2023 NEC Article 110
Article 110 is the part of the NEC that follows the equipment after it is bought. It asks whether the product is suitable, whether the wire fits the terminal, whether a person can safely stand in front of the panel, and whether the labels tell the truth. Inspectors check these items in every commercial building, usually in the electrical room, and they are among the cheapest to get right on paper and the most expensive to fix after the walls are closed. The figure shows the working space rules for a panelboard in plan and in elevation.
Section 110.3(A) tells the inspector what to weigh when judging equipment: suitability for the use, mechanical strength, wire-bending and connection space, insulation, heating, and arcing effects, among others. Equipment may be new, reconditioned, refurbished, or remanufactured, but it must be suitable. Section 110.3(B) requires listed or labeled equipment to be installed and used the way its listing instructions say, and those instructions may arrive as paper, a QR code, or a web address. Section 110.21(A) requires the manufacturer's name or trademark on all electrical equipment, and reconditioned equipment must also carry the reconditioner's name, the date, and the word reconditioned, with the original listing mark removed.
Section 110.14(C) says the conductor's temperature rating used for ampacity must not exceed the lowest rating of any connected termination, conductor, or device. In practice the terminal is the weak link. For circuits rated 100 amperes or less, or terminals marked for 14 AWG through 1 AWG, the conductor must be rated 60 degrees C, or a higher-rated conductor may be used only at its 60 degree C ampacity, unless the equipment is listed and identified for more. For circuits over 100 amperes, or terminals marked for conductors larger than 1 AWG, the same idea applies at 75 degrees C. So 90 degree C insulation on a 60-ampere breaker with 60 degree C terminals does not raise the allowable current, although the higher rating still helps when you apply adjustment and correction factors. Section 110.14(D) adds that terminals must be tightened to the manufacturer's torque value using an approved means, such as a calibrated torque tool or a shear-bolt lug.
Working space is the clear area in front of equipment that will need examination, adjustment, servicing, or maintenance while energized. It applies to equipment at 1000 volts nominal or less to ground. The depth, measured from the enclosure front or from exposed live parts, comes from Table 110.26(A)(1) and depends on the voltage to ground and on what is across from the equipment.
| Nominal voltage to ground | Condition 1 | Condition 2 | Condition 3 |
|---|---|---|---|
| 0 to 150 V | 3 ft | 3 ft | 3 ft |
| 151 to 600 V | 3 ft | 3 ft 6 in. | 4 ft |
| 601 to 1000 V | 3 ft | 4 ft | 5 ft |
Condition 1 means live parts on one side and nothing live or grounded on the other, or live parts guarded by insulation. Condition 2 means live parts on one side and grounded parts on the other, and the Code treats concrete, brick, and tile walls as grounded. Condition 3 means live parts on both sides. Three exceptions matter: dead-front equipment needs no space at the back or sides if every connection is reachable from the front, low-voltage equipment may get smaller spaces by special permission, and in existing buildings Condition 2 may be allowed across an aisle between replaced equipment under written procedures.
The working space must be as wide as the equipment or 30 inches, whichever is greater, and it must let equipment doors and hinged panels open at least 90 degrees. It must be clear from the floor to 6 1/2 feet or the height of the equipment, whichever is greater. Other equipment above or below, such as a concrete pad, may extend no more than 6 inches into the space. Where the installation instructions or the equipment's function require a space with limited access, such as above a lay-in ceiling, 110.26(A)(4) calls for an opening of at least 22 by 22 inches, or 22 by 30 inches in a crawl space, plus the same width, door swing, and depth rules.
At least one entrance to the working space must be provided. For large equipment that holds overcurrent, switching, or control devices, 110.26(C)(2) requires an entrance at each end of the working space, at least 24 inches wide and 6 1/2 feet high. That applies to equipment rated 1200 amperes or more and over 6 feet wide, and to service disconnects of the same combined rating and width. A single entrance is allowed where egress is continuous and unobstructed, or where the working space is twice as deep as the table requires. Where equipment rated 800 amperes or more has a personnel door less than 25 feet from the working space, the door must swing at least 90 degrees in the direction of egress and use listed panic or fire exit hardware. Lighting is required for the working space about indoor service equipment, switchboards, switchgear, enclosed panelboards, and motor control centers, and automatic controls cannot switch off all of it.
