Upon completing this chapter, you will be able to:
6.Identify the service point, service conductors, and service equipment for various occupancy types, and apply the correct disconnecting means and overcurrent protection requirements.
7.Calculate loads for three-phase, four-wire systems, including line-to-line and line-to-neutral loads, and apply demand factors correctly.
8.Distinguish between a separately derived system (transformer or generator) and a non-separately derived system, and apply grounding, bonding, and overcurrent requirements for each.
9.Size feeders and service conductors using the correct ampacity adjustment and correction factors, and verify compliance with the 80% continuous load rule.
10.Navigate the NEC efficiently to locate code requirements for motors, generators, transformers, and commercial/industrial installations.
11.Identify common code traps and inspection points that appear on the Wyoming Master exam and in field supervision.
1.1 The Service and Service Equipment (Article 230)
As a Master, you are responsible for the entire service from the utility point of attachment to the final branch circuit. The NEC defines the service point as the interface between the utility and the premises wiring. The service conductors run from this point to the service disconnecting means.
Key Distinctions for the Master:
Service Conductors vs. Feeders: Service conductors are under the exclusive control of the utility up to the service point; beyond that, they are the electrician's responsibility. Feeders are the conductors between the service disconnecting means and the final branch-circuit overcurrent device.
Number of Services (230.2): A building can have only one service unless specific exceptions apply (fire pumps, emergency systems, separate occupancy, utility requirements, etc.). Each service requires a disconnecting means.
Disconnecting Means (230.70 – 230.80): The service disconnecting means must disconnect all ungrounded conductors. It must be a clearly marked, readily accessible switch or circuit breaker. For a multi-occupancy building, each occupant must have access to their own disconnecting means.
Service Overcurrent Protection (230.90): Each ungrounded service conductor must have an overcurrent device. The rating of this device must not exceed the ampacity of the conductor, unless the next standard size (per 240.6) is used and is not over 800A. This is a classic exam trap: you may size a conductor at 400A and use a 400A OCPD, but if the calculated load is 390A, you must check if the conductor is protected against overload.
Supervision Point: On site, verify that the service disconnecting means is not located in a bathroom, is not within 3 ft of a window that opens into the service equipment, and is accessible without climbing over obstacles.
1.2 Three-Phase Systems and Calculations
A Master must be fluent in calculating loads for three-phase systems, especially 208Y/120V and 480Y/277V systems.
Voltage and Current Relationships:
For a balanced three-phase system: I = VA / (√3 × V_L-L) where V_L-L is the line-to-line voltage.
For line-to-neutral loads (e.g., 120V lighting on a 208Y/120V system), the phase current is I = VA / V_L-N.
Total VA for a three-phase load is √3 × V_L-L × I.
Load Calculations (Article 220):
General Lighting and Receptacles (220.14): Use 3 VA per square foot for dwelling units, but for commercial/industrial, use the actual connected load or Table 220.12 for general lighting (e.g., 1 VA/sq ft for banks, 2 VA/sq ft for hospitals).
Demand Factors (Table 220.42): For dwelling units, the first 3,000 VA of lighting load is at 100%, the next 117,000 VA at 35%, and the remainder at 25%. This is a standard calculation.
Dryers and Cooking Equipment (220.54, 220.55): Use Table 220.54 for household dryers (5,000 VA each with demand factors). For cooking equipment, use Table 220.55, which provides demand factors based on the number of units.
Commercial Loads (220.56): For commercial cooking equipment, the demand factors are based on the number of units, but you must add the nameplate rating of the largest unit at 100% plus the demand factor for the rest.
Exam Trap: When calculating a 208Y/120V panel with both 3-phase (208V) motors and single-phase (120V) lighting, you cannot simply add the currents. You must calculate the VA per phase. For a 120V load on phase A, the current is VA/120. For a 208V load, the current is VA/208. Sum the currents per phase, and the highest phase current determines the feeder size.
