NEC 705 and 706 for Apprentices: Connecting Solar and Batteries to a Panel, Supply-Side Taps, the 120% Rule and Disconnects

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, follow the inverter and battery manufacturer’s instructions and the utility’s interconnection requirements, and follow your supervisor and the local inspector. A panel with an interconnected source can stay energized from more than one direction.

Where the rules live

Topic	Where to look
The PV array, its circuits and rapid shutdown	Article 690
Connecting any parallel source to the premises wiring	Article 705
Battery and other energy storage systems	Article 706
Generators	Article 445
Emergency and standby systems	Articles 700, 701 and 702

Supply side or load side

• Supply-side connection. The source connects on the supply side of the service disconnecting means, often between the utility meter and the service disconnect (NEC 705.11). The exact point depends on the service arrangement and the utility’s requirements. It does not use the panel busbar, so the busbar limit does not apply. The conductors and the disconnect for the source have their own rules, similar to service conductors, and the connection method must be suitable, for example a listed connector or lugs provided for it.
• Load-side connection. The source connects through a breaker in a panelboard or to a feeder (NEC 705.12). Here the busbar and feeder conductors can be fed from two directions, so the code limits how much current the sources can add.

The 120% rule for a panel busbar

For a load-side connection in a panelboard, the code gives a few options (NEC 705.12). The ones apprentices use most:

• The 100% option. The sum of 125% of the inverter output current and the rating of the breaker protecting the busbar does not exceed the busbar rating.
• The 120% option. The sum of 125% of the inverter output current and the rating of the breaker protecting the busbar does not exceed 120% of the busbar rating, and the inverter breaker is at the opposite end of the busbar from the main supply. The panel gets a warning label not to relocate that breaker.

Other options exist, such as a method based on the sum of the overcurrent device ratings with the exclusions the code lists, or a listed power control system that limits current (NEC 705.13). Each option has its own conditions. Check the full list in your 2026 NEC.

Worked example: how big a solar inverter fits on a 200 A panel

A house has a 200 A busbar and a 200 A main breaker. The inverter is 240 V and will be connected with a backfed breaker at the opposite end of the busbar from the main. Assuming the panel qualifies for the 120% option, what is the largest continuous inverter output current this calculation allows?

1. 120% of the busbar. 200 A × 1.20 = 240 A.
2. Room left after the main. 240 A − 200 A = 40 A. This is the limit for 125% of the inverter output current.
3. Inverter output current. 40 A ÷ 1.25 = 32 A.
4. Inverter size. 32 A × 240 V = 7,680 W, about 7.68 kW of inverter output.
5. Breaker. 125% of 32 A is 40 A, and a 40 A breaker is a standard size.
6. Check. 40 A + 200 A = 240 A, which equals 120% of 200 A and does not exceed it.

Main breaker derate. If the load calculation and the equipment listing allow the 200 A main to be replaced with a compatible 175 A breaker, the room becomes 240 − 175 = 65 A, so the calculation allows up to 65 ÷ 1.25 = 52 A of inverter output, about 12.5 kW at 240 V. The breaker, conductors and equipment still have to meet their own rules.

Disconnects and loss of the utility

• Disconnecting means. The power sources need a means to disconnect them from all ungrounded conductors of the premises wiring, and the code allows some sources to share a disconnect (NEC 705.20). It must meet the accessibility and marking rules, and where the line and load terminals can both stay energized in the open position, it gets a warning marking.
• Loss of the primary source. When the utility goes down, an interactive inverter must automatically stop sending power to the utility and must not reconnect until the utility is restored (NEC 705.40). This protects line workers. Systems that keep the house running in an outage do so through equipment that separates the house from the utility first.
• Backfed breakers. Backfed plug-in breakers generally need an additional fastener under the panelboard rules, but Article 705 has an exception for some breakers backfed by interactive inverters. Check whether it applies before you order a hold-down kit.

Energy storage systems: Article 706

Article 706 covers energy storage systems, including home battery systems. For most installers the key points are:

• Listing. The energy storage system is listed for its use, and it is installed per its listing and the manufacturer’s instructions.
• Disconnecting means. The system needs a disconnect that isolates its ungrounded conductors from the wiring it connects to. It can be built into the system or installed separately, and Article 706 sets its location, accessibility and marking rules. Check them and the equipment instructions before you pick the spot.
• Connection. When a battery inverter runs in parallel with the utility, the interconnection rules of Article 705 apply to it, including the busbar options above.

Location, working space and spacing rules for batteries also come from Article 706, the manufacturer and the fire code. Check them before you plan the wall.

Check yourself

1. For the 120% option, where does the inverter breaker go? At the opposite end of the busbar from the main supply.
2. A 200 A busbar has a 200 A main. What is the largest inverter output current under the 120% option? 32 A.
3. What must an interactive inverter do when the utility goes down? Stop sending power to the utility automatically and not reconnect until the utility is restored.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 690, 705 and 706