Master Electrician Exam Prep

Exam blueprint

Sourced from NEC NFPA 70 (2023 edition) + Common State Master Electrician Exam Outlines (PSI/Prov/Block)

• NEC definitions + general requirements (Art. 100, 110)10%
• Wiring methods + overcurrent protection (Art. 240, 250, 300)15%
• Services + service sizing (Art. 230)12%
• Branch circuits + feeders (Art. 210, 215, 220)15%
• Grounding + bonding (Art. 250)13%
• Motors + motor circuit calculations (Art. 430)10%
• Special occupancies + locations (Art. 500-590, 700)10%
• Business + jurisdiction-specific law15%

1. 01Service entrance + service load calculations

   ~120min

   Sizing the service is the single most common calculation question on the master exam. Standard vs. optional method, demand factors, and the sequence the NEC expects you to follow.

   • Service entrance terms (Article 230)

     SERVICE = the conductors and equipment connecting the serving utility to the wiring of the premises served. SERVICE DROP = overhead service conductors from the last utility pole to the service point. SERVICE LATERAL = underground service conductors. SERVICE-ENTRANCE CONDUCTORS (overhead OR underground) = from service point to service equipment. SERVICE EQUIPMENT = the main disconnect + overcurrent device. SERVICE POINT = the boundary between utility and customer wiring (often the meter base on residential). NEC 230 governs every component from the utility to the load side of the main breaker.

     Reference: NEC 2023 Article 230 + Art. 100 definitions

   • Standard load calculation — Article 220 Part III

     Sequence per 220.40-220.61: (1) GENERAL LIGHTING + RECEPTACLE LOAD per 220.12 (3 VA/sq ft for dwellings); (2) SMALL APPLIANCE + LAUNDRY (1500 VA each x at least 2 small-appliance circuits + 1 laundry); (3) DEMAND FACTOR per Table 220.42 (first 3000 VA at 100%, next 117,000 VA at 35%, remainder at 25% for dwelling); (4) APPLIANCE LOADS per 220.53 (75% demand allowed if 4 or more); (5) DRYER per 220.54; (6) RANGE per Table 220.55; (7) HEATING/AC larger of 220.51; (8) MOTOR loads at 125% of largest plus sum of others. Total VA / 240 V = service amperage.

     Reference: NEC 2023 220.40 - 220.61

   • Optional dwelling-unit calculation — 220.82

     Section 220.82 provides an OPTIONAL simplified calculation for one-family or individual unit of two-family/multifamily dwellings served by a 100 A or larger service. Add: (a) 3 VA/sq ft general lighting + receptacles; (b) 1500 VA per small-appliance + laundry circuit; (c) NAMEPLATE rating of all appliances (range, dryer, water heater, dishwasher, etc.); (d) 25% of largest motor. Apply demand: FIRST 10 KVA at 100%, REMAINDER at 40%. Add HVAC (largest of 220.82(C) selections). The optional calc usually produces a SMALLER service size than the standard calc — use it when the dwelling allows.

     Reference: NEC 2023 220.82

   • Worked example — 2,400 sq ft single family

     OPTIONAL method: General lighting/receptacle = 2,400 x 3 = 7,200 VA. Two SA + laundry = 3 x 1500 = 4,500 VA. Range nameplate 12,000 VA. Dryer nameplate 5,500 VA. Water heater 4,500 VA. Dishwasher 1,200 VA. Disposal 0.5 HP = ~600 VA. Sum (without HVAC): 35,500 VA. Apply demand: 10,000 at 100% + 25,500 at 40% = 10,000 + 10,200 = 20,200 VA. HVAC (largest): 6 ton AC at ~6,000 VA. Total = 26,200 VA. 26,200 / 240 = 109 A. Round up to next standard service: 125 A panel meets minimum (200 A is more common for resale value + future EVSE/heat pump).

