How to wire an inverter to a 120 vac mains/breaker panel Richard Perez Getting the inverter's power output into a conventional 120 vac mains panel can be a problem. Unless the wiring is properly connected and sized, the inverter's power will not be effectively transferred. So here's the straight dope about wiring your inverter to a mains panel that was designed for conventional grid power input. These panels are found in all "electrically standard" homes and manufactured housing. The Marriage of Inverter to Mains If you're reading Home Power, then you probably are already familiar with inverters. These marvelous devices change the low voltage DC stored in our batteries into 120 vac, 60 cycle, power. They allow us to use PV produced and battery stored energy in conventional appliances. The inverter's power output, while not an exact replica of that supplied by the power company, is close enough to run almost all conventional 120 vac appliances. Just like downtown. Now the mains panel is a different matter. This piece of electrical equipment lurks in basements, closets, and other dark, unfrequented places. The function of the mains panel is to connect your building with the conventional commercial power grid. It provides a terminus for your building's wiring. Within the mains panel each 120 vac circuit, via its individual circuit breaker, connects with the main power input. Hence its name, mains panel. Our mission is to wed the inverter, from the world of renewable energy, with the mains panel, from the world of costly, pollution ridden, commercial grid electricity. Maybe not a marriage made in heaven, but certainly one made in the sunshine. Consider yourself an ecological/electrical match maker. Getting the Power out of the Inverter All high quality inverters offer us two ways to connect to their output- via a plug or via hardwired terminals. Let's look at plugs first. The male plugs are a standard 3 prong, grounding, 20 Ampere plugs known in electrical jargon as "cord caps" (don't ask me why). You can use just about any male plug, but get one that is of high quality. This means strong prongs, anti-corrosion plating, and a solid case. A high quality cord cap will cost around $5 to $7 and is worth it. The inverter's entire output is passing through this plug, so it's not the place to save a buck. Connect the plug as follows. The GOLD colored terminal of the plug is HOT and connected to the BLACK wire in the output cable. The SILVER colored terminal of the plug is COMMON and connected to the WHITE wire in the output cable. The GREEN colored terminal of the plug is GROUND and connected to the BARE copper wire in the output cable. You may also have a hardwire output for your inverter. This output consists of three electrical terminals that will accept either bare wire ends, or ring connectors. Wire these according to the manufacturer's instructions on your particular inverter. Here's some info on two of the most common types of inverters. The Trace inverters offer their hardwired output via a barrier strip under their Plexiglas window. The terminus is located in the upper right hand corner of the window just below the standby input line. Trace supplies ring connectors and an Allen wrench with every inverter. These allow you to install lightweight wire into the barrier strip. The Heliotrope inverter supplies three large & easily used connectors on the lower left hand side of their main PC board. These connectors will accept 10 gauge wire ends directly. Once again the wiring scheme is the same: HOT to BLACK wire, COMMON to WHITE wire, and GROUND to BARE wire. Inverter to Mains Panel Wiring The wire transferring the inverter's power to the mains panel must be of sufficient size to handle the current over the distance without excessive losses. If the inverter to mains panel wiring distance is less than 70 feet, then 12 gauge copper will will do the job at 98% or better efficiency. If the inverter to mains panel wiring distance is about 120 feet, then 10 gauge copper wire will be 98% efficient. These facts are computed on the round trip wiring distance (two conductors) and half of these distances is the actual physical distance between the inverter and the mains panel. Use conventional ROMEX cable for this purpose, like NM12/2 with Ground. This cable contains three solid 12 gauge copper wires: one black insulation, one with white insulation, and one without insulation. For longer distances, use the 10 gauge equivalent, NM10/2 with Ground. If the cable is exposed to sunlight or buried, then use cable with USE (Underground Service Entrance) insulation. The USE insulation on the outside of the cable will not photodegrade in sunlight, or rot in moisture. Connecting to the Mains Panel Route the wire into the mains panel from its top. Connect the hot (BLACK) to the main input breaker(s). Connect the Common (WHITE) and the Ground (BARE) to the wiring terminal beside the rows of circuit breakers. See the diagram below. INSERTDIAGRAM The mains panel is designed for commercial power input. Each row of breakers (and there are two), is connected to a 120 vac leg of the grid input power. Together, these two 120 vac legs make 240 vac. Well, the inverter just makes 120 vac. In order to energize the second set of breakers, we must add a jumper between the two main input terminals as shown in the diagram. This effectively converts the mains panel from 120/240 vac operation to just 120 vac operation. The terminals where the common and ground wires are all connected should be grounded. This is the main system ground for the 120 vac distribution system. This terminus should be connected, with 6 gauge bare copper wire, to a metallic rod driven at least six feet into the ground. And they lived happily ever afterÉ The inverter is now wedded to the mains panel, and all 120 vac circuits are energized. As with many weddings there are leftovers. Most weddings produce an excess of toasters and cuisinarts, but in this case the leftovers are circuit breakers. The inverter contains its own output circuit breaker, there are the main breakers at the top of the panel as well as the individual breakers for each ac circuit. There are actually three circuit breakers in series with every circuit. More than enough to please even the fussiest building inspector.