NEC Article 300 outlines the general requirements for wiring methods and materials. This article addresses various aspects, including conductor spacing, protection, installation environments, and more. Below are the key points covered in NEC Article 300:

Scope of NEC Article 300

NEC Article 300 specifies general wiring requirements for materials and methods, excluding conductors that are part of factory-assembled or listed equipment.

Limitations on Wiring Methods and Materials

Wiring methods in NEC Article 300 apply to systems operating up to 1000 volts AC or 1500 volts DC unless exceptions apply, while conductors must have insulation that withstands high and low temperatures without degrading. For example, conductors installed near industrial equipment or in cold climates may require specialized insulation like THHN, XHHW, or XLPE.

Conductor Requirements

• Single conductors must be installed as part of recognized wiring methods (e.g., raceways or cable assemblies).
• Conductors of the same circuit must be grouped within the same raceway, trench, or cable unless exceptions apply.
• Conductors with different voltage ratings may share a raceway if all have insulation rated for the highest voltage present.

Conductor and raceway protection against physical damage

• Conductors routed through wooden structures must be offset at least 1 ¼ inches from edges or shielded with steel plates to prevent penetration by nails or screws.
• A minimum clearance of 1 ½ inches between conductors and metal roof decking must be maintained unless concrete encasement provides adequate protection.

Depth for underground installations

Direct-buried cables must meet depth requirements based on the location and wiring method:

• Streets, highways, and parking lots: A minimum burial depth of 24 inches is required to protect against heavy vehicular loads.
• Residential yards: Burial depths can be reduced to 6 inches when GFCI protection or cables are placed beneath a concrete slab.
• Under buildings: Cables beneath buildings must be installed in a raceway or meet specific standards like Type MC or MI cables.
• Low-voltage circuits: Circuits for irrigation or landscape lighting, operating at 30 volts or less, require a minimum depth of 6 inches when using UF cables.
• Airport runways and restricted areas: A minimum burial depth of 18 inches is required in airport runways or adjacent restricted zones.

Protection against corrosion via raceways and metal enclosures

Metal raceways, such as rigid metal conduits (RMC) and intermediate metal conduits (IMC), require corrosion-resistant coatings in environments prone to moisture or chemical exposure. Common coatings include:

• Zinc coatings are hot-dip galvanized or electroplated zinc for outdoor or industrial use.
• Epoxy coatings provide resistance to chemicals and abrasions in extreme environments.
• PVC coatings are used for enhanced durability in corrosive or wet locations.
• Nonmetallic raceways like PVC or HDPE must be sunlight-resistant and withstand exposure to chemicals like oils or solvents.

Fire safety and environmental air spaces

• Electrical penetrations in fire-rated walls must be sealed to maintain the fire rating.
• Only approved wiring methods, such as Type MC or MI cables, are permitted in ducts or plenums, ensuring low smoke and heat release properties.

Securing and supporting wiring systems

• Wiring systems must be securely fastened, with independent supports for suspended ceilings.
• Raceway systems cannot support other wiring methods or equipment unless specifically approved.

Vertical raceway support requirements

Conductors in vertical raceways need adequate support to prevent sagging or strain. Examples of required support intervals include:

• 18 to 8 AWG: Support every 100 feet for both copper and aluminum conductors.
• 6 AWG to 1/0 AWG: Copper requires support every 100 feet; aluminum every 200 feet.
• 2/0 AWG to 4/0 AWG: Copper requires support every 80 feet; aluminum every 180 feet.
• Larger conductors (350 kcmil to 500 kcmil): Copper must be supported every 50 feet, while aluminum requires support every 120 feet.