Master Electrician Practice study guide with diagrams.
Feeders
Learning Objectives
Upon completing this chapter, the candidate will be able to:
4.Define a feeder per the NEC and distinguish it from a branch circuit and service conductor.
5.Apply the minimum feeder sizing rules, including the 125% continuous load factor and the 100% neutral rule exceptions.
6.Calculate feeder conductor ampacity for 3-phase systems, accounting for voltage drop, ambient temperature, and conductor bundling.
7.Size the feeder overcurrent protective device (OCPD) and apply the next-size-up rule (NEC 240.4(B)).
8.Identify feeder requirements for separately derived systems (transformers and generators), including grounding and bonding.
9.Navigate the NEC efficiently to locate feeder-related sections for design, installation, and inspection.
1.1 Code Navigation: Where to Find Feeder Rules
For the open-book exam, speed and accuracy in locating code sections are paramount. The term "feeder" is defined in Article 100. The primary installation and sizing requirements are in Article 215 (Feeders). However, a master must cross-reference multiple articles:
Sizing & Load Calculations: Article 220 (Branch-Circuit, Feeder, and Service Calculations), specifically Part III for feeders.
Conductor Ampacity: Article 310 (Conductors for General Wiring), specifically Table 310.16 (allowable ampacities) and the adjustment/correction factors in 310.15.
Overcurrent Protection: Article 240 (Overcurrent Protection), specifically 240.4 (Protection of Conductors) and 240.21 (Location in Circuit).
Grounding & Bonding: Article 250 (Grounding and Bonding) – critical for separately derived systems (250.30).
Services (related): Article 230 (Services) – for understanding the point of demarcation.
Transformers: Article 450 (Transformers) – for primary/secondary protection and conductor sizing.
Motors: Article 430 (Motors, Motor Circuits, and Controllers) – for feeder conductors supplying multiple motors (430.24).
Generators: Article 445 (Generators) – for sizing conductors and protection.
1.2 Feeder Definition and Scope
NEC Article 100 defines a feeder as 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.
Key Distinctions for the Master:
Service Conductors run from the utility point of attachment to the service disconnecting means.
Feeders run from the service disconnect (or the secondary of a transformer, or the output of a generator) to the panelboard or the branch-circuit OCPD.
Branch Circuits run from the final OCPD to the outlets or loads.
A common field error is mislabeling a conductor that runs from a panel to a sub-panel as a branch circuit. If it supplies a panel that contains branch-circuit OCPDs, it is a feeder.
1.3 Feeder Sizing: Minimum Ampacity (NEC 215.2)
NEC 215.2(A)(1) requires that feeder conductors have an ampacity sufficient to carry the load, computed per Article 220. The minimum ampacity must be no less than the larger of the following two calculations:
35.The non-continuous load plus 125% of the continuous load.
36.The load after the application of demand factors (if applicable, per 220.60 or other sections).
Critical Threshold: The 125% factor for continuous loads is a minimum. It applies to the feeder conductor ampacity before any adjustment for ambient temperature or conductor bundling. This is a common exam trap: you must first size the conductor to the 125% continuous load, then check if the adjusted ampacity (after temperature and bundling corrections) is still sufficient.
Example Calculation:
A 3-phase, 208Y/120V feeder supplies a panelboard with a continuous lighting load of 80 A and a non-continuous receptacle load of 30 A.
Minimum ampacity = (80 A × 1.25) + 30 A = 100 A + 30 A = 130 A.
You must select a conductor with an ampacity of at least 130 A before any adjustment factors.
Neutral Conductor (NEC 215.2(A)(2)):
The grounded (neutral) conductor must be sized to carry the maximum unbalanced load. For a 3-phase, 4-wire system, this is the maximum load between the neutral and any one ungrounded conductor. The neutral is not required to be sized to 125% of the continuous load unless it serves that load directly (e.g., a feeder supplying a panel that feeds line-to-neutral loads). However, if the neutral carries only the unbalanced current from line-to-line loads (e.g., a 3-phase motor), it can be sized smaller.
1.4 Conductor Ampacity and Adjustments (NEC 310.15)
A master must understand the difference between terminal ampacity and conductor ampacity.
Terminal Ampacity: The rating of the terminals (e.g., a 100 A breaker has 75°C terminals). You use the 75°C column of Table 310.16 for terminations unless the equipment is specifically listed for 90°C terminations (rare).
Conductor Ampacity: The actual current-carrying capacity of the conductor based on its insulation rating (90°C column for THHN/THWN-2) after applying adjustment and correction factors.
The 3-Step Sizing Process:
51.Calculate the minimum ampacity (per 215.2, including 125% continuous).
