Wiring Methods & Materials

Learning Objectives

Upon completing this chapter, you will be able to:

4.Apply the general wiring methods of NEC Chapter 3 to commercial and industrial installations, including the correct use of conduit, cable, and raceway systems.

5.Size service conductors and service equipment for 3-phase systems, including calculating the minimum rating of the service disconnecting means.

6.Identify and properly ground separately derived systems (transformers and generators), including the required bonding and grounding electrode connections.

7.Perform feeder sizing calculations for continuous and non-continuous loads, including the application of demand factors and adjustment factors.

8.Apply the specific requirements for motor and generator installations, including conductor sizing, overcurrent protection, and disconnecting means.

9.Coordinate overcurrent protection devices to achieve selective coordination where required by the Code.

10.Navigate the NEC efficiently to locate requirements for specific wiring methods and materials during the open-book exam.

1.1 General Wiring Methods (Article 300)

Article 300 is the foundation for all wiring installations. A master electrician must understand the scope and the specific rules that apply across all occupancy types.

General Requirements (300.4, 300.5, 300.7):

Protection Against Physical Damage: All conductors and cables must be protected where subject to physical damage. This includes running boards for cables in accessible attics (300.4(C)) and the requirement for cables to be secured within 300 mm (12 in.) of a box or fitting.

Minimum Cover Depths (Table 300.5): Direct-buried cables and conduits must be installed at specific depths. For example, direct-buried UF cable under a residential driveway requires 24 in. of cover, while rigid metal conduit (RMC) only requires 6 in. A master must know these depths for site supervision, especially for services and feeders to outbuildings.

Raceways Exposed to Different Temperatures (300.7(A)): Where a raceway passes from a warm to a cold area (e.g., a walk-in freezer), it must be sealed to prevent air circulation. This is a common inspection point in commercial refrigeration.

Conductors in Parallel (300.10, 310.10(G)):

This is a critical master-level concept. For circuits over 600V, and for larger ampacity circuits, conductors are often paralleled.

Requirements for Parallel Installation: Conductors of the same phase, polarity, neutral, or grounded conductor must be the same length, same conductor material (copper or aluminum), same cross-sectional area (size), and have the same insulation type.

Ampacity Adjustment: When conductors are paralleled, the ampacity of each conductor is determined by its own cross-section, and the total ampacity is the sum of the paralleled conductors. However, if more than three current-carrying conductors are in a single raceway, the ampacity of each conductor must be adjusted per Table 310.15(C)(1).

1.2 Conductors and Ampacity (Article 310)

Conductor Types and Insulation (310.4, Table 310.4(A)):

The exam will test your ability to select the correct conductor for an application. Key insulation types include:

THHN/THWN-2: The most common thermoplastic insulation, rated 90°C dry and 75°C wet. The "2" rating means it is 90°C in both wet and dry locations.

XHHW-2: Cross-linked polyethylene, rated 90°C wet or dry. Often preferred for industrial applications due to its toughness.

USE-2: Underground Service Entrance cable, rated 90°C for wet locations.

Ampacity Tables (Table 310.15(B)(16)):

This is the primary table for standard ampacities. A master must understand the three temperature columns (60°C, 75°C, 90°C) and when to use each.

Termination Temperature Limitations (110.14(C)): The ampacity of a conductor is limited by the lowest temperature rating of any termination, device, or conductor in the circuit. For circuits rated 100A or less, or for #14 AWG through #1 AWG, the 60°C column is used unless the equipment is specifically listed for 75°C or higher. For circuits over 100A, or for conductors larger than #1 AWG, the 75°C column is typically used. The 90°C column is only used for derating purposes (e.g., ambient temperature or conduit fill), not for the final ampacity at the termination.

Adjustment and Correction Factors (310.15(B)(1), (C)(1)):

Ambient Temperature Correction: Table 310.15(B)(2)(a) provides multipliers for ambient temperatures other than 30°C (86°F). For example, a conductor rated at 75°C installed in a 50°C ambient must have its ampacity multiplied by 0.75.

Conduit Fill Adjustment: When more than three current-carrying conductors are in a raceway or cable, Table 310.15(C)(1) requires an adjustment factor. For 4-6 conductors, the factor is 0.80; for 7-9, it is 0.70; for 10-20, it is 0.50.

Order of Application: The correction factor for ambient temperature is applied first, followed by the adjustment factor for conduit fill. The final adjusted ampacity must not exceed the termination temperature rating.

1.3 Services and Service Equipment (Article 230)

This is a high-yield area for the master exam. The master is responsible for the design and installation of the service, which is the point of connection between the utility and the premises wiring.

Service Conductors (230.42):

Minimum Size: Service conductors must have an ampacity of at least the sum of the non-continuous loads plus 125% of the continuous loads, based on the maximum load served.

