Master Electrician Practice study guide with diagrams.
General Electrical Knowledge
Learning Objectives
Upon completing this chapter, you will be able to:
Calculate and apply 3-phase power formulas, including balanced and unbalanced loads, and correct for power factor.
Define the components of services and service equipment, and apply NEC requirements for service disconnects, grounding, and overcurrent protection.
Distinguish between separately derived systems (transformers, generators) and non-separately derived systems, and apply grounding and bonding rules for each.
Size feeders and branch circuits using the correct demand factors, continuous load multipliers, and voltage drop considerations.
Apply NEC rules for commercial and industrial installations, including multi-tenant buildings, common areas, and kitchen equipment.
Select motor and generator overcurrent protection, conductors, and disconnects per Articles 430 and 445.
Perform basic overcurrent protection coordination (selective coordination) for emergency and legally required systems.
Navigate the NEC efficiently using article structure, tables, and informational notes.
1.1 Three-Phase Systems and Power Calculations
A master electrician must be fluent in 3-phase calculations, not just for design but for verifying existing installations and troubleshooting.
Basic Formulas:
Real Power (kW): P = √3 × V_LL × I × PF (for balanced 3-phase)
Apparent Power (kVA): S = √3 × V_LL × I
Reactive Power (kVAR): Q = √3 × V_LL × I × sin(θ)
Power Factor (PF): PF = P / S = cos(θ)
Where V_LL is line-to-line voltage, I is line current, and θ is the phase angle between voltage and current.
Key Relationships:
For a wye system: V_LL = √3 × V_LN. Line current equals phase current.
For a delta system: V_LL = V_phase. Line current = √3 × phase current.
Unbalanced Loads: When loads are not equal across phases, you must sum the individual phase powers. The neutral conductor in a wye system carries the vector sum of the unbalanced phase currents. For a 4-wire, 3-phase wye system, the neutral current can be calculated using phasor addition, not simple arithmetic. In practice, a master checks for excessive neutral current on shared neutrals, especially with nonlinear loads (see 310.15 and 220.61 for neutral sizing allowances).
Power Factor Correction: Adding capacitors in parallel with inductive loads raises the PF. The required kVAR is: Q_c = P × (tan θ_1 – tan θ_2), where θ_1 and θ_2 are the original and desired phase angles. Capacitors must be rated for the system voltage and installed with discharge resistors per 460.28.
Exam Trap: Do not use √3 for single-phase loads. Also, remember that kVA is the unit used for transformer and generator sizing, while kW is used for energy billing and motor output. Always convert kW to kVA using the PF before sizing transformers.
1.2 Services and Service Equipment
Article 230 governs services. A service is the conductors and equipment that deliver power from the utility to the service disconnecting means.
Service Components:
Service Point: The point of connection between utility and premises wiring.
Service Conductors: From the service point to the service disconnecting means.
Service Equipment: The disconnecting means and overcurrent devices, typically a main breaker panelboard or switchboard.
Service Disconnect Requirements (230.70 – 230.71):
Each service must have a disconnecting means that is readily accessible, plainly marked, and capable of being locked in the open position.
For a service with multiple disconnects, the 2023 NEC requires a single main service disconnecting means for all buildings or structures (230.71). Exception: for fire pumps, emergency systems, and legally required standby systems, additional disconnects are permitted.
The disconnect must be located at a readily accessible point nearest the point of entrance of the service conductors (230.70).
Service Overcurrent Protection (230.90):
Each ungrounded service conductor must have an overcurrent device. The rating must not exceed the ampacity of the conductor.
Exception: For services with multiple disconnects, each disconnect can have its own overcurrent device, provided the sum of the ratings does not exceed the conductor ampacity.
Grounding and Bonding of Services (250.24):
The grounded (neutral) conductor must be connected to the grounding electrode system at the service. This is the single point of grounding for the building.
The main bonding jumper connects the grounded conductor to the equipment grounding conductor and the service enclosure.
Critical Rule: The grounded conductor must not be bonded to the equipment grounding conductor on the load side of the service disconnect (250.142(B)). This is a common violation and a frequent exam question.
Inspection Point: Verify that the service entrance conductors are protected from physical damage (230.50), that the service panel is properly bonded, and that the grounding electrode conductor is sized per 250.66.