   Practice questions (2)

   1. 1. Per the optional calculation in 220.82, the demand factor applied to the first 10 kVA of the general load is:

      • A.25%
      • B.35%
      • C.40%
      • D.100%✓ correct

      220.82(B) applies 100% demand to the first 10 kVA, then 40% to the remainder. The 25% and 35% values are distractors borrowed from Table 220.42 (the STANDARD method demand factors); 40% is the rate applied to the remainder under the optional method, not the first 10 kVA.

   2. 2. A single-family dwelling has a calculated load of 95 A. Per 230.79, what is the minimum service-disconnect rating allowed?

      • A.60 A
      • B.100 A✓ correct
      • C.125 A
      • D.200 A

      230.79(C) sets a 100 A minimum for one-family dwellings, regardless of calculated load. A calc below 100 A does not let you install a 60 A service. 125 A and 200 A are common upgrades but not the code minimum.

2. 02Overcurrent protection + conductor sizing

   ~90min

   Article 240 + 310 — selecting the right wire and the right breaker. Ampacity tables, temperature termination ratings, and the 80% continuous-load rule.

   • Conductor ampacity — Table 310.16

     Table 310.16 gives ampacities for not more than three current-carrying conductors in raceway/cable, in 30°C ambient. Three temperature columns: 60°C, 75°C, 90°C. CRITICAL RULE: even if your conductor is 90°C-rated, you must use the COLUMN MATCHING THE LOWEST-RATED TERMINATION in the circuit. Most circuit breakers and panelboards are 75°C-listed, so for typical residential/commercial circuits you read the 75°C column even with THHN/THWN-2 (which is 90°C-rated). The 90°C column is used for ampacity ADJUSTMENTS (correction factors for ambient temp, conduit fill) before checking termination temp.

     Reference: NEC 2023 110.14(C) + Table 310.16

   • The 80% continuous-load rule

     Per 210.20(A) and 215.3, branch circuits and feeders supplying CONTINUOUS LOADS (operating 3 hours or more) must be sized at 125% of continuous load + 100% of non-continuous load. Equivalently: the breaker may load to 80% of its rating with continuous load. Example: a 100 A continuous load requires a feeder + breaker rated 100 / 0.80 = 125 A minimum. The conductor minimum follows the same 125% rule unless the assembly is listed for 100% operation (rare, but exists for some industrial breakers).

     Reference: NEC 2023 210.20(A) + 215.3

   • Adjustment + correction factors

     When more than 3 current-carrying conductors are in a raceway/cable, ampacity is adjusted DOWN per Table 310.15(C)(1): 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%, 21-30 = 45%, 31-40 = 40%, 41+ = 35%. AMBIENT TEMPERATURE correction is per Table 310.15(B)(1) — for example, in 41-45°C ambient, multiply by 0.87 (75°C column). Apply BOTH factors to the 90°C ampacity (the maximum ampacity allowed by insulation), THEN compare to the 75°C termination column to find the final allowable ampacity. The lower of the two governs.

     Reference: NEC 2023 310.15

   Practice questions (2)

   1. 1. A 75°C-rated breaker terminates a 90°C-rated #6 THHN copper conductor (90°C ampacity = 75 A, 75°C ampacity = 65 A). What ampacity governs at the termination?

      • A.75 A
      • B.65 A✓ correct
      • C.55 A (60°C)
      • D.The higher of the two

      Per 110.14(C), termination temperature governs. The 75°C breaker forces use of the 75°C column = 65 A — even though the wire insulation tolerates 90°C. The 90°C rating is used for adjustment-factor calculations only, not the final termination ampacity. "Higher of the two" is the most common wrong intuition.

   2. 2. A continuous load draws 80 A. Minimum breaker size?

      • A.80 A
      • B.90 A
      • C.100 A✓ correct
      • D.125 A

      80 x 1.25 = 100 A minimum (210.20(A) for branch / 215.3 for feeder). 80 A would violate the 80% rule. 90 A is not a standard size in 240.6 anyway. 125 A is the next standard size up but is not the minimum required.

