General Electrical Knowledge

General Electrical Knowledge

Learning Objectives

Upon completing this chapter, you will be able to:

1.1 Three-Phase Systems and Voltage Drop

A master electrician must understand three-phase power fundamentals to properly size conductors and equipment. In a balanced three-phase system, total power is calculated as P = √3 × V × I × power factor (for line-to-line voltage). For line-to-neutral calculations, use P = 3 × V × I × power factor.

Voltage drop is a performance consideration, not a safety requirement, except where specifically mandated (e.g., 210.19 for branch circuits, 215.2 for feeders, and 250.122 for equipment grounding conductors). The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total (feeder plus branch circuit). For three-phase circuits, voltage drop is calculated using:

VD = (√3 × K × I × L) / CM

where K is the conductor resistivity constant (approximately 12.9 for copper, 21.2 for aluminum), I is the current in amperes, L is the one-way length in feet, and CM is the circular mil area of the conductor.

Exam trap: When calculating voltage drop for three-phase systems, do not use the single-phase formula (2 × K × I × L / CM). The √3 factor is often forgotten, leading to undersized conductors.

1.2 Services and Service Equipment (Article 230)

A service is the conductors and equipment that deliver electric power from the utility to the service disconnecting means. The service point is the interface between utility and premises wiring.

Service conductors must be sized per 230.42 to carry the computed load per Article 220, with a minimum size of 8 AWG copper or 6 AWG aluminum for overhead service conductors (230.23). Underground service conductors have no minimum size but must be sized for the load.

Service disconnecting means (230.70–230.85) must:

Service overcurrent protection (230.90) must protect the service conductors against overload and short circuit. The rating must not exceed the ampacity of the conductors, except where the next standard size (per 240.6) is permitted when the ampacity does not correspond to a standard rating.

Exam trap: For services with multiple disconnects, each disconnect must be rated for the load it serves, but the total of all disconnects must not be less than the computed load. The 6-disconnect rule counts each handle, not each enclosure.

Inspection point: Verify that the service disconnecting means is marked "Suitable for Use as Service Equipment" when it is the only disconnecting means, per 230.66.

1.3 Separately Derived Systems (Article 250 Part III)

A separately derived system is a premises wiring system whose power is derived from a source of electrical energy (e.g., transformer, generator) that has no direct electrical connection to the supply conductors from another system.

Bonding requirements (250.30):

Grounding electrode conductor (250.30(A)(4)): For a separately derived system, a grounding electrode conductor must connect the grounded conductor to a grounding electrode. The size is based on Table 250.66, using the area of the largest ungrounded conductor. The grounding electrode can be a building steel electrode, a concrete-encased electrode, or a ground rod, but the connection must be made at the same point as the system bonding jumper.

Generator applications: A generator with a transfer switch that opens the neutral is a separately derived system. If the transfer switch does not switch the neutral (solid neutral), the generator is not separately derived, and the neutral remains bonded at the service.

Exam trap: For a transformer supplying a panelboard, the neutral must be bonded to the equipment grounding conductor at the transformer (or first disconnect) and must NOT be bonded again at the panelboard. A common code violation is installing a bonding screw in the panelboard neutral bar when the transformer is the source.

1.4 Feeder Sizing and Load Calculations (Article 220)

The standard method (220.40–220.61) applies to all installations unless the optional method is permitted.

General lighting load (Table 220.12): For dwelling units, 3 VA/ft²; for commercial (banks, offices), 3.5 VA/ft²; for warehouses, 1.25 VA/ft². These are minimum values.

Receptacle loads (220.14): For non-dwelling occupancies, receptacles are calculated at 180 VA each. The first 10 kVA of receptacle load is permitted at 100% demand factor; the remainder at 50% (220.44).

Feeder sizing (215.2): The feeder must have an ampacity not less than the computed load, and must be sized to accommodate voltage drop. The minimum feeder size must also be adequate for the largest motor plus 125% of the remaining load (430.24).

Optional method for feeders (220.86): For existing dwelling units with electric space heating, air conditioning, or both, the feeder can be sized using the optional calculation. The computed load is the larger of the heating or cooling load, plus the first 8 kVA of remaining load at 100%, and the remainder at 40%.