1.3 Separately Derived Systems (SDS)
A separately derived system has no direct electrical connection to the supply system, other than through bonding and grounding. Examples: a transformer secondary, a generator with a transfer switch, or an inverter output.
Grounding Requirements (250.30):
The system must have a grounding electrode conductor connected to the system's grounding electrode. This is typically at the first disconnecting means or at the source (transformer).
The grounded conductor (neutral) must be bonded to the equipment grounding conductor at the SDS source or at the first disconnecting means, but not at both.
A system bonding jumper is required at the source or the first disconnecting means.
Transformer Installations (Article 450):
Transformer overcurrent protection per 450.3. Primary protection can be sized at 125% of rated primary current. If the primary OCPD is sized at 250% or less, secondary protection is not required (for single transformers). If primary is over 250%, secondary protection is required.
Exam Trap: The 125% factor applies to continuous loads on the secondary. A transformer feeding a 100 A continuous load must be sized at 125 A minimum.
Generator Installations (Article 445):
Generators are SDS when they have a transfer switch that opens the neutral. If the transfer switch does not switch the neutral, the generator is not an SDS and must be grounded as a separately derived system only if the neutral is bonded.
Grounding: The generator frame must be bonded to the equipment grounding conductor. The neutral must be bonded at the generator only if it is an SDS.
Inspection Point: Check that the neutral-to-case bond is removed in the generator panel if the transfer switch is a 3-pole (switching only phases) type. A 4-pole transfer switch switches the neutral and creates an SDS.
1.4 Feeder Sizing and Calculations
Feeder sizing is a core master-level skill. The process involves:
65.Calculate the load per Article 220.
66.Apply demand factors (e.g., 220.42 for lighting, 220.44 for dryers, 220.54 for cooking equipment).
67.Add continuous loads at 125% (210.19(A)(1) for branch circuits, 215.2(A)(1) for feeders).
68.Select conductor size based on ampacity per 310.15 and 310.16 (or 310.17 for free air).
69.Check voltage drop (informational notes in 210.19 and 215.2 recommend 3% for branch circuits and 5% total).
Continuous Loads: A continuous load is one where the maximum current is expected to continue for 3 hours or more (100). Examples: lighting in commercial buildings, motors running continuously, and electric heat.
Demand Factors:
General lighting (220.42): For dwelling units, 3 VA/ft². For commercial, use Table 220.12 (e.g., 1.2 VA/ft² for banks, 1.5 VA/ft² for offices).
Receptacles: For general-purpose receptacles in commercial buildings, 180 VA per receptacle (220.14(I)). The first 10 kVA at 100%, the remainder at 50% (220.44).
Kitchen equipment (220.56): For commercial kitchens, the load can be calculated at the nameplate rating, but demand factors apply if there are more than 3 pieces of equipment.
Feeder Neutral Sizing (220.61):
The neutral must be sized for the maximum unbalanced load. For 3-phase wye systems, the neutral carries the unbalanced current.
Nonlinear loads: If the load is more than 50% nonlinear (e.g., electronic ballasts, computers), the neutral must be counted as a current-carrying conductor (310.15(E)). This often requires a larger neutral or derating of the conductors.
Exam Trap: Do not forget to multiply continuous loads by 125% before comparing to the conductor ampacity. Also, remember that the 125% factor applies to the overcurrent device rating as well (210.20(A)).
1.5 Commercial and Industrial Installations
Multi-Tenant Buildings (230.72(C), 230.82):
Each tenant must have access to their own disconnecting means. For multi-occupancy buildings, each occupant must have a means to disconnect their own service.
Common areas (hallways, parking lots) must have their own service or feeder.
Commercial Kitchen Equipment (Article 422, Part II):
Fixed appliances must have disconnecting means (422.31). For cord-and-plug connected appliances, the plug is the disconnect.
Ventilation: Exhaust hoods and duct systems must be interlocked with the cooking equipment (see NFPA 96, but the NEC requires the interlock in 422.53).
Industrial Machinery (Article 670):
Industrial machinery must have a disconnecting means that is readily accessible and can be locked in the open position (670.4).
The machine must be supplied by a circuit with a short-circuit current rating (SCCR) that is at least the available fault current at the machine's terminals.
Inspection Point: For commercial kitchens, verify that all equipment is properly grounded and that the neutral is not shared between different phases if nonlinear loads are present.
