Services & Service Equipment

Wyoming Master Electrician Exam — 2023 NEC (NFPA 70) Study Chapter

Learning Objectives

By the end of this chapter, you will be able to:

6.Define the components of a service and distinguish between service conductors, service equipment, and service-entrance conductors.

7.Apply the 2023 NEC rules for service disconnects, overcurrent protection, and grounding/bonding at the service.

8.Calculate minimum service size for 3-phase commercial/industrial loads using the standard and optional calculation methods.

9.Identify the requirements for separately derived systems (transformers and generators) and their grounding/bonding differences from services.

10.Navigate the NEC efficiently to locate service-related tables and exceptions during the open-book exam.

11.Recognize common inspection failures and exam traps related to services and service equipment.

1.1 Scope and Definitions: What Constitutes a "Service"

The NEC defines a Service as the conductors and equipment for delivering electric energy from the utility supply system to the wiring system of the premises served. A Service Point is the interface between the utility and the premises wiring. This is a critical distinction: the NEC generally has no jurisdiction on the utility side of the service point, but everything on the premises side must comply.

Service Conductors are the conductors from the service point to the service disconnecting means. They are further subdivided:

Service-Entrance Conductors (Overhead): From the last pole or aerial support to the service disconnect.

Service-Entrance Conductors (Underground): From the utility termination point (e.g., transformer or pedestal) to the service disconnect.

Service Equipment (Article 100) includes the necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to constitute the main control and cutoff of the supply. This is the first disconnecting means.

Key Master-Level Distinction: The neutral (grounded conductor) is only permitted to be bonded to the grounding electrode system at one point — the service. This is the first point of disconnect. Downstream, the neutral must be isolated from equipment grounding conductors (EGCs) to prevent objectionable current flow on grounding paths.

1.2 Service Disconnects and Overcurrent Protection (Article 230)

1.2.1 Number of Disconnects (230.71)

The 2023 NEC permits a maximum of six circuit breakers or six sets of fuses to serve as the service disconnecting means for a single service. These must be grouped in one location. This rule allows for multiple disconnects (e.g., for separate loads like a fire pump, HVAC, and general lighting) without a single main breaker, provided they are all in a common enclosure or group.

Master Trap: The "six-handle rule" counts every pole of a multi-pole switch as a single disconnect. A 3-pole breaker for a 3-phase service counts as one disconnect. However, a single enclosure with six individual single-pole breakers feeding separate 120V circuits counts as six disconnects.

1.2.2 Location and Rating (230.72, 230.79)

Disconnects must be installed at a readily accessible location nearest the point of entrance of the service conductors.

The minimum rating for a service disconnect is 100 amperes for a one- or two-family dwelling, but for commercial/industrial, the rating must be adequate for the calculated load per Article 220.

Service Overcurrent Protection (230.90): Each ungrounded service conductor must have a protective device. The rating of the service OCPD must not exceed the ampacity of the conductor, except for specific allowances for motor loads and fire pumps.

1.2.3 Disconnect Construction (230.80)

The service disconnecting means must be a circuit breaker, a molded-case switch, a general-use switch, or an isolating switch. Knife switches are generally prohibited unless they are part of a listed motor starter or are of the enclosed safety-switch type.

1.3 Grounding and Bonding at the Service (Article 250)

This is the most heavily tested area for the master exam. The service is the only place where the grounded conductor (neutral) and the equipment grounding conductor (EGC) are intentionally connected together.

1.3.1 The Grounding Electrode System (250.50)

All grounding electrodes present at the building must be bonded together to form the grounding electrode system. This includes:

Metal underground water pipe (minimum 10 ft in contact with earth)

Metal frame of the building

Concrete-encased electrode (Ufer ground) — minimum 20 ft of 4 AWG bare copper in concrete

Ground ring — minimum 20 ft of 2 AWG bare copper

Rod and pipe electrodes — minimum 8 ft in contact with soil

Master Rule: If a concrete-encased electrode (CEE) is present, it must be used. You cannot rely solely on ground rods if a CEE exists.

1.3.2 Sizing the Grounding Electrode Conductor (GEC) — Table 250.66

The GEC is sized based on the largest ungrounded service-entrance conductor. For example, if you have 500 kcmil copper service conductors, the GEC must be at least 1/0 AWG copper. Note the table's upper limit: for conductors larger than 1100 kcmil, the GEC is capped at 3/0 AWG copper.

