Equipment & Devices

Learning Objectives

Upon completing this chapter, you will be able to:

4.Apply the NEC requirements for service conductors, service equipment, and grounding electrode systems for 3-phase, 4-wire services.

5.Differentiate between separately derived systems (transformers, generators) and non-separately derived systems, and correctly size and protect their conductors.

6.Perform feeder sizing calculations for commercial and industrial loads, including demand factors and continuous load adjustments.

7.Select and apply overcurrent protective devices (OCPDs) with proper coordination for motor and generator applications.

8.Identify code-required clearances, working space, and identification requirements for equipment installations.

9.Navigate the NEC efficiently using article, section, and table numbers to verify requirements during an open-book exam.

1.1 Services and Service Equipment (Article 230)

A service is the conductors and equipment that deliver electric power from the utility supply system to the service disconnecting means. For a master electrician, the critical distinction is between service conductors (from the utility point of connection to the service disconnecting means) and feeder conductors (from the service disconnecting means to branch-circuit OCPDs).

Service Disconnecting Means (230.70 – 230.71):

Each service shall have a single disconnecting means, unless the service is for multiple occupancy or the load exceeds the rating of a single disconnect. In that case, you may use up to six disconnects (switches or circuit breakers) grouped in one location. This is the classic "six-handle rule." For a master exam, remember that these six disconnects must be capable of being operated independently, but they must all be grouped.

The disconnecting means must have a rating of not less than the calculated load, and must be capable of interrupting the maximum fault current available at its terminals. For a 3-phase, 4-wire service, the disconnect must be a 3-pole switch or breaker, simultaneously opening all ungrounded conductors.

Service Overcurrent Protection (230.90):

Each ungrounded service conductor must have an OCPD. The rating of the OCPD must not exceed the ampacity of the conductor, except as permitted for motor circuits or where the next higher standard size (240.6) does not exceed 800 amperes. For services, the OCPD can be a single device or multiple devices in parallel (e.g., two 400 A breakers in parallel to serve an 800 A service).

Ground-Fault Protection (230.95): For solidly grounded wye services of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase, where the service disconnecting means is rated 1000 amperes or more, you must provide ground-fault protection. This is a critical threshold. The GFP must be set to trip at a maximum of 1200 amperes, and the time delay must be coordinated with the feeder and branch-circuit OCPDs. This is a common inspection point for commercial/industrial work.

Service Conductor Sizing (230.42):

Service conductors must have an ampacity sufficient to carry the calculated load. For a 3-phase service, the calculation uses the formula: I = VA / (√3 × V_LL) where V_LL is the line-to-line voltage (e.g., 208 V, 480 V). Remember to multiply the calculated load by 125% for continuous loads (loads expected to operate for 3 hours or more) before comparing to the conductor ampacity.

Minimum Size: Service conductors must not be smaller than 8 AWG copper or 6 AWG aluminum for a 120/240 V, 3-wire service, but for a 3-phase, 4-wire service, the minimum is typically governed by the calculated load, not the minimum size rule.

1.2 Grounding and Bonding for Services (Article 250)

This is the most heavily tested area for the master exam. The distinction between grounding (connecting to earth) and bonding (connecting metallic parts to establish electrical continuity) is fundamental.

Grounding Electrode System (250.50):

All grounding electrodes present at the building must be bonded together to form the grounding electrode system. These include: metal underground water pipe (at least 10 feet in contact with earth), metal frame of the building, concrete-encased electrode (Ufer ground – minimum 20 feet of 4 AWG bare copper in the footing), ground ring, and rod/plate electrodes.

Resistance: If a single rod electrode has a resistance to earth greater than 25 ohms, you must add a second rod (250.53). This is a field-testing point.

Grounding Electrode Conductor (GEC) Sizing (250.66):

The GEC is sized based on the largest ungrounded service conductor. For a 3-phase service with 500 kcmil copper service conductors, the GEC must be at least 1/0 AWG copper. Use Table 250.66. Note: The GEC is never required to be larger than 3/0 copper for rod or plate electrodes, but for a concrete-encased electrode, the GEC can be as small as 4 AWg copper (250.66(B)).

