Article 110 general requirements

If Article 100 is the dictionary of the Code, Article 110 is its rulebook for every job. Its requirements apply to all electrical installations covered by the Code, from a single receptacle in a house to a substation in a factory. Master exams draw on Article 110 heavily, partly because its rules apply everywhere and partly because several of them, especially working space and terminal temperature ratings, involve numbers that are easy to test. Dana tabs Article 110 before any other article in her book, and she reads it from start to finish twice in her first two weeks.

This lesson walks through the sections of Article 110 that master exams use most, in the order they appear. As always, the rules are explained in our own words; read each section in your own code book as you go.

The shape of Article 110

Article 110 is divided into parts. Part I contains general requirements that apply to everything. Part II covers installations of 1000 volts, nominal, or less, and is where the working space rules for most equipment live. Part III covers installations over 1000 volts. Further parts cover tunnel installations and manholes and other enclosures that people can enter. For most master exam questions you will be in Parts I and II.

Approval and the listing instructions: 110.2 and 110.3

Section 110.2 says that conductors and equipment required or permitted by the Code are acceptable only if approved, and approved, as lesson 3 showed, means acceptable to the authority having jurisdiction. Section 110.3 then tells the AHJ, and you, how to judge equipment. Its first subdivision lists factors to consider, such as suitability for the purpose, mechanical strength, wire-bending space, heating, arcing and classification by type, size and voltage. Listing or labeling is a strong basis for that judgment.

The most important part of 110.3 for everyday work is the rule that listed or labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling. This single sentence makes the manufacturer's installation instructions enforceable. If a luminaire's instructions say it must not be in contact with insulation, if a breaker panel's label lists the only breakers that may be installed in it, or if a heat pump's nameplate gives a maximum overcurrent device rating, those instructions carry the force of the Code. On the exam, an answer that cites the manufacturer's instructions is often correct because of 110.3(B).

Wiring integrity and conductor material: 110.5 and 110.7

Section 110.5 says that conductors are copper unless the Code says otherwise. When the Code gives a conductor size without saying which metal, it means copper; where aluminum is permitted, a separate size is usually given. Section 110.7 requires completed wiring to be free from short circuits, ground faults and connections to ground other than those the Code requires or permits. Before energizing, the system should be tested to show this. These short sections show up on exams as quick definition-style questions.

Fault current: 110.9, 110.10 and 110.24

Two of the most important safety rules in the Code deal with what happens during a short circuit. Section 110.9 requires equipment intended to interrupt current at fault levels, such as fuses and circuit breakers, to have an interrupting rating at least equal to the current available at its line terminals. Section 110.10 requires the overcurrent devices, the total impedance and the short-circuit current ratings of equipment to be selected and coordinated so that the circuit protective devices can clear a fault without extensive damage to the electrical equipment of the circuit.

Section 110.24 makes the available fault current visible. In other than dwelling units, service equipment must be legibly marked in the field with the maximum available fault current, the date the calculation was done, and the marking must be durable enough for the environment. When changes to the electrical installation affect the available fault current, the marking must be updated. On the Delgado service upgrade, the utility's new transformer raises the available fault current, so Dana's engineer recalculates it and Brightwater updates the label.

Workmanship and connections: 110.12 and 110.14

Section 110.12 requires electrical equipment to be installed in a neat and workmanlike manner. It also requires unused openings in boxes, raceways, cabinets and equipment cases to be closed with protection substantially equivalent to the wall of the equipment, and it protects internal parts from paint, plaster, cleaners and other contaminants. Exams tend to ask about the unused-openings rule because it has a clear answer.

Section 110.14 covers electrical connections, and two of its subdivisions are heavily tested. The first, 110.14(C), sets temperature limitations: the temperature rating of the conductor used for ampacity must be coordinated so it does not exceed the lowest temperature rating of any connected termination, conductor or device. In practice, this means a conductor with 90-degree insulation cannot be loaded to its 90-degree ampacity if the breaker or lug it lands on is rated only for 75 degrees.

A master-level subtlety: the higher-temperature ampacity of a conductor is still useful. When ampacity must be reduced for high ambient temperature or for more than three current-carrying conductors in a raceway, the adjustment can start from the 90-degree column, as long as the final result does not exceed the termination-limited ampacity. MELU-103 works through those calculations.

