NEC Article 430 Practice Questions: Motors

The four-step motor problem — table FLC, conductors at 125%, short-circuit device from Table 430.52, overloads from nameplate — drilled with real exam-style questions.

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61 Article 430 questions in the bankWritten in the PSI computer-based exam formatEvery answer cites the NEC section

What Article 430 Covers on the Exam

Article 430 is the highest-value calculation article after 220. Every motor question follows the same skeleton: take the full-load current from the tables (430.247–430.250, never the nameplate, except for overloads), size branch-circuit conductors at 125% of FLC (430.22), size the short-circuit/ground-fault device from Table 430.52 percentages (250% for inverse-time breakers on most AC motors), and size overload protection from the nameplate current (430.32 — 125% for motors with a service factor of 1.15 or more). Learn the skeleton and every variation becomes plug-and-chug.

Tested subtopics from Article 430

Full-load current tables for single-phase and three-phase motors — 430.248 and 430.250
Branch-circuit conductor sizing at 125% of FLC — 430.22
Short-circuit and ground-fault protection percentages — 430.52 and Table 430.52
Overload protection from nameplate current — 430.32
Feeder sizing for multiple motors: largest FLC × 125% + sum of others — 430.24

Drill a different article

61 questions in our bank cite Article 430. Practice the other high-yield articles too:

Art. 110 — General Requirements Art. 210 — Branch Circuits Art. 220 — Load Calculations Art. 230 — Services Art. 240 — Overcurrent Protection Art. 250 — Grounding and Bonding Art. 300 — Wiring Methods Art. 310 — Conductors & Ampacity Art. 314 — Boxes & Box Fill Art. 450 — Transformers Art. 680 — Swimming Pools & Spas

How to Study Article 430

Memorize which number comes from where: table FLC for conductors and breakers, nameplate current for overloads only. Then drill the Table 430.52 percentages for inverse-time breakers (250%) and dual-element fuses (175%), and the 430.24 multi-motor feeder rule (125% of the largest motor FLC plus the full FLC of the rest). Those three facts, plus the FLC tables tabbed, solve nearly every motor question on the Texas exam.

Then put it under exam conditions: the free diagnostic scores you across every category and shows whether motors is actually a weak area — or whether your study time belongs somewhere else.

Sample Questions (From Our Real Question Bank)

Here are 4 real questions from our question bank. The free diagnostic quiz has 10 more.

NEC NEC 430.6(A)(1)Equipment & Devices

1. Per NEC 430.6(A)(1), when determining the ampere rating of motor branch-circuit conductors, what value should be used instead of the motor nameplate rating?

AThe motor service factor
BThe values from the appropriate NEC table (Tables 430.247-430.250)
C125% of the nameplate rating
DThe locked-rotor current

Show explanation

NEC 430.6(A)(1) requires using the values in NEC Tables 430.247-430.250 for motor FLC, not the nameplate rating.

NEC NEC 430.22(A)Calculations - Motor Branch Circuit

2. Per NEC Table 430.250, a 10 HP, 230-volt, 3-phase motor has a full-load current (FLC) of 28 amperes. What is the minimum branch-circuit conductor ampacity per NEC 430.22(A)?

A28 amperes
B35 amperes
C42 amperes
D56 amperes

Show explanation

NEC 430.22(A) requires conductor ampacity of at least 125% of FLC: 28A × 1.25 = 35A.

NEC NEC 430.52(C)(1) and Table 430.52Calculations - Motor Protection

3. A 5 HP, 230V, single-phase motor has an NEC table FLC of 28 amperes. What is the maximum standard size inverse-time circuit breaker permitted for branch-circuit short-circuit and ground-fault protection per NEC Table 430.52?

A35 amperes
B50 amperes
C70 amperes
D90 amperes

Show explanation

For inverse-time breaker on single-phase motor: 250% of FLC = 28A × 2.50 = 70A (standard size per 240.6(A)).

NEC NEC 430.6(A)(1), Table 430.247Equipment & Devices

4. The ampacity of a 3-horsepower, 120-volt DC motor branch circuit conductor shall be based on:

AThe horsepower rating divided by the voltage
B125% of the motor full-load current from NEC Table 430.247
CThe nameplate current rating if it is marked on the motor
D100% of the horsepower rating in amperes

Show explanation

Per NEC 430.6(A)(1) and 430.22, the branch-circuit conductor for a single DC motor shall have an ampacity of not less than 125% of the motor's full-load current as listed in Table 430.247 (DC motors). Table 430.248 applies to single-phase AC motors and shall not be used for DC motor calculations.

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Frequently Asked Questions

Do I use nameplate current or table current for motor calculations?

Use the NEC full-load current tables (430.247–430.250) for sizing conductors and the branch-circuit short-circuit device. Use the motor nameplate current only for sizing overload protection per 430.6(A). Mixing these up is the most common motor-question mistake on the exam.

How do I size a breaker for a motor under 430.52?

Take the table FLC and apply the Table 430.52 percentage for the device type — typically 250% for an inverse-time circuit breaker or 175% for dual-element time-delay fuses on AC motors. If that value does not match a standard rating, 430.52(C)(1) Exception 1 permits the next higher standard size.

How are conductors sized for several motors on one feeder?

Per 430.24, feeder conductors supplying multiple motors are sized at 125% of the full-load current of the highest-rated motor plus the sum of the full-load currents of the other motors (plus any other loads served).