Equipment & Devices

Learning Objectives

By the end of this chapter, you will be able to:

4.Apply the correct NEC requirements for sizing and protecting conductors and equipment in 3-phase systems, including line and phase relationships.

5.Identify the specific requirements for services, service equipment, and grounding electrode systems for commercial and industrial installations.

6.Differentiate between separately derived systems (transformers, generators) and non-separately derived systems, and apply the required grounding, bonding, and overcurrent protection rules.

7.Perform feeder sizing calculations that account for continuous loads, derating, and voltage drop, and select the appropriate overcurrent protective device (OCPD) with proper coordination.

8.Apply the specialized rules for motor and generator circuits, including branch-circuit, feeder, and short-circuit/ground-fault protection.

9.Recognize common code traps and inspection points that a Master Electrician must verify before signing off on an installation.

1.1 Three-Phase Systems and Conductor Identification

A Master Electrician must be fluent in the mathematics and code implications of 3-phase systems. The NEC does not mandate a specific system voltage, but it does require that all conductors and equipment be rated for the actual system voltage and current. For a 3-phase wye system (e.g., 208Y/120 or 480Y/277), the line-to-line voltage is √3 (1.732) times the line-to-neutral voltage. For a delta system (e.g., 240 or 480), there is no neutral unless a center tap is provided.

Key Code References:

Article 100 – Definitions for "Grounded Conductor," "Grounding Conductor," and "Separately Derived System."

Article 200 – Use and identification of the grounded conductor. The grounded conductor (neutral) must be identified with white or gray insulation, or with three continuous white stripes on other than green insulation.

Article 210.5(C) – For branch circuits, an ungrounded conductor identification method must be documented and posted. This is a common inspection point for multi-voltage systems.

Master-Level Consideration: When sizing conductors for a 3-phase feeder, you must use the line current, not the phase current. For a balanced wye load, line current equals phase current. For a delta load, line current is 1.732 times the phase current. The NEC tables (Table 310.16) are based on the ampacity of the conductor, which is the current-carrying capacity under specified conditions. A common error is to use the kVA formula incorrectly: kVA (3-phase) = V (line-to-line) × I (line) × 1.732 / 1000.

1.2 Services and Service Equipment

Service equipment is the first overcurrent protection device and disconnecting means on the premises. For a Master, the critical distinction is between a service and a feeder. A service starts at the utility point of attachment and ends at the service disconnecting means. Everything after that is a feeder or branch circuit.

Key Code References:

Article 230 – Services. Section 230.70 requires the service disconnecting means to be at a readily accessible location nearest the point of entrance of the service conductors.

230.71 – Maximum number of disconnects. The rule allows up to six disconnects to serve as the service disconnecting means, but each must be grouped. This is a classic exam trap: six switches or six circuit breakers are permitted, but they must be in a single enclosure or grouped.

230.79 – Rating of service disconnecting means. For one-family dwellings, the minimum is 100 A. For commercial and industrial, the rating must be not less than the computed load per Article 220.

230.95 – Ground-fault protection of equipment. For solidly grounded wye services of more than 150 V to ground but not exceeding 600 V phase-to-phase (e.g., 480Y/277 V), the service disconnecting means must be provided with ground-fault protection if the rating is 1000 A or more. This is a mandatory requirement, not an option.

Inspection Point: Verify that the service disconnecting means is marked "Suitable for Use as Service Equipment" (unless it is a panelboard with a main breaker). Check that the grounding electrode conductor is sized per Table 250.66 and is connected to the grounding electrode system per Article 250.50.

1.3 Separately Derived Systems (SDS)

A separately derived system is a premises wiring system whose power is derived from a source of energy (e.g., a transformer, generator, or solar inverter) that has no direct electrical connection, including a solidly connected grounded circuit conductor, to the supply conductors originating from another system.

Key Code References:

Article 100 – Definition of SDS.

Article 250.30 – Grounding and bonding of SDS. This is a high-yield area for the Master exam.

250.30(A) – The system must be grounded by connecting the grounded conductor (neutral) to the grounding electrode conductor and the equipment grounding conductor at the source (e.g., the transformer) or at the first disconnecting means. The neutral-to-case bond is made at only one point.

