Electrical Equipment and Devices

Electrical Equipment and Devices

Learning Objectives

Upon completing this chapter, you will be able to:

1.1 Services and Service Equipment (Article 230)

The service point is where the utility ends and the premises wiring begins. For a master electrician, the critical distinction is between the service conductors and the service equipment. Service conductors can be overhead (230.24) or underground (230.31), but the rules for clearance and protection are absolute. Overhead service conductors must have 10 ft (3.0 m) vertical clearance above finished grade, sidewalks, or residential property, and 12 ft (3.7 m) above residential driveways and commercial areas subject to truck traffic. The 18 ft (5.5 m) clearance applies only over public streets, alleys, or roads.

Service disconnecting means (230.70–230.71) must be installed at a readily accessible location nearest the point of entrance of the service conductors. For a commercial building with multiple services, each service must have its own disconnecting means. The 2026 NEC continues to require that a service disconnect be capable of being locked in the open position, but the lock need not be part of the disconnect itself.

Ground-fault protection for services is a master-level concern. Per 230.95, a grounded wye service rated 1000 A or more, with a line-to-ground voltage exceeding 150 V, must have ground-fault protection. This is not optional. The GFP must be set to open all ungrounded conductors at a maximum of 1200 A, with a maximum time delay of 1 second for fault currents of 3000 A or greater. This requirement drives the coordination study you will perform on larger jobs.

Exam Trap: Do not confuse the 1000 A service GFP threshold with the 1000 V system voltage threshold. The rule applies to current rating, not voltage.

1.2 Separately Derived Systems (Article 250.30)

A separately derived system (SDS) is a premises wiring system whose power is derived from a battery, a photovoltaic system, a generator, or a transformer, and that has no direct electrical connection—including a solidly connected grounded circuit conductor—to the supply conductors originating in another system.

Transformer-derived systems are the most common SDS you will supervise. The key rule is that the system must have a system bonding jumper installed at the source (the transformer) or at the first disconnecting means, but not both. The grounding electrode conductor (GEC) for an SDS must be sized per Table 250.66 based on the derived phase conductors, and it must terminate to a grounding electrode that is as close as practicable and preferably in the same area as the transformer.

For a generator used as an SDS, the transfer switch must open the grounded conductor if the generator is a separately derived system. If the generator is not separately derived (i.e., it uses a solidly grounded neutral from the utility), the transfer switch must be a 3-pole switch for a three-phase, 4-wire system, and the generator neutral must be bonded to the generator frame only if it is a separately derived system.

Master Point: When a transformer supplies a 120/208 V panelboard from a 480 V service, the neutral of the secondary must be bonded to the equipment grounding conductor at the transformer enclosure. The transformer enclosure must also be bonded to the grounding electrode system. Failure to install the system bonding jumper at the correct location is a frequent inspection failure.

1.3 Three-Phase Systems and Conductor Sizing (Articles 210, 215, 220)

Three-phase calculations are the bread and butter of the master exam. You must be fluent with the square root of three (1.732) in voltage-drop and load calculations.

Feeder sizing (Article 215) requires that the feeder conductor ampacity be not less than the sum of the noncontinuous loads plus 125% of the continuous loads. For a 225 A continuous load and a 50 A noncontinuous load, the minimum feeder ampacity is (225 × 1.25) + 50 = 331.25 A. You would then select a conductor with an ampacity of at least 332 A at the termination temperature rating (typically 75 °C for equipment rated 100 A and larger).

Branch circuits (Article 210) follow the same rule for continuous loads. However, a branch circuit supplying a single motor is an exception—it is sized at 125% of the motor full-load current, not 125% of the continuous load, because motor rules (Article 430) take precedence.

Demand factors for feeders are found in Article 220. Table 220.42 permits a demand factor of 100% for the first 3000 VA of general lighting, then 35% for the remainder from 3001 to 120,000 VA, and 25% for the excess over 120,000 VA. For dwelling units, Table 220.42 also allows a 75% demand factor for the first 20 kW of general loads, but this does not apply to commercial occupancies.

Voltage drop is a recommendation, not a mandate, in the NEC (210.19 Informational Note No. 4). However, for motors, the 2026 NEC requires that the voltage at the motor terminals be within 10% of the motor nameplate rating (430.6). For feeders, the informational note suggests 3% for feeders and 5% total for feeders and branch circuits, but the master electrician must calculate this for performance, not just code compliance.

Exam Trap: When sizing a feeder for a panelboard that supplies both continuous and noncontinuous loads, apply the 125% factor to the continuous portion only. Do not multiply the total load by 1.25.

