6 realistic master-level open-book NEC questions for the Texas Master Electrician exam (TDLR · PSI) — service sizing, feeders, transformers, motors, grounding, and renewable/EV — with the exact code reference for every answer. Click to reveal the answer and explanation. Free practice starts with the first chapter of the TX Master exam in the app.
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These 6 sample questions represent the type of master-level questions you will encounter on the Texas Master Electrician exam, administered by PSI for the Texas Department of Licensing and Regulation (TDLR). The exam has two separately scored portions: the NEC Knowledge portion (TX-MST-KNOW — 75 items in 150 minutes: 70 scored + 5 unscored pretest) and the Calculations portion (TX-MST-CALC — 33 items in 170 minutes: 30 scored + 3 unscored). Both are open book against the NEC 2026, and both test your ability to find and apply code rules — not to memorize them.
The sample set above maps to the official blueprint: service sizing and grounding come from the Services/Service Equipment/SDS subject area (11 scored KNOW items — the biggest on that portion), feeder and motor questions from the Calculations portion (8 and 6 scored CALC items respectively), and transformer and EV questions from the Equipment & Devices and Renewable Energy areas. Unless a question says otherwise, all conductors are considered copper — just like the real exam.
Click the Show Answer button under each question to see the correct answer, a detailed explanation, and the code reference. Use these questions to practice your code-navigation skills — chapter map, section numbering, and the index — and to identify areas that need more study. Sign up for free to start with the first chapter of the TX Master exam.
A 120/240-volt, single-phase service supplies 180 amperes of continuous load and no noncontinuous load. Per NEC 230.42, what is the minimum required ampacity of the ungrounded service-entrance conductors?
✅ Correct Answer: C: 225 amperes
NEC 230.42 requires service-entrance conductors to be sized at 100% of the noncontinuous load plus 125% of the continuous load: 180 A × 1.25 = 225 A. Services are the single largest subject area on the Texas Master exam — 11 scored items on the NEC Knowledge portion and 8 on the Calculations portion.
Reference: NEC 230.42
A feeder supplies 40 amperes of continuous load. Per NEC 215.2(A)(1), what is the minimum required feeder conductor ampacity before any adjustment or correction factors?
✅ Correct Answer: C: 50 amperes
NEC 215.2(A)(1) sizes feeder conductors at 100% of the noncontinuous load plus 125% of the continuous load: 40 A × 1.25 = 50 A. This 125% continuous-load rule reappears throughout the Calculations portion — services, feeders, and branch circuits all use it.
Reference: NEC 215.2(A)(1)
A 75 kVA, three-phase transformer is fed from a 480-volt primary. Per NEC 450.3(B), what is the maximum standard rating permitted for the primary overcurrent device?
✅ Correct Answer: D: 125 amperes
Primary full-load current = 75,000 VA ÷ (480 V × 1.732) = 90.2 A. NEC 450.3(B) permits primary protection up to 125% of rated current for a transformer over 9 amperes — 90.2 A × 1.25 = 112.8 A — and because that is not a standard rating, the next higher standard size from NEC 240.6, 125 A, is allowed.
Reference: NEC 450.3(B), 240.6
A 25 hp, 460-volt, three-phase continuous-duty motor is supplied by a branch circuit. Per NEC 430.22 and Table 430.250 (full-load current 34 A), what is the minimum size 75°C copper branch-circuit conductor required?
✅ Correct Answer: B: 8 AWG
Per NEC 430.6(A)(1), motor calculations use the table full-load currents, not the nameplate: Table 430.250 lists 34 A for a 25 hp, 460 V three-phase motor. NEC 430.22(A) requires branch-circuit conductors rated at 125% of that value — 42.5 A — and 8 AWG THWN (50 A at 75°C) is the smallest conductor with sufficient ampacity.
Reference: NEC 430.22, Table 430.250
The ungrounded service-entrance conductors for a dwelling are 3/0 AWG copper. Per Table 250.66, what is the minimum size copper grounding electrode conductor required?
✅ Correct Answer: C: 4 AWG
Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service-entrance conductor: 2 AWG or smaller takes 8 AWG, 1/0–1 AWG takes 6 AWG, and 2/0–3/0 AWG takes 4 AWG copper. Grounding and bonding questions are staples of the Services/SDS subject area on the TX Master exam.
Reference: NEC Table 250.66
A 40-ampere, 240-volt electric vehicle supply equipment (EVSE) unit is connected to a dedicated branch circuit. Per NEC 625.40 and 625.41, what is the minimum branch-circuit rating required for this continuous load?
✅ Correct Answer: C: 50 amperes
NEC 625.40 requires EVSE to be supplied by a dedicated branch circuit, and NEC 625.41 requires the EVSE to be treated as a continuous load. 40 A × 1.25 = 50 A, so the branch circuit must be rated at least 50 A. Renewable energy and EV equipment carries 3 scored items on the TX Master NEC Knowledge portion.
Reference: NEC 625.40, 625.41
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