A 14-section interactive reference guide covering NEC 2023 (NFPA 70) topics used daily in electrical engineering design. Includes ampacity tables, load calculation methods, motor circuit formulas, grounding conductor sizing, conduit fill rules, and voltage drop calculations.
Each section targets a core NEC chapter or design discipline: load calculations (Art. 220 Standard and Optional methods), branch circuit rules and GFCI/AFCI locations (Art. 210), conductor ampacity and derating (Table 310.16), overcurrent protection sizing and next-standard-size rule (Art. 240), service entrance clearances and disconnect rules (Art. 230), motor circuit MCA/MOCP/FLA values (Art. 430 Tables), grounding electrode systems and EGC/GEC sizing tables (Art. 250), conduit types and fill calculations, panelboard 42-circuit limits and working space requirements (110.26), GFCI/AFCI/SPD requirements, voltage drop formulas with K-factor examples, one-line diagram symbols and hierarchy, and a master quick-reference table.
Use the Prev / Next buttons at the bottom, or press the arrow keys on your keyboard. Click the ☰ menu button in the top-right to open the table of contents and jump to any section. The gold progress bar at the top tracks your position through all 14 sections.
This guide references NEC 2023 (NFPA 70-2023). Many jurisdictions adopt NEC 2020, 2017, or earlier editions — confirm the adopted edition with your local AHJ before applying code-specific values. AFCI scope, GFCI locations, and SPD requirements have expanded significantly between editions.
Ampacity values from Table 310.16 assume not more than 3 current-carrying conductors in a conduit at 30°C ambient. Apply the bundling derating factor (310.15(C)) for 4+ conductors and the temperature correction factor (Table 310.15(B)(1)(a)) for ambient temperatures above 30°C. Always use the 75°C ampacity column for conductor sizing unless you have verified the terminal rating supports 90°C.
The Standard Method (NEC 220.40) applies universally and is required for additions or alterations. The Optional Method (220.82 for single-family, 220.84 for multifamily) can be used for new construction and typically yields a lower calculated demand, which is useful when a smaller service size is desired. The Optional Method applies only to dwelling units and requires all loads to be on the same service.
Per NEC 240.4(B), when a conductor's ampacity does not correspond to a standard OCPD rating, the next higher standard size rating is permitted — but only if the OCPD does not exceed 800A, the conductors are not part of a branch circuit supplying receptacles for cord-and-plug equipment, and the conductors are not #14, #12, or #10 AWG (which have absolute maximum OCPD ratings regardless of this rule).
EGC size is based on the rating of the OCPD protecting the circuit, per NEC Table 250.122. For example, a 20A circuit requires a minimum #12 AWG copper EGC. If the overcurrent device is rated 100A, the EGC must be at least #8 AWG copper. The EGC is not sized based on load current — it's sized to handle fault current long enough for the OCPD to clear.
NEC recommends (but does not mandate) a maximum 3% voltage drop for branch circuits (210.19 informational note) and 3% for feeders (215.2 informational note), with a combined maximum of 5%. Many project specifications and energy codes treat these as mandatory requirements. For sensitive equipment (medical, data center, lab instruments), design to 1–2% total to avoid equipment malfunctions.