8.7 Motors and Generators Case Lab

Case method

Mixed equipment questions look intimidating because they name several devices at once: motor, controller, disconnect, generator, transfer switch, transformer, fire alarm panel, or standby load. The solution is to split the fact pattern into layers. Do not try to hold the whole system in your head.

Use this worksheet:

Write the article path before the calculation. A few seconds spent on the path can save several minutes of table hunting.

Case 1: Single motor in a shop

A 480 V, three-phase, 10 hp motor supplies a dust collector. The question gives an NEC table FLC and a lower nameplate full-load current. It asks for minimum branch-circuit conductor ampacity.

Path: Article 430 motor conductor rule, motor FLC table, Article 310 conductor ampacity if a conductor size is required.

Setup:

minimum ampacity = 1.25 x NEC table FLC

Do not use the lower nameplate current unless the specific conductor rule tells you to. Do not size the breaker first. Do not choose the overload size. The asked-for output is conductor ampacity.

Trap answer: an overload value based on nameplate current. That value may be useful later, but it does not answer the conductor question.

Case 2: Motor breaker seems too large

A motor branch circuit has conductors with 35 A ampacity and an inverse-time breaker larger than 35 A. The stem says separate overload protection is installed. It asks whether the breaker size is automatically a violation.

Path: Article 430 branch-circuit short-circuit and ground-fault protection, overload rules, conductor sizing.

Answer logic: not automatically. Motor fault protection may be larger than conductor ampacity where Article 430 permits it, because the breaker must ride through starting current. The separate overload device protects the motor from overload heating. The final answer still depends on the maximum device permitted by the Article 430 table and any allowed increase.

Trap answer: applying ordinary small-conductor branch-circuit rules without motor context.

Case 3: Optional standby generator for a dwelling

A homeowner installs a standby generator feeding a selected-load panel with refrigerator, furnace blower, lights, and a sump pump. The transfer switch does not switch the neutral. The question asks how to classify the system and what grounding issue to check.

Path: Article 702 optional standby, Article 445 generator, Article 250 grounding and bonding.

Classification: optional standby, assuming the AHJ has not required these loads as emergency or legally required standby. Grounding issue: because the neutral is not switched, the generator is commonly nonseparately derived. Check that the neutral is not bonded again at the generator in a way that creates parallel neutral current paths.

Trap answer: calling it emergency because the owner considers the sump pump important.

Case 4: Neutral-switched generator transfer

A commercial generator feeds a transfer switch that switches all phase conductors and the grounded conductor. The question asks whether separately derived system rules may apply.

Path: transfer equipment article for system type, Article 250 separately derived systems, Article 445 generator.

Logic: switching the grounded conductor can make the generator system separately derived during generator operation. Then the system bonding jumper and grounding electrode conductor rules become central. The bonding point must be intentional, accessible as required, and coordinated with the rest of the grounding system.

Trap answer: using the same grounding answer as a solid-neutral transfer switch.

Case 5: Transformer feeding a panel

A 75 kVA, three-phase transformer has a 480 V primary and 208Y/120 V secondary. The question asks for secondary full-load current.

Path: transformer current formula first. Article 450 and Article 240 only if overcurrent protection or secondary conductors are asked.

Setup:

I = 75,000 / (1.732 x 208) = 208.2 A

Stop if the answer choices are current values. If the question asks for conductor size or overcurrent protection, continue to the appropriate article. Many exam mistakes come from continuing past the asked-for output and selecting a standard breaker size when the question only asked for current.

Case 6: Fire alarm and normal power in one raceway

A power-limited fire alarm circuit is proposed in the same raceway as 120 V lighting conductors. The question asks whether that is automatically allowed because the fire alarm cable is low voltage.

Path: Article 760 fire alarm, Article 725 if class circuits are involved, wiring method and separation rules.

Logic: low voltage is not enough. Check whether the circuit is power-limited or non-power-limited, conductor insulation ratings, separation permissions, and whether the cable type is listed for the location. Fire alarm circuits have system integrity and separation concerns.

Trap answer: approving it because both circuits are under 600 V.

Final exam checklist

Before choosing an answer, ask four questions:

1. What exactly is being requested: ampacity, breaker size, overload setting, classification, grounding point, or disconnect location?
2. Which article owns the equipment: 430, 440, 445, 450, 700, 701, 702, 725, 760, or another special article?
3. Which current value belongs in the formula: table FLC, nameplate current, calculated kVA current, marked MCA, or connected load?
4. Is there a special marking, neutral-switching fact, system classification, or AHJ-required load that changes the default answer?

That checklist is the chapter in compact form. It also matches the reality of an ICC open-book exam: you have four-option multiple-choice questions, no guessing penalty, and limited time. You cannot look up every concept from zero. Build the map, use the tables deliberately, and answer the question asked.