Service, Feeders & Branch Circuits

Service, Feeders & Branch Circuits

Learning Objectives

Upon completing this chapter, you will be able to:

1.1 The Big Picture: System Architecture

The NEC organizes power distribution from the utility point to the outlet. A Service is the conductors and equipment that deliver electric power from the utility supply system to the service disconnecting means. A Feeder comprises all circuit conductors between the service equipment, the source of a separately derived system, or other power supply source, and the final branch-circuit overcurrent device. A Branch Circuit is the conductors and components between the final overcurrent device protecting the circuit and the outlet(s).

A master electrician must visualize this hierarchy on every job. The rules governing each segment are distinct, and mixing them up is a classic exam error. For instance, the ampacity adjustment factors of Article 310 apply to feeders and branch circuits, but service conductors have their own specific rules under Article 230.

1.2 Services (Article 230)

Article 230 is the master’s guide to the service entrance. Key areas of focus for the exam and field supervision include the number of services, disconnects, and the specific rules for service conductors.

Number of Services (230.2): A building can be served by only one service, unless specific conditions are met. These include:

Service Disconnects (230.71): The service disconnecting means must consist of not more than six switches or circuit breakers grouped in one location. This is the "six-handle rule." Each disconnect must be suitable for the maximum fault current available at its terminals. For a master, this means verifying the interrupting rating of the breakers matches the available fault current calculation.

Service Conductor Sizing (230.42): Service conductors must be sized to carry not less than the sum of the non-continuous loads plus 125% of the continuous loads. The minimum size for a service is 8 AWG copper or 6 AWG aluminum, but in practice, commercial services are far larger. The ampacity of the service conductors must be based on Table 310.16 (at 75°C for typical terminations) and adjusted for ambient temperature and conductor bundling per Article 310.15.

Service Grounding (250.24): The grounded conductor (neutral) from the utility must be connected to the grounding electrode system at the service. This is the single point of grounding. The grounded conductor must be routed with the phase conductors and is not permitted to be used for any other purpose. A common violation is bonding the neutral at a subpanel downstream of the service — this is strictly prohibited (250.142(B)).

Inspection Point: Verify that the service neutral is bonded to the service equipment enclosure via the main bonding jumper. Check that the grounding electrode conductor (GEC) is properly sized per Table 250.66 and connected to an acceptable electrode (ground rod, concrete-encased electrode, etc.) per 250.52.

1.3 Feeders (Article 215)

Feeders are the workhorses of a commercial building, distributing power from the service to panelboards. The master is responsible for ensuring feeder conductors are properly sized for voltage drop and load.

Feeder Sizing (215.2): The minimum feeder conductor size must have an ampacity of not less than the non-continuous load plus 125% of the continuous load. This is the same calculation as for services. However, the minimum size is 14 AWG for copper and 12 AWG for aluminum, per 215.2(A)(1).

Voltage Drop (215.2(A)(1) Informational Note): While not a mandatory percentage, the FPN recommends that feeders be sized so the voltage drop does not exceed 3%, and the total drop for feeders and branch circuits combined does not exceed 5%. For a master, this is a practical design criterion. Long runs of feeders to rooftop units or remote panels are prime candidates for voltage drop issues. Oversizing the feeder by one or two sizes is a standard field solution.

Feeder Overcurrent Protection (215.3): Feeders must be protected against overcurrent. The rating of the feeder overcurrent device must not be less than the ampacity of the feeder conductors, but it can be higher if the next standard size (per 240.6) is used, provided the conductors are protected against overload.

Neutral Conductor (215.2(A)(2)): The neutral must be sized to carry the maximum unbalanced load. For a 3-phase, 4-wire wye system, this is the maximum load on any one phase. In many cases, the neutral can be smaller than the phase conductors, but it must never be smaller than the grounding conductor required by Table 250.122.

Exam Trap: Do not confuse feeder sizing with branch-circuit sizing. A feeder supplying a panelboard that feeds multiple branch circuits must be sized on the sum of the loads, not the rating of the main breaker in the panelboard.

1.4 Branch Circuits (Article 210)

Branch circuits are the final conductors. The master must understand the classification of branch circuits and the rules for multiwire branch circuits.

Classifications (210.3): Branch circuits are classified by the rating of the overcurrent device protecting them. Common ratings are 15, 20, 30, 40, and 50 amperes. The circuit’s rating dictates the minimum conductor size and the maximum connected load.

Continuous Loads (210.19(A)(1)): The ampacity of a branch-circuit conductor must be not less than the non-continuous load plus 125% of the continuous load. A continuous load is one where the maximum current is expected to continue for 3 hours or more. Lighting in commercial spaces is almost always considered continuous.