3. 03Grounding + bonding (Article 250)

   ~90min

   The most-cited and most-misunderstood NEC article. Grounding electrode systems, equipment grounding conductors, and the bonding jumper that ties them together.

   • Grounding vs. bonding — definitions matter

     GROUNDED = connected to earth or to a conducting body that extends the ground connection. BONDED = connected to establish electrical continuity and conductivity. GROUNDING ELECTRODE CONDUCTOR (GEC) = conductor used to connect the system grounded (neutral) point or equipment to a grounding electrode. EQUIPMENT GROUNDING CONDUCTOR (EGC) = conductive path that provides a ground-fault current path. MAIN BONDING JUMPER = the unspliced conductor/screw/strap connecting the EGC to the grounded conductor inside service equipment — this is where the neutral and ground first touch on the customer side. After the main bonding jumper, neutrals and grounds are SEPARATE — never bonded again on the load side.

     Reference: NEC 2023 Article 100 (definitions) + 250.24

   • Grounding electrode system — 250.50, 250.52

     Section 250.50 requires that ALL of the following PRESENT AT A BUILDING be bonded into a Grounding Electrode SYSTEM (per 250.52(A)): (1) metal underground water pipe in direct contact with earth for 10+ ft, (2) effectively grounded metal frame of building, (3) concrete-encased electrode (Ufer) at least 20 ft of #4 bare copper or 1/2" reinforcing rod in concrete contact with earth, (4) ground ring of 20+ ft of #2 bare copper. If NONE of these exist, install one or more of the made electrodes in 250.52(A)(4)-(8): rod/pipe (8 ft minimum, 25 Ω OR augmented with second), plate, or other listed electrode. The water-pipe electrode CANNOT be used alone — must be supplemented (250.53(D)(2)).

     Reference: NEC 2023 250.50 - 250.53

   • EGC sizing — Table 250.122

     The Equipment Grounding Conductor is sized per Table 250.122 based on the OVERCURRENT DEVICE rating ahead of the equipment, NOT the conductor size. Examples: 15 A breaker = #14 cu EGC; 20 A = #12; 60 A = #10; 100 A = #8; 200 A = #6; 400 A = #3; 600 A = #1; 800 A = 1/0; 1200 A = 3/0. EXCEPTION: when ungrounded conductors are upsized for voltage drop, the EGC must be PROPORTIONALLY UPSIZED per 250.122(B). EGC need not be larger than the ungrounded conductors. Wire-type EGC is not required where metallic conduit/raceway qualifies as the equipment grounding conductor under 250.118.

     Reference: NEC 2023 Table 250.122

   Practice questions (2)

   1. 1. Per 250.122, an EGC for a 200 A circuit (copper, no upsizing) is sized at:

      • A.#10
      • B.#8
      • C.#6✓ correct
      • D.#4

      Table 250.122: 200 A overcurrent device → #6 copper EGC. #10 is for 60 A; #8 is for 100 A; #4 is for 300 A. The EGC is sized from the breaker rating, not from conductor ampacity.

   2. 2. A house has a metallic underground water pipe (15 ft of contact with earth) and no other electrode. Per 250.53(D)(2), you must:

      • A.Use the water pipe alone
      • B.Supplement the water pipe with at least one additional electrode✓ correct
      • C.Replace the water pipe with a ground rod
      • D.Use a 50 ft ground ring

      250.53(D)(2) requires the water-pipe electrode to be SUPPLEMENTED by another electrode (typically a ground rod). The water pipe alone is non-compliant due to the risk of the homeowner replacing a section with PVC. Replacement is not required; rings are an option but not the only one.

4. 04Motor branch circuits + sizing

   ~60min

   Article 430 has its own peculiar sizing rules — conductors at 125% of motor FLA, breakers at much higher percentages. Don't apply Article 240 logic blindly.