1.4 Overcurrent Protection and Coordination (Articles 240, 430)

Overcurrent protection is not just about interrupting capacity; it is about coordination. A master must understand the difference between a fuse and a circuit breaker in terms of time-current curves.

Fuses (240.60) must be rated for the voltage of the circuit. A 250 V fuse cannot be used on a 480 V circuit. Class RK1 fuses have a current-limiting capability that can reduce the let-through energy, which is critical for protecting downstream equipment.

Circuit breakers (240.80) must be listed and have an interrupting rating sufficient for the available fault current. The available fault current at the service must be calculated or obtained from the utility. If the available fault current exceeds the breaker's interrupting rating, the breaker must be replaced with a higher-rated unit or protected by a current-limiting device upstream.

Motor overcurrent protection (Article 430) is a separate system. The branch-circuit short-circuit and ground-fault protective device (SCGFP) is sized per Table 430.52. For a NEMA Design B motor, the maximum rating of an inverse-time breaker is 250% of the motor full-load current. For a time-delay fuse, it is 175%. These percentages are maximums, but you may use the next higher standard size (240.6) if the calculated value does not correspond to a standard rating.

Motor overload protection (430.32) is separate from short-circuit protection. Overloads must be sized at no more than 125% of the motor nameplate full-load current for motors with a service factor of 1.15 or more, or a temperature rise of 40 °C or less. For all other motors, the maximum is 115%.

Coordination for emergency systems (Article 700) and legally required standby systems (Article 701) mandates that the overcurrent devices be selectively coordinated. This means that when a fault occurs, only the device nearest the fault opens, not the upstream feeder breaker. This requires a time-current coordination study, which is a master-level responsibility.

Exam Trap: Do not use the motor nameplate current for sizing the branch-circuit conductors. Use the full-load current from Tables 430.247 through 430.250. The nameplate is only used for overload sizing.

1.5 Motors and Generators (Article 430, 445)

Motor controllers (430.83) must have a horsepower rating not less than the motor's horsepower rating at the applied voltage. A controller rated for a 10 hp motor at 230 V cannot control a 10 hp motor at 460 V unless it is marked for dual voltage.

Motor disconnecting means (430.102) must be located within sight of the motor and the driven machinery. "Within sight" means visible and not more than 15 m (50 ft) apart. The disconnect must be capable of being locked in the open position.

Generators (Article 445) have specific rules for nameplate marking, overcurrent protection, and grounding. The generator's ampacity must be based on the nameplate current rating. The overcurrent device must protect the generator conductors, but it must not be set lower than 115% of the generator's rated current. For a generator rated 1000 kVA or more, the overcurrent device may be set at 115% of the nameplate current, but the conductors must be sized for the generator output.

Generators used for standby power must comply with Article 702 (optional standby) or Article 700 (emergency systems). Emergency system generators must be able to supply power within 10 seconds of the normal power failure. The transfer switch must be listed for emergency use and must be mechanically held, electrically operated.

1.6 Commercial and Industrial Installations (Articles 210, 220, 240, 250)

Receptacle placement in commercial occupancies (210.52) requires that receptacles be installed so that no point along the floor line is more than 1.8 m (6 ft) from a receptacle. This is the same rule as for dwellings, but the load calculation is different. Commercial receptacle loads are calculated at 180 VA per receptacle strap (220.14).

Multi-wire branch circuits (210.4) are permitted in commercial buildings, but they must have a means to simultaneously disconnect all ungrounded conductors. In a 3-phase, 4-wire system, a multi-wire branch circuit can share a neutral, but the neutral must be sized for the maximum unbalanced load.

Grounding and bonding (Article 250) is where most inspection failures occur. The equipment grounding conductor (EGC) must be sized per Table 250.122 based on the rating of the overcurrent device protecting the circuit. For a 100 A circuit, the EGC must be a minimum of #8 copper or #6 aluminum. Do not confuse the EGC with the grounded conductor (neutral). The neutral is a current-carrying conductor; the EGC is not.

Bonding of metal raceways and enclosures is required at both ends of the raceway. A master must verify that the locknut-bushing connections are tight and that no paint or non-conductive coating prevents a low-impedance path.

1.7 Code Navigation: Where to Find It

1.8 Inspection and Supervision Points

As a master, you are responsible for the work of your journeymen and apprentices. On any commercial job, verify the following:

1.9 Common Exam Traps

Summary

Mastering the NEC for the Texas Master Electrician exam requires not just memorization, but a working knowledge of how the articles interact. Service equipment, separately derived systems, and motor circuits are the three most heavily tested areas in the commercial/industrial domain. Always start with the load calculation, then select the conductor, then the overcurrent device, and finally verify the grounding and bonding. The code is a minimum standard—your job as a master is to ensure the installation is safe, reliable, and code-compliant.