Multiwire Branch Circuits (210.4): A multiwire branch circuit consists of two or more ungrounded conductors that share a common grounded conductor (neutral). They must originate from the same panelboard. For a master, the critical rule is that all conductors of a multiwire branch circuit must be disconnected simultaneously by the branch-circuit overcurrent device. This means a 2-pole or 3-pole breaker is required, not individual single-pole breakers, unless the breakers are handle-tied.

Inspection Point: In a commercial kitchen or office, verify that multiwire circuits share a common trip breaker. A common violation is using two single-pole breakers with a handle tie, which does not provide common trip for a fault on one phase.

Branch Circuit Ratings (210.23): The total load on a branch circuit must not exceed the branch circuit rating. For a 15- or 20-amp circuit supplying multiple outlets, the load is limited to 80% of the rating if the load is continuous. For a circuit supplying a single appliance, the load can be up to 100% of the rating.

1.5 Separately Derived Systems (Article 250.30)

A separately derived system is a source of power that has no direct connection to the supply conductors, except through the grounding and bonding path. Common examples are transformers and generators.

Grounding (250.30(A)): The system must be grounded by connecting the grounded conductor (neutral) to a grounding electrode system at the source. This is a new grounding point, separate from the service. The GEC is sized per Table 250.66 based on the derived phase conductors.

Bonding (250.30(A)(2)): The grounded conductor must be bonded to the system enclosure (the transformer or generator housing) and to the equipment grounding conductor. This is done with a system bonding jumper.

Overcurrent Protection (240.21(C)): Transformer secondary conductors must be protected by an overcurrent device on the secondary side. The primary side overcurrent device can protect the secondary if the primary-to-secondary voltage ratio is considered, but the most common practice is a secondary breaker or fused disconnect.

Inspection Point: For a 480V-to-208Y/120V transformer, check that the neutral is bonded to the transformer case and a GEC is run to the nearest grounding electrode. A common error is treating the transformer like a subpanel and floating the neutral, which is a serious safety hazard.

Generators (702.4): For an optional standby generator, the transfer switch must be listed for the purpose. If the generator is a separately derived system (i.e., it has a transfer switch that opens the neutral), it must be grounded per 250.30. If it is a non-separately derived system (solidly connected neutral), the generator frame is bonded to the service neutral.

1.6 Motor and Generator Applications (Article 430)

Motors are a major load in commercial and industrial settings. The master must apply the specific rules of Article 430, which differ significantly from general lighting circuits.

Conductor Sizing (430.22): Motor branch-circuit conductors must have an ampacity of not less than 125% of the motor’s full-load current (FLC). The FLC is found in Tables 430.247 through 430.250, not on the motor nameplate. The nameplate is used for overload protection, but the tables are used for conductor sizing and short-circuit protection.

Overload Protection (430.32): Motors must be protected against overload. The overload device (heaters in a starter or an electronic overload relay) must be sized at not more than 115% to 125% of the motor nameplate current rating, depending on the motor’s service factor and temperature rise.

Short-Circuit and Ground-Fault Protection (430.52): The motor branch circuit must be protected by a fuse or breaker sized per Table 430.52. The maximum rating is typically 250% of the FLC for a time-delay fuse and 800% for an instantaneous trip breaker. If the maximum rating is not sufficient to start the motor, the next higher standard size is permitted.

Disconnecting Means (430.102): A disconnecting means must be located in sight from the motor and the driven machinery. "In sight" means visible and not more than 50 feet away. The disconnect must open all ungrounded conductors.

Exam Trap: Do not use the motor nameplate current for conductor sizing. Use the table values. The nameplate current is often lower than the table value, leading to undersized conductors.

1.7 Overcurrent Protection Coordination

Selective coordination is a design requirement for specific systems. It ensures that when a fault occurs, only the nearest overcurrent device opens, leaving the rest of the system energized.

Requirements (700.28, 701.27, 708.54): Selective coordination is mandatory for emergency systems (Article 700), legally required standby systems (Article 701), and critical operations power systems (Article 708). The master must ensure that the overcurrent devices are coordinated so that a fault on a branch circuit does not take down the entire feeder.

Practical Application: This often requires the use of current-limiting fuses or circuit breakers with adjustable trip settings. A master must review the time-current curves of the devices to verify coordination. This is a design function that is often overlooked but is a key inspection point for AHJs on healthcare and high-rise projects.

1.8 Code Navigation

1.9 Inspection & Supervision Points

1.10 Common Exam Traps