Electrical Services, Service Equipment, and Separately Derived Systems

Learning Objectives

Upon completing this chapter, you will be able to:

4.Define the precise boundaries of a service, service conductors, and service equipment, and apply the NEC rules governing their installation, sizing, and overcurrent protection.

5.Calculate service loads for 3-phase commercial and industrial occupancies, including demand factors, motor loads, and neutral loads, using the standard and optional calculation methods.

6.Distinguish between service equipment and separately derived systems (SDS), and correctly apply grounding, bonding, and overcurrent protection requirements for each.

7.Size feeders and service conductors for continuous and non-continuous loads, including the 125% rule, voltage drop considerations, and the minimum ampacity rules of 240.4(B).

8.Identify the critical code sections, tables, and thresholds required to supervise and sign off on service and SDS installations, including inspection checkpoints and common code traps.

1.1 Defining the Service: Point of Attachment to the First Disconnect

The NEC defines a service as the conductors and equipment for delivering electric energy from the utility supply system to the service point. For a master electrician, the critical boundary is the service point — where the utility ownership ends and the premises wiring begins. Everything downstream of the service point, up to and including the service disconnecting means, is considered service equipment.

Key components:

Service conductors: The conductors from the service point to the service disconnecting means. These can be service-drop conductors (overhead) or service-lateral conductors (underground).

Service-entrance conductors: The portion of the service conductors from the point of attachment (or termination of the service lateral) to the service disconnecting means. This is the conductor set you will size and protect.

Service equipment: The necessary equipment (enclosures, disconnects, overcurrent devices, metering) connected to the load end of service conductors, intended to constitute the main control and cutoff of the supply. This is usually the main breaker panelboard or a separately mounted disconnect switch.

Critical rule (230.70): Each service shall have a single service disconnecting means, unless multiple disconnects are permitted for specific reasons (e.g., multiple occupancies, multiple services for fire pumps, emergency systems). When multiple disconnects are used, they must be grouped and each must be suitable for the load. The disconnecting means must be at a readily accessible location nearest the point of entrance of the service conductors.

Master-level nuance: The service disconnecting means must be suitable for use as service equipment. This means it must be rated for the available fault current and be capable of interrupting the maximum fault current at its line terminals. For services rated over 1000 volts, additional requirements apply, but for the master exam, focus on the 1000-volt-and-below rules.

1.2 Service Sizing and the 125% Continuous Load Rule

The most common calculation error on the master exam involves continuous loads. A continuous load is a load where the maximum current is expected to continue for 3 hours or more (Article 100). The NEC requires that service and feeder conductors be sized at 125% of the continuous load plus 100% of the non-continuous load (210.19(A)(1), 215.2(A)(1), 230.42(A)(1)).

Formula:

Minimum ampacity = (1.25 × Continuous Load) + (1.00 × Non-continuous Load)

Example: A 3-phase, 208/120V panelboard serves a lighting load of 80A continuous and a receptacle load of 30A non-continuous.

Minimum ampacity = (1.25 × 80) + 30 = 100 + 30 = 130A.

You would select a conductor with an ampacity of at least 130A (e.g., 1/0 AWG copper at 75°C, which is rated 150A).

Master-level nuance: The 125% factor applies to the conductor ampacity and the overcurrent device rating. However, the overcurrent device must be the next standard size above the calculated ampacity (240.6(A)). For a calculated ampacity of 130A, the next standard size is 150A. But if the calculated ampacity is 131A, you still use a 150A breaker — you cannot round down.

Voltage drop (210.19(A) Informational Note): While not mandatory for service conductors, the NEC recommends that feeders and branch circuits be sized to limit voltage drop to 3% for the feeder and 3% for the branch circuit, with a total of 5% for both. For long runs, you must calculate voltage drop using the formula:

VD = (2 × Length × Current × Resistance per 1000 ft) / 1000 for single-phase, or VD = (1.732 × Length × Current × Resistance) / 1000 for 3-phase.

1.3 3-Phase Systems and Neutral (Grounded Conductor) Sizing

For a master, understanding the neutral conductor is not optional. The grounded conductor (neutral) carries the unbalanced load in a 3-phase, 4-wire system.

Neutral sizing rule (220.61): The neutral must be sized to carry the maximum unbalanced load, but it is not required to be larger than the largest ungrounded conductor. For a 3-phase wye system, the neutral current is calculated as the vector sum of the phase currents. In a perfectly balanced system, the neutral current is zero.

Demand factors for neutrals (220.61(B)): A demand factor of 70% may be applied to the portion of the neutral load that exceeds 200A. This is a common exam trap: for a service with 300A of neutral load, the first 200A is at 100%, and the remaining 100A is at 70%, giving a calculated neutral load of 200 + 70 = 270A.

Master-level nuance: The neutral must be switched only if the service disconnecting means is a 3-pole breaker and the system is a corner-grounded delta (rare). In a standard 3-phase wye, the neutral is not switched.