1.3 Separately Derived Systems (Article 250.30)
This is a high-yield topic for the Master exam. A separately derived system is a premises wiring system whose power is derived from a battery, a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection (including a solidly grounded circuit conductor) to the supply conductors originating in another system.
Examples: A 480V-to-208Y/120V transformer, an emergency generator with a transfer switch, or an isolation transformer.
Grounding and Bonding Requirements (250.30):
System Grounding: The system must be grounded by connecting the grounded conductor (neutral) to a grounding electrode conductor (GEC) at the source (the transformer or generator) or at the first disconnecting means.
Grounding Electrode: The GEC must be connected to a grounding electrode (e.g., building steel, ground ring, or a concrete-encased electrode). The size of the GEC is per Table 250.66.
Bonding the Neutral: The neutral must be bonded to the equipment grounding conductor (EGC) and the grounding electrode at the source. This is the single point of bonding for the derived system.
Switching the Neutral: In a separately derived system, the neutral is switched with the phase conductors in the transfer switch (if used). This is critical for generators.
Non-Separately Derived System (250.32): If a generator is used as a backup and the neutral is solidly connected to the utility neutral (no transfer switch that opens the neutral), it is not a separately derived system. In this case, the generator frame must be bonded to the EGC, but the neutral is not re-grounded at the generator.
Supervision Point: On a job site, check that the neutral-to-case bond is only at the transformer or generator, and not at the downstream panelboard. A missing or double bond is a common violation.
1.4 Feeder Sizing and Overcurrent Protection
Feeder sizing is not just about ampacity; it is about coordination and protection.
Conductor Ampacity (Article 310):
Use Table 310.16 for standard ampacities at 30°C ambient.
Adjustment Factors (Table 310.15(C)(1)): For more than 3 current-carrying conductors in a raceway, apply the adjustment factors (e.g., 80% for 4-6 conductors, 70% for 7-9).
Correction Factors (Table 310.15(B)(1)): For ambient temperatures above 30°C, apply the correction factors.
Critical Exam Trap: The order of operations. First, apply the correction factor (temperature) to the base ampacity. Then, apply the adjustment factor (number of conductors). The final value must be ≥ the calculated load.
Overcurrent Protection (240.4):
General Rule: The OCPD must protect the conductor against overcurrent. The next standard size up (240.6) is permitted if it does not exceed 800A.
Motor Circuits (430.52): For motors, the OCPD can be much higher than the conductor ampacity to allow for starting current. The branch-circuit short-circuit and ground-fault protection (SCGFP) is sized per Table 430.52 (e.g., 250% for inverse-time breakers). The conductors are sized at 125% of the motor FLC (430.22).
Feeder Taps (240.21): Taps are permitted if they are not longer than 10 ft (240.21(B)(1)) or 25 ft (240.21(B)(2)) under specific conditions. The tap conductors must have ampacity not less than the load, and the OCPD at the tap location must protect the tap.
Coordination (240.12, 700.28, 701.27): For emergency and legally required standby systems, overcurrent devices must be selectively coordinated. This means that a fault on a branch circuit should not take down the entire feeder. As a Master, you must verify that the OCPD ratings and time-current curves are coordinated.
1.5 Motors and Generators (Articles 430, 445)
Motor Circuits (Article 430):
Branch Circuit: Conductors must be sized at 125% of the motor's full-load current (FLC) from Table 430.247-430.250 (not the nameplate). The nameplate is used for overload protection (430.32).
Overload Protection (430.32): Sized at 115% to 125% of the nameplate current rating. If the motor is marked with a service factor of 1.15 or more, or a temperature rise of 40°C or less, the overload can be 125%.
Short-Circuit and Ground-Fault Protection (430.52): Use Table 430.52 to size the OCPD. For an inverse-time breaker, it is 250% of FLC for most motors. If the motor will not start, you can increase the size, but not beyond 400% for inverse-time breakers.
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 50 ft away.
Generators (Article 445):
Ampacity of Conductors (445.13): The conductors from the generator terminals must have an ampacity of at least 115% of the generator's nameplate current rating.