Exam trap: When calculating a feeder for a commercial kitchen, the demand factors of Table 220.56 apply. Do not apply the dwelling unit demand factors of 220.52 to commercial occupancies.

1.5 Overcurrent Protection and Coordination (Article 240)

Standard ampere ratings (240.6): 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, 6000.

Overcurrent device location (240.21): Overcurrent devices must be located where conductors receive their supply, with exceptions for taps (240.21(B) for feeders, 240.21(C) for transformer secondaries).

Transformer secondary protection (240.21(C)): The primary protection must not exceed 125% of the primary rated current (for transformers with 9% or less impedance). If primary protection exceeds this, secondary protection is required within 10 feet of the transformer.

Coordination (240.12): Where required by other articles (e.g., 700.27 for emergency systems, 701.27 for legally required standby systems), overcurrent devices must be coordinated so that a fault on a branch circuit clears only the branch circuit device, not the feeder or main.

Short-circuit current rating (SCCR): Equipment must be rated for the available fault current at its terminals (110.10). A master must verify that the interrupting rating of the overcurrent device exceeds the available fault current.

Exam trap: The 10-foot tap rule (240.21(B)(1)) requires the tap conductor ampacity to be at least 10% of the rating of the overcurrent device protecting the feeder. A common error is using 25% (which applies to the 25-foot tap rule).

1.6 Motors and Generators (Articles 430 and 445)

Motor branch-circuit conductors (430.22): Must have an ampacity of at least 125% of the motor full-load current (FLC). For continuous duty motors, use Table 430.247 (DC), 430.248 (single-phase), or 430.250 (three-phase) to determine FLC.

Motor branch-circuit short-circuit and ground-fault protection (430.52): The rating must not exceed the percentages in Table 430.52:

If these values do not correspond to standard ratings, the next higher standard size is permitted (430.52(C)(1) Exception 1).

Motor feeder conductors (430.24): Must be sized at 125% of the largest motor FLC plus the sum of the FLC of all other motors on the feeder.

Motor feeder overcurrent protection (430.62): The feeder protection must be sized at the largest branch-circuit protection device plus the sum of the FLC of all other motors.

Generators (Article 445): Generator conductors must be sized at 115% of the generator nameplate current rating (445.13). Overcurrent protection must be provided per 445.12, but if the generator is a separately derived system, the bonding and grounding rules of 250.30 apply.

Exam trap: For a motor with a nameplate full-load current that differs from the table value, use the table value for conductor sizing and the nameplate value for overload protection (430.32). Mixing these up is a classic error.

1.7 Commercial and Industrial Installations

Panelboards (408.36): Must have overcurrent protection on the supply side, with the protection rating not exceeding the panelboard rating. Exception: a panelboard supplied by a transformer with primary protection per 240.21(C) may be protected at the primary.

Switchboards and switchgear (408.3): Must have bus bars rated for the available fault current. The equipment grounding conductor must be sized per Table 250.122.

Wiring methods (Article 300): In commercial occupancies, conduit fill must not exceed 40% for three or more conductors (Chapter 9, Table 1). For industrial installations, cable trays (Article 392) have specific ampacity adjustment factors per 392.80.

Receptacle placement (210.52 for dwellings; 210.62 for commercial): Commercial occupancies must have receptacles within 25 feet of permanently installed equipment (210.63). Service areas require at least one receptacle.

Inspection point: Verify that all equipment grounding conductors are properly terminated and that no bonding screw or jumper is installed in a panelboard that is supplied by a separately derived system (other than the source).

1.8 Code Navigation

1.9 Inspection and Supervision Points

When supervising an installation, a master electrician must verify:

1.10 Common Exam Traps

Summary

Master-level knowledge of general electrical theory requires fluency in three-phase calculations, service and separately derived system rules, feeder sizing, overcurrent coordination, and motor applications. The NEC is organized to support these calculations, but the master must know where to look and how to apply the tables and exceptions correctly. On the exam, always verify the article number, check the table notes, and confirm whether an exception applies before selecting a final answer.