Master Electrician Practice study guide with diagrams.
General Knowledge & Plan Reading
Learning Objectives
Upon completing this chapter, you will be able to:
4.Interpret commercial and industrial electrical plan symbols, schedules, and one-line diagrams.
5.Apply NEC requirements for services, service equipment, and grounding to real-world installations.
6.Perform code-compliant sizing for feeders, transformers, and motors in multi-load commercial settings.
7.Distinguish between separately derived systems and non-separately derived systems for grounding and bonding.
8.Apply overcurrent protection coordination principles, including selective coordination requirements.
9.Navigate the 2023 NEC efficiently by Article, Section, and Table to verify installations for permit sign-off.
1.1 The Master’s Role: From Plan to Permit
The Kentucky Master Electrician is responsible for the means and methods of an installation, not just its final appearance. Your signature on a permit certifies compliance with the adopted edition of the NEC (2023 in most KY jurisdictions) and all local amendments. Plan reading for a master is not about tracing circuits; it is about load verification and system coordination.
When reviewing plans, you must verify:
Service size vs. calculated load (NEC Article 220).
Voltage drop for feeders and branch circuits (NEC 210.19(A) Informational Note No. 4, and 215.2(A) Informational Note No. 2). While not a strict code requirement for most circuits, it is a design standard (3% feeder, 3% branch, 5% total) that a master should enforce on plans.
Overcurrent protection device (OCPD) ratings vs. conductor ampacity and equipment ratings.
Short-circuit current rating (SCCR) of equipment vs. the available fault current at the service (NEC 110.10).
1.2 Three-Phase Systems and Calculations
1.2.1 System Configurations
A master must instantly recognize the implications of system types:
Wye (Y): 208Y/120V (3-phase, 4-wire) and 480Y/277V (3-phase, 4-wire). The line-to-neutral voltage is the line-to-line voltage divided by √3 (1.732).
Delta (Δ): 240V (3-phase, 3-wire) or 240/120V (3-phase, 4-wire, center-tapped "high-leg" delta). The high-leg (B phase) is 208V to ground and must be identified (NEC 110.15) — orange or tagged. No loads may be connected to the high leg to neutral.
1.2.2 Power Calculations
Apparent Power (kVA): For 3-phase, kVA = (V × I × 1.732) / 1000.
Real Power (kW): kW = kVA × Power Factor (PF).
Current (Amperes): I = (kVA × 1000) / (V × 1.732).
Exam Trap: When sizing a feeder for a 3-phase panelboard, use the line-to-line voltage (e.g., 208V or 480V), not the line-to-neutral voltage (120V or 277V). Using the wrong voltage will result in a current calculation that is off by a factor of 1.732.
1.3 Services and Service Equipment (Article 230)
1.3.1 Number of Services
A building is permitted to have only one service, except for specific conditions (NEC 230.2). These include:
Fire pumps, emergency systems, legally required standby, or optional standby systems.
Different voltages, frequencies, or phase characteristics.
Multiple occupancies with specific needs.
Capacity requirements (over 2000A) where multiple services are more practical.
1.3.2 Service Disconnects
Rating: The service disconnecting means must have a rating of at least the calculated load (NEC 230.79). For one-family dwellings, the minimum is 100A. For all other occupancies, the minimum is 60A.
Number of Disconnects: The service disconnecting means must consist of not more than six switches or sets of circuit breakers (NEC 230.71). This is a grouping requirement; they must be in a single enclosure or a group of separate enclosures.
Location: Must be at a readily accessible location nearest the point of entrance of the service conductors (NEC 230.70). The "nearest point" rule is a common inspection failure — conductors cannot run through a building for hundreds of feet before reaching the disconnect.
1.3.3 Service Conductors
Minimum Size: Service conductors must have an ampacity of at least the calculated load (NEC 230.42). For 120/240V, 3-wire, single-phase services, the minimum is 100A (dwelling) or 60A (others).
Grounding Conductor: The grounded (neutral) conductor must be sized per NEC Table 250.102(C)(1) for the sum of the phase conductors' circular mil areas.
1.4 Separately Derived Systems (Article 250.30)
This is a critical distinction for a master. A separately derived system (SDS) has no direct electrical connection to the supply conductors except through a transformer, motor-generator set, or other equivalent device. Examples: a 480V-to-208Y/120V transformer, a generator with a transfer switch, or an uninterruptible power supply (UPS).
