General Knowledge of the Trade & Calculations

Arkansas Master Electrician Exam (AR-MST) – 2023 NEC

Learning Objectives

Upon completing this chapter, you will be able to:

6.Calculate service and feeder loads for commercial and industrial occupancies using the standard and optional methods.

7.Apply the NEC requirements for services, service equipment, and grounding/bonding of separately derived systems (SDS).

8.Size feeders and branch circuits for motors, generators, and continuous loads with correct ampacity adjustments.

9.Understand overcurrent protection coordination (selectivity) and the master’s responsibility for system documentation.

10.Navigate the 2023 NEC efficiently by Article, Section, and Table number for open-book problem solving.

1.1 Three-Phase Systems and Voltage Drop

A master electrician must think in three phases. Most commercial and industrial loads are three-phase, 208Y/120V or 480Y/277V. The key formulas:

Line-to-line voltage = Line-to-neutral × √3 (1.732)

Three-phase power (kW) = V (line-to-line) × I × 1.732 × PF ÷ 1000

Three-phase current (amperes) = kVA × 1000 ÷ (V × 1.732)

Voltage drop is not a mandatory calculation in the NEC for general circuits, but 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 recommend limiting drop to 3% for branch circuits and feeders, with a total of 5% from service to the farthest outlet. For a master, this is a design responsibility.

Formula for three-phase voltage drop:

VD = (2 × L × I × K) ÷ (Cmils) for single-phase; for three-phase, multiply by 0.866 (or use 1.732 in the numerator with line-to-line voltage). Use K = 12.9 for copper, 21.2 for aluminum at 75°C.

Exam Trap: Do not use the single-phase formula for a three-phase circuit. The three-phase multiplier is 1.732, not 2. Also, always use the actual conductor length (round trip) and the ambient temperature correction factors from Table 310.16 (now Table 310.12 in the 2023 NEC for 0–2000V).

1.2 Services and Service Equipment (Article 230)

A service is the conductors and equipment that deliver power from the utility to the service disconnecting means. The master must know the difference between a service and a feeder.

Key requirements:

Service disconnecting means (230.70): Must be at a readily accessible location, outside or inside nearest the point of entrance. Each service shall have only one disconnecting means, except for up to six switches (230.71).

Service overcurrent protection (230.90): Each ungrounded service conductor shall have protection. The rating of the service OCPD shall not exceed the ampacity of the conductor, except for specific motor and other load conditions.

Service conductors (230.42): Minimum size shall be 100A for residential (230.79) but for commercial, size per calculated load. The minimum service for commercial is not fixed by the NEC; it is load-driven.

Ground-fault protection (230.95): For grounded wye services over 150V to ground but not exceeding 600V phase-to-phase, and with the service disconnecting means rated 1000A or more, ground-fault protection is required. This is a classic master-level item. The setting must not exceed 1200A, and the time delay must be coordinated with downstream devices.

Inspection Point: On a 480Y/277V service with a 1200A main breaker, verify the ground-fault relay is installed and tested. Check the label indicating the GFPE setting. Missing GFPE on a 1000A+ service is a common violation.

Exam Trap: Do not confuse ground-fault protection (equipment protection, 230.95) with ground-fault circuit interrupters (personnel protection, 210.8). They are entirely different.

1.3 Separately Derived Systems (Article 250.30)

Transformers and generators that have no direct electrical connection to the supply system are separately derived systems (SDS) . The master must ensure proper grounding and bonding.

Key rules for SDS (250.30):

The system must have a grounding electrode conductor (GEC) connected to a grounding electrode (building steel, ground ring, etc.).

The system bonding jumper connects the equipment grounding conductor (EGC) and the grounded conductor (neutral) at the SDS source (transformer secondary or generator).

The grounded conductor (neutral) must be bonded to the equipment grounding conductor at only one point — the source or the first disconnecting means, but not both (250.30(A)(1)).

For a transformer, the secondary neutral must be grounded. For a generator, if it is an SDS, the neutral is grounded at the generator.

Impedance grounded systems (250.36): For 480V systems, you may use a high-resistance grounded (HRG) system. The neutral is grounded through a resistor, limiting ground-fault current. This is common in industrial plants to avoid shutdowns on the first ground fault. The master must know that HRG systems require a ground detection system.

Inspection Point: On a 75 kVA, 480-208Y/120V transformer, check that the secondary neutral is bonded to the transformer case and the GEC is connected to the building steel. Verify the primary and secondary grounding electrode conductors are sized per Table 250.66.

Exam Trap: A generator with a transfer switch that switches the neutral is not an SDS if the neutral is solidly connected to the utility neutral. If the transfer switch does not switch the neutral, the generator is not separately derived. This changes the grounding rules.

