NEC 408 Panelboards for Apprentices: Circuit Directories, Overcurrent Protection, Backfed Breakers and the High Leg

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 panel manufacturer’s instructions, and follow your supervisor and the local inspector. Before working inside a panel, follow your employer’s electrical safety program, put the panel in an electrically safe work condition and verify the absence of voltage.

What Article 408 covers

Topic	Where to look
Circuit directory and source marking	408.4
Bus and phase arrangement, including the high leg	408.3
Unused openings	408.7
Panelboard overcurrent protection and backfed devices	408.36
Grounding terminals and wire bending space	Article 408 and Article 312
Cabinets and the enclosures themselves	Article 312
Working space in front of the panel	Section 110.26

The circuit directory

Every circuit and every circuit modification must be legibly identified on a circuit directory at the panel (NEC 408.4). The description has to be clear and specific enough to tell one circuit from another, and it cannot depend on things that change, such as “Bob’s room” or “spare bedroom” when the room has no fixed name. Installed breakers held for future use are identified as spares.

Useful habits on the job:

• Write the directory as you terminate, not at the end of the day.
• Use room names and fixed equipment names, for example “Kitchen counter receptacles, east wall” or “Water heater.”
• Update the directory whenever you change a circuit, even on service calls.

In other than one- and two-family dwellings, a switchboard, switchgear or panelboard supplied by a feeder is also marked to show where its power comes from. Check the exact wording in your 2026 NEC.

Protecting the panel itself

As a general rule, each panelboard is protected by an overcurrent device rated no more than the panelboard (NEC 408.36). The protection can be a main breaker in the panel or an overcurrent device on the supply side, such as the feeder breaker that supplies it. The section has exceptions, for example for some existing panelboards used as service equipment.

So a 125 A panelboard fed by a 100 A feeder breaker is protected by that breaker. A 100 A panelboard fed by a 125 A feeder breaker is not, unless it has its own main rated 100 A or less.

Backfed breakers and unused openings

• Backfed plug-in breakers. When a plug-in breaker is backfed and terminates field-installed supply conductors, for example as the main in some panels or for a generator connection, it must be secured with an additional fastener that takes more than a pull to release (NEC 408.36). Use the hold-down kit the panel manufacturer lists.
• Unused openings. Unused openings for circuit breakers and switches must be closed with identified closures or other approved means that give protection substantially equivalent to the wall of the enclosure (NEC 408.7). Use the manufacturer’s filler plates, not tape or cardboard.

The high leg on a 4-wire delta

On a 120/240 V three-phase 4-wire delta system, one phase measures about 208 V to the neutral. That phase is called the high leg, or wild leg. It is not suitable for 120 V loads.

• Location. In a panelboard, the high leg is generally connected to the B phase (NEC 408.3). The section has an exception for equipment in the same section as the metering equipment, which may follow the meter’s phase arrangement. Check it before you apply the B-phase rule.
• Marking. The high leg conductor is identified by an outer finish orange in color, or by other effective means, wherever a connection is made and the neutral is also present. The marking rule is in Article 110.
• Danger. If a single-pole breaker for a 120 V load is snapped onto the high leg, the load sees about 208 V.

Worked example: why the high leg reads about 208 V

A 120/240 V three-phase 4-wire delta is center-tapped on one winding. The neutral sits at the midpoint of that 240 V winding. Find the voltage from the high leg to the neutral.

1. The three corners of the delta form an equilateral triangle with 240 V sides.
2. The neutral is at the midpoint of one side, so it is 120 V from each of the two corners on that side.
3. The high leg is the opposite corner. The distance from a corner of an equilateral triangle to the midpoint of the opposite side is the side × 0.866.
4. 240 V × 0.866 = 207.8 V, about 208 V.
5. Check. 120² + 207.8² = 14,400 + 43,181 = 57,581. The square root is about 240, the line-to-line voltage, which matches the right triangle formed by the high leg, the neutral and one corner.

Check yourself

1. Can a circuit directory entry say “Bob’s room”? No. It must be clear and specific and not depend on changing conditions.
2. A 100 A panelboard is fed from a 125 A feeder breaker and has no main. Is it protected? No. The protection must not exceed the panelboard rating.
3. On a 120/240 V 4-wire delta, which phase is generally the high leg in a panelboard? The B phase.

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 110, 312 and 408