1.4.1 Grounding Requirements
For an SDS, you must:
50.Bond the system neutral to the grounding electrode system at the source (transformer) or at the first disconnecting means of the SDS.
51.Install a system bonding jumper (SBJ) — sized per Table 250.102(C)(1) — connecting the grounded conductor to the equipment grounding conductor.
52.Provide a grounding electrode — the SDS must have its own grounding electrode system connection (NEC 250.30(A)(4)). The electrode can be the nearest effectively grounded structural metal member or water pipe, but it must be within the immediate area.
1.4.2 The "Neutral-Ground Bond" Trap
In a standard service, the neutral and ground are bonded at the main service disconnect. In a separately derived system, they are bonded again at the SDS source. However, downstream of the SDS, the neutral must be isolated from ground (no parallel paths). A common site error is a panelboard fed from a transformer that has a neutral-to-ground bond screw installed — this creates a parallel neutral path and violates NEC 250.30(A)(1) and 250.6.
Inspection Point: Check the transformer enclosure and the first panelboard of the SDS. There must be exactly one neutral-to-ground bond for the SDS, and it must be at the source or the first disconnecting means, not both.
1.5 Transformer Sizing and Feeder Protection
1.5.1 Sizing
Transformers are rated in kVA. To size a transformer for a load:
60.Calculate the total connected load in kVA (including demand factors).
61.Add growth margin (typically 25%).
62.Select the next standard size (e.g., 15, 25, 37.5, 50, 75, 112.5, 150, 225, 300, 500 kVA).
1.5.2 Primary and Secondary Protection
Primary Protection: NEC 450.3(B) allows the primary OCPD to protect the transformer if it is sized at 125% of the primary full-load current (FLC). If the primary is 9A or more, the OCPD can be set at 125% (next standard size up). If the primary is less than 9A, the OCPD can be set at 167%.
Secondary Protection: If the secondary OCPD is present and sized at 125% of the secondary FLC, the primary OCPD can be larger (up to 250% for transformers over 600V, or 125% for ≤600V). For transformers ≤600V, if the primary OCPD is >125%, you must have secondary protection.
Exam Trap: The 2023 NEC removed the "next standard size up" rule for transformer primary protection in some cases. Verify the exact language of 450.3 — for primary-only protection, the OCPD must be sized at 125% and cannot exceed that rating unless the secondary is protected.
1.5.3 Feeder Conductors
Primary Feeder: Sized at 125% of the transformer primary FLC (NEC 210.19(A)(1) for branch circuits, but for feeders, use 215.2).
Secondary Feeder: Sized at 125% of the transformer secondary FLC, unless the load is less and the feeder is protected at the panelboard.
1.6 Motor and Generator Applications (Article 430 & 445)
1.6.1 Motor Branch Circuit
A master must verify the entire motor circuit, not just the breaker:
75.Motor FLC: Use NEC Tables 430.247 through 430.250, not the nameplate rating. The nameplate is for overload protection; the tables are for conductor and OCPD sizing.
76.Branch Circuit Conductor: Sized at 125% of the motor FLC (NEC 430.22).
77.Branch Circuit OCPD: Sized per NEC 430.52. The maximum rating is typically 250% of FLC for inverse-time breakers and 300% for fuses (for most AC motors). If the motor will not start under these limits, you may increase the OCPD, but never beyond 400% for breakers or 300% for fuses (for certain motors).
78.Overload Protection: Sized per NEC 430.32. Typically 125% of the nameplate full-load current for motors with a service factor of 1.15 or more, or a temperature rise of 40°C or less. Otherwise, 115%.
Exam Trap: Do not confuse the motor FLC table value with the nameplate value. The table value is used for the branch circuit conductor and the OCPD; the nameplate value is used for the overload relay setting.
1.6.2 Generators (Article 445)
Generators are treated as a source. Key requirements:
Rating: The generator must be rated to supply the connected load (NEC 445.10).
Overcurrent Protection: Conductors from the generator terminals must be protected per 445.12. If the generator is a separately derived system, the neutral must be bonded per 250.30.
