Solar panels are now a common sight on American rooftops, and the rules that govern them are part of the NEC. A grid-tied solar system turns sunlight into electricity, uses what the home needs, and sends the rest back to the utility grid. This post explains how the system works, how net metering pays you for the extra power, and what NEC Article 690 requires.
How a Grid-Tied Solar System Works
Sunlight hits the solar panels and the photovoltaic cells inside produce direct current, or DC. An inverter converts that DC into 120/240-volt AC, the same power the home uses. The AC flows into the main panel, where it supplies the home's loads first. Any extra power flows backwards through the utility meter and out to the grid. When the sun is not shining, the home simply draws power from the grid as normal.
Because the system is tied to the grid, it shuts down automatically during a power cut. This is called anti-islanding, and it protects line workers who may be repairing the grid.
The Parts of the System
Solar panels. Modules of photovoltaic cells, usually 350 to 450 watts each. A typical home system is 5 to 10 kW, or about 12 to 25 panels.
The inverter. Converts DC from the panels into AC for the home. It can be one central string inverter or a small microinverter under each panel.
The AC disconnect. A switch between the inverter and the main panel that lets the system be isolated for service.
The production meter. Measures how much electricity the system generates.
Diagram: A grid-tied solar system: panels, inverter, AC disconnect, main panel, utility meter and the grid.
Net Metering: Getting Paid for Extra Power
Net metering is the billing arrangement that makes rooftop solar pay. When your panels make more power than your home uses, the extra power flows to the grid and your meter runs backwards, building up a credit. At night, or on cloudy days, you draw power from the grid and use up that credit. You are billed only for the net energy you use over the billing period.
Diagram: How net metering works: daytime export builds a credit, nighttime import uses it up.
NEC Article 690: The Rules for Solar
Article 690 covers solar photovoltaic systems. A few rules matter most. Section 690.12 requires rapid shutdown: when the system is shut down, the conductors on the roof must drop to 30 volts or less within 30 seconds, so firefighters can work safely on a roof. Section 690.31 requires DC conductors to be in metal raceways or listed cable where they run inside a building. Section 690.8 sizes the conductors and overcurrent devices at 125% of the maximum current, because solar is a continuous source.
There is also the 120% rule in Section 705.12: the main breaker plus the solar backfeed breaker may not exceed 120% of the panel busbar rating. For a 200-amp panel, that allows a 40-amp solar breaker (200 + 40 = 240 = 120% of 200). If the system is bigger, the main breaker must be downsized or the panel upgraded.
Microinverters vs String Inverters
String inverter: one large inverter for the whole array. It is cheaper, but the whole string performs as well as its weakest panel, so shade on one panel drags the whole string down.
Microinverters: one small inverter under each panel. Each panel works independently, so shade on one panel does not affect the others, and you can monitor each panel separately. They cost more per watt but are common on residential roofs.
Key Takeaways
A grid-tied solar system turns sunlight into DC, an inverter turns it into AC, the home uses what it needs and the rest flows to the grid. Net metering credits you for the exported power, so you pay only for your net use. NEC Article 690 requires rapid shutdown, proper DC wiring and 125% sizing, and the 120% rule limits the solar breaker a panel can accept.
Objective Questions
1. What does the inverter do in a grid-tied solar system?
a) It stores electricity in a battery
b) It converts DC from the panels into 120/240-volt AC for the home
c) It disconnects the panels at night
d) It measures the voltage of the grid
Answer: (b) It converts DC from the panels into 120/240-volt AC for the home
Explanation: Solar panels produce DC, and the inverter converts it into the AC the home uses.
2. Why does a grid-tied solar system shut down during a power cut?
a) To save the battery
b) Anti-islanding, to protect line workers repairing the grid
c) Because the panels stop working at night
d) Because the inverter overheats
Answer: (b) Anti-islanding, to protect line workers repairing the grid
Explanation: Anti-islanding shuts the system down during an outage so it cannot energise a grid that workers believe is dead.
3. Under NEC Section 690.12, what must rapid shutdown do?
a) Stop the inverter within 5 minutes
b) Drop roof conductors to 30 volts or less within 30 seconds
c) Disconnect the utility meter
d) Turn off all the lights in the house
Answer: (b) Drop roof conductors to 30 volts or less within 30 seconds
Explanation: Section 690.12 requires rapid shutdown to bring conductors on the roof to 30 V or less within 30 seconds, so firefighters can work safely.
4. A home has a 200-amp panel with a 200-amp main breaker. Under the 120% rule, what is the largest solar backfeed breaker allowed?
a) 20 amps
b) 30 amps
c) 40 amps
d) 60 amps
Answer: (c) 40 amps
Explanation: 120% of a 200-amp busbar is 240 amps, so 240 - 200 = 40 amps is the largest solar breaker allowed.
5. What is the main advantage of microinverters over a string inverter?
a) They are much cheaper
b) Each panel works independently, so shade on one panel does not affect the others
c) They need no AC disconnect
d) They work during a power cut without batteries
Answer: (b) Each panel works independently, so shade on one panel does not affect the others
Explanation: With a microinverter under each panel, shade on one panel does not drag down the whole array, and each panel can be monitored separately.
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