Short answer: For motors, size conductors, breakers and fuses from the table full-load current (Table 430.250 three-phase, Table 430.248 single-phase), not the nameplate, and size overloads from the nameplate (430.6(A)(1)). Conductors are 125% of table current (430.22), overloads 125% or 115% of nameplate (430.32(A)(1)), and the short-circuit device is the Table 430.52(C)(1) percent times table current. For transformers, full-load current is kVA x 1,000 / V single-phase or kVA x 1,000 / (1.732 x V) three-phase, at the voltage of the side being protected, then apply Table 450.5(B).
Motor and transformer questions are step chains, and one wrong current at the start sinks every step after it. The Practice Pack has 299 step-by-step calculation solutions, plus 10 full timed two-part mocks (59 questions in 130 minutes plus 26 in 110 minutes) with a score by part and by topic and a line showing which part you would fail. Code-book navigation aids and a ranked tab-these-first list come with it. US$39, one payment.
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Motor: which current goes where
The first decision on every motor question is which current to start from. The rule in 430.6(A)(1) splits it cleanly.
| What you are sizing | Current to start from | Percent | Where |
|---|---|---|---|
| Branch circuit conductors | Table full-load current | 125% | 430.22 |
| Overload, service factor 1.15 or more, or marked rise 40 C or less | Nameplate current | 125% | 430.32(A)(1) |
| Overload, all other motors | Nameplate current | 115% | 430.32(A)(1) |
| Inverse time breaker | Table full-load current | 250% | Table 430.52(C)(1) |
| Dual-element time-delay fuse | Table full-load current | 175% | Table 430.52(C)(1) |
| Several motors on one feeder | Table full-load current | 125% of the largest plus 100% of the others | 430.24 |
Step by step, with round example numbers that are not table values:
- Read the table current for the motor’s horsepower and voltage from Table 430.250 or Table 430.248. Example: 10 A.
- Conductors: 10 x 125% = 12.5 A minimum ampacity. Then pick the conductor from Table 310.16 in your book.
- Overload: use the nameplate current from the question. Example nameplate 10 A with a service factor of 1.15: 10 x 125% = 12.5 A.
- Breaker: 10 x 250% = 25 A before rounding. 430.52(C)(1)(a) permits the next higher standard rating, which you read from 240.6(A).
- Feeder for several motors: example table currents of 10, 20 and 30 A give 30 x 125% + 10 + 20 = 67.5 A.
The trap the workbook flags most: sizing conductors from nameplate current. Nameplate is for overloads only.
Transformer: current first, then the percent
- Pick the side. Use the voltage of the side being protected. Using the secondary voltage to find primary current is the classic trap.
- Find full-load current. Single-phase: kVA x 1,000 / V. Three-phase: kVA x 1,000 / (1.732 x V).
- Open Table 450.5(B) for 1000 V and less (formerly Table 450.3(B)). Over 1000 V, it is Table 450.5(A) (formerly Table 450.3(A)).
- Apply the right row.
| Protection method | Percent | Rounding |
|---|---|---|
| Primary only, primary current 9 A or more | 125% | Note 1 allows the next standard size up |
| Primary and secondary: primary side | Up to 250% | Does not round up |
| Primary and secondary: secondary side | 125% | Rounds up |
Worked example with round example numbers: an example 50 kVA three-phase transformer with a 480 V primary has 50 x 1,000 / (1.732 x 480) = 60.14 A of primary current. With primary-only protection, 60.14 x 125% = 75.18 A, then Note 1 lets you go to the next standard size up from 240.6(A). With primary and secondary protection, the primary limit is 60.14 x 250% = 150.36 A, and that one does not round up.
Where these sit on the exam
The PSI outline gives Motors and Generators 5 Knowledge items and 2 Calculation items, and Article 430 is the third most cited article in our workbook’s 486 questions, with 35 citations. The Calculations part gives you 110 minutes for 26 items, about 4 minutes 14 seconds each, and a motor or transformer chain fits in that if you write the steps in order. Use the plain method: the PSI bulletin says not to consider exceptions or optional calculation methods unless the question directs you to.
Generator overcurrent questions use 445.19(A), formerly 445.18(B).
Run the numbers
Motor and transformer sizing steps
All defaults are examples to replace. Read the motor table current and the breaker percent from your code book, and take the nameplate current, kVA and voltage from the question.
- Motor full-load current from Table 430.250 or Table 430.248 (your book)
- Motor nameplate current (from the question)
- Service factor 1.15 or more, or marked rise 40 C or less? 1 = yes, 0 = no
- Device percent from Table 430.52(C)(1) (250 inverse time breaker, 175 dual-element fuse)
- Transformer kVA (from the question)
- Voltage of the side being protected
- Phases (1 or 3)
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Texas Journeyman Electrician Exam Prep 2026 NEC (paperback)
Our own workbook has separate motor and transformer lessons among its 8 calculation method lessons, and each calculation answer shows the givens, the formula, every step, where to round and what mistake leads to each wrong choice.
Do you use nameplate or table current for motor wire size?
Table current. Under 430.6(A)(1), conductors, breakers and fuses are sized from Table 430.250 or Table 430.248, and conductors are 125% of that current (430.22). Nameplate current is used for overloads.
What percent is a motor breaker on the journeyman exam?
Read it from Table 430.52(C)(1) and multiply by the table full-load current. An inverse time breaker is 250% and a dual-element time-delay fuse is 175%, and 430.52(C)(1)(a) permits the next higher standard rating.
How do I find transformer full-load current?
Single-phase is kVA x 1,000 divided by the voltage. Three-phase is kVA x 1,000 divided by 1.732 x the voltage. Always use the voltage of the side you are protecting.
