2023 NEC Articles 404, 406, 408, 410, 422, 424 and 450
Most of what an inspector touches in a finished commercial building is general-use equipment: the switch by the door, the receptacle in the lobby, the panelboard in the electrical room, the troffer in the ceiling, and the dry-type transformer that makes 120 volts out of 480. Each has a short list of rules that show up again and again in plan review and in the field. This lesson walks through the ones that matter most, then closes with a transformer protection example. Where a rule has a number, the section is given so you can find it in the adopted edition. Local adoption governs which edition applies (see Lesson 1).
A switch or a breaker used as a switch must be reachable from a readily accessible place, and the center of the operating handle, in its highest position, may not be more than 6 ft 7 in. above the floor or working platform (2023 NEC 404.8(A)). That number matters for the top breaker of a tall panelboard as well as for wall switches. Three exceptions cover busway devices operated from the floor, switches and breakers installed next to the motor or appliance they serve (reached by portable means), and hookstick-operated isolating switches. Two more switch rules turn up in plan review. Where a switch controls lighting on a general-purpose branch circuit serving bathrooms, hallways, stairways, or habitable and occupiable spaces, the grounded (neutral) conductor must be brought to the switch box, with listed exceptions such as automatically controlled lighting or a switch that controls a receptacle (404.2(C)). Snap switches and dimmers must also be connected to an equipment grounding conductor (404.9(B)).
Section 406.12 requires listed tamper-resistant receptacles for 15 and 20 ampere, 125 and 250 volt nonlocking receptacles in a defined list of places. The list is broader than dwellings, and it is the part most often missed on commercial and community projects. Excepted are receptacles more than 5 1/2 ft above the floor, receptacles that are part of a luminaire or appliance, and the receptacle behind a heavy appliance that is not easily moved and is not readily accessible.
| Location (406.12) | Tamper-resistant required? |
|---|---|
| Child care facilities, preschools, and education facilities | Yes |
| Hotel and motel guest rooms and their common areas | Yes |
| Clinic and medical office lobbies, waiting spaces, and public business offices | Yes |
| Gymnasiums, fitness centers, skating rinks, auditoriums, and places awaiting transportation | Yes |
| Dormitories, assisted living, group homes, foster care, and nursing homes | Yes |
| Any listed location, receptacle more than 5 1/2 ft above the floor | No |
Outdoors, the enclosure rules depend on exposure (406.9). In a damp location, such as under a roofed canopy, the enclosure must be weatherproof when the cover is closed, and the receptacle must be a listed weather-resistant type. In a wet location, the 15 and 20 ampere receptacle needs an enclosure that is weatherproof even with a plug inserted, the familiar in-use cover.
A panelboard must have a rating not less than the minimum feeder capacity for the load calculated under Article 220 (408.30). It must also be protected by an overcurrent device rated no more than the panelboard, located in or ahead of it (408.36). A panelboard fed by two mains with a combined rating within the panel rating is a narrow exception, and only that exception carries the limit of 42 overcurrent devices. Separately, every panelboard needs a physical stop that prevents installing more devices than it was designed, rated, and listed for, and a 2-pole breaker counts as two devices and a 3-pole breaker as three (408.54). Plug-in breakers that are back-fed to terminate field supply conductors need an extra fastener that takes more than a pull to release (408.36(D)).
Every circuit and every circuit change needs a legible, permanent description in the directory (408.4(A)). A panelboard supplied by a feeder, other than in a one- or two-family dwelling, must also carry a permanent label naming where its power originates and its location, and that label may not be handwritten (408.4(B)). On a 3-phase bus the phases run A, B, C from front to back, top to bottom, or left to right, and on a 4-wire delta system the B phase is the high leg. A panelboard with a high leg must be marked to warn of the voltage to ground (408.3(E)(1), 408.3(F)(1)). Panelboards are mounted in cabinets and are dead-front (408.38). Metal cabinets and frames are bonded to the equipment grounding conductor (408.40), and each grounded conductor lands under its own terminal, except an identified terminal for parallel conductors (408.41).
All luminaires, lampholders, and retrofit kits must be listed (410.6). A luminaire in a wet location must be marked suitable for wet locations, and one in a damp location must be marked suitable for wet or damp (410.10(A)). Recessed luminaires that are not identified as Type IC need at least 1/2 in. from combustible material at the recessed parts, and thermal insulation may not go above them or within 3 in. of the enclosure, wiring compartment, or driver (410.116). In a suspended ceiling, the grid must be attached to the structure at suitable intervals, and each luminaire is fastened to a grid member by bolts, screws, rivets, or listed clips (410.36(B)).
Certain listed appliances at 150 volts or less to ground and 60 amperes or less need Class A GFCI protection built in or provided at the circuit: drinking water coolers and bottle fill stations, vending machines, sump pumps, dishwashers, tire inflation machines, vacuum machines, and cord-and-plug pressure washers (422.5(A)). A fixed storage-type water heater of 120 gallons or less needs a branch-circuit device and conductors rated at least 125 percent of the heater (422.13). A permanently connected appliance over 300 VA can use its branch-circuit breaker as the disconnect if the breaker is in sight or lockable open (422.31). Fixed electric space heating, such as baseboard or unit heaters, needs conductors of at least 125 percent of the heater and associated motor load (424.4(B)).
