Feeders and services

In residential work (ELEC-130), the electrician typically installs one service, one panel and a set of branch circuits. Commercial and industrial buildings are larger and more varied: a utility transformer or customer-owned substation feeds switchboards, which feed distribution panels, motor control centers and transformers through feeders, which in turn feed branch circuit panels throughout the building. This course applies the code to commercial and industrial installations. This first lesson covers calculating loads for non-dwelling occupancies, sizing services and feeders, continuous loads, voltage drop, neutral sizing, feeder taps, parallel conductors and service equipment requirements, including ground fault protection. The section at the end of this lesson shows how the twelve lessons fit together.

References in this course are to the 2023 edition of the National Electrical Code (NFPA 70), the edition most widely adopted in 2026. The 2026 edition, published in 2025, reorganized some provisions, so always check the edition and local amendments in force where you work.

The distribution system

A typical commercial or industrial distribution system includes:

• The service: the conductors and equipment that deliver energy from the utility to the building's wiring, ending at the service disconnecting means.
• Main switchboard or switchgear: the service disconnect, metering and overcurrent devices for the feeders.
• Feeders: circuit conductors between the service equipment (or a separately derived system) and the final branch circuit overcurrent devices.
• Distribution and branch circuit panelboards, motor control centers, and dry-type transformers that step down voltage for lighting and receptacles.
• Branch circuits: the conductors between the final overcurrent device and the outlets.

Common US commercial systems are 480Y/277 V three-phase four-wire, with 277 V lighting and 480 V motors, plus 208Y/120 V from step-down transformers for receptacles; smaller buildings use 208Y/120 V services directly. Industrial plants may take service at 4.16 kV, 13.2 kV or other medium voltages.

Calculated loads for non-dwelling occupancies

For occupancies other than dwellings, Article 220 sets the minimum calculated load. Part II requires a general lighting load based on floor area from Table 220.12 (for example, 1.3 VA/ft² for office buildings), plus receptacle loads at 180 VA per receptacle outlet (220.14(I)), with a minimum of 1 VA/ft² for receptacles in banks and offices where the number is not known (220.14(K)). Receptacle loads over 10 kVA may be reduced to 100% of the first 10 kVA plus 50% of the remainder (220.44). Motor, heating, air-conditioning and other loads are added at their actual ratings.

Continuous loads

A continuous load is one where the maximum current is expected to continue for three hours or more, such as lighting in a store, office HVAC or a data center load. Because overcurrent devices and their terminations heat up under sustained current, the code limits their continuous loading:

215.2(A)(1) and 215.3 require feeder conductors and overcurrent devices to be rated at least 100% of the noncontinuous load plus 125% of the continuous load, unless the assembly including the overcurrent device is listed for operation at 100% of its rating. The same rule applies to services (230.42) and branch circuits (210.19 and 210.20).

Voltage drop

Conductors that are adequate for heating may still cause excessive voltage drop on long feeders, starving motors and lighting of voltage. For a three-phase circuit:

where \(R_L\) is the resistance of the conductor length (ohms per unit length times one-way length).

The NEC recommends, in informational notes to 210.19 and 215.2, a maximum voltage drop of 3% in a feeder or branch circuit and 5% overall; these notes are not mandatory in most cases, but energy codes and specifications often make them so, and voltage drop limits are mandatory for some systems such as fire pumps.

Neutral sizing and harmonics

In a three-phase four-wire system with balanced linear loads, the neutral carries little current, and the code allows the neutral to be sized for the maximum unbalanced load, with a demand reduction for the portion above a threshold. However, nonlinear loads such as LED drivers, computer power supplies and variable-frequency drives produce triplen harmonic currents (3rd, 9th, ...) that add in the neutral rather than canceling (ELEC-210), so the neutral may carry more current than the phases.

220.61 permits the feeder or service neutral load to be the maximum unbalance, with a 70% demand factor on the portion above 200 A, but not for the portion consisting of nonlinear loads supplied from a 4-wire wye system. 310.15(E) requires the neutral to be counted as a current-carrying conductor when a major portion of the load is nonlinear.

Feeder taps

A tap is a conductor with an ampacity smaller than the rating of the overcurrent device protecting the feeder it connects to. The code permits short taps without overcurrent protection at the point of connection, provided they are short, protected from damage and end at an overcurrent device:

240.21(B) allows feeder taps without overcurrent protection at the tap point, including the 10 ft tap rule (ampacity not less than the combined calculated load and not less than the rating of the device it supplies, and not less than one-tenth of the feeder overcurrent device rating for field installations leaving the enclosure) and the 25 ft tap rule (ampacity at least one-third of the feeder overcurrent device rating, terminating in a single overcurrent device, protected from physical damage). Further rules cover taps supplying transformers and high-bay manufacturing buildings.

Parallel conductors

Large feeders and services carry currents too large for a single conductor per phase, so several conductors are run in parallel. To share current equally they must be the same length, conductor material, size, insulation type and termination, and each parallel set in separate raceways must contain all phases and the neutral, to avoid induced heating in metal raceways.

310.10(G) permits conductors of size 1/0 AWG and larger to be run in parallel under these conditions, and 250.122(F) requires a full-size equipment grounding conductor in each parallel raceway.

Service equipment and ground fault protection

Large solidly grounded wye services can have arcing ground faults that draw too little current to trip the service overcurrent device quickly but release enough energy to destroy switchgear and start fires. Ground fault protection (GFP) senses the vector sum of the phase and neutral currents and trips the service disconnect when ground fault current exceeds its setting.

230.95 requires ground fault protection of equipment for each service disconnect rated 1000 A or more on a solidly grounded wye system with more than 150 V to ground but not more than 600 V phase-to-phase (such as 480Y/277 V), with a maximum setting of 1200 A and a maximum time of 1 s for currents of 3000 A or more. Fire pump services and continuous industrial processes where a shutdown would be more hazardous are exempt. 215.10 requires similar protection for large feeders in some cases, and 240.87 requires arc energy reduction (such as zone-selective interlocking or energy-reducing maintenance switches) for breakers rated or adjustable to 1200 A or more.

Service equipment must also be rated for the available fault current at its line terminals, marked with that value, and installed with the required working space in front of it (ELEC-130). In large services, the utility's available fault current can exceed 50 kA, requiring equipment and series ratings chosen accordingly.

How this course is organized

The twelve lessons cover the code requirements that set commercial and industrial work apart:

• Lesson 1 (this lesson): loads, feeders, services, voltage drop, taps, parallel conductors and ground fault protection.
• Lesson 2: wiring methods: raceways, cables, conduit and box fill, cable trays, busways and firestopping.
• Lesson 3: motor circuits, protection, disconnects, controls and drives.
• Lesson 4: overcurrent protection, available fault current, equipment ratings, transformers and arc flash.
• Lesson 5: hazardous locations.
• Lesson 6: grounding and bonding in commercial and industrial systems.
• Lesson 7: special occupancies and equipment: health care, pools, EV charging, solar and storage, fire pumps and IT rooms.
• Lesson 8: industrial control panels, machine safety and lockout.
• Lesson 9: emergency and standby power systems.
• Lesson 10: fire alarm, communication and low-energy systems.
• Lesson 11: commercial lighting, controls and energy codes.
• Lesson 12: permits, inspection, testing and a commercial design project.