The second heavily tested subdivision, 110.14(D), deals with tightening torque. Where the equipment or its instructions give a tightening torque value, connections must be tightened to that value using an approved means, such as a calibrated torque tool, and the 2023 edition kept and refined this requirement. Annex I gives recommended torque values for use where the manufacturer gives none. Loose connections are a leading cause of electrical fires, which is why the rule exists.

Markings: 110.15, 110.16, 110.21 and 110.22

On a 4-wire delta-connected system where the midpoint of one phase winding is grounded, one phase conductor has a higher voltage to ground than the other two. Section 110.15 requires that high leg to be durably marked orange, or by another effective means, wherever a connection is made and the grounded conductor is also present. Other sections specify the high leg's position in panelboards and meter sockets.

Section 110.16 requires equipment such as switchboards, panelboards, industrial control panels and meter socket enclosures in other than dwelling units, where it is likely to need examination, adjustment, servicing or maintenance while energized, to be marked to warn qualified persons of potential arc-flash hazards. For larger service equipment the marking must include more detail, such as the nominal voltage, available fault current, clearing time of the service overcurrent devices and the date the label was applied. Section 110.21 covers manufacturer markings and field-applied hazard markings, and 110.22 requires each disconnecting means to be legibly marked to show its purpose unless the purpose is evident.

Working space: 110.26

Working space is the single most tested topic in Article 110, and Dana practices it until she can answer without opening the book. Section 110.26 requires sufficient access and working space around equipment that is likely to need examination, adjustment, servicing or maintenance while energized. The working space has three dimensions: depth, width and height. It must be kept clear and must not be used for storage.

The depth required depends on two things: the nominal voltage to ground and the conditions on the far side of the space. Voltage to ground, not line-to-line voltage, is what counts. A 120/208-volt system has 120 volts to ground; a 480Y/277-volt system has 277 volts to ground. The three conditions describe what the worker could touch behind them: Condition 1 has exposed live parts on one side only and no grounded parts on the other side, or the other side is insulated; Condition 2 has exposed live parts on one side and grounded parts on the other, where concrete, brick or tile walls count as grounded; Condition 3 has exposed live parts on both sides.

The width must be at least 30 inches or the width of the equipment, whichever is greater, and the space must allow equipment doors or hinged panels to open at least 90 degrees. The height must be at least 6.5 feet or the height of the equipment, whichever is greater, with limited exceptions for existing dwelling services and small equipment. Separately, 110.26(E) requires dedicated equipment space for switchboards and panelboards: the space directly above and below the equipment, as wide and deep as the equipment, from the floor to 6 feet above the equipment or to the structural ceiling, kept free of foreign piping, ducts and equipment.

For large equipment, 110.26(C)(2) requires an entrance at each end of the working space where the equipment is rated 1200 amperes or more and over 6 feet wide, with exceptions for unobstructed exits and doubled working space. A separate rule, 110.26(C)(3), covers personnel doors near large equipment: within a set distance of the working space they must open in the direction of egress and have listed panic or fire exit hardware. Check the ampere threshold and distance in your edition.

# Minimum working space depth from Table 110.26(A)(1), summarized for Part II systems
def depth_ft(volts_to_ground, condition):
    if volts_to_ground <= 150:
        return 3.0
    return {1: 3.0, 2: 3.5, 3: 4.0}[condition]

jobs = [("Copper Kettle panel, 120/208 V, concrete wall behind", 120, 2),
        ("Delgado switchboard, 480Y/277 V, concrete wall behind", 277, 2),
        ("Delgado MCC facing another MCC, 480Y/277 V", 277, 3)]
for name, vg, cond in jobs:
    print(f"{name}: {depth_ft(vg, cond)} ft")

Copper Kettle panel, 120/208 V, concrete wall behind: 3.0 ft
Delgado switchboard, 480Y/277 V, concrete wall behind: 3.5 ft
Delgado MCC facing another MCC, 480Y/277 V: 4.0 ft

Guarding and enclosures: 110.27 and 110.28

Section 110.27 requires live parts of electrical equipment operating at 50 volts or more to be guarded against accidental contact by approved enclosures or by location, such as in a room accessible only to qualified persons, on a suitable balcony or platform, or at a sufficient elevation. Section 110.28 and its table explain enclosure type numbers, such as Type 1 for general indoor use, Type 3R for outdoor rain, and Type 4X for washdown and corrosion. An exam may give an environment and ask which enclosure type is suitable.