Harmonic loads: For nonlinear loads (e.g., electronic ballasts, VFDs, computers), the neutral can carry more current than the phase conductors due to triplen harmonics. The NEC requires that the neutral be sized to carry the harmonic current, and in some cases, you must derate the neutral or use a larger neutral conductor. For a master, this is a design consideration, not a code minimum, but you must be aware of it when supervising installations.

1.4 Service Equipment: Disconnects, Overcurrent Protection, and the 6-Handle Rule

230.71(A) — Maximum Number of Disconnects: Each service shall have no more than six disconnects to disconnect all ungrounded conductors. This is the famous "six-handle rule." Each disconnect must be a separate enclosure or a single enclosure with multiple disconnects.

230.79 — Rating of Service Disconnect: The service disconnecting means must have a rating of not less than the calculated load. The minimum rating for a one-family dwelling is 100A, but for commercial/industrial, the minimum is based on the calculated load per Part III of Article 220.

230.90 — Overcurrent Protection: Each ungrounded service conductor must have overcurrent protection. The rating of the service overcurrent device must not exceed the ampacity of the conductor, except as permitted for motor loads (430.52) or where the next standard size is used (240.4(B)).

Master-level nuance: The service overcurrent device is not required to protect the service conductors against overload if the conductors are protected by the utility transformer's primary protection. However, the service conductors must be protected against short-circuit and ground-fault conditions. In practice, the service breaker is sized to the calculated load and the conductor ampacity.

Inspection point: Verify that the service equipment is bonded to the grounded conductor (neutral) at the service point only. Downstream of the service disconnect, the neutral must be isolated from the equipment grounding conductor (250.24(A)(5), 250.142(B)).

1.5 Separately Derived Systems (SDS): Transformers and Generators

A separately derived system is a premises wiring system whose power is derived from a source of electric energy or from a transforming device, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system (Article 100).

Common SDS examples:

A 480V to 208Y/120V transformer

An engine-generator with a transfer switch that opens the neutral

A UPS with an isolation transformer

Grounding and Bonding (250.30): The SDS must have its grounded conductor (neutral) connected to a grounding electrode at the source or at the first disconnecting means. The system bonding jumper connects the grounded conductor to the equipment grounding conductor at the SDS source.

Critical rule (250.30(A)(1)): The grounding electrode for the SDS must be as close as practical to the source. For a transformer, this is typically the nearest effectively grounded structural metal or a ground rod. The size of the grounding electrode conductor is based on the size of the largest ungrounded conductor of the SDS, per Table 250.66.

Master-level nuance: The neutral of the SDS must be bonded to the equipment grounding conductor at the SDS source, but it must be isolated from the equipment grounding conductor downstream. This is the same rule as for a service. If you fail to isolate the neutral, you create a parallel path for neutral current on the equipment grounding conductors, which is a safety hazard and a code violation.

Overcurrent protection for transformers (450.3): The primary and secondary of a transformer must have overcurrent protection. The primary protection can be sized at up to 125% of the primary rated current (or 250% for certain conditions). The secondary protection is required if the primary protection does not provide adequate protection for the secondary conductors.

Feeder sizing for transformer secondary (240.21(C)): The secondary conductors of a transformer are considered protected if the primary overcurrent device is sized at no more than 125% of the primary rated current, and the secondary conductors have an ampacity of at least the secondary rated current. This is the primary protection rule that allows you to avoid a secondary breaker in some cases.

1.6 Generator Systems and Transfer Switches

Generators are SDS only when the transfer switch opens the neutral conductor. If the neutral is solidly connected through the transfer switch, the generator is not an SDS, and the grounding rules differ.

For a separately derived generator system (250.30):

The generator must have its neutral bonded to the equipment grounding conductor at the generator.

A grounding electrode is required at the generator location.

The transfer switch must be a 4-pole switch (opening all ungrounded conductors and the neutral) to maintain the separation of the neutral and ground.

For a non-separately derived generator (250.35):

The generator is connected to the same grounded system as the utility.

The neutral is not switched.

The generator frame must be bonded to the equipment grounding conductor of the system.

Master-level nuance: The generator's overcurrent protection must be sized per Article 445. The generator's rated output current is the basis for sizing the conductors and the overcurrent device. For a generator rated at 100kW, 3-phase, 480V, the rated current is:

I = (100,000 VA) / (1.732 × 480) = 120.3A.

The conductors must be sized at 125% of this value (150.4A), and the breaker must be sized accordingly.

1.7 Motor and Generator Applications: Feeder Sizing and Coordination

For a master, motor loads are a significant portion of commercial and industrial services. The NEC requires that motor feeders be sized at 125% of the largest motor plus the sum of the full-load currents of all other motors, plus the calculated load of other loads (430.24).

Formula for motor feeder:

Feeder ampacity = (1.25 × FLA of largest motor) + (Sum of FLA of all other motors) + (Other loads)

Example: A feeder serves three motors: 10A, 15A, and 20A.

