Branch Circuits & Conductors

Wyoming Master Electrician Exam — 2023 NEC (NFPA 70)

Learning Objectives

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

6.Apply the general requirements for branch circuits, including conductor sizing, overcurrent protection, and voltage drop considerations for commercial and industrial loads.

7.Distinguish between multiwire branch circuits, individual branch circuits, and feeders, and apply the specific code rules for each in a 3-phase, 4-wire system.

8.Calculate minimum branch-circuit conductor ampacity using the correction and adjustment factors from Table 310.16 and associated notes.

9.Identify the requirements for grounding and bonding of branch circuits supplied from separately derived systems (transformers and generators).

10.Navigate the NEC efficiently to locate code sections for branch circuits, conductors, overcurrent protection, and services during the open-book exam.

11.Recognize common inspection failures and exam traps related to conductor identification, derating, and overcurrent device sizing.

1.1 General Branch-Circuit Requirements — Article 210

Article 210 is your foundational map for branch circuits. As a master, you are responsible for the design and supervision of these circuits, not just their installation.

210.3 Rating of Branch Circuits. The rating of a branch circuit is determined by the rating of the overcurrent device (OCPD), not the conductor ampacity. This is a critical distinction. A circuit with #10 AWG conductors (rated 30 A at 75°C) protected by a 20 A breaker is a 20-ampere branch circuit. The conductor is oversized; the circuit is not.

210.4 Multiwire Branch Circuits. This is a high-liability area for a supervising electrician. A multiwire branch circuit consists of two or more ungrounded conductors that share a common grounded (neutral) conductor. In a 3-phase, 4-wire wye system, you can have three phase conductors (A, B, C) and one neutral.

210.4(B) Disconnecting Means. All ungrounded conductors of a multiwire circuit must have a means to disconnect them simultaneously. This means a 2-pole or 3-pole breaker, not individual single-pole breakers with a handle tie (unless the tie is identified for the purpose).

210.4(C) Line-to-Neutral Loads. The neutral conductor carries the unbalanced load. You must not connect loads that produce line-to-line voltage (e.g., a 208 V load) on a multiwire circuit that shares a neutral, unless the overcurrent protection is specifically designed to open all ungrounded conductors simultaneously.

210.4(D) Grouping. When you have more than one multiwire circuit in a single enclosure, you must group the ungrounded conductors of each circuit by wire ties, cable ties, or similar means. This is a common inspection point — sloppy grouping is a frequent violation.

210.5 Identification for Branch Circuits. For voltages greater than 277 V to ground (e.g., 480Y/277 V systems), the ungrounded conductors must be identified by phase (A, B, C) using color coding or tagging. For 120/208 V or 120/240 V systems, you must identify the grounded conductor (neutral) — white or gray — and the equipment grounding conductor (green or bare). A master must ensure the color-coding scheme is consistent throughout the installation.

1.2 Conductor Sizing and Ampacity — Article 310

This is where the math gets serious. The 2023 NEC uses a two-step process for conductor sizing:

26.Base Ampacity: Start with Table 310.16 for conductors rated 0–2000 V. This table gives ampacities for specific conductor sizes and insulation types at an ambient temperature of 30°C (86°F).

27.Correction and Adjustment: Apply the appropriate factors to the base ampacity.

310.15(B)(1) Ambient Temperature Correction. If the ambient temperature where the conductor runs exceeds 30°C, you must apply the correction factors from Table 310.15(B)(1). For example, a 90°C-rated THHN conductor in a 40°C ambient environment has a correction factor of 0.91. You multiply the 90°C column ampacity by 0.91.

310.15(C)(1) Adjustment Factors. When more than three current-carrying conductors are bundled together, you must apply the adjustment factors from Table 310.15(C)(1). For 4–6 conductors, the factor is 0.80. For 7–9 conductors, it is 0.70.

Critical Exam Trap: The adjustment and correction factors are applied to the insulation temperature rating column (e.g., 90°C for THHN), not the termination temperature rating (usually 75°C). However, the final ampacity after derating cannot exceed the termination temperature rating of the terminals (per 110.14(C)). This is the classic "round trip" calculation:

Step 1: Start with the 90°C column ampacity.

Step 2: Apply correction and adjustment factors.

Step 3: The result must be ≥ the calculated load.

Step 4: The OCPD must protect the conductor at its final ampacity, but you cannot use a 90°C rating for the termination.

Example: A 20 A continuous load on a circuit with 4 current-carrying conductors in a 45°C ambient. Using #12 THHN (90°C column = 30 A): 30 A × 0.80 (4–6 conductors) × 0.82 (45°C correction) = 19.68 A. This is less than 20 A, so #12 is too small. You must use #10.

310.15(B)(3)(c) Neutral as a Current-Carrying Conductor. For a 3-phase, 4-wire wye system supplying nonlinear loads (e.g., electronic ballasts, VFDs, computers), the neutral is considered a current-carrying conductor because of harmonic currents. This means you must count the neutral when applying the adjustment factors of Table 310.15(C)(1). This is a common miss in commercial office fit-outs.

