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Master Electrician Exam — Quick Reference

Formulas · NEC Articles · Ampacity · Exam Tips

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⚡ Ohm's Law & Power
V = I × R
I = V ÷ R
R = V ÷ I
P = V × I = I² × R = V² ÷ R
P = power (W) · V = volts · I = amps · R = ohms
kW → A (1φ)I = kW×1000 ÷ (V×PF)
kW → A (3φ)I = kW×1000 ÷ (√3×V×PF)
kVA → kWkW = kVA × PF
📉 Voltage Drop (NEC 210.19)
VD = (2 × K × I × L) ÷ CM  [1φ]
VD = (1.732 × K × I × L) ÷ CM  [3φ]
K (copper)12.9
K (aluminum)21.2
Lone-way length (ft)
CMcircular mils of wire
Max VD: 3% branch circuit · 5% total (feeder + branch)
%VD = (VD ÷ Vsource) × 100
🔌 Wire Ampacity (Cu, 75°C, NEC 310)
#14 AWG15 A
#12 AWG20 A
#10 AWG30 A
#8 AWG50 A
#6 AWG65 A
#4 AWG85 A
#2 AWG115 A
#1/0 AWG150 A
#2/0 AWG175 A
#3/0 AWG200 A
#4/0 AWG230 A
90°C rating used for derating; termination temp governs final ampacity
📦 Conduit Fill (NEC Ch.9, Table 1)
1 conductor53 %
2 conductors31 %
3+ conductors40 %
Common Conduit IDs (EMT)
½" EMT0.622" ID
¾" EMT0.824" ID
1" EMT1.049" ID
1¼" EMT1.380" ID
1½" EMT1.610" ID
2" EMT2.067" ID
Fill area = π/4 × ID² · Wire area from NEC Annex C
⚙️ Motor Circuits (NEC Art. 430)
Always use NEC Table FLC — NOT nameplate!
Branch conductorsFLC × 125%
Overload (SF ≥ 1.15)FLA × 125%
Overload (SF < 1.15)FLA × 115%
SCPD (inverse time)FLC × 250%
SCPD (dual-element)FLC × 175%
Feeder conductors125% × largest + others
HP = (V × I × Eff × PF) ÷ 746  [3φ]
🔄 Transformer Calculations
V₁/V₂ = N₁/N₂ = I₂/I₁
Primary ampsI₁ = kVA×1000 ÷ V₁
Secondary ampsI₂ = kVA×1000 ÷ V₂
Turns ratioa = N₁ ÷ N₂
NEC 450.3 — OCPD for transformers based on impedance %
Primary OCPD (Z ≤ 6%)125% of FLC
Secondary OCPD125% of FLC
🏠 Dwelling Load Calc (NEC Art. 220)
General lighting3 VA/sq ft
Small appliance circuits2 × 1,500 VA
Laundry circuit1 × 1,500 VA
First 3,000 VA100% demand
3,001–120,000 VA35% demand
Over 120,000 VA25% demand
A/C vs heat — use larger, not both (220.60)
Service amps = Total VA ÷ (V × 1.732)  [3φ]
🌍 Grounding & Bonding (NEC Art. 250)
GEC — 200A service#4 AWG Cu
GEC — 100A service#8 AWG Cu
EGC with #12 ckts#14 AWG Cu
EGC with #10 ckts#10 AWG Cu
Ground rod electrode8 ft × ⅝" rod
Neutral bonded to GNDat service only
GEC = Grounding Electrode Conductor
EGC = Equipment Grounding Conductor
📋 Key NEC Article Numbers
90 Introduction & Scope
100 Definitions
110 Installation Requirements
200 Grounded Conductor ID
210 Branch Circuits
215 Feeders
220 Load Calculations
225 Outside Branch Circuits
230 Services
240 Overcurrent Protection
250 Grounding & Bonding
280 Surge Arresters
300 Wiring Methods — General
310 Conductors
330 MC Cable
334 NM Cable (Romex)
340 UF Cable
344 Rigid Metal Conduit (RMC)
348 Flexible Metal Conduit
350 LFMC
352 Rigid PVC Conduit
358 EMT
404 Switches
406 Receptacles
408 Panelboards
410 Luminaires
422 Appliances
430 Motors
440 A/C & Refrigeration
445 Generators
450 Transformers
480 Batteries
500 Hazardous Locations
517 Healthcare Facilities
625 EV Charging Equipment
680 Swimming Pools
690 Solar PV Systems
700 Emergency Systems
702 Optional Standby
705 Interconnected Power
💡 Must-Know Exam Tips
✦ "Shall" = mandatory. "Should" = advisory only.
✦ Motor FLC: use NEC tables, NOT the nameplate.
✦ Neutral bonded to ground at the SERVICE only — never in subpanels.
✦ White/gray = neutral. Green/bare = EGC.
✦ GFCI required: bathrooms, kitchens, garages, outdoors, crawl spaces, unfinished basements, rooftops, boathouses (NEC 210.8).
✦ AFCI required: all 120V 15/20A branch circuits in dwellings (NEC 210.12).
✦ #14 → 15A max OCPD · #12 → 20A · #10 → 30A
✦ Two ground rods required unless first rod ≤ 25Ω (NEC 250.53).
✦ Derating: >3 current-carrying conductors in conduit applies.
✦ 6 disconnects max at service entrance (NEC 230.71).
✦ Transformer vaults: 3-hour fire rating, drain, ventilation (NEC 450.42).
✦ Working space: 3 ft min depth, 30 in width, 6 ft 6 in headroom (NEC 110.26).
✦ Feeder neutral = largest unbalanced load.
✦ PV systems: max array voltage = Voc × 1.25 temperature correction.
✦ Hazardous areas: Class I (gas), Class II (dust), Class III (fibers).
✦ Pool bonding: #8 solid Cu to all metal within 5 ft (NEC 680.26).

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