250.30(B) – For ungrounded SDS, a grounding electrode conductor must be connected to the equipment grounding conductor, but there is no neutral bond.

Master-Level Consideration: A transformer secondary is an SDS. The primary side has a neutral that is bonded to the primary equipment grounding conductor. The secondary side must have its own system bonding jumper. If you fail to install the system bonding jumper at the transformer, you create a parallel path for neutral current on the equipment grounding conductors, which is a violation of 250.6 and a safety hazard.

Generator Consideration: A generator with a transfer switch that switches the grounded conductor is an SDS. If the transfer switch does not switch the neutral (a "3-pole" switch for a 3-phase, 4-wire system), the generator is not an SDS, and the neutral must remain bonded to the generator frame only if the generator is the first point of disconnect. For a portable generator supplying a building via a transfer switch, the neutral must be switched to avoid parallel paths.

1.4 Feeder Sizing and Overcurrent Protection

Feeder sizing is not just about ampacity; it is about the minimum circuit ampacity and the maximum OCPD rating.

Step 1: Calculate the load. Use Article 220. For continuous loads (e.g., lighting for 3+ hours), the load must be calculated at 125% (210.20(A)). For non-continuous, use 100%.

Step 2: Determine the minimum conductor ampacity. The conductor must have an ampacity of not less than the calculated load, after applying adjustment and correction factors from Table 310.15(B)(1)(a) (ambient temperature) and Table 310.15(C)(1) (number of current-carrying conductors).

Step 3: Select the OCPD. The OCPD must protect the conductor per 240.4. The general rule is that the OCPD rating must not exceed the conductor ampacity. However, 240.4(B) allows the next higher standard OCPD rating (e.g., 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) if the conductor ampacity is not less than the calculated load and the OCPD is a standard rating. This is a common trap: you cannot use the next higher rule if the calculated load exceeds the conductor ampacity.

Master-Level Consideration: For feeders supplying multiple motors, the feeder conductor must be sized at 125% of the largest motor (430.24) plus the sum of the full-load currents of all other motors. The feeder OCPD must be sized per 430.62, which is the largest branch-circuit short-circuit/ground-fault protective device plus the sum of the full-load currents of the other motors. This is not a simple sum; it requires careful reading of 430.62(A).

1.5 Motor and Generator Applications

Motor circuits are governed by Article 430. The Master must know the difference between the motor full-load current (FLC) from Tables 430.247 through 430.250 and the nameplate rating. The FLC is used for sizing conductors and OCPDs; the nameplate is used for overload protection.

Branch Circuit:

430.22 – Single motor: conductor ampacity must be at least 125% of the motor FLC.

430.52 – Short-circuit and ground-fault protection: the maximum rating is based on the type of motor and the OCPD. For example, an inverse-time circuit breaker for a squirrel-cage motor can be up to 250% of FLC. If the OCPD trips on start, you can use the next higher standard size, but not exceeding 400% for inverse-time breakers (430.52(C)(1) Exception 1).

Overload Protection:

430.32 – Overload devices (heaters, solid-state) must be sized at not more than 115% to 125% of the motor nameplate current, depending on the service factor and temperature rise. If the overloads are not sufficient to start the motor, you may use a higher rating, but not exceeding 140% for motors with a service factor of 1.15 or more.

Generator Applications:

Article 445 – Generators. Section 445.13 requires the ampacity of the conductors from the generator terminals to the first OCPD to be not less than 115% of the generator nameplate current rating. This is a specific percentage that differs from motor rules.

445.18 – Generators must have a disconnecting means that is lockable in the open position, unless the generator is a portable unit with a cord-and-plug connection.

Inspection Point: Verify that the motor controller (disconnect) is within sight of the motor (430.102). "Within sight" means visible and not more than 15 m (50 ft) from the motor. If not, a separate disconnecting means must be installed at the motor.

1.6 Overcurrent Protection Coordination

Coordination is the selection of OCPDs such that a fault on a downstream circuit is cleared by the nearest upstream device, minimizing the outage area. The NEC requires coordination for specific systems:

Article 700 – Emergency systems. 700.28 requires that emergency system overcurrent devices be selectively coordinated with all supply-side OCPDs.

