Equipment for General Use

Delaware Master Electrician Exam — Open Book (2023 NEC)

Learning Objectives

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

6.Apply NEC Article 430 rules to size branch-circuit conductors, short-circuit and ground-fault protection, and overload protection for single and multiple motors.

7.Distinguish between "separately derived systems" and "non-separately derived systems" and apply grounding (bonding) requirements per Article 250.30.

8.Calculate service and feeder loads for commercial/industrial occupancies using the standard method (Article 220) and identify when demand factors apply.

9.Select overcurrent protection devices with proper interrupting ratings and achieve selective coordination for emergency and legally required systems.

10.Identify the specific inspection points for motors, transformers, generators, and service equipment that a master electrician must verify before signing off.

1.1 Motors — Article 430

For a master, motor work is not just about wiring a contactor. It is about the entire circuit — from the branch-circuit overcurrent device back to the feeder and transformer.

1.1.1 Motor Full-Load Current (FLC) vs. Nameplate

The most common field error is using the motor nameplate current to size conductors and short-circuit protection. You must use the FLC tables (Tables 430.247 through 430.250) for conductor sizing and branch-circuit protection. The nameplate is used only for overload relay selection (430.32) and for the motor disconnect rating.

Single-phase: Table 430.248

Direct-current: Table 430.247

Three-phase: Table 430.250

Master trap: For a 3-phase, 460 V motor, the FLC from Table 430.250 is often lower than the nameplate. If you size the branch circuit from the nameplate, you will oversize conductors and protection — which is legal but often unnecessary. The reverse (undersizing) is a violation.

1.1.2 Branch-Circuit Conductors (430.22)

Conductors supplying a single motor must have an ampacity of not less than 125% of the motor FLC. For a motor used in a continuous-duty application, this is the minimum.

For multiple motors on one branch circuit (430.24), the conductor ampacity must be the sum of:

125% of the highest-rated motor FLC, plus

100% of the FLC of all other motors, plus

100% of the non-motor loads served.

1.1.3 Overload Protection (430.32)

Overload devices protect the motor, the conductor, and the controller from excessive heating due to running overloads. They are not short-circuit protection.

Motors of 1 HP or more: The overload device must be set at no more than 125% of the motor nameplate current (for motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less).

For all other motors: 115% of nameplate.

If the required setting is not sufficient to start the motor, you may increase it, but never above 140% for motors with a 1.15 service factor, or 130% for others (430.32(C)).

Master trap: Overload relays are sized from the nameplate, not the table FLC. A motor with a 1.0 service factor and a 40°C rise requires a lower overload setting than a "standard" motor.

1.1.4 Short-Circuit and Ground-Fault Protection (430.52)

The branch-circuit fuse or breaker protects the conductors and equipment against short circuits. The maximum permitted rating is based on a percentage of the FLC from the tables:

Non-time-delay fuse: 300%

Dual-element (time-delay) fuse: 175%

Inverse-time breaker: 250%

If these values do not permit the motor to start, the code allows an increase, but the absolute maximums are:

400% for non-time-delay fuses

225% for dual-element fuses

400% for inverse-time breakers (for motors not over 100 A)

Coordination trap: For a motor with an FLC of 20 A, an inverse-time breaker is permitted at 50 A (250%). If the motor stalls on start, you may go up to 80 A (400%). But increasing the breaker reduces the protection for the conductors — the conductors must still be protected by the breaker's rating.

1.1.5 Motor Feeder Protection (430.62)

The feeder overcurrent device protecting two or more motors must be sized at the sum of the branch-circuit protection ratings of all motors, plus the sum of the FLCs of any other loads. However, you may apply a demand factor — the feeder device can be sized at the largest branch-circuit device plus the sum of the FLCs of all other motors.

Master trap: Do not size the feeder protection from the sum of the motor FLCs. You must use the branch-circuit protection ratings (fuse/breaker sizes), not the conductor ampacities.

1.1.6 Disconnecting Means (430.102)

A disconnect must be located in sight from the motor and the driven machinery. "In sight" means within 15 m (50 ft) and visible. The disconnect must open all ungrounded conductors simultaneously. For a motor over 100 HP, a motor-circuit switch is not sufficient — you need a controller rated for the motor.

1.2 Transformers — Article 450

Transformers are the heart of commercial and industrial systems. A master must verify the installation from the primary side to the secondary.

1.2.1 Overcurrent Protection (450.3)

The rules depend on whether the transformer is primary-only protected or has primary and secondary protection.