1.4 Feeder and Service Load Calculations (Article 220)

The master must perform load calculations for services and feeders. The NEC provides two paths: Standard (Part III) and Optional (Part IV).

Standard Method (220.10-220.23):

General lighting load (Table 220.12): For commercial (banks, offices), 3.5 VA/sq ft. For warehouses, 1.25 VA/sq ft. For hospitals, 2 VA/sq ft.

Receptacle loads: 180 VA per receptacle strap (220.14(I)). For multi-outlet assemblies, 180 VA per 5 ft (220.14(H)).

Demand factors (Table 220.44): Receptacle loads over 10 kVA can take a 50% demand factor for general-purpose receptacles in non-dwelling occupancies.

Continuous loads (210.19(A)(1)): Branch circuits and feeders must be sized at 125% of the continuous load. This is the single most common calculation error.

Example (Standard): A 10,000 sq ft office with 3.5 VA/sq ft = 35,000 VA. Add 5,000 VA for receptacles (10,000 VA × 50% demand). Add HVAC at 15,000 VA. Total = 55,000 VA. Service at 208Y/120V three-phase: I = 55,000 ÷ (208 × 1.732) = 152.7A. Feeder OCPD must be ≥ 125% of continuous portion. If HVAC is continuous, that portion is 15,000 × 1.25 = 18,750 VA. Total for OCPD sizing = 35,000 + 5,000 + 18,750 = 58,750 VA → I = 163A. Use a 200A service.

Optional Method (220.82 for dwellings, 220.86 for commercial): For commercial, the optional method applies to feeders with a calculated load of 100% of the total connected load, with demand factors from Table 220.86. This is only for feeders with at least three separately controlled loads.

Exam Trap: The 125% factor applies to the continuous load only. Do not multiply the entire calculated load by 125%. Also, the neutral conductor is sized for the maximum unbalanced load, not the full phase load (220.61).

1.5 Motor and Generator Applications (Articles 430 and 445)

Motors are the most calculation-intensive area of the NEC. The master must know the difference between branch-circuit, feeder, and motor protection.

Motor branch circuit (430.52):

Branch-circuit short-circuit and ground-fault protection (OCPD) is sized per Table 430.52. For a standard squirrel-cage motor, the maximum is 250% of the full-load current (FLC) for inverse-time breakers.

The motor overload protection (430.32) is separate. It is sized at 115% to 125% of the motor nameplate current, not the table FLC.

Feeder conductors (430.24): The feeder for multiple motors shall be sized at 125% of the largest motor FLC plus the sum of the FLCs of all other motors.

Motor full-load currents: Use Tables 430.247 through 430.250. Do not use the motor nameplate for conductor sizing; use the table values. Nameplate is only for overloads.

Generators (Article 445): Generators are treated similarly to motors in reverse. The ampacity of the conductors from the generator terminals must be at least 115% of the nameplate current rating (445.13). Overcurrent protection is per 445.12.

Inspection Point: For a 50 HP, 460V three-phase motor (FLC = 65A per Table 430.250), the branch circuit conductors must be sized at 125% of 65A = 81.25A → use #3 AWG copper at 75°C (85A). The inverse-time breaker can be up to 250% × 65A = 162.5A → use a 150A breaker. The overload relay is set at 125% of the nameplate current.

Exam Trap: Do not use the motor nameplate FLC for conductor sizing. The tables are the authority. Also, do not confuse the branch-circuit OCPD (for short circuits) with the overload relay (for running protection). They serve different purposes.

1.6 Overcurrent Protection Coordination (Selectivity)

A master is responsible for system coordination. Selectivity (240.12) means that an overcurrent device closest to the fault opens first, without interrupting upstream devices. This is a requirement for emergency systems (700.28), legally required standby (701.27), and critical operations (708.54).

Key concepts:

Series ratings (240.86): A downstream device with a lower interrupting rating can be used if the upstream device is series-rated. The combination must be tested and labeled.

Fuse coordination: Fuses are current-limiting. A 200A fuse will clear a fault before a 400A fuse, provided the total clearing curves do not overlap.

Breaker coordination: Adjustable trip units allow the master to set long-time, short-time, and instantaneous pickups. For selectivity, the upstream breaker's short-time pickup must be set above the downstream breaker's instantaneous trip.

Inspection Point: In a switchboard with multiple feeder breakers, verify that the main breaker's short-time pickup is not set below the largest feeder breaker's instantaneous trip. If it is, a fault on a feeder will trip the main, causing a total outage.

Exam Trap: The NEC does not require full coordination for all systems. Only specific systems (emergency, legally required standby, critical operations) have mandatory coordination. For general power systems, coordination is a design goal, not a code requirement.