Minimum Size: The minimum size for service conductors is 8 AWG copper or 6 AWG aluminum (230.42(B)).

Service Disconnecting Means (230.70, 230.71, 230.79):

Location: The service disconnecting means must be at a readily accessible location nearest the point of entrance of the service conductors.

Number of Disconnects: The 2023 NEC now requires a single service disconnecting means for each service (230.71). This is a significant change from previous editions that allowed up to six. Each occupant must have access to their own disconnecting means.

Rating: The service disconnecting means must have a rating of not less than the calculated load. For a one-family dwelling, the minimum is 100A (230.79(C)). For all other installations, the minimum is 60A (230.79(D)).

Ground-Fault Protection (230.95):

For solidly grounded wye electrical services of more than 150V to ground, but not exceeding 600V phase-to-phase, the service disconnecting means must be provided with ground-fault protection. The maximum setting is 1200A, and the maximum time delay is 1 second for fault currents of 3000A or more. This is a critical safety requirement for large commercial and industrial services.

1.4 Feeders and Branch Circuits (Articles 210, 215)

Branch Circuit Requirements (210.19, 210.20):

Conductor Sizing: Branch circuit conductors must have an ampacity of not less than the maximum load served. For continuous loads, the ampacity must be at least 125% of the continuous load.

Overcurrent Protection: The rating of the branch circuit overcurrent device must not be less than the non-continuous load plus 125% of the continuous load.

Feeder Requirements (215.2, 215.3):

Conductor Sizing: Feeder conductors must be sized to supply the total load of the connected branch circuits. The minimum feeder ampacity is the sum of all non-continuous loads plus 125% of all continuous loads.

Feeder Overcurrent Protection: The feeder overcurrent device must be rated to protect the feeder conductors. However, where the feeder supplies a panelboard with multiple branch circuits, the feeder overcurrent device must not exceed the rating of the panelboard (408.36).

Feeder Taps (240.21(B)):

This is a common source of exam questions. A feeder tap is a conductor connected to a feeder that is protected by the feeder overcurrent device, not by its own device.

10-Foot Tap Rule (240.21(B)(1)): A tap conductor can be up to 10 ft long if its ampacity is not less than the combined load, and not less than 10% of the rating of the overcurrent device protecting the feeder.

25-Foot Tap Rule (240.21(B)(2)): A tap conductor can be up to 25 ft long if its ampacity is not less than one-third of the rating of the overcurrent device protecting the feeder.

1.5 Separately Derived Systems (Article 250.30)

A separately derived system (SDS) is a premises wiring system whose power is derived from a battery, solar photovoltaic system, or a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. This is a critical concept for transformers and generators.

Grounding and Bonding (250.30(A)):

System Bonding Jumper: A system bonding jumper must connect the grounded conductor to the equipment grounding conductor at the source (e.g., the transformer secondary) or at the first disconnecting means of the SDS.

Grounding Electrode Conductor: The grounded conductor must be connected to a grounding electrode at the source or the first disconnecting means. The size of the grounding electrode conductor is based on the size of the largest ungrounded conductor of the SDS, per Table 250.66.

Equipment Grounding Conductor: An equipment grounding conductor must be run with the supply conductors from the SDS to the first disconnecting means.

Transformer Installations (450.3):

Overcurrent Protection: Transformers must be protected against overcurrent. Primary-only protection is allowed if the primary overcurrent device is rated at not more than 125% of the transformer primary current. If the primary device is rated at more than 125% but not more than 250%, secondary protection is also required.

Ventilation: Transformer rooms must be ventilated to dissipate heat. The ventilation must not be obstructed.

Generator Installations (Article 445):

Disconnecting Means: Generators must have a disconnecting means that is readily accessible and located within sight of the generator.

Overcurrent Protection: Generators must be protected from overcurrent. The overcurrent device must be rated to protect the generator from overload.

Transfer Switches: Where a generator is used as a backup power source, a transfer switch is required to prevent backfeeding. The transfer switch must be listed for the purpose and must be capable of interrupting the maximum fault current available.

1.6 Motors and Generators (Article 430)

Motor circuits are a complex area requiring careful application of multiple articles.

Motor Circuit Conductors (430.22):

Single Motor: The branch-circuit conductors supplying a single motor must have an ampacity of not less than 125% of the motor's full-load current (FLC).

Several Motors: The conductors supplying two or more motors must have an ampacity of not less than 125% of the largest motor FLC plus the sum of the FLCs of all other motors.

Motor Overload Protection (430.32):

Thermal Protection: Motors must be protected against overload. The overload device (e.g., a heater in a starter) must be sized at not more than 115% to 125% of the motor's nameplate full-load current, depending on the motor's service factor and temperature rise.