Overcurrent Protection (445.12): Generators must be protected against overcurrent. If the generator is a separately derived system, the neutral must be bonded and grounded per 250.30.
Transfer Switches (Article 700, 701): For emergency systems, the transfer switch must be listed for the purpose and must open all ungrounded conductors. The neutral is switched only if the system is separately derived.
Exam Trap: For a generator feeding a load via a transfer switch, do not forget the 115% rule for the generator conductors. Also, remember that the generator's OCPD is sized to protect the generator, not necessarily the feeder conductors.
1.6 Commercial and Industrial Installations
Panelboards (Article 408):
Each panelboard must have a main overcurrent device if it has more than 6 disconnecting means (408.36).
The phase arrangement must be such that the loading is balanced (408.3).
Neutral Bushing: The neutral must be insulated from the panelboard enclosure unless it is the service disconnecting means (where the neutral is bonded to the enclosure).
Switchboards and Switchgear (Article 408.3): Clearances and working space must comply with 110.26. For a Master, this is a common inspection point. Working space must be at least 30 inches wide, 36 inches deep (for 0-150V to ground), and 6.5 ft high.
Transformers (Article 450):
Overcurrent Protection (450.3): Primary-only protection is permitted if the primary OCPD is sized at 125% of the primary current (for 600V or less). If the transformer has secondary protection, the primary can be sized up to 250%.
Disconnecting Means (450.14): A disconnecting means must be located within sight of the transformer, unless the transformer is in a locked room.
Ventilation (450.9): Transformers must be ventilated to dissipate heat.
Supervision Point: Check that the transformer's secondary neutral is bonded to the enclosure and the grounding electrode, and that the secondary panelboard has a separate neutral and EGC bus.
1.7 Code Navigation: Where to Find It
Topic
NEC Location
Service disconnecting means
Article 230.70 – 230.80
Load calculations (general)
Article 220
Lighting load demand factors
Table 220.12, 220.42
Motor FLC tables
Table 430.247 – 430.250
Motor OCPD sizing
Table 430.52
Separately derived systems
Article 250.30
Grounding electrode conductor size
Table 250.66
Conductor ampacity tables
Table 310.16
Adjustment/correction factors
Table 310.15(B)(1), (C)(1)
Transformer OCPD
Article 450.3
Working space clearances
Article 110.26
Emergency systems
Article 700
Generators
Article 445
Panelboards
Article 408
Feeder taps
Article 240.21
1.8 Inspection and Supervision Points
As a Master supervising an installation, your final walk-through should include:
91.Service: Verify the service disconnect is accessible, the neutral is bonded only at the service, and the GEC is properly sized and connected to an approved electrode.
92.Grounding: Check that all metal enclosures, raceways, and equipment are bonded to the EGC. Look for missing bonding bushings on concentric or eccentric knockouts.
93.Overcurrent Protection: Verify that the OCPD ratings match the drawings and that the conductor ampacity is sufficient after derating.
94.Motor Circuits: Check that the motor overloads are sized per the nameplate, and the SCGFP is per Table 430.52.
95.Separately Derived Systems: Confirm the neutral-to-case bond is at the source only, and the GEC is connected to the correct electrode.
Common Exam Traps
Trap 1: Using the motor nameplate current instead of the FLC from Table 430.250 for conductor sizing.
Trap 2: Forgetting to apply the 125% factor for a single motor or a continuous load.
Trap 3: Applying the adjustment factor (number of conductors) before the correction factor (temperature) — the order matters.
Trap 4: Treating a generator with a solidly connected neutral as a separately derived system, leading to an improper bond.
Trap 5: Sizing the transformer primary OCPD at 125% when secondary protection is present, which allows a 250% rating.
Trap 6: Overlooking the 6-disconnect rule for a panelboard that is not a service.
This chapter provides the theoretical foundation. For the exam, practice applying these rules to multi-family, commercial, and industrial scenarios. Know your tables, and always verify the specific article number for the exact requirement.
Preparing for the Wyoming Master Electrician license?
See the full licensing path, exam format, eligibility and application steps.