Feeder ampacity = (1.25 × 20) + 15 + 10 = 25 + 25 = 50A.

Motor branch circuit (430.22): The branch circuit conductors for a single motor must be sized at 125% of the motor's full-load current (not the nameplate current, but the FLA from Tables 430.247 through 430.250).

Overcurrent protection coordination (430.52): The motor branch circuit short-circuit and ground-fault protective device must be sized per Table 430.52. For a standard squirrel-cage motor, the maximum rating is 250% of the FLA for an inverse-time breaker. If the motor will not start, you may increase the size, but not above 400% for a standard motor.

Master-level nuance: The motor overload protection (430.32) is separate from the short-circuit protection. Overloads protect the motor from running overcurrent, while the branch circuit breaker protects the conductors from short circuits. The overloads are typically sized at 115% to 125% of the motor nameplate current.

1.8 Overcurrent Protection Coordination and Selective Coordination

Selective coordination is the requirement that when an overcurrent occurs, only the nearest upstream device opens, leaving the rest of the system energized. This is mandatory for life safety systems (700.28), legally required standby systems (701.27), and critical operations power systems (708.54).

For a master, this means: When you have a panelboard fed from a main distribution panel, and a fault occurs on a branch circuit, the branch circuit breaker must open — not the feeder breaker, and not the main breaker. This requires careful selection of breaker trip curves.

Exam trap: The NEC does not require selective coordination for normal power systems. It is only required for emergency, legally required standby, and critical operations systems. However, for a master supervising a commercial installation, it is good practice to coordinate breakers to minimize downtime.

Coordination study: For large services (typically over 1200A), a coordination study is often required by the engineer. The master must be able to read the study and verify that the installed breakers match the specified trip settings.

1.9 Code Navigation: Where to Find It

Topic	NEC Article/Section
Definitions (Service, SDS, Continuous Load)	Article 100
Service Conductors and Equipment	Article 230
Service Disconnecting Means	230.70 – 230.80
Service Overcurrent Protection	230.90 – 230.95
Grounding and Bonding (Services)	Article 250, Part I, II, III
Grounding Electrode System	250.50 – 250.66
Separately Derived Systems	250.30
Branch Circuit Sizing	210.19, 210.20
Feeder Sizing	215.2, 215.3
Load Calculations (Standard)	Article 220, Part III
Load Calculations (Optional)	Article 220, Part IV
Neutral Sizing	220.61
Transformer Protection	450.3
Motor Circuits	Article 430
Motor Tables (FLA)	430.247 – 430.250
Generator Systems	Article 445
Selective Coordination	700.28, 701.27, 708.54
Standard Breaker Sizes	240.6(A)
Conductor Ampacity Tables	Table 310.16 (and 310.15)

1.10 Inspection and Supervision Points

When you are the master on site, you are responsible for the final sign-off. Verify the following:

94.Service disconnect location: Is it readily accessible and nearest the point of entrance?

95.Neutral-ground bond: Is the neutral bonded to the equipment grounding conductor only at the service or SDS source? Check for a bonding screw or strap in the panelboard.

96.Grounding electrode conductor: Is it sized per Table 250.66 and connected to a code-compliant electrode (ground rod, concrete-encased electrode, water pipe)?

97.Conductor ampacity: Verify the conductor size matches the calculated load, including the 125% continuous load factor.

98.Transformer secondary protection: Is the secondary protected per 240.21(C) or 450.3?

99.Motor overloads: Are the overload heaters or electronic overloads sized to the motor nameplate, not the table FLA?

100.Transfer switch: Is it a 4-pole switch for a separately derived generator? Is the neutral switched?

1.11 Common Exam Traps

The 125% rule is not optional. It applies to continuous loads on services, feeders, and branch circuits. Do not forget to apply it to both the conductor and the overcurrent device.

The neutral is not always the same size as the phase conductors. Use the unbalanced load calculation and the 70% demand factor for neutrals over 200A.

The six-handle rule counts disconnects, not breakers. A single panelboard with a main breaker is one disconnect. A panelboard with six branch breakers and no main is six disconnects — which is the maximum allowed.

A generator is not always an SDS. If the neutral is not switched, it is not separately derived.

Motor FLA is from the tables, not the nameplate. The nameplate current is for overload sizing; the table current is for conductor and breaker sizing.

Voltage drop is a recommendation, not a code requirement. Do not fail an installation for voltage drop, but do flag it as a design concern.

The next standard breaker size is only permitted if the conductor ampacity is at least the calculated load. You cannot use a 150A breaker on a conductor rated 130A if the calculated load is 135A — you must increase the conductor size.

Summary

As a master electrician, you are the final authority on code compliance for services and separately derived systems. Your ability to navigate the NEC, apply the correct calculation methods, and verify the critical bonding and grounding points will determine whether an installation is safe, code-compliant, and ready for inspection. Master these concepts, and you will be prepared for both the exam and the responsibility of running your own jobs.

Electrical Services, Service Equipment, and Separately Derived Systems