Main Bonding Jumper (250.28):

The main bonding jumper connects the grounded conductor (neutral) to the equipment grounding conductor at the service. It must be sized per Table 250.102(C)(1), based on the largest ungrounded service conductor. For a 500 kcmil copper service, the main bonding jumper is 1/0 AWG copper.

Critical Trap: The neutral must be bonded to the enclosure ONLY at the service (or at the source of a separately derived system). It must be isolated everywhere downstream. This is a frequent violation found during inspections.

Separately Derived Systems (SDS) – 250.30:

A transformer secondary or a generator is an SDS if there is no direct electrical connection (metallic path) between the supply side and the load side conductors. The grounded conductor (neutral) of the SDS must be bonded to the equipment grounding conductor at the source (or at the first disconnecting means).

The grounding electrode for the SDS must be as close as practicable to the source. You must run a GEC from the SDS to the nearest grounding electrode (e.g., building steel, water pipe). The size of the GEC for the SDS is based on the size of the largest ungrounded secondary conductor (not the primary conductor). This is a classic exam trap.

1.3 Transformers and Generator Installations (Articles 450 and 445)

Transformer Overcurrent Protection (450.3):

The primary OCPD for a transformer must be sized per Table 450.3(B). For a transformer with a primary current of 9 amperes or more, the primary OCPD can be set at 125% of the rated primary current. If the next standard size is not sufficient, you may use the next higher standard size.

Secondary Protection: If the secondary conductors are protected by a single OCPD at the secondary, the primary OCPD can be larger. However, if you rely on the primary OCPD to protect the secondary, the primary device must be sized at 125% of the primary current, and the secondary conductors must terminate in an OCPD rated at 125% of the secondary current. For a master, you must know the difference between "primary only protection" and "primary and secondary protection."

Example: A 75 kVA, 480 V primary, 208Y/120 V secondary transformer. Primary current = 75,000 / (480 × √3) = 90.2 A. Primary OCPD = 90.2 × 1.25 = 112.8 A → use 125 A breaker (next standard size). Secondary current = 75,000 / (208 × √3) = 208 A. Secondary OCPD = 208 × 1.25 = 260 A → use 300 A breaker.

Transformer Enclosures and Ventilation (450.9):

Transformers must be readily accessible for inspection and maintenance. The ventilation openings must not be obstructed. For dry-type transformers rated over 112.5 kVA, the installation must comply with the separation from combustible materials (450.21).

Generators (Article 445):

Generators are treated as SDS if they have a transfer switch that opens the neutral. If the generator is a portable unit with a bonded neutral, it is NOT an SDS and must be treated as a feeder source.

Overcurrent Protection (445.12): Generators must be protected from overloads. The OCPD must be rated not less than 115% of the generator's full-load current rating. This is a specific code minimum. For a 100 kW, 480 V, 3-phase generator, full-load current = 100,000 / (480 × √3) = 120.3 A. The minimum OCPD = 120.3 × 1.15 = 138.3 A → use 150 A breaker.

Conductor Sizing (445.13): The conductors from the generator terminals must have an ampacity of not less than 115% of the generator nameplate current rating. This is a common trap – you cannot size the conductors to the calculated load; you must use the generator nameplate.

1.4 Feeder Sizing and Load Calculations (Articles 215 and 220)

Feeder Conductor Sizing (215.2):

Feeders must have an ampacity of not less than the calculated load, plus 125% of the continuous load. The minimum feeder size for a 3-wire, 120/240 V system is 10 AWG copper, but for a 3-phase, 4-wire system, the minimum is governed by the load.

Neutral Conductor (215.2(A)(2)): The neutral must be sized to carry the maximum unbalanced load. For a 3-phase, 4-wire wye system, the neutral carries the unbalanced current. If the load is a nonlinear load (e.g., electronic ballasts, VFDs), the neutral must be counted as a current-carrying conductor (310.15(E)), which may require derating.

Load Calculations (Article 220):

General Lighting (220.14): For commercial buildings, the general lighting load is calculated at 1.2 VA per square foot (Table 220.12). For a 10,000 sq ft office, the lighting load = 12,000 VA.

Receptacle Loads (220.14(H)): For general-purpose receptacles in commercial buildings, the load is calculated at 180 VA per receptacle. For a space with 20 receptacles, the load = 3,600 VA.

Demand Factors (220.44): For receptacle loads exceeding 10 kVA, you can apply a demand factor of 50% to the portion above 10 kVA.