1.3.3 Bonding the Neutral (250.24)

The main bonding jumper connects the grounded conductor (neutral) to the service equipment enclosure and the GEC. This creates the single-point ground.

Inspection Point: Verify that the neutral bar is bonded to the enclosure via the main bonding jumper only at the service. In a panel downstream, the neutral must be floating (isolated from the enclosure) and the EGC bar must be bonded to the enclosure.

1.3.4 Separately Derived Systems (SDS) — 250.30

Transformers and generators (standby or backup) that have no direct connection to the service neutral are separately derived systems. They require their own grounding electrode system and their own bonding jumper.

Critical Difference from a Service:

For a service, the neutral is bonded to the enclosure.

For an SDS (e.g., a 480V-to-208Y/120V transformer), the neutral of the secondary must be bonded to the transformer enclosure and connected to a grounding electrode (typically the building steel or a CEE). The neutral is then run to the first disconnect of the SDS as an equipment grounding conductor and neutral combined, but after that first disconnect, they must be separated.

Master Trap: A generator with a switched neutral (transfer switch opens the neutral) is not an SDS. A generator with a solidly connected neutral (transfer switch does not switch the neutral) is an SDS. This affects whether you need a new ground rod at the generator.

1.4 Sizing Services for 3-Phase Commercial/Industrial (Article 220)

The master must calculate the minimum service size. The standard method (Part III) and optional method (Part IV) are both used.

1.4.1 Standard Method (220.40 – 220.61)

The general lighting load is calculated at 125 volt-amperes (VA) per square foot for commercial occupancies (Table 220.12). This is a minimum — actual connected load may be higher.

Add demand factors for:

Receptacle loads (Table 220.44): 10,000 VA at 100%, remainder at 50%.

Fixed appliances, motors, and HVAC (Article 220.50, 220.51, 220.60).

Neutral Load (220.61): The neutral must be sized for the maximum unbalanced load. For 3-phase 4-wire wye systems, the neutral carries the unbalanced current. There is a demand factor of 70% permitted for the neutral on dwelling units, but not for commercial.

1.4.2 Optional Method for Commercial (220.86)

This method is permitted if the total load is served by three or more feeders or services. It allows a demand factor of 100% for the first 10 kVA, 50% for the next 40 kVA, and 40% for the remainder. This often results in a smaller service than the standard method.

Master Calculation Example (3-Phase):

A 10,000 sq ft office building:

Lighting: 10,000 sq ft × 1.5 VA/sq ft = 15,000 VA (Table 220.12 for office)

Receptacles: 15,000 VA (assumed)

HVAC: 20,000 VA

Total connected: 50,000 VA

Using standard method: Lighting at 100% (15,000) + Receptacles (10,000 + 50% of 5,000 = 12,500) + HVAC at 100% (20,000) = 47,500 VA.

Using optional method (220.86): 10,000 + 50% of 40,000 = 30,000 VA. The optional method is significantly lower.

Converting VA to Amps (3-Phase):

I = VA / (E × √3)

For 208V 3-phase: I = 47,500 / (208 × 1.732) = 47,500 / 360 = 132 A.

Minimum service size: 150 A (next standard size per 240.6).

1.5 Feeders and Overcurrent Protection Coordination

1.5.1 Feeder Sizing (215.2)

Feeders must have an ampacity of not less than the sum of the noncontinuous loads plus 125% of the continuous loads. A continuous load is one where the maximum current is expected to continue for 3 hours or more.

Master Trap: The 125% factor applies to the load, not the conductor. If a feeder carries 100 A of continuous load, the conductor must be rated for 125 A. You cannot simply use a 100 A breaker and a 100 A conductor.

1.5.2 Selective Coordination (240.87, 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 the fuse or breaker closest to the fault opens, while the upstream device remains closed to keep power on for other loads.

Inspection Point: For a master, this often requires reviewing fuse curves or breaker trip settings. In practice, this means using current-limiting fuses or adjustable-trip breakers with specific settings. You cannot simply rely on standard inverse-time breakers "downstream" being smaller than "upstream" — you must verify the time-current curves do not overlap.

1.6 Motor and Generator Applications (Articles 430, 445)

1.6.1 Motor Feeder and Branch Circuit Sizing (430.22, 430.24)

Single motor branch circuit: Conductor ampacity must be at least 125% of the motor's full-load current (FLC) from Tables 430.247–430.250.

Motor feeder: Must be sized for 125% of the largest motor FLC plus the sum of the FLCs of all other motors on the feeder, plus any other loads.

