66 questions
A 208-V, three-phase feeder supplies a 120,000-VA load. What is the calculated feeder current?
- a.577 A
- b.288 A
- c.333 A✓
- d.200 A
Three-phase current = 120,000 / (1.732 x 208) = 333 A.
A 225-A busbar is fed by a 225-A main breaker at one end. Using the 120% rule, what is the maximum back-fed inverter (PV) supply breaker at the opposite end?
- a.60 A
- b.45 A✓
- c.40 A
- d.30 A
2023 NEC 705.12(B)(3): 120% of the 225-A busbar = 270 A; subtract the 225-A main: 270 - 225 = 45 A maximum for the back-fed supply breaker.
A feeder is protected by a 500-A overcurrent device. What is the minimum copper equipment grounding conductor?
- a.#2 AWG✓
- b.#3 AWG
- c.#4 AWG
- d.#1 AWG
2023 NEC Table 250.122: a 500-A overcurrent device requires a #2 Cu equipment grounding conductor.
A feeder is protected by an 800-A overcurrent device. What is the minimum copper equipment grounding conductor?
- a.1/0 AWG✓
- b.#2 AWG
- c.#1 AWG
- d.2/0 AWG
2023 NEC Table 250.122: an 800-A overcurrent device requires a 1/0 Cu equipment grounding conductor.
A feeder is protected by a 1,200-A overcurrent device. What is the minimum copper equipment grounding conductor?
- a.3/0 AWG✓
- b.4/0 AWG
- c.#1 AWG
- d.2/0 AWG
2023 NEC Table 250.122: a 1,200-A overcurrent device requires a 3/0 Cu equipment grounding conductor.
A service uses 1/0 AWG copper ungrounded conductors. What is the minimum copper grounding electrode conductor?
- a.#4 AWG
- b.#2 AWG
- c.#6 AWG✓
- d.#8 AWG
2023 NEC Table 250.66: service conductors of 1 or 1/0 Cu require a #6 Cu grounding electrode conductor.
A service uses 750-kcmil copper ungrounded conductors. What is the minimum copper grounding electrode conductor?
- a.2/0 AWG✓
- b.3/0 AWG
- c.1/0 AWG
- d.#1 AWG
2023 NEC Table 250.66: over 600 through 1,100 kcmil Cu requires a 2/0 Cu grounding electrode conductor.
A separately derived system (transformer) has 600-kcmil copper derived conductors. What is the minimum copper supply-side bonding jumper?
- a.1/0 AWG✓
- b.#1 AWG
- c.2/0 AWG
- d.#2 AWG
2023 NEC 250.30(A)(2) with Table 250.102(C)(1): over 350 through 600 kcmil Cu requires a 1/0 Cu bonding jumper.
Service-entrance conductors supply a 150-A continuous load. What minimum conductor ampacity is required?
- a.187.5 A✓
- b.150 A
- c.200 A
- d.165 A
2023 NEC 230.42(A): service conductors must carry at least 125% of the continuous load: 150 x 1.25 = 187.5 A.
A service has 3/0 AWG copper ungrounded conductors. What is the minimum copper bonding jumper for the metal underground water pipe?
- a.#2 AWG
- b.#6 AWG
- c.#4 AWG✓
- d.#8 AWG
2023 NEC 250.104(A)(1) sizes the water-pipe bonding jumper from Table 250.66: 2/0-3/0 Cu service conductors require a #4 Cu bonding jumper.
Under 2023 NEC 110.9, a circuit breaker or fuse intended to interrupt fault current must have an interrupting rating that is:
- a.125% of the continuous load
- b.equal to the connected load current
- c.at least the available fault current at its line terminals✓
- d.the same as the largest downstream conductor ampacity
2023 NEC 110.9 requires the device's interrupting rating to be at least the available fault current where it is applied; ampacity and load current size the device only for normal operation.
A downstream circuit breaker has an interrupting rating below the available fault current. It may still be applied only if:
- a.the connected load is noncontinuous
- b.it is a molded-case type
- c.it is derated 20% for continuous operation
- d.it is part of a listed, tested series combination with the line-side device✓
2023 NEC 240.86 permits a lower-rated downstream breaker only as part of a tested, marked series combination with the line-side OCPD (or under engineering supervision). Derating does not raise interrupting rating.
Service phase conductors total 1,100 kcmil Cu per phase. Using the bonding table, what is the minimum copper main bonding jumper?
- a.1/0 AWG
- b.2/0 AWG✓
- c.#1 AWG
- d.3/0 AWG
2023 NEC 250.28(D)(1) with Table 250.102(C)(1): over 600 through 1,100 kcmil Cu requires a 2/0 Cu main bonding jumper.
What is the maximum rating of the overcurrent device that may protect a 400-A-rated panelboard?
- a.350 A
- b.450 A
- c.400 A✓
- d.500 A
2023 NEC 408.36: a panelboard must be protected by an overcurrent device rated no more than the panelboard rating, so 400 A is the maximum.
A single made electrode (ground rod) must be supplemented by a second electrode unless its resistance to earth does not exceed:
- a.1 ohm
- b.5 ohms
- c.25 ohms✓
- d.10 ohms
2023 NEC 250.53(A)(2): a single rod, pipe, or plate electrode must be supplemented unless it has a resistance to earth of 25 ohms or less.
A branch circuit is protected by a 30-A overcurrent device. What is the minimum copper equipment grounding conductor?
- a.#12 AWG
- b.#10 AWG✓
- c.#14 AWG
- d.#8 AWG
2023 NEC Table 250.122: overcurrent devices from 30 through 60 A require a #10 Cu equipment grounding conductor.
这门考试有多难？
电工技师（Master）执照由各州主办、以 NEC 为依据，因此格式因州而异。例如在德州，考试分两个开卷部分（2023 NEC）——75 道 NEC 知识题与 33 道计算题（共 108 题）——各须 70% 方可通过，费用 78 美元。电工年薪中位数约 62,350 美元（BLS，2024 年 5 月）。
- 推荐学习时间
- 多数人 80-150 小时——技师考试比熟练工增加更重的计算与规范应用深度。
- 官方公布的通过率
- 19.08% （来源未说明统计的是哪些考次）（n = 3,946）；22.21% （来源未说明统计的是哪些考次）（n = 3,472） —— Texas TDLR，FY 2025。请把它们看作两场考试而非一场：TDLR 并没有名为「Master Electrician」的行，只有「Master Calculations」（第一个数字）和「Master NEC」（第二个数字）。任何引用单一「德州 master 通过率 19%」的说法，都悄悄丢掉了一半考试。执照按州发放，因此这只适用于德州。来源： Texas TDLR — Electrician Exam Statistics, Fiscal Year 2025
- 重点学习方向
- 服务、服务设备与单独派生系统是最重的板块之一（德州约 18%），外加负载计算部分。
费用与薪资为近似值，会随时间变动。上方的通过率引自旁边链接的来源，并限于该来源覆盖的期间——凡是我们尚未核实来源的，都会直接说明并且不给数字。