Continuous vs. Non-continuous: The sum of all continuous loads must be multiplied by 125% before adding to the non-continuous loads. This is the single most common calculation error.

Feeder Taps (215.2(A)(2) and 240.21(B)):

A feeder tap is a conductor connected to a feeder that is protected by the feeder OCPD, not by its own OCPD. The tap rules are strict:

Tap length ≤ 10 feet: The tap must have an ampacity not less than the load served, and must terminate in a single OCPD. The tap must be enclosed in a raceway.

Tap length ≤ 25 feet: The tap must have an ampacity of not less than one-third of the feeder OCPD rating. For a 400 A feeder, the tap must be at least 133 A (use 1/0 copper).

Tap length ≤ 100 feet: The tap must have an ampacity of not less than one-third of the feeder OCPD rating, AND the tap must terminate in a single OCPD. This is for industrial installations only.

1.5 Overcurrent Protection and Coordination (Article 240)

Standard Ampere Ratings (240.6): The standard sizes for fuses and breakers are: 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.

Conductor Protection (240.4):

Conductors must be protected against overcurrent. The OCPD rating must not exceed the conductor ampacity, EXCEPT for the "next higher standard size" rule (240.4(B)): if the conductor ampacity does not correspond to a standard OCPD rating, you may use the next higher standard size, provided the OCPD rating does not exceed 800 A.

Example: A 3 AWG copper conductor with 75°C insulation has an ampacity of 100 A. You can protect it with a 100 A breaker. If the ampacity were 95 A (due to derating), you could still use a 100 A breaker (next higher standard size).

Motor Overcurrent Protection (430.52):

Motor branch-circuit short-circuit and ground-fault protection is sized per Table 430.52. For a standard squirrel-cage motor, the maximum rating is 250% of the motor full-load current (for inverse-time breakers). For a 20 A motor, the maximum breaker = 20 × 2.5 = 50 A. If the motor will not start with a 50 A breaker, you may increase to the next standard size, but not exceeding 400% for inverse-time breakers (430.52(C)(1) Ex. 1).

Motor Overload Protection (430.32): The overload relay must be sized at 125% of the motor full-load current for motors with a service factor of 1.15 or more. For a 20 A motor, the overload = 20 × 1.25 = 25 A. You must use a heater or solid-state relay rated at 25 A, or the next higher available size.

Coordination (240.12 and 240.87):

For emergency systems and legally required standby systems, overcurrent devices must be coordinated so that a fault on a branch circuit does not take out the feeder. This is selective coordination.

For service equipment rated 1200 A or more, you must provide arc-energy reduction (240.87). This can be achieved via zone-selective interlocking, differential relaying, or an arc-flash reduction maintenance switch. This is a modern requirement that a master must know for industrial installations.

1.6 Motor and Generator Applications (Article 430)

Motor Feeder Sizing (430.24):

The feeder conductors supplying multiple motors must have an ampacity of not less than 125% of the largest motor plus the sum of the full-load currents of all other motors. For a feeder supplying a 20 A motor and a 10 A motor: Feeder ampacity = (20 × 1.25) + 10 = 35 A → use 8 AWG copper.

Motor Disconnecting Means (430.102):

A disconnecting means must be located within sight of the motor and the driven machinery. "Within sight" means visible and not more than 50 feet away. The disconnect must open all ungrounded conductors.

Motor Controllers (430.83):

The controller must have a horsepower rating not less than the motor's horsepower. For a 10 HP motor, the controller must be rated at least 10 HP. You cannot use a "general use" switch for a motor unless it is specifically rated for the motor.

Generators as Motors (445.10):

When a generator is used as a motor (e.g., for starting a synchronous generator), the conductors must be sized for the motor current, not the generator current. This is a rare but tested scenario.

1.7 Equipment Clearances and Working Space (Article 110)

Working Space (110.26):

Working space must be provided around all electrical equipment rated 600 V or less. The depth of the working space depends on the voltage and the condition of the opposite surface:

Condition 1 (exposed live parts on one side, grounded parts on the other): 3 feet for 0–150 V, 3.5 feet for 151–600 V.

Condition 2 (exposed live parts on both sides): 3.5 feet for 0–150 V, 4 feet for 151–600 V.

