5.7 Feeders, Branch Circuits, and Conductor Case Lab

Integrated case method

Master-level feeder and branch-circuit questions rarely test only one table. They combine a load description, wiring method, conductor material, temperature rating, terminals, overcurrent device, neutral, equipment type, and sometimes an occupancy rule. The correct response comes from a repeatable case method. Do not chase answer choices first. Build the circuit, classify it, calculate it, and then compare the choices to your result. This section provides a lab-style workflow that can be used in ICC R16, T16, G16, and jurisdiction-specific NEC-centered exams.

Use the seven-line case sheet. Line 1: source and system voltage. Line 2: equipment and overcurrent devices in load-flow order. Line 3: conductor segment classification as service, feeder, branch circuit, tap, or equipment secondary. Line 4: loads, including continuous, noncontinuous, motor, HVAC, nonlinear, receptacle, lighting, and special occupancy tags. Line 5: conductor facts, including copper or aluminum, insulation, raceway or cable, ambient temperature, and conductor count.

Line 6: protection facts, including terminal ratings, standard overcurrent device sizes, nameplate maximums, tap length, and simultaneous disconnecting. Line 7: final checks for GFCI, AFCI, required outlets, grounding, bonding, voltage drop, and documentation.

Case 1: A 120/240 volt single-phase feeder supplies a detached workshop panel with receptacle and lighting branch circuits. The conductors from the dwelling service equipment to the workshop panel are feeders because they supply branch-circuit overcurrent devices. The conductors from the workshop panel breakers to receptacles and lights are branch circuits.

Before sizing the feeder, calculate the workshop load under the applicable rules, decide whether any loads are continuous, select conductors based on material and insulation, apply adjustment and correction if several conductors share a raceway or the route is hot, compare to terminal ratings, and select overcurrent protection. Then address feeder equipment grounding, grounding electrodes at the detached structure where required, neutral isolation, and any building disconnect requirement. A question may focus on conductors, but a master answer sees the entire feeder installation.

Case 2: A commercial panel supplies four two-wire 120 volt receptacle circuits in one raceway using individual neutrals and one equipment grounding conductor. The current-carrying conductor count is normally eight: four ungrounded conductors and four neutrals that carry circuit current. The equipment grounding conductor does not count. If the conductors are copper THHN/THWN-2, the 90 C column may be the starting point for adjustment and correction, but terminals still impose 60 C or 75 C limits depending on the equipment.

The candidate must not count only four conductors because there are four circuits, and must not count the equipment grounding conductor as a ninth current-carrying conductor. After derating, check the small conductor overcurrent rule if 14, 12, or 10 AWG conductors are involved.

Case 3: A 25 foot set of conductors taps a 400 amp feeder to supply a 100 amp fused disconnect. The first question is not conductor ampacity; it is whether the installation qualifies under a specific tap rule. Identify the length, location, physical protection, ampacity relative to the upstream and downstream devices, and termination in a single overcurrent device. If the rule requires the tap conductors to terminate in a device that limits the load to their ampacity, a simple junction box continuation is not enough.

If the conductors are too long or routed without required protection, the design fails even if the math looks close. Tap questions reward candidates who treat the exception as a checklist.

Case 4: A packaged rooftop unit nameplate shows minimum circuit ampacity and maximum overcurrent protection. Use minimum circuit ampacity to size the branch-circuit conductors after considering conductor conditions. Use maximum overcurrent protection as the ceiling for the breaker or fuse type allowed by the nameplate. Do not use a larger breaker because the conductor table allows it. Do not use a smaller conductor because the running load seems lower than the nameplate minimum.

If the equipment is on a rooftop, ambient temperature, sunlight, raceway location, service receptacle requirements, disconnecting means, working clearance, and conductor insulation suitability may also appear in the problem.

Case 5: A dwelling kitchen remodel adds countertop receptacles, a dishwasher, a disposal, lighting, and an island. First identify required receptacle outlets and small-appliance branch circuits under the adopted NEC edition. Then assign fixed appliances and lighting to permitted circuits. Then check GFCI and AFCI requirements, receptacle ratings, tamper resistance where applicable, box fill, cable protection, and equipment grounding. The wrong answer often sizes a 20 amp circuit correctly but places loads on a circuit not permitted to serve them or misses protection.

Required outlet design is as much about circuit purpose as conductor size.

Case 6: A long 20 amp branch circuit supplies equipment at the far end of a warehouse. First size the branch-circuit conductors by ampacity and protection. Then evaluate voltage drop with the given distance, current, voltage, phase, and conductor size. If voltage drop is excessive for the design goal, increase conductor size and recheck terminations, box fill, conduit fill, and equipment grounding conductor requirements. Do not call the original installation a code violation solely because an informational voltage drop recommendation is exceeded unless the adopted rule or specification makes it mandatory.

In contract work, however, performance specifications may make the design target enforceable.

Case 7: A shared-neutral circuit trips a new protective breaker immediately after a panel upgrade. The likely issue may be that all conductors of the protected circuit do not pass through the sensing device, or that neutrals from separate circuits are tied together downstream. Troubleshoot by identifying the multiwire branch circuit, verifying phase relationship, verifying simultaneous disconnecting means, tracing neutral continuity, and checking the product instructions for the GFCI or AFCI device. Do not defeat the protective device to stop nuisance tripping.

The installation must be corrected so the protection monitors the complete circuit current path.

Use a written rejection habit on exam practice. For each answer choice, ask why it is too small, too large, wrong article, wrong circuit type, wrong edition, wrong conductor count, wrong terminal column, or wrong equipment marking. Master exams are timed open-book tests, so you will not look up every word. The advantage comes from recognizing the code neighborhood, using a compact workflow, and avoiding traps. In field supervision, the same method creates better submittals, cleaner inspections, and safer troubleshooting.

It also respects the real licensing structure: ICC provides exams used by licensing agencies, but state and local authorities determine actual license requirements.

Structured Decision Aid

• Build the case from load list, occupancy, continuous-load status, conductor material, and termination rating.
• Run ampacity, derating, OCPD, grounding, and voltage-drop checks in a fixed sequence.
• Compare field shortcuts against code-minimum results before choosing the answer.
• Record each missed step in the error log as formula, table, article, or assumption failure.