The working space may not be used for storage. Section 110.26(E) then reserves a dedicated space for service equipment, switchboards, switchgear, panelboards, and motor control centers. Indoors, that zone is the width and depth of the equipment and rises from the floor to 6 feet above the equipment or to the structural ceiling, whichever is lower. No piping, ducts, leak protection apparatus, or other equipment foreign to the electrical system may occupy it. A suspended ceiling that adds no strength to the building is not a structural ceiling, so the zone continues above it. Foreign systems are allowed above the zone if protected against leaks and condensation, and sprinkler piping is permitted in it if it meets the section. A locked electrical room is considered accessible to qualified persons.
Section 110.16(A) requires an arc-flash warning marking on switchboards, switchgear, enclosed panelboards, industrial control panels, meter socket enclosures, and motor control centers, other than in dwelling units, that are likely to be serviced while energized. Under 110.16(B), service equipment and feeder-supplied equipment rated 1000 amperes or more must carry a permanent arc-flash label that includes the date it was applied. Section 110.24(A) requires service equipment other than in dwelling units to be marked in the field with the available fault current and the date the calculation was done, and the calculation must be documented. Section 110.22(A) requires each disconnect to be marked for its purpose and, other than in one- and two-family dwellings, to identify where its power comes from.
A worked example on working space: a 208Y/120 V panelboard has 120 volts to ground, so it uses the 0 to 150 volt row and needs 3 feet of depth in every condition. A 480Y/277 V panelboard has 277 volts to ground and uses the 151 to 600 volt row. If an unfinished concrete block wall is across from it, that is Condition 2 and the depth is 3 feet 6 inches. If a second 480Y/277 V panel sits on the opposite wall with exposed live parts, that is Condition 3 and each panel needs 4 feet. A 20-inch-wide panelboard needs a 30-inch-wide space, while a 48-inch-wide switchboard needs 48 inches.
In the field
A small church wants to convert a janitor's closet into the building's electrical room. The panel is 480Y/277 V with a block wall across from it, so the inspector measures 3 feet 6 inches from the panel front. Three problems show up: mop buckets and a shelf stand in the space, a water line runs across the ceiling directly above the panel, and the closet has one light on a motion sensor. The fix is a cleared and marked floor, a rerouted water line, and a switched light, all before the drywall is finished.
1. A 208Y/120 V panelboard in a small retail store has a bare concrete block wall opposite. What is the minimum depth of working space in front of it under Table 110.26(A)(1)?
2. A 480Y/277 V panelboard in an office building electrical room has an unfinished concrete wall across from it, and nothing else within the space. What is the minimum working space depth?
3. Two 480Y/277 V panelboards face each other across an aisle in a clinic electrical room, both with exposed live parts when opened. What depth applies to each?
4. A 20-inch-wide enclosed panelboard is installed in a community building storage room. What is the minimum width of its working space?
5. An inspector measures a 480Y/277 V panelboard in a Group B building. The working space is clear to a height of 6 ft 2 in., and the panel is 6 ft tall. Under 110.26(A)(3), the space is:
6. A shelf in a restaurant's back hallway holds paper goods in front of a panelboard's working space. Under 110.26(B), the working space:
7. A switchboard rated 1600 A and 8 ft wide sits in the electrical room of a warehouse. How many entrances to the working space does 110.26(C)(2) generally require, and what size?
8. Above a panelboard in a school kitchen wing, a plumber plans to run a hot water line through the space directly overhead, 4 ft above the panel top. The structural ceiling is 10 ft above the floor. Under 110.26(E)(1), this line:
9. A tenant improvement uses a suspended acoustic tile ceiling in a room with a panelboard. For the dedicated equipment space, the suspended ceiling is:
10. A contractor terminates 90 degrees C rated conductors on a 60 A breaker whose terminals are rated 60 degrees C. Under 110.14(C), the conductor ampacity used for this circuit is:
11. For a circuit rated 200 A with terminals marked for conductors larger than 1 AWG, unless the equipment is listed otherwise, the termination temperature rating generally is:
12. An inspector visits a new 1200 A service in a Group B building and finds no label listing the available fault current. Under 110.24(A), service equipment other than in dwelling units must be marked with:
13. A new 1200 A switchboard is installed as service equipment in a strip shopping center. Under 110.16(B), what label does it need in addition to the general arc-flash warning?
14. A disconnect in a restaurant is fed from a panel two rooms away, and the disconnect is not marked with its source. Under 110.22(A), in other than one- and two-family dwellings, the marking must: