NEC Worked Examples
10 calculations worked all the way through against the National Electrical Code (NEC) 2020 tables, with every lookup and every multiplication shown rather than summarized. Each one names the table it reads and links to the calculator that runs the same math on your numbers.
Conduit and box fill
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How Many #12 THHN Fit in 3/4 EMT?
The Chapter 9 Table 1 40% fill worked end to end for the most-asked conduit question on the job.
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Conduit Fill for Mixed Wire Sizes
Summing Table 5 conductor areas across different AWG in one raceway, where the single-size shortcut stops working.
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How Many Wires in a Single-Gang Box?
Every 314.16(B) allowance counted in order, including the ones installers routinely forget.
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Box Fill Example: 4-Inch Square Box
A full 314.16 count on the most common commercial box, device and clamp allowances included.
Conductor sizing and voltage drop
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Calculating Voltage Drop Percentage
The 3% branch and 5% total figures are informational notes, not code. What they actually govern.
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Wire Size for a 200 ft, 30 Amp Run
Where a long run makes voltage drop, not ampacity, the value that picks the conductor.
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What Size Wire for a 50 Amp Breaker?
6 AWG copper derived from Table 310.16 and 240.4, and why reaching for 8 AWG is the trap.
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Sizing a Breaker for a Continuous Load
The 125% rule applied in the right order against the 240.6(A) standard sizes.
Ampacity, derating and correction
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Derating 8 Current-Carrying Conductors
The Table 310.15(C)(1) adjustment at the count where it first bites, and what counts as current-carrying.
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Ampacity Correction in High Ambient
Attic and rooftop temperatures applied against the right column, plus the 110.14(C) termination cap.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Get the full guide
NEC Code Quickstart
This guide covers one calculation. NEC Code Quickstart walks all twelve exam calculations start to finish, each with a worked example and the exact code reference: ampacity and derating, breaker sizing, voltage drop, box and conduit fill, grounding, dwelling load, range and dryer demand, motor circuits, and transformer current. Written for the 2023 NEC with notes for the 2026 edition.