PV Output Circuit Voltage Drop Requirements

[Farlander]

Planning to run 15kW PV into four outback flexmax80 charge controllers. The home runs from the array combiners to the house are about 200 feet. I was hoping to get by with a #8 awg copper thwn-2, but I end up with over 7% voltage drop. Unfortunately there's 15 strings of 3 modules each, operating voltage about 100.

Ampacity of #8 looks ok when using the Isc @ NOCT and correcting for underground temps, but the Vdrop of 7%-8% is bothersome. Will that raise current in the circuit? Is there an NEC requirement for PV output voltage drop? It wouldn't be bad to shed some volts and power over the home run, the system will be overcharging batts as is.

Advice and suggestions appreciated,

http://www.calculator.net/voltage-drop-calculator.html

[AzRoute66]

Will that raise current in the circuit?

It will not.

Is there an NEC requirement for PV output voltage drop?

There is not.

[kb58]

"Less voltage drop is better" from an electrical stand point, but economically, minimum investment over 20 years will dictate a smaller wire size than what's most efficient.

[Mike90250]

try running the calc with #2 aluminum wire (or maybe #4)
usual goal is < 3% loss

If you have not bought gear, look at the 600vdc solar charge controllers (Morningstar & Schneider) or the high voltage Midnight. higher voltage, fewer controllers, less runs.

Aluminum is safe, make long runs, wired directly to breakers, don try to splice aluminum. Use the #8 copper for the short interconnects, size breakers for the least wire.

[BackwoodsEE]

I second Mike's suggestion to look into the 600V charge controllers. I'm delighted with how my two Morningstar TS-MPPT-600-60 charge controllers are working with 150 feet of a pair of 10 AWG conductors for each of two strings. I used 10x9 tray cable with 600V THHN conductors and an outer jacket that gives the whole thing a direct burial rating. Four pairs of conductors (two spare pairs for a future array) and an EGC.

So far the most I've seen it produce is 5 kW, but I'm expecting 6 kW on a regular basis once the sun starts shining again here in gloomy WA.

[Farlander]

I would have preferred to go with a 600V controller(s), but the customer was keen on the the larger capacity of the radian inverter, and it's what I'm familiar with. Ultimately between Schneider and Outback it came down to Outbacks' online interface - I can control and tweak every setting remotely, whereas with Schneider, you need a dedicated VPN with port forwarding. The OB Flexmax100 300V max is set to be out soon I think, not soon enough for me it looks like.

In regards to the voltage drop, in a normal branch circuit, does a voltage drop correlate to amp increase? My electrician was arguing that we should be testing amp load at the source rather than at the load, and I told him the amps through the circuit would be the same at either end.

[Mike90250]

..... My electrician was arguing that we should be testing amp load at the source rather than at the load, and I told him the amps through the circuit would be the same at either end.

fire him before he damages something/someone

[AzRoute66]

In regards to the voltage drop, in a normal branch circuit, does a voltage drop correlate to amp increase? My electrician was arguing that we should be testing amp load at the source rather than at the load, and I told him the amps through the circuit would be the same at either end.

A voltage drop indicates the presence of amps, not an 'amp increase'.

In a typical branch circuit, the sum of the load amperages will equal the source amperage since most loads are wired in parallel. In most cases the source amperage will not be equal to any individual load amperage. If you are talking about a device with a dedicated circuit, the amperage should be the same at the source and load. There are many benefits of checking amperage at the source, including detecting unanticipated paths.

[sensij]

A voltage drop indicates the presence of amps, not an 'amp increase'.

A load with a fixed power requirement will draw more amps from a lower voltage source than from a higher voltage source. Maybe that fact planted a seed of confusion in the electrician?

[AzRoute66]

Maybe that fact planted a seed of confusion in the electrician?

I suspect the electrician is just fine.

[BackwoodsEE]

I would have preferred to go with a 600V controller(s), but the customer was keen on the the larger capacity of the radian inverter, and it's what I'm familiar with.

The Outback Radian inverter is what I've got. Works just fine with my Morningstar CCs. They communicate exclusively via a 48V bidirectional analog interface, i.e., big red and black wires going to a common battery.

[Mike90250]

The Outback Radian inverter is what I've got. Works just fine with my Morningstar CCs. They communicate exclusively via a 48V bidirectional analog interface, i.e., big red and black wires going to a common battery.

And that's a splendid wrap-up. There is no need for brand loyalty. Any well programmed gear with correct setpoints (ya gotta look up the specs for your batteries anyway - before you program anything) is just going to work fine with other gear. Adding Li-Ion to the mix means you need a couple more decimal points of accuracy, but it all boils down to basic electrical measurements and understanding how things work, how they fail and you being the brains behind it all.

[Bob Johnson]

[QUOTE=sensij;n365985]

A load with a fixed power requirement will draw more amps from a lower voltage source than from a higher voltage source. Maybe that fact planted a seed of confusion in the electrician?

But if the voltage drops at the load, then power is being lost in the circuit due to its resistance. The higher the resistance the more the voltage drop at the load end compared with the source and consequently less current . So More resistance = less Volts x Amps therefore power loss. A good idea to measure the voltage at both ends. The current has to be the same. or something is shorted or open circuit. The only variable is the resistance of the circuit between source and load.

[Bob Johnson]

A question I seem to get conflicting answers to all with logical explanations as to why! Can I drive a grid tie inverter with a sine wave inverter as a reference source or will the grid tie inverter try and dump excess power back to the reference source and fry it? Naturally the grid is not connected !!!

[Mike90250]

A question I seem to get conflicting answers to all with logical explanations as to why! Can I drive a grid tie inverter with a sine wave inverter as a reference source or will the grid tie inverter try and dump excess power back to the reference source and fry it? Naturally the grid is not connected !!!

Generally, NO. One of the tests a GT inverter makes is a Line Impedance Test. Specifically, to see if the "grid" is real or fake. Another test is Frequency stability, a good inverter will pass the test at 60.000 Hz, but a cheap generator that wobbles around 59.5 - 60.5 Hz will fail to qualify