   • Motor FLC vs. nameplate FLA

     For motor BRANCH-CIRCUIT CONDUCTOR sizing (430.22) and BRANCH-CIRCUIT SHORT-CIRCUIT/GROUND-FAULT PROTECTION sizing (430.52), use the FULL-LOAD CURRENT (FLC) FROM TABLES 430.247-430.250 — NOT the motor nameplate FLA. The tables give "typical" current draws by horsepower + voltage. The nameplate FLA is used ONLY for: motor OVERLOAD (running) protection sizing per 430.32, and for ampacity of separately-supplied control transformers. Mixing these up is the most common motor-circuit calculation error.

     Reference: NEC 2023 430.6(A)(1)

   • Motor branch-circuit conductor — 125% of FLC

     Per 430.22, conductors supplying a single continuous-duty motor are sized at 125% of motor FLC (from Table). Multiple motors on one feeder per 430.24: sum of all FLCs PLUS 25% of largest motor FLC. The 25% accounts for the inrush current acceptance, not continuous load — different rule from 210.20(A). A 10 HP 3-phase 230 V motor has a Table FLC of 28 A. Conductor ampacity = 28 x 1.25 = 35 A → #8 cu (75°C col).

     Reference: NEC 2023 430.22, 430.24

   • Branch-circuit OCPD — 250% non-time-delay fuse, 175% inverse-time breaker

     Per Table 430.52(C)(1), motor branch-circuit short-circuit + ground-fault protection percentages are MUCH HIGHER than the conductor ampacity — they are sized to allow inrush current. For ordinary (not Design B/C/D, not wound-rotor) motors: NON-TIME-DELAY FUSE = 300% of FLC; TIME-DELAY (DUAL-ELEMENT) FUSE = 175% of FLC; INVERSE-TIME BREAKER = 250% of FLC; INSTANTANEOUS-TRIP BREAKER = 800% of FLC. If the calculated value does not correspond to a standard size in 240.6, you MAY round up to next standard size (430.52(C)(1) Exception 1). Up to 400% is allowed for inverse-time and 225% for time-delay fuse if inrush requires (Exception 2).

     Reference: NEC 2023 Table 430.52(C)(1)

   Practice questions (2)

   1. 1. For sizing the motor branch-circuit conductors, you should use:

      • A.The motor nameplate FLA
      • B.The full-load current from NEC Tables 430.247-430.250✓ correct
      • C.The motor service factor
      • D.The locked-rotor current

      430.6(A)(1) explicitly directs use of the Table FLC for branch-circuit conductor and OCPD sizing. Nameplate FLA is for OVERLOAD (running) protection — a separate device. Service factor adjusts overload trip points. Locked-rotor current relates to short-circuit calculations, not ampacity.

   2. 2. A standard inverse-time circuit breaker for an ordinary AC motor is sized at what percentage of motor FLC per Table 430.52(C)(1)?

      • A.125%
      • B.175%
      • C.250%✓ correct
      • D.300%

      250% inverse-time breaker. 125% is for conductor ampacity. 175% is the time-delay (dual-element) fuse value. 300% is the non-time-delay fuse value. The breaker percentage allows motor starting inrush without nuisance trips.

External resources

• Official
  NEC NFPA 70 — National Electrical Code (2023 edition) ↗

  The code itself. Master exam is open-book on the NEC; tab and highlight before exam day. Articles 100, 110, 210, 220, 230, 240, 250, 310, 314, 408, 430 are responsible for ~80% of master-exam questions.

• Official
  NFPA 70 Handbook (2023) ↗

  NFPA-published commentary edition: every article reproduced with explanatory text, illustrations, and worked examples. Not allowed in most exams (handbooks are usually prohibited as too aiding) but the best at-home study companion.

• Third-party
  State licensing-board candidate handbook (PSI / Prov / Block) ↗

  Each state's candidate handbook lists exam blueprint, allowed reference materials, calculator type, and the business-law portion specific to that jurisdiction. Three biggest national vendors are PSI, Prov, and Block — your state board page tells you which administers your exam.