52.Select a conductor from Table 310.16 using the 75°C column (for termination rating) that meets the minimum ampacity.
53.Verify the conductor's 90°C ampacity (if using 90°C insulation) after applying:
Ambient Temperature Correction Factor (Table 310.15(B)(1)) – based on the ambient temperature of the installation location.
Adjustment Factor for More Than 3 Current-Carrying Conductors (Table 310.15(B)(3)(a)) – based on the number of conductors in the raceway or cable.
Exam Trap: If the adjusted 90°C ampacity drops below the required minimum, you must increase the conductor size. The 75°C column is only for the initial selection; the 90°C column is for the adjusted ampacity check.
Voltage Drop (Informational Note):
The NEC does not mandate a specific voltage drop percentage for feeders, but NEC 215.2(A)(4) Informational Note No. 2 recommends a maximum of 3% for feeders and a total of 5% for feeders plus branch circuits. A master should always calculate voltage drop for long runs. Use the formula:
VD = (2 × L × I × R) / 1000 for single-phase, or VD = (1.732 × L × I × R) / 1000 for 3-phase, where R is the conductor resistance (Ω/kFT) from NEC Chapter 9, Table 8.
1.5 Feeder Overcurrent Protection (NEC 240.4 and 215.3)
NEC 215.3 requires that feeders be protected against overcurrent per Article 240.
The Next-Size-Up Rule (NEC 240.4(B)):
If the calculated ampacity of the conductor does not correspond to a standard OCPD rating (listed in 240.6(A)), you may use the next higher standard size OCPD, provided:
The conductor ampacity is not less than the non-continuous load plus 125% of the continuous load.
The OCPD rating does not exceed 800 A.
Example: A feeder conductor has an ampacity of 115 A. Standard breaker sizes include 110 A and 125 A. You may use a 125 A breaker because 115 A is not a standard size, and 125 A is the next higher standard size.
Exam Trap: The next-size-up rule applies to standard sizes only. You cannot use it to jump from 115 A to a 150 A breaker. Also, the rule does not apply to motor circuits (Article 430) or transformer secondary conductors (240.21(C)) in the same way.
Feeder Taps (NEC 240.21(B)):
A master must know the rules for tapping a feeder without an OCPD at the tap point. The key taps are:
10-Foot Tap (240.21(B)(1)): The tap conductor must have an ampacity not less than the combined load, and not less than the rating of the device supplied. The tap must terminate in a single OCPD.
25-Foot Tap (240.21(B)(2)): The tap conductor must have an ampacity of at least one-third of the rating of the OCPD protecting the feeder.
100-Foot Tap (240.21(B)(4)): For high-rise buildings, the tap must be at least one-third the ampacity of the feeder OCPD, and the tap must terminate in a single OCPD.
1.6 Feeders for Separately Derived Systems (Transformers and Generators)
This is a critical area for the master exam. A separately derived system (SDS) is a source of power with no direct electrical connection to the supply conductors (e.g., a transformer secondary or a generator output).
Transformer Feeders (NEC 450.3 and 240.21(C)):
Primary Protection: The transformer primary OCPD is sized per Table 450.3(B). For a transformer with a primary current of 9 A or more, the primary OCPD can be set at 125% of the primary current. If 125% does not correspond to a standard size, you may use the next higher size.
Secondary Protection: If the secondary is protected by a primary OCPD at 125% (or next higher), the secondary conductors can be protected by the primary device if the primary OCPD rating multiplied by the transformer turns ratio does not exceed the secondary conductor ampacity.
Secondary Conductors (240.21(C)): If you are using a secondary OCPD, the secondary conductors can be sized per the load, but the tap rules (10-foot or 25-foot) apply if the OCPD is not at the transformer.
Generator Feeders (NEC 445.13):
The conductors from the generator terminals to the first OCPD must have an ampacity of at least 115% of the generator's nameplate current rating. This is a minimum; the actual load may require larger conductors.
Grounding: The generator must be grounded per 250.30 if it is a separately derived system. This requires a system bonding jumper at the generator and a grounding electrode conductor.
Grounding and Bonding (NEC 250.30):
For an SDS, you must:
86.Connect the grounded conductor (neutral) to the equipment grounding conductor at the source (the transformer or generator) via a system bonding jumper.
87.Provide a grounding electrode conductor to connect the grounded conductor to a grounding electrode (e.g., a ground rod or building steel).
88.Do not bond the neutral again at the downstream panelboard (this creates a parallel path for neutral current, which is a violation).
Inspection Point: On site, a master must verify that the neutral and ground bus are separate in the downstream panelboard of an SDS. This is a common inspection failure.
