The Next Array

MarkC · March 2016

I'm planning to "fill" the remaining roof area of my solar shed with approximately 3000 watts of grid-tied panels. I have a SMA Sunny Boy 4000 TL-US that has worked well for me, so I'll add a SMA 3000TL-US for the new panels. I also want the ability to have two secure power supplies available during power outages that can charge my golf cart battery banks for limited energy during power outages.

The existing breaker box that is mounted in the solar shed will have individual sets of breakers for the inverters. There is another breaker in the main grid panel that will be sized for the entire solar shed production. Is there any concern feeding the outputs of two grid-tied inverters together? Both will have their own meters and disconnects.

I have all the wiring, connectors and AC components left over from ongoing work, so the cost is the panels, panel mounts and inverter. All AC wiring was sized for this addition. Since my power company co-op pays back at the same rate as my energy cost, this addition should pay back in around 7 years - with current costs of panels and the Fed rebate.

Any suggestions to improve this plan?

BB. · March 2016

One suggestion... On a nice spring day--Check the AC voltage at the GT inverter. Adding more GT inverter can cause "voltage rise" (aka voltage drop) on your wiring. And could run your GT inverter outputs upwards of 260 volts ("drop" from GT inverter, through wiring, and out to the utility transformer).

If you AC power is running 240-255 VAC maximum or so--Then you are probably OK.

-Bill

MarkC · March 2016

Good suggestion - already seeing upwards of 3500 watts production. I'll check when grid usage is low at the same time.

Combined outputs will be nominal 240 volt split phase (of course) with a maximum current of around 27 amps, based on the maximum wattages logged to date of the 3850 watt existing system, projecting that for the new 3000 watts addition. The total distance from the "solar shed" to the main panel is around 100 feet. Wire size is 8 AWG for both phases and neutral, so I guess it might be getting a little close on voltage drop?

My concern was potentially two close-tied inverters might fight each other - especially when they are out at a "remote" location as I have.

BB. · March 2016

No--The GT inverters "don't care" about each other. Just the line voltage they see. If over ~260-264+ VAC, they will shut down for five minutes and "try again" to reconnect.

You can use a voltage drop calculator like this one to estimate the "drop" (or voltage rise) in your wiring.

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

-Bill

MarkC · March 2016

Thanks Bill - Out of curiosity - when dealing with 240 V split phase (one conductor per 120 volt phase), what is the best way to input these calculators? Is it 120 volt, full amperage and two conductors, or use 240 volt single phase, full amperage and one (or two?) conductor?

BB. · March 2016

240 vac and full current. The current goes out one wire and comes back on the other.

There is no sharing on 240 vac.

-Bill

MarkC · March 2016

Good - believe I'm fine with less than 4 volts drop - assuming 240v at solar shed.

MarkC · March 2016

BB. said:

One suggestion... On a nice spring day--Check the AC voltage at the GT inverter. Adding more GT inverter can cause "voltage rise" (aka voltage drop) on your wiring. And could run your GT inverter outputs upwards of 260 volts ("drop" from GT inverter, through wiring, and out to the utility transformer).

If you AC power is running 240-255 VAC maximum or so--Then you are probably OK.

-Bill

Turned out to be an excellent suggestion. Last week, on a semi-cloudy, day the wattage went above 4000 for the first time that I have ever seen - what timing. The sub-panel in the solar shed measured 247 volts max. Very little load on the main panel, so I'm thinking this is about as high as the sub panel will see if the grid voltage is stable. I'd expect that the additional amps on the wiring from the solar shed to the main panel will cause an additional 3-4 volts max. So, the tandem inverters might actually see 250 +/- volts at the sub-panel and at highest productions. In addition, there are some losses thru the PV disconnect/meter/wiring, so maybe 255+/- volts max at both inverter outputs with the new system? What do you think?

BB. · March 2016

Sounds like it should be OK... Our utility (PG&E) seems to set their maximum line voltage at ~252 VAC (and they tend to run their lines hot in the couple places I have checked--Close to 250-252 volts at times). And the GT inverter will probably run OK to ~260 VAC (technically, 264 VAC +/- or 264 VAC - ; no + error).

-Bill

MarkC · March 2016

I'll continue to check line voltages under no PV production at the solar shed to confirm what variations occur on the grid - at minimal main panel usages.
Thanks for your help.

The Next Array

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