1.3 Overcurrent Protection — Article 240

The master must understand the hierarchy of overcurrent protection: branch-circuit OCPD protects the branch-circuit conductors and the connected equipment; feeder OCPD protects the feeder conductors; service OCPD protects the service conductors.

240.4 Protection of Conductors. Conductors must be protected against overcurrent in accordance with their ampacity, unless specific sections permit the use of the next higher standard size OCPD.

240.4(B) Next Higher Standard Size. If the conductor ampacity does not correspond to a standard OCPD rating (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000 A), you may use the next higher standard size, provided the conductor ampacity is not less than the noncontinuous load plus 125% of the continuous load.

240.4(D) Small Conductors. For #14, #12, and #10 AWG copper, the OCPD is limited to 15 A, 20 A, and 30 A respectively, regardless of the conductor's 90°C rating. You cannot put a 40 A breaker on #10 THHN just because the 90°C column says 40 A.

240.6(A) Standard Ampere Ratings. You must memorize the standard ratings listed above. The exam will test your ability to select the correct OCPD size without looking up the table.

240.21 Location in Circuit. Overcurrent devices must be located where the conductor receives its supply (the point of tap), unless specific tap rules apply. This is critical for feeder taps (240.21(B)) and transformer secondary taps (240.21(C)). For a master supervising a job, knowing the 10-ft tap rule (240.21(B)(1)) and the 25-ft tap rule (240.21(B)(2)) is essential for industrial installations where you tap a large feeder to feed a small panel.

1.4 Services and Service Equipment — Article 230

Article 230 governs the service conductors and equipment from the utility point of connection to the service disconnecting means.

230.42 Minimum Size of Service Conductors. Service conductors must have an ampacity of not less than the sum of the noncontinuous loads plus 125% of the continuous loads. This is the same 125% rule that applies to feeders and branch circuits.

230.71 Number of Disconnects. The 2023 NEC permits a maximum of six service disconnecting means for each service, grouped in one location. This is the "six-handle rule." If you have more than six, you must provide a single main disconnecting means.

230.95 Ground-Fault Protection of Equipment. For grounded wye services of more than 150 V to ground but not exceeding 600 V phase-to-phase (e.g., 480Y/277 V), and with a service disconnect rated 1000 A or more, you must provide ground-fault protection of equipment (GFPE). This is a mandatory safety feature for large commercial and industrial services. The GFPE must be set to open at a maximum of 1200 A, with a maximum time delay of 1 second for fault currents of 3000 A or more.

Inspection Point: Verify that the GFPE is installed on the line side of the main disconnect, and that it is tested and labeled. A master must ensure the coordination study is performed if there are multiple levels of GFPE to prevent nuisance tripping.

1.5 Feeders and Separately Derived Systems — Articles 215, 250, and 450

Feeders are the conductors between the service equipment and the branch-circuit OCPD. Article 215 covers feeder requirements.

215.2(A)(1) Minimum Feeder Ampacity. Feeder conductors must have an ampacity of not less than the noncontinuous load plus 125% of the continuous load. This is the same formula as for services.

215.12 Identification. Feeders must have the grounded conductor identified (white or gray) and the equipment grounding conductor identified (green or bare). For 3-phase feeders, phase identification must be consistent.

Separately Derived Systems (SDS). A transformer secondary or a generator is a separately derived system if there is no direct electrical connection between the supply side and the load side (i.e., they are isolated by a transformer winding or by the generator's stator/rotor).

250.30 Grounding and Bonding of SDS. The system must have a grounding electrode conductor connected to the system's grounding electrode (e.g., a ground rod or building steel). The grounded conductor (neutral) must be bonded to the equipment grounding conductor at the first disconnecting means of the SDS, or at the source (the transformer or generator) if there is no disconnecting means.

250.30(A)(2) Bonding Jumper. A main bonding jumper must connect the grounded conductor to the equipment grounding conductor at the SDS source or first disconnect.

450.3 Transformer Overcurrent Protection. The primary and secondary of a transformer must be protected against overcurrent. For a transformer with a primary current of 9 A or more, the primary OCPD can be set at 125% of the primary current. For a transformer with a primary current of less than 9 A, the primary OCPD can be set at 167%. The secondary OCPD (if provided) can be set at 125% of the secondary current.

Exam Trap: For a 75 kVA, 480 V primary, 208Y/120 V secondary transformer:

Primary current = 75,000 VA / (480 V × √3) = 90.2 A. Primary OCPD = 90.2 A × 1.25 = 112.8 A → use 125 A (next standard size).

Secondary current = 75,000 VA / (208 V × √3) = 208.2 A. Secondary OCPD = 208.2 A × 1.25 = 260.2 A → use 300 A.

Generator Applications — Article 445. Generators are treated similarly to transformers for grounding purposes. The generator's neutral must be bonded to the equipment grounding conductor at the generator only if it is a separately derived system. If the generator is a non-separately derived system (i.e., it is a portable generator with a direct connection to the load via a transfer switch that switches the neutral), the neutral must not be bonded at the generator — it must be bonded at the service.