1.7 Commercial and Industrial Installations

Receptacle placement (210.52 for dwellings; 210.60 for guest rooms): In commercial, there is no general receptacle spacing requirement. Receptacles are placed per the design. However, in guest rooms (hotels), receptacles must be installed per 210.60, which references dwelling unit requirements.

Lighting (Article 410): Recessed luminaires must be marked "IC" (insulation contact) or "non-IC." Non-IC fixtures require 3-inch clearance from insulation. The master must check for thermal protection.

Cable trays (Article 392): Industrial installations often use cable tray. The fill requirements are in 392.22. For multiconductor cables, the sum of the cross-sectional areas must not exceed 40% of the tray cross-section for ladder tray.

Busways (Article 368): Busway is a common industrial feeder. Plug-in busway allows for tap connections. The busway must be marked with its ampere rating. Overcurrent protection is required at the point of supply (368.17).

Inspection Point: In a commercial kitchen, verify that all 120V, 15A and 20A receptacles are GFCI-protected (210.8(B)(2)). In an industrial plant, check that all 15A and 20A, 125V receptacles are GFCI-protected (210.8(B)(1)).

1.8 Code Navigation: Where to Find It

Topic	NEC 2023 Location
Service requirements	Article 230 (230.70, 230.71, 230.90, 230.95)
Grounding & bonding (SDS)	Article 250 (250.30, 250.36, 250.66)
Load calculations	Article 220 (220.12, 220.14, 220.44, 220.61, 220.82, 220.86)
Branch circuits	Article 210 (210.8, 210.19, 210.20)
Feeders	Article 215 (215.2, 215.3)
Motors	Article 430 (430.24, 430.32, 430.52, Tables 430.247-250)
Generators	Article 445 (445.12, 445.13)
Transformers	Article 450 (450.3)
Overcurrent protection	Article 240 (240.12, 240.86)
Emergency systems	Article 700 (700.28)
Legally required standby	Article 701 (701.27)
Critical operations	Article 708 (708.54)
Ampacity tables	Table 310.12 (0-2000V), Table 310.15
Voltage drop (informational)	210.19(A) IN No. 4, 215.2(A) IN No. 2
Receptacle loads	220.14(I)
Lighting loads	Table 220.12
Motor feeder demand	430.24
Busways	Article 368
Cable tray	Article 392

1.9 Inspection and Supervision Points

As a master, you are responsible for the work of others. Your site inspection checklist should include:

89.Service Entrance: Verify the service disconnect is within sight of the meter or is lockable. Check for proper working clearance (110.26) — 36 inches in front, 30 inches wide.

90.Grounding Electrode System: Confirm the GEC is continuous, properly sized (Table 250.66), and connected to the building steel and ground rod(s). Check that the ground rod is driven to 8 feet (250.53(G)).

91.Bonding: Verify the neutral is bonded to the equipment grounding conductor at the service only. Check for a bonding jumper on the water meter (250.53(E)).

92.GFCI and AFCI: Test all GFCI devices with a test button. Verify AFCI protection in dwelling unit bedrooms, living rooms, and similar areas (210.12).

93.Conductor Identification: Check that the neutral is white or gray, and the equipment grounding conductor is green or bare (200.6, 250.119).

94.Torque Requirements: Verify that all terminations are torqued to the manufacturer's specifications (110.14(D)). This is a common cause of fires and a frequent inspection finding.

1.10 Common Exam Traps

Continuous Loads: Forgetting the 125% factor on continuous loads. Always ask: "Is this load running for more than 3 hours?"

Neutral Sizing: The neutral is sized for the unbalanced load, not the phase load. In a 3-phase, 4-wire system with linear loads, the neutral may be smaller.

Motor Nameplate vs. Table: Always use Table 430.250 for conductor sizing, not the nameplate. The nameplate is only for overloads.

GFPE vs. GFCI: Ground-fault protection of equipment (230.95) is for 1000A+ services. GFCI is for personnel. Do not mix them up.

SDS Grounding: A generator with a 3-pole transfer switch (neutral not switched) is not an SDS. If the neutral is switched, it is an SDS and requires a GEC.

Voltage Drop: The NEC recommends, but does not require, voltage drop limits. However, the master must design for proper operation. A motor with low voltage will draw more current and overheat.

Conductor Ampacity: Use the 75°C column for terminations unless the equipment is marked for 90°C. The 90°C column is only for derating purposes (110.14(C)).

Summary

The master electrician is the person who signs the permit and takes responsibility for the installation. This requires not just knowing the code, but understanding the physics of three-phase power, the logic of overcurrent protection, and the practical realities of commercial and industrial equipment. Master the calculations in this chapter, memorize the key Article numbers, and always verify your work against the 2023 NEC. On the Arkansas exam, you will be open-book — but you must know where to look and what the code means when you find it.

General Knowledge of the Trade & Calculations