Example: A feeder supplies a 25 hp, 3-phase, 460V motor (FLC = 34 A) and a 10 hp motor (FLC = 14 A).

Feeder ampacity = (34 × 1.25) + 14 = 42.5 + 14 = 56.5 A.

Use a 60 A conductor (75°C column).

1.6.2 Motor Overload Protection (430.32)

The overload device (heaters or electronic) must be sized at no more than 115% of the motor nameplate full-load current for motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less. For all other motors, it's 125%. If these values don't correspond to standard heater sizes, you may go up to the next size, but never exceed 130% (or 140% for the 1.15 SF case).

1.6.3 Generators (Article 445)

Generators are treated similarly to motors but in reverse. The generator's output terminals are the source. The ampacity of the conductors from the generator to the first disconnect must be at least 115% of the generator's rated output current (445.13).

Master Trap: A generator rated 100 kW at 480V 3-phase has an output current of:

I = 100,000 W / (480 × 1.732) = 100,000 / 831 = 120 A.

Conductors must be sized for 120 × 1.15 = 138 A. Use a 150 A conductor.

1.7 Code Navigation: Where to Find It

Concept	NEC 2023 Location
Definitions (Service, SDS)	Article 100
Service Disconnects	230.70 – 230.80
Service Overcurrent Protection	230.90 – 230.95
Grounding & Bonding (Service)	250.24, 250.28, 250.50 – 250.66
Grounding Electrode Table	Table 250.66
Separately Derived Systems	250.30
Load Calculations (Standard)	220.40 – 220.61
Load Calculations (Optional)	220.86
Lighting Load Densities	Table 220.12
Receptacle Demand Factors	Table 220.44
Motor FLC Tables	Tables 430.247 – 430.250
Motor Sizing Rules	430.22, 430.24, 430.32
Generator Sizing	445.13
Selective Coordination	240.87, 700.28, 701.27
Standard OCPD Sizes	Table 240.6

1.8 Inspection & Supervision Points

As a master, you are responsible for the work being code-compliant before the AHJ inspects. Check these on every service:

108.Single Point of Ground: Verify no bonding screw or strap exists in any panel downstream of the service. Use a continuity tester to confirm the neutral bar is isolated from the enclosure.

109.GEC Continuity: Ensure the GEC is protected in conduit where subject to physical damage (typically requires Schedule 80 PVC or metal conduit) and is properly connected with irreversible compression fittings (acorn clamps are not permitted for direct burial).

110.Working Clearance (110.26): The service equipment must have at least 36 inches of clearance in front, 30 inches of width, and 6.5 feet of headroom. Check for stored materials or piping that intrudes into this zone.

111.Service Conductor Splices: No splices are permitted in service conductors unless they are made with a listed splicing device (230.46). You cannot use a split-bolt and tape in a service raceway.

112.Neutral Isolation: In a 3-phase 4-wire system, verify that the neutral is not used as an EGC for any branch circuit. Each circuit must have its own EGC.

1.9 Common Exam Traps

Trap 1: The 6-Disconnect Rule. The six disconnects must be grouped. If they are in separate enclosures across a large wall, they must still be adjacent. They cannot be on opposite sides of a building.

Trap 2: 125% vs. 100%. The 125% factor for continuous loads applies to the conductor and OCPD sizing. It does not apply to the load calculation for the service if you are using the optional method (which already includes a demand factor).

Trap 3: Transformer Grounding. A 480V-to-208Y/120V transformer requires a GEC sized per Table 250.66 based on the secondary conductors, not the primary. Many journeymen incorrectly size it off the primary feeder.

Trap 4: Motor FLC vs. Nameplate. Always use the FLC from the NEC tables (430.247–250) for conductor and OCPD sizing. Use the nameplate current only for overload relay sizing. Mixing these up will result in wrong answers.

Trap 5: The Neutral is Not Always a Current-Carrying Conductor. For derating purposes (310.15), the neutral of a 3-phase 4-wire wye system supplying nonlinear loads is a current-carrying conductor. For standard 3-phase loads, it is not.

Summary

Mastering services requires understanding the service point, the single-point ground, and the calculation methods. The 2023 NEC is strict about the separation of the neutral and the EGC after the service disconnect. For the Wyoming Master exam, be prepared to calculate a 3-phase service size, size the GEC, and apply the motor and SDS rules with precision. Use the Code Navigation table above to jump directly to the correct article during the open-book exam, and always verify the exceptions — the exam loves to test whether you know when the rule does not apply.

Services & Service Equipment