Transfer Switches: For standby systems, the transfer switch must be listed and must switch the grounded conductor if the system is a separately derived system (NEC 700.5(C) and 701.5(C)).
1.7 Overcurrent Protection Coordination
1.7.1 Selective Coordination (NEC 700.28, 701.27)
For emergency systems (Article 700) and legally required standby systems (Article 701), the overcurrent devices must be selectively coordinated. This means that when a fault occurs on a branch circuit, only the OCPD closest to the fault opens — the upstream feeder or service OCPD must not trip.
Inspection Point: On a job with a generator and an automatic transfer switch (ATS), check the feeder breaker feeding the emergency panelboard. It must be coordinated with the branch breakers. This often requires a time-current curve (TCC) study, which a master should request from the engineer or perform using software.
1.7.2 The 2023 NEC Change
The 2023 NEC expanded selective coordination requirements to include optional standby systems (Article 702) in some cases, specifically for fire pump and life safety loads. Verify the scope of 702.10.
1.8 Code Navigation: Where to Find It
Concept
NEC 2023 Location
General Requirements (voltages, SCCR)
Article 100, 110.9, 110.10
Branch Circuits
Article 210
Feeders
Article 215
Services
Article 230
Overcurrent Protection
Article 240
Grounding & Bonding
Article 250
Wiring Methods
Articles 300 – 398
Motors & Controllers
Article 430
Transformers & SDS
Article 450, 250.30
Generators
Article 445
Emergency Systems
Article 700
Legally Required Standby
Article 701
Optional Standby
Article 702
Load Calculations
Article 220
Conductor Ampacity
Table 310.16 (with 310.15(C)(1) adjustments)
Conductor Sizing for Grounding
Table 250.122
Grounding Electrode Conductor
Table 250.66
Motor FLC Tables
Tables 430.247 – 430.250
Box Fill & Conduit Fill
Article 314, Chapter 9 Tables
1.9 Inspection and Supervision Points
As a master, you are the final authority on the job site. Verify these items before calling for inspection:
99.Service Disconnect Grouping: Are the six disconnects grouped in one location? Is the main bonding jumper installed correctly?
100.High-Leg Delta: Is the B phase (208V to ground) properly identified with orange tape at every termination point?
101.SDS Bonding: Is there exactly one neutral-to-ground bond at the transformer/generator? Are the downstream panelboards free of bond screws?
102.Conductor Sizing: Are the neutrals sized for the unbalanced load and harmonic content (e.g., 200% for high harmonic loads per 310.15(E))?
103.Working Clearance: Is the working space in front of the panelboard at least 36 inches deep, 30 inches wide, and 6.5 feet high (NEC 110.26)?
104.Torque Requirements: Are all terminations torqued to the manufacturer's specifications (NEC 110.14(D))? This is a common cause of failures and fires.
1.10 Common Exam Traps
The 125% Rule: When sizing a continuous load (e.g., lighting, motors), the OCPD must be at least 125% of the continuous load. The conductor must also be sized at 125% of the continuous load before any adjustment factors are applied.
Neutral as a Phase Conductor: In a 3-phase, 4-wire wye system, the neutral carries the unbalanced current. It is not a phase conductor, but it must be counted for conduit fill and derating purposes.
Voltage Drop is Not a Code Requirement (Usually): The NEC recommends voltage drop limits but does not require them for general circuits. However, the calculated load must be based on the actual voltage at the load terminals.
SCCR vs. Interrupting Rating: The interrupting rating (e.g., 10kAIC) is the maximum fault current the OCPD can interrupt. The SCCR is the maximum fault current the equipment can withstand. Both must be greater than the available fault current.
Table 310.16 Adjustments: Always apply the ambient temperature correction factor (Table 310.16) first, then the conduit fill adjustment factor (310.15(C)(1)). The order matters for the final ampacity.
Summary
Mastery of general knowledge and plan reading is the foundation of the Kentucky Master Electrician exam. You must be able to look at a one-line diagram and immediately identify the service type, the SDS boundaries, the required OCPD ratings, and the potential code violations. Use the NEC as your primary tool — know the structure of the code (Articles, Parts, Sections) so you can flip to the correct page in seconds. The open-book exam rewards speed and accuracy; both come from knowing where to look, not from memorizing every table.
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