1.6 Motors and Generators
Motor Circuits (Article 430):
Branch Circuit Conductors (430.22): Must be sized at 125% of the motor's full-load current (FLC), not the nameplate current. Use Tables 430.248 (single-phase) and 430.250 (3-phase) for FLC values.
Overload Protection (430.32): Must protect the motor against running overloads. Typically sized at 125% of the motor nameplate current for motors with a service factor of 1.15 or more, or a temperature rise of 40°C or less. For other motors, use 115%.
Short-Circuit and Ground-Fault Protection (430.52): The motor branch circuit OCPD (fuse or breaker) must be sized per Table 430.52. For example, a 3-phase squirrel-cage motor can have an inverse-time breaker at 250% of FLC, or a non-time-delay fuse at 300%.
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 15 m (50 ft) away.
Motor Feeder Sizing (430.24):
The feeder must be sized for the sum of all motor FLCs, plus 125% of the largest motor FLC.
Generators (Article 445):
Generator conductors must be sized at 115% of the generator's rated output (445.13). The overcurrent device must be rated at 115% of the generator's rated current (445.12).
Exam Trap: Do not use the 125% factor for generators; it is 115% for both conductors and OCPD.
1.7 Overcurrent Protection Coordination
Selective Coordination (Articles 700, 701, 708):
For emergency systems (700.28), legally required standby systems (701.27), and critical operations power systems (708.54), overcurrent devices must be selectively coordinated. This means that when a fault occurs, only the device nearest the fault opens, not the upstream device.
This is achieved by ensuring that the time-current curves of the upstream device are above and to the right of the downstream device.
Coordination Study: A master may be required to review a coordination study. The study must show that the total clearing time of the downstream device is less than the minimum melting time of the upstream fuse, or the unlatching time of the upstream breaker.
Inspection Point: Check that the OCPD ratings and types match the coordination study. A common field issue is replacing a fuse with a higher-rated one, which destroys coordination.
1.8 Code Navigation
Efficient navigation is essential for the open-book exam. Memorize the following map:
Topic
NEC Location
Definitions
Article 100
Branch Circuits
Article 210
Feeders
Article 215
Services
Article 230
Overcurrent Protection
Article 240
Grounding & Bonding
Article 250
Wiring Methods
Articles 300–398
Conductors for General Wiring
Article 310
Motors, Motor Circuits
Article 430
Generators
Article 445
Transformers
Article 450
Emergency Systems
Article 700
Legally Required Standby
Article 701
Optional Standby
Article 702
Load Calculations
Article 220
Ampacity Tables
Table 310.16 (60/75/90°C)
Motor FLC Tables
Tables 430.248, 430.250
Demand Factors
Tables 220.42, 220.44, 220.54
Exam Strategy: When a question asks for a specific code section, first identify the topic (e.g., motor overloads), then go to the article (430), then the part (Part III for overloads), then the section (430.32). Use the index in the back of the NEC for less common terms.
1.9 Inspection and Supervision Points
As a master, you are responsible for the work of others. Key checks:
Service Panels: Verify the main bonding jumper is in place, the neutral is isolated from the enclosure on the load side, and all terminations are torqued to specification.
Transformers: Check that the enclosure is bonded, the secondary neutral is grounded at the first disconnecting means, and the overcurrent protection matches the primary and secondary ratings.
Motor Installations: Confirm the overload relay is set to the nameplate current, not the FLC. Verify the disconnecting means is within sight and can be locked.
Generators: Ensure the transfer switch is the correct type (3-pole vs. 4-pole) and that the neutral bonding is correct for the system type.
1.10 Common Exam Traps
Continuous Loads: Forgetting the 125% multiplier on continuous loads.
Motor vs. Nameplate: Using the motor nameplate current instead of the Table FLC for conductor sizing.
Generator vs. Motor: Using 125% for generator conductors (it is 115%).
Neutral as Current-Carrying: Ignoring the requirement to count the neutral as a current-carrying conductor for nonlinear loads.
Multiple Disconnects: Assuming six disconnects are still allowed for services (the 2023 NEC requires a single main disconnect for most services).
SDS Bonding: Bonding the neutral at both the transformer and the first disconnect, creating a parallel path for neutral current.
Voltage Drop: Treating the informational notes in 210.19 and 215.2 as mandatory requirements (they are recommendations, not code requirements).
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