1.7 Feeders for Motors and Multiple Loads (NEC 430.24)
Feeder conductors supplying two or more motors must have an ampacity of at least 125% of the full-load current of the highest-rated motor plus the sum of the full-load currents of all other motors on the same feeder, plus the calculated load of any other loads.
Formula:
Feeder Ampacity = (1.25 × Largest Motor FLA) + (Sum of all other Motor FLAs) + (Other Loads)
Example:
A feeder supplies three motors: 10 HP (14 A FLA), 5 HP (7.6 A FLA), and 2 HP (3.4 A FLA).
Feeder ampacity = (1.25 × 14 A) + 7.6 A + 3.4 A = 17.5 A + 11 A = 28.5 A.
Select a conductor with an ampacity of at least 28.5 A (per 75°C column).
Feeder OCPD for Motors (NEC 430.62):
The feeder OCPD must be sized to allow the largest motor to start without opening the feeder. The rating is typically the largest motor's branch-circuit OCPD rating plus the sum of the full-load currents of all other motors. This is often much larger than the conductor ampacity, which is why the feeder OCPD is permitted to exceed the conductor ampacity for motor loads (per 240.4(G)).
1.8 Commercial and Industrial Feeder Considerations
Demand Factors (NEC 220.61):
For feeders supplying panelboards in commercial buildings, you may apply demand factors to the neutral load (220.61) and to certain types of loads (e.g., lighting in 220.42). A master must know when to apply these factors to avoid oversizing feeders unnecessarily.
Kitchen Equipment (NEC 220.56):
For commercial kitchens, the feeder can be sized based on the sum of the nameplate ratings of all appliances, but you may apply a demand factor of 90% for 4 or more appliances, and 80% for 10 or more appliances, provided the appliances are not all used simultaneously.
Continuous vs. Non-Continuous:
In commercial settings, many loads (lighting, HVAC, water heaters) are considered continuous (operating for 3 hours or more). A master must correctly classify loads to avoid undersizing feeders.
High-Rise Feeders (NEC 240.21(B)(4)):
For high-rise buildings, the 100-foot tap rule allows feeders to be tapped at each floor without a main OCPD on the feeder, provided the tap conductors are at least one-third the ampacity of the feeder OCPD and terminate in a single OCPD.
1.9 Inspection and Supervision Points
As a master, you are responsible for the final sign-off. On site, verify the following:
115.Conductor Identification: The grounded (neutral) conductor must be white or gray (200.6). The equipment grounding conductor must be green or bare (250.119). Ungrounded conductors must be identified per the phase color code.
116.Termination Torque: All feeder terminations must be torqued to the manufacturer's specifications. Loose connections are a leading cause of feeder failures.
117.Bonding at the Service: The neutral must be bonded to the equipment grounding conductor at the first disconnecting means only. Downstream, the neutral must be isolated.
118.SDS Bonding: Verify that the system bonding jumper is installed at the SDS source (transformer or generator) and that the downstream panel has an isolated neutral.
119.Conduit Fill and Support: Verify that the raceway is properly supported (per Article 358 for EMT, Article 344 for RMC, etc.) and that conduit fill does not exceed the limits of Chapter 9, Table 1.
120.Working Clearance: Ensure that the feeder panelboard has proper working clearance (110.26) – at least 30 inches wide, 36 inches deep, and 6.5 feet high.
1.10 Common Exam Traps
The 125% Factor: Do not apply the 125% continuous load factor to the neutral unless the neutral serves the continuous load directly.
Terminal vs. Conductor Ampacity: Always use the 75°C column for initial conductor selection, then check the 90°C column after adjustments.
Next-Size-Up Rule: The rule in 240.4(B) applies to standard OCPD sizes only, and it does not apply to motor or transformer secondary conductors in the same manner.
Motor Feeder OCPD: The feeder OCPD for motors is based on the branch-circuit OCPD of the largest motor, not the motor FLA.
SDS Grounding: Do not bond the neutral at the downstream panel of an SDS. This is a violation and a common inspection failure.
Voltage Drop: The NEC does not mandate a specific percentage for feeders, but the informational note recommends 3% for feeders and 5% total. A master should always calculate for long runs.
Summary
Feeder design is a core competency for the master electrician. It requires a synthesis of load calculations (Article 220), conductor ampacity (Article 310), overcurrent protection (Article 240), and specific rules for motors, transformers, and generators. The master must not only know the code sections but also understand the intent behind them—to protect conductors from overheating and to ensure system coordination. On the exam, focus on the 125% continuous load factor, the 75°C vs. 90°C column distinction, and the unique rules for separately derived systems. In the field, your ability to verify proper bonding, conductor sizing, and OCPD coordination will define your competence as a master.
Preparing for the Kentucky Master Electrician license?
See the full licensing path, exam format, eligibility and application steps.
