ASK THE EXPERTS: Grid-Tie Connection

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PV System power Wall
Why can’t you just connect an inverter directly to the closest 120-volt household outlet you can find?

I have read many of your articles detailing the design and installation of solar-electric systems and am very inspired to start planning one of my own. I want to mount PV modules on my garage because it faces due south and has an excellent pitch. However, it’s about as far as you can get from my home’s main breaker panel. This means that I would have to run an expensive and, more importantly, a long and difficult conduit run.

I’ve never understood why you can’t just connect the inverter directly to the closest 120-volt household outlet that you can find. Of course, that would mean using a 120-volt inverter, such as the SWR1800U from SMA America. Would I be making a mistake by connecting it to the closest outlet I can locate?

Jakob Speksnijder • West Chester, Pennsylvania

From a strictly electrical perspective, there is nothing incorrect or inherently unsafe about making a grid-tie inverter’s AC connection into an AC outlet circuit. But from a National Electrical Code (NEC) viewpoint, it is not acceptable. There also might be some performance problems caused by connecting a grid-tie inverter this way.

Where the inverter is connected really makes no difference to the electrons. But the NEC requires that a grid-tie inverter be connected to its own dedicated wiring circuit—connecting it to a 120 VAC outlet circuit does not meet this requirement.

One potential problem with using the outlet circuit for the inverter has to do with the possibility of excessive voltage drop in the existing wiring when the grid-tie inverter is operating at high power levels. Underwriters Laboratories Standard 1741 requires the operation of a grid-tie inverter to fall within a very tight voltage window to satisfy safety and power quality requirements. Often a long wire run like you described should be installed with a larger wire size than the #12 cable typically used in home outlet circuits. The heavier-gauge wiring will have a higher current rating, minimize voltage drop, and ensure optimal inverter performance.

In addition, if the wiring feeding the outlet circuit were heavily loaded with appliances, the AC voltage might drop below the grid-tie inverter’s lower voltage limit, especially when an appliance starts up. This could result in the inverter frequently shutting off and restarting, which will reduce the system’s performance and possibly affect inverter reliability over time.

Finally, many utilities will require an externally mounted AC inverter disconnect located nearby the service entrance and utility meter. With the inverter located in the garage and connected to an AC outlet, this would not be possible. If your garage has a subpanel, it might be possible to tie into a breaker there, depending on local utility requirements.

Christopher Freitas

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