your house. It doesn't do anything useful.

Ground mount uses a local ground b/c its a separate structure, although it depends on if you are using PVC or RMC. Almost nobody does RMC outside of industrial.

Same rules apply for outbuildings attached to your service on a sub-panel if there's not a bond due to the conduit.

See this article for the breakdown of various situations: https://eepower.com/technical-artic...de-2023-basics-grounding-and-bonding-part-8/#

Note specifically that PVC vs Metal or possible alternate current paths makes a huge difference in how you bond.

Figure 4 (mostly) applies to my situation b/c I used PVC pipe. Again, NEC is very AC centric.
 
There's a reason the PoCo doesn't run a bonding conductor from the transformer to your house. It doesn't do anything useful.
Actually the reason is cost. POCO's in other countries do provide the ground conductor. North American companies save the money and force the customer to create the grounding system.
Ground mount uses a local ground b/c its a separate structure, although it depends on if you are using PVC or RMC. Almost nobody does RMC outside of industrial.

Same rules apply for outbuildings attached to your service on a sub-panel if there's not a bond due to the conduit.
Also incorrect.
An EGC is required with feeders to separate structures. An auxiliary ground rod is also required at the separate structure. Which must be connected to the EGC from the source structure.
But we aren't talking about a structure. We are talking about a solar array.
Although I have heard of some local AHJ's that think it's a structure, and force the auxiliary ground rod. But that doesn't remove the EGC requirement.

You are quoting code from 20 years ago. None of which applies, now.
It used to be either/ or , now it's both, for a separate "structure".
 
Actually the reason is cost. POCO's in other countries do provide the ground conductor. North American companies save the money and force the customer to create the grounding system.

Also incorrect.
An EGC is required with feeders to separate structures. An auxiliary ground rod is also required at the separate structure. Which must be connected to the EGC from the source structure.
But we aren't talking about a structure. We are talking about a solar array.
Although I have heard of some local AHJ's that think it's a structure, and force the auxiliary ground rod. But that doesn't remove the EGC requirement.

You are quoting code from 20 years ago. None of which applies, now.
It used to be either/ or , now it's both, for a separate "structure".
So running a ground wire from my array back the the same rod that my house is grounded on, not ok?
 
So running a ground wire from my array back the the same rod that my house is grounded on, not ok?
You are trying to create a low resistance path for fault current, back to the N/G bond (not earth) through the grounding system. If the ground rod at the house is connected to the grounding system, (as it should be) then technically, connecting to it does connect you to the grounding system. But it wouldn't be my preferred way of getting there. It's a connection that would be exposed to the weather.
 

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