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

Service Conductors and Ratings

Service conductors must be sized to carry the calculated load per Article 230 and Part III of Article 220. For a master electrician, the critical distinction is between service conductors (from the utility point of attachment to the service disconnecting means) and feeder conductors (from the service disconnect to downstream panels). Service conductors are not required to have an overcurrent device at the point of supply if the service disconnect provides that protection, but the ampacity must be at least the rating of the service disconnect.

Minimum service size for a single-family dwelling is 100 A at 120/240 V (230.79). For commercial and industrial occupancies, there is no absolute minimum; the service must be sized to the calculated demand load per Article 220. However, the service disconnecting means for any occupancy must have a rating of at least the computed load, and never less than 100 A for a one- or two-family dwelling.

Service Disconnecting Means

Each service must have a disconnecting means that:

For services with more than one disconnect, the rule changed significantly in recent cycles. The 2026 NEC continues to allow up to six disconnects for a service, but each must be grouped and each must be rated to disconnect the entire service if it is the only disconnect. In practice, most commercial work uses a single main breaker or a single service disconnect switch.

Exam trap: A service disconnect for a 3-phase, 4-wire service must open all ungrounded conductors. A 3-pole breaker is required — never a 2-pole breaker with a neutral that is switched or fused.

Grounding and Bonding at the Service

The service is the single point of grounding for the entire premises. The grounding electrode conductor (GEC) must be sized per Table 250.66 based on the size of the largest ungrounded service conductor. The GEC must connect to a grounding electrode system per 250.50 — which includes metal underground water pipe (if 10 ft or more in contact with earth), concrete-encased electrode (Ufer), ground ring, and driven rods.

Key master-level point: The main bonding jumper at the service connects the grounded (neutral) conductor to the equipment grounding conductor and the service enclosure. This is the ONLY place where the neutral and ground are intentionally bonded in a separately derived or service-supplied system. Downstream, the neutral must be insulated and isolated from ground.

Inspection point: Verify that the neutral bus in the service panel is bonded to the enclosure (via the main bonding jumper) and that the equipment grounding bus is separate. In a subpanel, the neutral must float — no bonding screw, no neutral-to-ground connection.

1.2 Separately Derived Systems

Definition and Identification

A separately derived system (SDS) is a premises wiring system whose power is derived from a source of electric energy or from a transformer that has no direct electrical connection (metallic or solid) to the supply conductors originating from another system. Common SDS sources include:

Per 250.30, an SDS must have its grounded conductor connected to a grounding electrode at the source or at the first disconnecting means. The grounding electrode conductor must be sized per Table 250.66 based on the largest ungrounded conductor of the SDS.

Grounding and Bonding for Transformers

For a transformer supplying a 3-phase, 4-wire system (e.g., 480 V delta primary to 208Y/120 V secondary):

Exam trap: The neutral of the secondary must be grounded, but the primary neutral (if a wye primary) must not be bonded to the secondary neutral. The transformer enclosure must be bonded to the primary equipment grounding conductor AND to the secondary equipment grounding conductor, but the two grounding systems must not be interconnected except through the earth.

Code navigation: 250.30(A) for grounded SDS, 250.30(B) for ungrounded SDS. Table 250.66 for GEC sizing.

Generators as Separately Derived Systems

A generator is an SDS only when the transfer switch opens the neutral conductor. If the neutral is solidly connected through the transfer switch (common in many residential and light commercial installations), the generator is NOT separately derived, and the generator frame must be bonded to the premises grounding system — but no additional grounding electrode is required at the generator.

For a true SDS generator:

Inspection point: Check the transfer switch. If it is a 3-pole switch (switching only phases) with a solid neutral, the generator is not an SDS. If it is a 4-pole switch (switching phases and neutral), the generator IS an SDS and requires its own grounding electrode.

1.3 Transformer Installations

Sizing and Overcurrent Protection

Transformer primary and secondary conductors must be protected per 240.21(C). The primary overcurrent device must not exceed 125% of the transformer primary rated current (or the next standard size up per 240.6). The secondary conductors may be protected by the primary device if the primary device is sized at 125% of the primary current AND the secondary conductors have an ampacity at least equal to the primary device rating divided by the transformer turns ratio.

Master-level calculation example:

For a 75 kVA, 480 V delta to 208Y/120 V transformer:

Exam trap: The 125% factor applies to the continuous load AND the transformer itself. Do not double-apply the 125% for continuous loads on the secondary if the transformer is already sized for the load.

Transformer Enclosures and Ventilation

Dry-type transformers rated over 112.5 kVA must be in a vault or have fire-resistant construction per 450.21. Transformers rated 112.5 kVA and below may be installed without a vault if they are separated from combustible materials by fire-resistant barriers or are of the dry-type with a temperature rise not exceeding 150°C.

Ventilation requirement: Transformer vaults must be ventilated to limit the temperature rise to 150°C above ambient per 450.45. In practice, most commercial transformers are dry-type, indoor, and require clearances per the manufacturer's instructions — typically 12 inches from walls and ceilings for units up to 112.5 kVA.

1.4 Feeder Sizing and Demand Factors

Continuous Loads

Per 210.19(A)(1) and 215.2(A)(1), branch circuits and feeders supplying continuous loads must have an ampacity of at least 125% of the continuous load, plus 100% of the noncontinuous load. A continuous load is defined in Article 100 as a load where the maximum current is expected to continue for 3 hours or more.

