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Voltage drop-system planning

Bluedog225

Texas
Joined
Nov 18, 2019
Messages
2,830
Good morning,

I’m thinking through a power shed instead of roof mount for solar panels. All off grid. The cabin is about 80 feet away.

I can house the inverter/charger/batteries either in the power shed or in the cabin.

I have a basic understanding of voltage drop and conductor sizing. Am I correct in thinking that the difference between AC and DC is quite small?

There is a mild preference for housing the equipment in the cabin for security reasons. Under this setup, I’d have an 80’ burried DC run.

It seems to me that I can plan my panel layout to accommodate a wide range of DC voltages (to include anticipated voltage drop) with no concerns because controllers and chargers seem to be able to accept a wide range of DC. This would allow smaller conductors if they have the correct ampacity.

If I use AC, the conductor size would be more stringently dictated (I think).

Am I on the right track? Or is there a general preference or custom for this type of run between panels and the ultimate loads? Or a preference between AC and DC in this application?

Thanks
 
AC in general suffers from less voltage drop over distance, but since AC is tightly standardized the voltage drop that does occur can be problematic.

In general though I think current thinking runs along the lines of ratcheting up DC solar voltage by adding more panels, then letting the controller deal with it. That's how I'm doing it with my own system, with DC runs up to 130 feet with insignificant voltage drop. That's running 7-8A of current at ~120VDC in 10 gauge copper wire.

Keep in mind that the controller will have to work a bit harder to transform the raw high solar voltage down to the battery charging voltage, but that is usually just a few percentage points. My controller company Midnight publishes a voltage efficency curve, so you can chose the best operating voltages for your particular system.

Use your manufacturer's curve (if there is one) along with a voltage drop calculator like this one to determine where you get the best bang for your buck so to speak. https://www.calculator.net/voltage-drop-calculator.html

As a general rule, you get the best compromise operating your solar strings at about 2-3X your battery charging voltage.
 
Good info. Thanks! I’m unreasonably interested in the Midnite B-17.

I’m guessing that DC voltage drop at a given distance and and conductor is fixed. Or does it vary with temperature?
 
I’m guessing that DC voltage drop at a given distance and and conductor is fixed. Or does it vary with temperature?
Most likely it does to a very slight degree, but not likely to be measurable with ordinary meters. The amperage running through the wire is far more important. The higher the amperage, the higher the voltage drop.
 
I guess that if you wire at least some of the panels in series, you'll have volts to spare... without having to spend a fortune on 80' of thick cable, if you lose a few fractions of an amp... a small panel or two to make up for that (in series) will very likely be cheaper. Or just lose them and be happy :·)
 
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