5.7 Feeders and Branch Circuits Case Lab
Key Takeaways
- Case questions should be reduced to load, circuit boundary, conductor ampacity, derating, terminal, and overcurrent checks.
- A feeder load calculation and a branch-circuit rating calculation are related but not identical.
- Tables are faster when you know the question's target before opening the code book.
- Most wrong numeric answers come from skipping a classification step or using a demand factor in the wrong place.
Case method
Mixed NEC questions are manageable when you force them into a sequence. Do not start by hunting answer choices. Start by naming the conductors and the load. Then decide which article family applies. A journeyman candidate who can classify the problem quickly will save minutes on the open-book exam.
Use this lab checklist:
- Boundary: service, feeder, or branch circuit.
- Load: continuous, noncontinuous, dwelling, motor, HVAC, appliance, lighting, receptacle, or mixed.
- Calculation: Article 220 or equipment article if load calculation is needed.
- Ampacity: material, insulation, table, ambient correction, conductor-count adjustment.
- Termination: 60 C or 75 C final ampacity limit unless marked otherwise.
- Protection: Article 240, small-conductor rule, standard sizes, and equipment-specific rules.
- Safety overlays: GFCI, AFCI, equipment grounding, bonding, and disconnecting means.
Case 1: small commercial feeder
A 120/208 V panelboard feeder supplies 42 A of noncontinuous lighting and receptacle load and 24 A of continuous lighting load. The question asks for the minimum feeder ampacity before conductor adjustment. Use the continuous-load rule:
42 A + (1.25 x 24 A) = 72 A
The minimum feeder ampacity is 72 A before any adjustment, correction, or terminal checks. If answer choices include 66 A, that is the simple sum and ignores continuous load. If choices include conductor sizes, move to Article 310 and terminal ratings. If choices include breaker sizes, move to Article 240 standard ratings after conductor ampacity is known.
Case 2: branch circuit in a shared raceway
A 20 A branch circuit uses 12 AWG copper THHN in a raceway containing eight current-carrying conductors total. Assume normal ambient and a 70 percent adjustment factor. The 90 C ampacity for 12 AWG copper THHN is commonly 30 A.
30 A x 0.70 = 21 A
The adjusted ampacity is 21 A, which can support a 20 A circuit before other checks. The final termination ampacity for 12 AWG copper is commonly 20 A under 60 C limitations, and the small-conductor rule also limits ordinary 12 AWG copper to 20 A protection. Therefore a 20 A breaker may be acceptable. A 25 A breaker is not acceptable for an ordinary branch circuit merely because the adjusted ampacity is 21 A.
Case 3: feeder to detached structure
A dwelling has a feeder from the service equipment to a detached workshop panel. The workshop panel contains branch-circuit breakers. The conductors to the workshop are feeders, not branch circuits. Article 225 may apply because the feeder supplies another building or structure. Article 250 grounding and bonding rules apply, including equipment grounding conductor and grounding electrode system considerations.
If the question asks for the minimum number of conductors, identify whether a neutral is needed for line-to-neutral loads and whether an equipment grounding conductor is required. Do not confuse the grounded neutral conductor with the equipment grounding conductor. For modern feeder wiring to a detached structure, treating the neutral and equipment grounding conductor as separate paths is the standard exam expectation unless a narrow legacy exception is stated by the applicable edition and conditions.
Case 4: dwelling kitchen circuits
A question asks how many 20 A small-appliance branch circuits are required for a dwelling kitchen and dining area. The answer is at least two. The same kitchen may also need GFCI protection for receptacles and AFCI protection depending on the NEC edition. Those protection requirements do not change the minimum number of small-appliance circuits.
If the question adds a dishwasher and disposal, do not automatically place them on the small-appliance circuits. Check appliance load, manufacturer instructions, and applicable branch-circuit rules. Countertop receptacles are the core small-appliance circuit issue. Lighting is a separate issue.
Case 5: HVAC nameplate
An outdoor condensing unit has a nameplate showing minimum circuit ampacity of 31 A and maximum overcurrent protective device of 50 A. In this type of problem, the nameplate values are not random information. Minimum circuit ampacity is used for conductor sizing. Maximum overcurrent protective device is the largest permitted breaker or fuse under the equipment rules.
A common wrong answer is to size the conductor from the 50 A breaker as if this were a general feeder. Another wrong answer is to reject a 50 A breaker because the conductor selected from minimum circuit ampacity is smaller than a normal 50 A general-purpose conductor. HVAC equipment has special rules, and the nameplate directs the sizing path.
Case 6: neutral count in a raceway
A raceway contains two 3-wire single-phase multiwire branch circuits with shared neutrals, plus equipment grounding conductors. If each multiwire branch circuit has two ungrounded conductors on opposite phases and one shared neutral carrying only imbalance, the neutrals may not count for adjustment. The equipment grounding conductors do not count. The current-carrying count may be four, not six or eight.
Change one fact and the answer may change. If the neutral is part of a 2-wire circuit, it counts. If the system is a 4-wire wye circuit with nonlinear loads, the neutral may count. Exam writers use these small wording changes to separate code readers from guessers.
Final exam traps table
| Trap | Better move |
|---|---|
| Starting with conductor size answer choices | Identify load and boundary first |
| Using demand factors to omit required dwelling circuits | Keep Article 210 requirements separate from Article 220 calculations |
| Using 90 C ampacity as final terminal ampacity | Use 90 C for derating only when permitted, then check terminal limits |
| Counting equipment grounding conductors for ampacity adjustment | Count them for fill, not as normal current-carrying conductors |
| Treating HVAC or motor circuits as general-purpose circuits | Use equipment article and nameplate values |
Open-book timing advice
ICC contractor/trades exams are multiple-choice and open book for R17, T17, and G17 journeyman electrician exams, but the time limit does not allow a fresh search for every answer. Mark the NEC index entries for branch circuit, feeder, ampacity, adjustment factors, temperature correction, overcurrent protection, small conductors, equipment grounding conductors, GFCI, and AFCI. Use permanent tabs and notes only as allowed by the current ICC bulletin and test center rules.
When practicing, force yourself to write the article path beside each answer: 210 for required branch circuits, 215 for feeders, 220 for load calculation, 240 for overcurrent, 250 for grounding, 310 for ampacity, and equipment articles when present. That habit turns a mixed case into a sequence instead of a memory contest.
A feeder supplies 42 A noncontinuous load and 24 A continuous load. What minimum ampacity is required before adjustment and correction?
An HVAC nameplate lists minimum circuit ampacity of 31 A and maximum overcurrent protective device of 50 A. What is the best sizing approach?
Which sequence is the best way to attack a mixed feeder and branch-circuit calculation question?