Primary-only protection: If the primary OCPD is rated at 125% or less of the transformer primary rated current, no secondary protection is required (for transformers over 600 V, the limit is 250%).

Primary and secondary protection: The primary device can be up to 250% of the primary current (for transformers over 9 A), and the secondary device can be up to 125% of the secondary current.

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

Primary current = 75,000 / (480 × 1.732) = 90.2 A

Primary protection at 125% = 112.8 A → next standard size up = 125 A (per 240.4(B))

Secondary current = 75,000 / (208 × 1.732) = 208 A

Secondary protection at 125% = 260 A → next standard size = 300 A

If you install a 125 A primary breaker, you still need secondary protection because the secondary conductors are not protected by the primary device.

1.2.2 Secondary Conductors (240.21(C))

The secondary conductors of a transformer are treated like a tap. They must terminate in a single OCPD, and the length of the secondary conductors cannot exceed 7.5 m (25 ft) unless they meet specific conditions. For a master, this means you cannot run secondary conductors 30 m to a panelboard without a main breaker at the transformer.

1.2.3 Grounding and Bonding (250.30)

A transformer secondary is a separately derived system. The system must have:

A grounding electrode conductor connected to the secondary neutral (X0 terminal).

The neutral must be bonded to the equipment grounding conductor at the transformer (or at the first disconnecting means).

The grounding electrode must be the nearest available electrode (building steel, water pipe, or ground ring).

Master trap: A transformer with a 480 V delta primary and a 208Y/120 V secondary requires a system bonding jumper at the transformer. If you bond the neutral at the panelboard instead, you create a parallel path for neutral current on the grounding conductors — a violation of 250.6 and a shock hazard.

1.2.4 Ventilation and Clearances

Transformers must be installed so they are accessible for inspection and maintenance. The working space requirements of 110.26 apply. Dry-type transformers must have ventilation openings that are not blocked. For transformers over 112.5 kVA, the surrounding clearances must be at least 300 mm (12 in.) from combustible materials.

1.3 Generators — Article 445

Generators are treated as separately derived systems when they have no direct connection to the utility source (i.e., they are not operating in parallel).

1.3.1 Nameplate and Ratings

Every generator must have a nameplate showing the rated voltage, current, power factor, and frequency. The ampacity of the conductors from the generator terminals to the first OCPD must be not less than 115% of the generator nameplate current (445.13).

1.3.2 Overcurrent Protection

Generators must be protected from overloads. The OCPD must be rated at not more than 115% of the generator's rated current. If the generator is a stand-alone unit with its own integral protection, the external OCPD may be omitted.

1.3.3 Transfer Switches

Generators used for standby power must be connected through a transfer switch that prevents backfeeding the utility. The transfer switch must be rated for the load and must be a listed device. For a master, the critical issue is the neutral switching:

For a separately derived generator (no utility neutral connection), the transfer switch must switch the neutral as well as the phase conductors.

For a non-separately derived system (generator neutral bonded to utility neutral), the transfer switch must be a 3-pole switch (switching only the phase conductors).

Master trap: If you use a 3-pole transfer switch on a separately derived generator, you create a parallel neutral path. The generator neutral must be switched to prevent the utility neutral from being connected to the generator's grounded conductor.

1.4 Services and Service Equipment — Article 230

Service work is the highest-liability work a master performs. Errors here affect the entire building.

1.4.1 Number of Services

A building can be served by only one service unless specific exceptions apply (230.2). Additional services are permitted for:

Fire pumps

Emergency systems

Optional standby systems

Different voltage characteristics (e.g., 480 V and 208 V)

Capacity requirements (when the total load exceeds 2,000 A)

1.4.2 Service Disconnects

The service disconnecting means must disconnect all ungrounded conductors. For a service with more than one disconnect, the disconnects must be grouped in one location. Each disconnect must be rated for the connected load.

Master trap: The six-disconnect rule (230.71) allows up to six disconnects for a service. However, if you have a 4,000 A service with six 800 A disconnects, each disconnect must have a rated interrupting capacity sufficient for the available fault current at the service.

1.4.3 Service Conductors

Service conductors must be sized per Article 220. The minimum size is based on the calculated load, but the ampacity must be sufficient for the load. For a service over 800 A, the conductors must be sized for the calculated load plus a demand factor.

Master trap: For a commercial building with a calculated load of 1,200 A, you cannot use parallel sets of 500 kcmil conductors unless you verify the ampacity adjustment for more than three current-carrying conductors in a raceway.

