Electrical Conductors

Conductor Materials

  • Copper and Aluminum are most commonly used for wiring.

Copper

  • Pros:
    • Low resistance, more current/area, smaller wire sizes.
  • Cons:
    • More expensive, price volatility.

Aluminum

  • Pros:
    • Less expensive, lighter, more flexible, easier to install.
  • Cons:
    • Greater thermal expansion/contraction, less current/area, larger wire sizes required.
    • Not for under 100A due to device ratings and arcing.

Conductor Types

  • Solid: Used for smaller wire sizes (#10 and smaller).
  • Stranded: Used for #8 and larger.

Conductor Insulation

  • Table 310.4(1) lists insulation types for conductors rated 600V.
  • Common types:
    • TW: Dry and wet locations, 60°C.
    • THW: Moisture and heat resistant, dry and wet locations, 75°C (special applications 90°C).
    • THHN: High heat resistant, dry and damp locations, nylon jacket, 90°C.
    • THWN: Moisture and heat resistant, dry and wet locations, nylon jacket, 75°C.
    • THWN-2: Moisture and heat resistant, dry and wet locations, nylon jacket, 90°C.
    • XHHW: Moisture resistant, dry and damp locations 90°C, wet locations 75°C.
  • Key:
    • T = Thermal resistant
    • W = Wet location
    • N = Nylon
    • X = Cross-linked polyethylene insulation

Conductor Ampacities

  • Function of Material (Copper vs Aluminum) and Type of insulation.
  • Function of:
    • Physical size (AWG or kcmil)
    • Total length of conductor (total ZZ of the circuit)
    • Location
  • Table 310.16 shows sizes and ampacities.

NEC 110.14(C) Temperature Limitations

  • Conductors connected to devices <= 100A: limited to 60°C.
  • Conductors connected to devices > 100A: limited to 75°C.
  • 90°C column may only be used for ampacity adjustments and corrections.

Guidelines

  • Copper:
    • Loads <= 100A: use 60°C column.
    • Loads > 100A: use 75°C column.
    • Minimum #12, Maximum 600kcmil
  • Aluminum:
    • Loads >= 100A only
    • Max 750kcmil

Multiple Sets

  • Large loads require multiple sets of conductors.
  • All conductors must be equal size.

Ambient Temperature Correction Factor

  • If ambient temperature > 30°C (86°F), use correction factor from Table 310.15 (B)(1)(1).
  • Rooftop: Add 33°C or 60°F to outdoor ambient temperature if conduit is < ¾-inches above the roof.

Adjustment Factor for More Than 3 Current Carrying Conductors

  • If more than 3 current carrying conductors are in one conduit, use Table 310.15(C)(1) to adjust ampacity.
  • Exclude neutral and grounding conductors in a 3-phase system.

Sizing the Equipment Grounding Conductor (EGC)

  • NEC Table 250.122: EGC is sized based on the OCPD protecting the feeder/branch circuit.

Identification - Industry Standards for Conductor Colors

  • 208Y/120V:
    • φA – Black, φB – Red, φC – Blue, Neutral – White, Equipment Ground – Green (or Bare)
  • 480Y/277V:
    • φA – Brown, φB – Orange, φC – Yellow, Neutral – White or Gray, Equipment Ground – Green (or Bare)

Conductor Types (Cables)

  • NEC Article 334: NM (Non-metallic sheathed cable) - "Romex", typically in residential.
  • NEC Article 340: UF (Underground feeder and branch-circuit cable) - Underground version of NM cable.
  • NEC Article 330: MC (Metal clad cable) - Wires in a metal casing from the factory. Casing acts as the conduit.