1.6 Voltage Drop — 210.19(A) Informational Note and 215.2(A) Informational Note

The NEC does not mandate a specific voltage drop percentage as a code requirement, but it is an informational note that recommends:

Feeders: 3% maximum voltage drop.

Branch circuits: 3% maximum voltage drop.

Combined feeder and branch circuit: 5% maximum total voltage drop.

Master Responsibility: While the code does not require you to calculate voltage drop, a master who signs off on an installation is responsible for the functional performance of the equipment. Motors and other inductive loads are sensitive to voltage drop. Use the formula:

Single-phase: VD = (2 × K × I × D) / CM

Three-phase: VD = (√3 × K × I × D) / CM

Where K = 12.9 for copper, I = current in amperes, D = distance in feet, and CM = circular mils of the conductor.

Practical Rule: For a 120 V circuit, a 3% drop is 3.6 V. For a 208 V circuit, it is 6.24 V. For a 480 V circuit, it is 14.4 V. If the run is long, increase the conductor size one or two sizes to compensate.

1.7 Code Navigation — Where to Find It

Topic	NEC 2023 Location
Branch-circuit general requirements	Article 210
Multiwire branch circuits	210.4
Branch-circuit ratings	210.3, 210.23
Conductor ampacity tables	Table 310.16
Ambient temperature correction	Table 310.15(B)(1)
Adjustment factors (bundle)	Table 310.15(C)(1)
Neutral as current-carrying	310.15(B)(3)(c)
Overcurrent protection general	Article 240
Standard OCPD ratings	240.6(A)
Protection of small conductors	240.4(D)
Tap rules	240.21(B), 240.21(C)
Service conductors	Article 230
Service disconnects (six-handle)	230.71
Ground-fault protection (services)	230.95
Feeder requirements	Article 215
Grounding and bonding of SDS	250.30
Transformer overcurrent protection	450.3
Generator requirements	Article 445
Voltage drop (informational)	210.19(A) IN, 215.2(A) IN
Conductor identification	210.5, 215.12

1.8 Inspection and Supervision Points

As a master, you are the final authority on the job site. Before you sign off on any rough-in or final, verify these items:

83.Termination Temperatures: Check that the conductor insulation rating matches the terminal rating. If the breaker is rated 75°C, you cannot use the 90°C ampacity for the final termination, even if the wire is THHN.

84.Derating in Conduit: Count the conductors in every conduit. If you have 4–6 current-carrying conductors, you must derate to 80%. If you have a neutral carrying harmonic current, count it.

85.Multiwire Circuit Grouping: Open the panel and verify that the phase conductors of each multiwire circuit are grouped with a tie or marker. This is a safety issue for future electricians.

86.GFPE on Large Services: For any 480Y/277 V service with a disconnect ≥ 1000 A, verify the GFPE is installed and the settings are documented.

87.Transformer Bonding: Verify that the neutral of a separately derived system is bonded to the equipment grounding conductor at the first disconnecting means, and that a grounding electrode conductor is run to an acceptable electrode.

88.Voltage Drop for Long Runs: If the distance from the panel to the load exceeds 100 ft, perform a voltage drop calculation. Do not rely on the minimum ampacity table alone.

1.9 Common Exam Traps

Trap 1: Using the 90°C column for terminations. The 90°C column is only for derating purposes. The final ampacity is limited by the 60°C or 75°C terminal rating.

Trap 2: Forgetting the 125% continuous load factor. For a 30 A continuous load, the conductor must be sized for 37.5 A (30 × 1.25), not 30 A. This often pushes you up one conductor size.

Trap 3: Counting the neutral. For a standard 3-phase, 4-wire wye circuit with balanced linear loads, the neutral is not counted as current-carrying. But for nonlinear loads, it is. The exam will specify "nonlinear" or "harmonic" loads to test this.

Trap 4: The six-disconnect rule. The six-handle rule applies to the service disconnecting means, not to feeders or branch circuits. A panelboard with 42 circuits is fine; a service with 7 disconnects is not.

Trap 5: Transformer secondary protection. Remember that the secondary conductors of a transformer are protected by the secondary OCPD, but the transformer itself is protected by the primary OCPD. If the primary OCPD is sized at 125% of the primary current, it does not protect the secondary conductors — you need a separate secondary OCPD or you must comply with the tap rules in 240.21(C).

Trap 6: Voltage drop is not a code requirement. The exam may ask you to calculate voltage drop, but it will not ask you to "violate the code" if you exceed 3%. The informational notes are recommendations, not mandates.

Final Master's Check

A master electrician does not just run conduit and pull wire — they design systems that are safe, code-compliant, and functional. When you approach a branch circuit or feeder problem on the exam, always ask: What is the load? Is it continuous? How many conductors are in the raceway? What is the ambient temperature? What is the termination temperature rating? Answer those five questions, and you will pass the calculation portion of the exam. The rest is code navigation — knowing where to look and what the table says.

Branch Circuits & Conductors