Motor Short-Circuit and Ground-Fault Protection (430.52):

Branch-Circuit Overcurrent Device: The motor branch-circuit overcurrent device (fuse or breaker) must be sized to allow the motor to start without opening, but must protect the circuit against short circuits. Table 430.52 provides the maximum percentages of FLC for different types of motors and overcurrent devices. For example, a NEMA Design B motor with an inverse-time breaker can be protected at up to 250% of FLC.

Motor Disconnecting Means (430.102, 430.109):

Location: A disconnecting means 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.

Type: The disconnecting means must be a listed motor-circuit switch, a molded-case circuit breaker, or another approved device.

1.7 Overcurrent Protection Coordination (Article 240)

Selective Coordination (240.12):

Selective coordination is the localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.

Where Required: Selective coordination is required for life safety systems, including emergency systems (Article 700), legally required standby systems (Article 701), and critical operations power systems (Article 708).

Application: For these systems, the overcurrent devices must be coordinated so that a fault on a branch circuit will not cause the feeder or service overcurrent device to open.

Coordination Study:

A master electrician must be able to read and understand a coordination study. This study plots the time-current curves of the overcurrent devices to ensure that the device closest to the fault opens first.

1.8 Code Navigation

A master must be able to find requirements quickly. Here is a quick reference guide:

Topic	Primary Article	Key Sections/Tables
General Wiring	300	300.4, 300.5, Table 300.5
Conductors	310	310.4, 310.10(G), 310.15(B)(16), 310.15(C)(1)
Services	230	230.42, 230.70, 230.71, 230.79, 230.95
Branch Circuits	210	210.19, 210.20
Feeders	215	215.2, 215.3
Overcurrent Protection	240	240.21(B), 240.24, 240.12
Grounding & Bonding	250	250.30, 250.66
Transformers	450	450.3
Generators	445	445.18, 445.20
Motors	430	430.22, 430.32, 430.52, Table 430.52
Emergency Systems	700	700.27 (Selective Coordination)
Haz Loc	500	500.5, 500.6, 501.10

1.9 Inspection and Supervision Points

As a master, you are responsible for the final inspection and sign-off. Here are key points to verify:

105.Service Entrance: Confirm the service disconnecting means is a single device, is readily accessible, and is properly rated. Verify that the grounding electrode conductor is properly sized and connected to an approved electrode.

106.Transformer Room: Check for proper ventilation, clearance around the transformer, and the correct application of the system bonding jumper and grounding electrode conductor on the secondary.

107.Motor Installation: Verify the motor is within sight of its disconnecting means. Check that the overload heaters are sized to the motor nameplate, not the FLC table. Confirm the branch-circuit overcurrent device does not exceed the Table 430.52 percentages.

108.Conduit Fill: Spot-check conduit fill calculations for feeders with multiple parallel conductors. Ensure the adjustment factors from Table 310.15(C)(1) have been applied.

109.Bonding: Verify that all metal raceways, boxes, and equipment are properly bonded to the equipment grounding conductor. Check for the presence of bonding bushings where concentric or eccentric knockouts are used.

1.10 Common Exam Traps

The 60°C Trap: Forgetting that the ampacity of a conductor is limited by the termination temperature. For a #6 AWG THHN conductor (90°C rating), the ampacity is 75A, but if terminated on a 75°C terminal, the ampacity is limited to 65A (the 75°C column).

The 125% Continuous Load Trap: Forgetting to multiply continuous loads by 125% when sizing conductors and overcurrent devices. This applies to branch circuits, feeders, and services.

The Motor FLC vs. Nameplate Trap: Using the motor nameplate current to size the branch-circuit conductors and overcurrent device. The FLC from Table 430.248 or 430.250 must be used for these calculations. The nameplate is only used for overload protection.

The Six-Disconnect Rule Trap: The 2023 NEC now requires a single service disconnecting means for each service. The old "six-handle" rule is gone for new installations.

The Neutral Conductor Trap: For a 3-phase, 4-wire wye system with nonlinear loads, the neutral conductor is considered a current-carrying conductor and must be counted when applying the conduit fill adjustment factors.

The Parallel Conductor Trap: Forgetting that paralleled conductors must be the same length, size, and material. You cannot parallel a 500 kcmil copper conductor with a 500 kcmil aluminum conductor.

Summary

This chapter has covered the core requirements for wiring methods and materials at the master level. The key to success on the NH Master Exam is not just memorizing rules, but understanding how the articles interact. For example, a motor feeder calculation requires the use of Article 430 (motor rules), Article 310 (ampacity), and Article 240 (overcurrent protection). By mastering the navigation of the Code and understanding the "why" behind the rules, you will be well-prepared to supervise complex installations and pass the exam.

Wiring Methods & Materials