Overcurrent protection follows Table 450.3(B). If the transformer has primary protection only, the primary device may be up to 125 percent of primary current for currents of 9 A or more. If both primary and secondary protection are provided, the primary may go to 250 percent and the secondary is limited to 125 percent for currents of 9 A or more. Where 125 percent is not a standard rating, the next higher standard rating is allowed. Other checks: ventilation must carry away the full-load heat losses (450.9); the transformer must be readily accessible to qualified persons, with relief for dry-type units in the open on walls or columns and for units up to 50 kVA in hollow spaces (450.13); a disconnect must be in sight or remote and lockable (450.14); and the nameplate must show kVA, voltages, clearances, and impedance for 25 kVA and up (450.11). Indoors, a dry-type unit of 112 1/2 kVA or less needs 12 in. from combustible material unless a fire-resistant barrier is used or the unit is 1000 V or less and completely enclosed except for ventilating openings, in which case the nameplate clearances govern, and a larger unit goes in a room with a 1-hour fire rating, with exceptions for Class 155 insulation (450.21).
A Group B medical office building has a 45 kVA, 480 V 3-phase to 208Y/120 V dry-type transformer that will have primary protection only. Primary current is 45,000 / (1.732 x 480) = 54.1 A. The largest device under Table 450.3(B) is 125 percent, or 54.1 x 1.25 = 67.6 A. That is not a standard size, so the next higher standard rating, 70 A, is permitted. Secondary current is 45,000 / (1.732 x 208) = 124.9 A. If the designer adds a secondary device instead, its limit is 125 percent of 124.9 A, which is 156 A, so 175 A is the largest size allowed, and the primary device may then go up to 250 percent of 54.1 A, which is 135 A, making 125 A the largest standard size not over that. The secondary conductors still need their own protection under Article 240 (Lesson 6).
In the field
A GCHS rehab converts a church annex into a clinic waiting room, and a new 45 kVA transformer goes in a storage room. At rough-in and final, the inspector checks the same short list. Are the waiting room receptacles tamper-resistant? Is the top breaker no higher than 6 ft 7 in.? Does the panelboard show where it is fed from, and is the directory filled in? Is the transformer clear of the wooden shelving by its nameplate clearance (12 in. if it is not enclosed), with its vents clear and a disconnect in sight? The breaker on the transformer primary is compared against the nameplate current, not against what happened to be on the truck.
1. A GCHS crew is finishing a new Group E elementary school classroom. Which 125-volt, 20-ampere nonlocking receptacles must be tamper-resistant under 2023 NEC 406.12?
2. In a community gymnasium (assembly use), a duplex receptacle for a scoreboard is mounted 6 ft above the floor. Is a tamper-resistant device required at that outlet?
3. An inspector finds a new wall switch in a Group M retail sales area with the center of its handle 7 ft above the floor. Nothing is nearby that the switch supplies. What is the finding under 404.8(A)?
4. The calculated load of a Group B office is 260 A, and the designer specifies a new 225 A panelboard for it. What does 408.30 say about that selection?
5. A 200 A panelboard in a Group B office is fed by a 250 A breaker at the switchboard, with no other device ahead of it. What does 408.36 require?
6. A panelboard listed for 42 overcurrent devices already has 40 poles used. An electrician wants to add one 3-pole breaker. What does 408.54 allow?
7. An inspector walks a new Group B office building and finds a subpanel fed by a feeder from the main switchboard with no label showing where its power comes from. What does 408.4(B) require?
8. A 3-phase, 4-wire, 240/120 volt delta panelboard is being checked in a Group F light-industrial shop. Which phase is the high leg, and what does 408.3 require at the panelboard?
9. A recessed downlight in a Group B office ceiling is not identified as Type IC, and a crew plans to blow insulation over the ceiling. What does 410.116(B) require?
10. A crew sets 2 ft by 4 ft troffers in a suspended T-bar grid in a Group B office and lets them rest on the grid with no fasteners. What does 410.36(B) require?
11. A Group A-3 community center installs a new cord-and-plug bottle fill station on a 20 A, 120 V circuit. Is GFCI protection required for it under 422.5(A)?
12. A 4,800 W, 240 V fixed electric unit heater in a Group B office has no associated motor. What is the minimum ampacity of its branch-circuit conductors under 424.4(B)?
13. A Group B office has a 30 kVA, 480 V 3-phase dry-type transformer with primary-only protection. What is the largest primary device allowed by Table 450.3(B), using the next-higher-size rule?
14. A 75 kVA dry-type transformer that is not enclosed is installed indoors in a Group S-1 storage room beside wood shelving, with no fire-resistant barrier between them. What separation does 450.21(A) require?