1.5 Overcurrent Protection Coordination

1.5.1 Selective Coordination (240.12)

For emergency systems, legally required standby systems, and critical operations power systems (COPS), overcurrent devices must be selectively coordinated. This means that when a fault occurs, only the device nearest the fault opens — not the upstream feeder breaker.

Master trap: A master must verify that the time-current curves of the branch breaker and the feeder breaker do not overlap. For example, a 100 A feeder breaker with a 20 A branch breaker: if the branch breaker's instantaneous trip is set at 500 A, and the feeder breaker's instantaneous trip is at 400 A, a 450 A fault will trip both breakers. This is a coordination failure.

1.5.2 Interrupting Rating (110.9)

Every overcurrent device must have an interrupting rating not less than the available fault current at its terminals. For a service with a utility transformer, the available fault current is often 10,000 A to 50,000 A. A standard 10 kAIC breaker is not sufficient for a service.

Master trap: When you replace a breaker in an existing panelboard, you must verify the panelboard's interrupting rating. If the available fault current has increased (due to a larger utility transformer), the existing panelboard may be inadequate.

1.6 Feeders and Branch Circuits — Article 215 and 210

1.6.1 Feeder Sizing (215.2)

Feeder conductors must have an ampacity of not less than 125% of the continuous load plus 100% of the non-continuous load. For a commercial kitchen with a 100 A continuous load, the feeder must be sized for 125 A.

1.6.2 Branch Circuits (210.19)

Branch-circuit conductors must be sized for the load they serve. For a continuous load (e.g., lighting for 3 hours or more), the branch circuit must be rated at 125% of the continuous load. This means a 20 A branch circuit can supply a maximum of 16 A of continuous load.

Master trap: A 2,000 W electric heater on a 120 V circuit draws 16.7 A. This is a continuous load (heating is considered continuous). The branch circuit must be rated at 125% of 16.7 A = 20.8 A → a 25 A branch circuit is required, not a 20 A.

1.7 Code Navigation — Where to Find It

Topic	NEC Reference
Motor FLC tables	Tables 430.247–430.250
Motor branch-circuit conductors	430.22
Motor overload protection	430.32
Motor short-circuit protection	430.52
Motor feeder protection	430.62
Motor disconnects	430.102
Transformer protection	450.3
Transformer secondary taps	240.21(C)
Separately derived systems grounding	250.30
Generator conductors	445.13
Number of services	230.2
Service disconnects	230.71
Selective coordination	240.12
Interrupting rating	110.9
Feeder sizing	215.2
Branch-circuit continuous loads	210.19(A)
Standard OCPD sizes	240.6(A)
Working space	110.26
Demand factors (commercial)	Table 220.44

1.8 Inspection and Supervision Points

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

122.Motor nameplate vs. table FLC: Check the overload relay setting against the nameplate, not the table.

123.Transformer bonding: Confirm the system bonding jumper is at the transformer, not at the panelboard.

124.Generator neutral: Verify the transfer switch poles match the system type (3-pole for non-separately derived, 4-pole for separately derived).

125.Service disconnect grouping: All service disconnects must be in one location and clearly marked.

126.Interrupting ratings: Check the label on the service breaker and the panelboard for the AIC rating.

127.Continuous load calculations: Verify that the branch circuit and feeder are sized at 125% of the continuous load.

128.Working space: Measure the clearance in front of the panelboard — it must be at least 900 mm (36 in.) deep and 750 mm (30 in.) wide.

1.9 Common Exam Traps

Using nameplate current instead of Table FLC for conductor and OCPD sizing.

Sizing feeder protection from motor FLCs instead of branch-circuit protection ratings.

Forgetting the 125% factor for continuous loads on feeders and branch circuits.

Bonding the neutral at the panelboard for a separately derived system.

Using a 3-pole transfer switch for a generator that requires a switched neutral.

Oversizing the transformer primary protection beyond 250% without secondary protection.

Ignoring the interrupting rating of breakers — a 10 kAIC breaker on a 22 kAIC fault is a fire hazard.

Applying the six-disconnect rule incorrectly — the disconnects must be grouped, not scattered.

Summary

This chapter covered the core equipment rules a Delaware Master Electrician must know to supervise installations. The key to the open-book exam is not memorizing every number but knowing which table or section to open and how to apply the adjustment factors. Practice navigating the NEC quickly: for any motor question, go to Article 430; for any transformer, Article 450; for any service, Article 230. The master's edge is in the details — the 125% factors, the FLC tables, and the coordination requirements.

Equipment for General Use