Common continuous loads: Lighting (general illumination), motors running continuously, HVAC equipment, and some process loads.

Exam trap: The 125% factor is applied to the load, not the conductor. A 20 A branch circuit can supply a maximum continuous load of 16 A (20 A ÷ 1.25), not 20 A.

Demand Factors for Commercial Loads

Table 220.44 allows demand factors for receptacle loads in other than dwelling units:

Table 220.42 covers lighting demand factors for dwelling units, but for commercial occupancies, lighting is typically calculated at 100% of the connected load unless specific demand factors apply (e.g., for continuous lighting in warehouses).

Table 220.56 provides demand factors for kitchen equipment in commercial occupancies — a critical calculation for restaurants and cafeterias.

Neutral Sizing

Per 220.61, the neutral conductor must be sized to carry the maximum unbalanced load. For 3-phase, 4-wire systems with nonlinear loads (electronic ballasts, VFDs, computers), the neutral must be counted as a current-carrying conductor per 310.15(E) — this triggers the 80% adjustment factor when there are 4 or more current-carrying conductors in a raceway.

Master-level point: In a 3-phase, 4-wire wye system with high harmonic content (triplen harmonics), the neutral can carry more current than the phase conductors. The NEC does not require the neutral to be oversized for harmonics, but the 310.15(E) requirement to count the neutral as current-carrying effectively derates the circuit. In practice, many engineers specify a 200% neutral for data centers and other high-harmonic environments.

1.5 Overcurrent Protection Coordination

Standard Ratings and Interrupting Ratings

Per 240.6, standard ampere ratings for fuses and inverse-time circuit breakers include: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000.

Interrupting rating (IR): All overcurrent devices must have an interrupting rating sufficient for the available fault current at their line terminals per 110.9. For services, the available fault current must be calculated and the service equipment must be rated accordingly. This is a critical master-level responsibility — the master must verify that the equipment label matches the calculated fault current.

Exam trap: A 20 A breaker with a 10 kA IR cannot be used where the available fault current is 22 kA, even if the breaker is in a panelboard that is rated 22 kA. The breaker itself must have the IR.

Selective Coordination

Per 240.12 and 700.28, selective coordination is required for emergency systems, legally required standby systems, and critical operations power systems (COPS). Selective coordination means that the overcurrent device closest to the fault opens while upstream devices remain closed, isolating only the faulted circuit.

Inspection point: For emergency systems, verify that the feeder and branch circuit OCPDs are coordinated — typically by using current-limiting fuses or breakers with adjustable trip settings. Documentation of coordination studies is required for COPS per 240.12.

Motor Overcurrent Protection

Per 430.52, motor branch circuit short-circuit and ground-fault protection must be sized at not more than:

These percentages may be increased if the motor cannot start without tripping, but never above 400% for fuses or 700% for breakers (430.52(C)(1) Exception).

Motor overload protection per 430.32 must be sized at not more than 125% of the motor nameplate full-load current for motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less. For other motors, the maximum is 115%.

Exam trap: The motor full-load current for branch circuit sizing comes from Tables 430.247 through 430.250, NOT from the motor nameplate. The nameplate is used only for overload protection.

1.6 Motor and Generator Applications

Motor Disconnects and Controllers

Each motor must have a disconnecting means per 430.102 that:

The motor controller must have a horsepower rating at least equal to the motor horsepower per 430.83. A controller rated for a higher horsepower may be used, but a controller rated for a lower horsepower cannot.

Inspection point: The disconnect must be within sight (within 50 ft) of the motor. If the disconnect is not within sight, the motor must have a lockable disconnect at the motor location, or the controller must be capable of being locked in the open position.

Motor Feeder Sizing

Motor feeders must be sized per 430.24 at not less than 125% of the largest motor full-load current plus the sum of the full-load currents of all other motors on the feeder. This is a common calculation error — the 125% applies only to the largest motor, not to all motors.

Example: A feeder supplies three motors: 10 hp (14 A), 15 hp (21 A), and 25 hp (34 A) at 460 V, 3-phase.

Feeder ampacity = (34 A × 1.25) + 21 A + 14 A = 42.5 + 21 + 14 = 77.5 A

Use 1 AWG THHN copper (75°C column = 130 A) or 3 AWG if derating applies.

Generator Installations

Generators must comply with Article 445. Key requirements:

Exam trap: The generator overcurrent device is sized at 115% of the generator rated current, NOT 125%. This is different from transformer and motor rules.

1.7 Code Navigation

1.8 Inspection and Supervision Points

When a master electrician signs off on an installation, the following must be verified:

1.9 Common Exam Traps

Summary

The master electrician must understand that equipment selection and installation go beyond simple ampacity calculations. Service equipment must be properly grounded and bonded at a single point; separately derived systems require careful attention to neutral switching and grounding electrode connections; transformers demand coordinated overcurrent protection on both primary and secondary; and motor circuits have unique rules for disconnects, controllers, and overload protection that differ from general branch circuit requirements. Mastery of these concepts — and the ability to navigate the NEC quickly to find the applicable section — is what separates a journeyman from a master.