National Electrical Code (NEC) Reference

The National Electrical Code (NEC), published by the National Fire Protection Association as NFPA 70, is the benchmark standard for electrical installation in the United States. It is revised on a three-year cycle and adopted - often with local amendments - by states and municipalities across the country. The NEC governs everything from conductor sizing and overcurrent protection to grounding, bonding and wiring methods, and most of the Voltix calculators that target the US market are built directly on its tables.

Key articles for calculations

A handful of articles do most of the heavy lifting when you are sizing a circuit. These are the ones worth knowing by their numbers:

Article 310 - Conductors for general wiring

Article 310 contains the ampacity tables - most notably Table 310.16- that tell you how much current a conductor of a given size, insulation rating and installation condition can safely carry. Ampacities are listed against the 60°C, 75°C and 90°C insulation columns, and you must respect both the conductor's temperature rating and the temperature rating of the terminations it lands on. Ambient temperature correction and conductor-bundling adjustment factors are also applied here.

Article 220 - Branch-circuit, feeder and service load calculations

Article 220 sets out how to total the connected load for a dwelling or other occupancy and convert it into a required service or feeder size. It defines general lighting loads per square foot, the demand factors that may be applied to large loads, and the rules for continuous loads (sized at 125% of the load, i.e. the familiar 80% rule).

Article 430 - Motors, motor circuits and controllers

Motors are sized differently from general loads. Article 430 uses the full-load current tables (such as Table 430.250) rather than the motor nameplate for conductor and overcurrent-device sizing, and it sets distinct rules for branch-circuit conductors, short-circuit protection and overload protection.

Article 250 - Grounding and bonding

Article 250 covers the grounding electrode system, equipment grounding conductors and bonding. Table 250.122 sizes the equipment grounding conductor against the rating of the overcurrent device protecting the circuit, while Table 250.66 sizes the grounding electrode conductor.

Chapter 9 - Tables (conduit fill)

Chapter 9 holds the dimensional tables used for conduit fill. The permitted fill depends on the number of conductors: 53% for one conductor, 31% for two and 40% for three or more. Table 1 gives those fill percentages, Table 4 gives the internal area of each conduit type and trade size, and Table 5 gives the cross-sectional area of each conductor by insulation type.

A note on voltage drop

Unlike BS 7671, the NEC does not mandate a hard voltage-drop limit in its enforceable text. It instead offers an informational note recommending a maximum of 3% on a branch circuit and 5% combined across feeders and branch circuits, primarily for efficiency. Many specifications and local rules adopt these figures as requirements.