Article 701 – Legally required standby systems. 701.27 has the same requirement.

Article 708 – Critical operations power systems (COPS). 708.54 requires coordination.

Master-Level Consideration: Selective coordination is not just about choosing a larger breaker upstream. It requires analyzing the time-current curves of both devices. A typical rule of thumb is a 2:1 ratio between the upstream and downstream breaker ratings, but this is not a code requirement and can be unreliable with electronic trip breakers. For the exam, know that the NEC mandates coordination for emergency and legally required systems, but not for normal commercial power systems, unless the engineer specifies it.

1.7 Code Navigation: Where to Find It

Topic	Primary Article	Key Sections/Tables
Definitions (SDS, Service, etc.)	100	Definitions
Grounded conductor identification	200	200.6, 200.7
Grounding and bonding (general)	250	250.30, 250.50, 250.66, 250.122
Services	230	230.70, 230.71, 230.79, 230.95
Branch circuits	210	210.20(A), 210.23
Feeders and calculations	215, 220	215.2, 220.5, 220.12, 220.14
Conductor ampacity	310	Table 310.16, 310.15(B)(1)(a), 310.15(C)(1)
Overcurrent protection	240	240.4, 240.6(A)
Motors	430	430.22, 430.24, 430.32, 430.52, 430.62, 430.102
Generators	445	445.13, 445.18
Emergency/Standby systems	700, 701	700.28, 701.27
Transformers (SDS)	450, 250.30	450.3, 250.30(A)

1.8 Inspection and Supervision Points

As a Master, you are responsible for the final sign-off. On every job, verify:

72.Neutral-to-case bond: Only at the service or at the SDS source. Never in a subpanel. Check for a bonding screw or strap in a main panel; it must be removed in a subpanel.

73.Grounding electrode conductor: Verify it is continuous, protected from physical damage (if exposed, in rigid metal conduit or Schedule 80 PVC), and connected to an acceptable electrode (ground rod, concrete-encased electrode, etc.).

74.Service disconnect: Confirm the number of disconnects is six or fewer and that they are grouped. Check for the "Suitable for Use as Service Equipment" label.

75.Motor disconnects: Ensure the disconnect is within sight of the motor and that the motor is not running without the overload protection being properly sized.

76.Conductor ampacity vs. OCPD: Verify that the breaker size does not exceed the conductor ampacity unless the next-higher rule (240.4(B)) is properly applied.

77.Ground-fault protection: For 480Y/277 V services rated 1000 A or more, confirm the presence of a ground-fault relay and that it is tested.

1.9 Common Exam Traps

The 125% factor: Do not apply 125% to the OCPD for a continuous load if the conductor is already sized at 125%. The OCPD is sized to protect the conductor, not the load. For a 100 A continuous load, the conductor must be sized for 125 A, and the OCPD can be 125 A (next higher is 125 A, but 125 A is a standard size). However, if the conductor is rated 110 A, you cannot use a 125 A breaker because the calculated load (125 A) exceeds the conductor ampacity.

Motor FLC vs. Nameplate: Use the tables for conductor and OCPD sizing. Use the nameplate only for overload sizing. Mixing these up will cost you points.

SDS Bonding: A transformer secondary is an SDS. The neutral must be bonded to the transformer case and to the grounding electrode conductor. If you treat it like a subpanel and float the neutral, you have a code violation.

Six-Disconnect Rule: The six disconnects are for the service, not for feeders. A feeder to a building must have a single disconnecting means (225.31).

Voltage Drop: The NEC recommends (not requires) a maximum voltage drop of 3% for branch circuits and 5% for feeders and branch circuits combined (210.19(A) Informational Note, 215.2(A) Informational Note). This is not a mandatory rule, but it is a design consideration. The exam will often ask if it is a requirement; the answer is no.

Summary

Mastering equipment and devices requires a shift from "how to wire it" to "why the code requires it this way." Focus on the definitions, the mandatory rules (shall), and the exceptions. Know where to find the tables and how to apply the adjustment factors. The open-book exam is a test of your navigation skills and your ability to apply the code to real-world scenarios. Practice looking up sections quickly, and always read the fine print notes and exceptions—they are often the source of the correct answer.

Equipment & Devices