NEC and Building Wiring
The NEC is a safety code for electrical installations. It is not merely an ampacity chart and it is not a substitute for engineering judgment. Conductor selection can require several code articles to be used together.
A useful NEC roadmap for wire
| NEC area | Why electricians look there |
|---|---|
| Article 110 | General requirements for electrical installations, equipment use, connections, and conductor termination considerations. |
| Article 200 | Use and identification of grounded conductors. |
| Article 210 | Branch circuits. |
| Article 215 | Feeders. |
| Article 240 | Overcurrent protection and conductor protection. |
| Article 250 | Grounding and bonding. |
| Article 300 | General requirements for wiring methods. |
| Article 310 | Conductors for general wiring, including ampacity rules and tables. |
| Article 334 | Nonmetallic-sheathed cable such as NM cable. |
| Article 400 | Flexible cords and flexible cables. |
| Chapter 9 tables | Conductor and raceway data used in calculations such as raceway fill and conductor properties. |
The exact section numbers inside these articles can change between NEC editions. That is another reason to learn to use the index, table of contents, article scope, and cross-references instead of memorizing only one edition's page numbers.
Ampacity is a process
A typical ampacity check may involve:
- Identifying the conductor and cable or raceway wiring method.
- Determining the load and any special load rules.
- Choosing the applicable conductor ampacity table.
- Applying ambient-temperature correction when required.
- Applying adjustment for multiple current-carrying conductors when required.
- Checking terminal temperature limitations.
- Checking special cable-article limitations.
- Coordinating conductor size with overcurrent protection.
- Checking voltage drop and equipment performance.
Grounded, grounding, and bonding are different jobs
Electricians often use the casual word “ground” for several different conductors. The NEC distinguishes them because they serve different purposes. The grounded conductor (often the neutral) normally carries circuit current. An equipment grounding conductor provides a low-impedance fault-current path. Bonding joins metal parts together so a fault can operate the overcurrent device. Grounding electrode conductors connect the electrical system to the grounding electrode system.
Article 250 is therefore worth learning as its own subject rather than treating every green, bare, white, or grounded connection as the same thing.
Voltage drop
NEC informational guidance has long encouraged design that limits voltage drop to about 3% on a branch circuit and about 5% total for feeder plus branch circuit. These are widely used design targets, but informational notes are not a blanket substitute for mandatory code text. Certain equipment or special applications can have their own voltage requirements.
Conductor identification
The code includes rules for identifying grounded conductors, equipment grounding conductors, high-leg conductors, multiple nominal-voltage systems, and other situations. Identification methods can depend on conductor size and application. A general teaching shortcut such as “white is neutral” is not enough for all field work.
Protection from physical damage
Article 300 and the individual wiring-method articles address support, securing, protection from nails or screws, raceway use, bending, boxes, fittings, and other mechanical issues. Electrical insulation is not intended to compensate for poor mechanical protection.
AHJ and local amendments
The Authority Having Jurisdiction is the final local authority interpreting and enforcing the adopted code. States and municipalities can amend NFPA 70, and utility rules, licensing rules, product listings, manufacturer instructions, and other standards can also apply.