Condition 3 (exposed live parts on both sides, with a concrete wall): 4 feet for 0–150 V, 5 feet for 151–600 V.

The working space must be at least 30 inches wide, or the width of the equipment, whichever is greater. The headroom must be at least 6.5 feet.

Entrance and Egress (110.26(C)):

Equipment rated 1200 A or more, or over 6 feet wide, must have an entrance at each end of the working space. The doors must open in the direction of egress and be equipped with panic hardware.

Identification (110.22):

All disconnecting means must be legibly marked to indicate its purpose. For a 3-phase panel, each circuit must be identified on a directory. The master is responsible for ensuring this is done before final inspection.

1.8 Code Navigation: Where to Find It

Topic	NEC Article / Table
Services – Disconnects	230.70 – 230.71
Services – OCPD & GFP	230.90, 230.95
Service Conductor Sizing	230.42, Table 310.16
Grounding Electrode System	250.50 – 250.53
GEC Sizing	Table 250.66
Main Bonding Jumper	250.28, Table 250.102(C)(1)
Separately Derived Systems	250.30
Transformer Protection	450.3, Table 450.3(B)
Generator Protection	445.12, 445.13
Feeder Sizing	215.2
Load Calculations	220.12, 220.14, 220.44
Feeder Taps	240.21(B)
Standard OCPD Ratings	240.6
Conductor Protection	240.4
Motor OCPD	430.52, Table 430.52
Motor Overload	430.32
Motor Feeders	430.24
Working Space	110.26
Arc-Energy Reduction	240.87

1.9 Inspection and Supervision Points

As a master, you are responsible for the final sign-off. On site, verify the following:

106.Neutral Isolation: Confirm that the neutral bus is isolated from the equipment grounding bus in all subpanels. Use a megger to verify no continuity between neutral and ground downstream of the service.

107.Torque Checks: Verify that all lugs are torqued to the manufacturer's specifications. Loose connections are the leading cause of equipment failure.

108.GFP Settings: For services over 1000 A, verify the ground-fault protection settings are not set above 1200 A and that the time delay is coordinated with downstream devices.

109.Working Space: Measure the actual clearances. A 42-inch clearance is required for a 277/480 V panelboard facing a grounded surface (Condition 2). Do not accept a 36-inch clearance.

110.Transformer Ventilation: Ensure that the transformer's ventilation openings are not blocked by stored materials. The clearance to combustible materials must be maintained.

111.Motor Overloads: Check the heater element size against the motor nameplate. A heater that is too large will not protect the motor; one that is too small will cause nuisance trips.

1.10 Common Exam Traps

The 125% Rule: Always apply 125% to continuous loads BEFORE comparing to the conductor ampacity. Do not apply it to the OCPD rating unless the OCPD is protecting a continuous load.

SDS Grounding: The GEC for a transformer is sized from the SECONDARY conductors, not the primary. A 480 V primary with a 208 V secondary – the GEC is based on the 208 V secondary conductors.

Generator Conductors: Use the generator nameplate current × 1.15, not the calculated load. If the generator is 100 kW but the load is only 50 kW, the conductors must still be sized for 115% of the generator rating.

Motor Feeder vs. Branch: The feeder for multiple motors uses 125% of the LARGEST motor, not 125% of all motors. The sum of the others is at 100%.

Six-Handle Rule: The six disconnects must be grouped. You cannot place three disconnects on one side of the building and three on the other.

Next Higher Standard Size: This rule applies ONLY when the conductor ampacity is not a standard rating. It does not allow you to oversize an OCPD for a motor unless specifically permitted by 430.52.

Working Space Width: The minimum width is 30 inches, but if the equipment is 36 inches wide, the working space must be 36 inches wide. The width is the greater of the two.

Summary

Mastering equipment and devices requires a synthesis of load calculations, conductor sizing, and overcurrent protection. The key is to understand the intent of the code: to protect life and property. For the Kentucky Master exam, focus on the thresholds (1000 A for GFP, 800 A for the next-size rule, 50 feet for "within sight") and the distinction between service, feeder, and branch-circuit rules. Use the code navigation table to quickly find the exact section during the exam. Remember that the NEC is a minimum standard – your field experience and judgment are what elevate you from a journeyman to a master.

Equipment & Devices