Understanding NEC Article 326: Integrated Gas Spacer Cable (Type IGS)

Introduction

NEC Article 326 governs the design, use, and installation of Integrated Gas Spacer (Type IGS) cables. Type IGS cables are specialized systems primarily used in underground installations. These cables consist of solid aluminum rod conductors insulated with dry kraft paper, housed within pressurized polyethylene conduits filled with sulfur hexafluoride (SF6) gas. This innovative design provides robust electrical performance, durability, and safety for high-capacity power distribution.

Scope

This article applies to the construction, installation, and operational standards for Type IGS cables. It specifies their use in underground applications such as service-entrance conductors, feeder circuits, and branch circuits while detailing prohibitions, bending requirements, and marking standards to ensure safety and efficiency.

Key Provisions

Permitted and Prohibited Uses

• Permitted Uses:
  1. Service-entrance conductors.
  2. Feeder or branch-circuit conductors.
  3. Underground service conductors, including direct burial in the earth.
• Prohibited Uses:
  • Cannot be used as interior wiring.
  • Prohibited from being installed in contact with buildings or exposed installations.

Installation Requirements

1. Bending Radius:
   • Type IGS cables must adhere to the minimum bend radius specified in Table 326.24 to maintain cable integrity.
   • Example: For a conduit size of trade 2 (metric 53), the minimum bend radius is 600 mm (24 inches).
2. Number of Bends:
   • A cable run cannot exceed the equivalent of four 90-degree bends (360 degrees total) between pull boxes or terminations.
3. Fittings:
   • Terminations and splices must be identified as suitable for maintaining gas pressure within the conduit.
   • Each conduit section must include valves and caps to monitor or inject SF6 gas as needed.
4. Ampacity Ratings:
   • Ampacity limits depend on conductor size and are specified in Table 326.80. For example:
     • 250 kcmil: 119 amperes.
     • 4750 kcmil: 519 amperes.

Construction Specifications

1. Conductors:
   • Solid aluminum rods ranging from 250 kcmil to 4750 kcmil.
   • Conductors are laid parallel, with a maximum of 19 rods per assembly.
2. Insulation:
   • Insulation consists of dry kraft paper tapes and SF6 gas. The kraft paper thickness varies by conductor size:
     • 250–1000 kcmil: 1.02 mm (0.040 inches).
     • 1250–4750 kcmil: 1.52 mm (0.060 inches).
3. Conduit:
   • Medium-density polyethylene conduits are used, suitable for natural gas-rated pipe applications.
   • Conduit sizes range from trade size 2 (metric 53) to trade size 4 (metric 103).
4. Marking:
   • The cable must be marked with voltage rating, temperature rating, and conductor specifications in compliance with Section 310.8.

Applications

1. Utility Systems:
   • Type IGS cables are ideal for underground utility lines, enabling safe and efficient power distribution over long distances.
2. Industrial Settings:
   • Used in high-demand installations requiring robust and reliable underground wiring.
3. Renewable Energy Projects:
   • Facilitates the connection of solar farms and wind turbines to the main grid.

Best Practices

1. Comply with Bend Requirements:
   • Ensure all bends are within specified radii to avoid damaging the cable or compromising gas pressure.
2. Monitor Gas Pressure:
   • Regularly inspect valves and caps for leaks or pressure loss to maintain system reliability.
3. Use Approved Components:
   • Only use fittings and splices specifically listed for Type IGS systems.

Conclusion

NEC Article 326 provides essential guidance for the safe and effective use of Integrated Gas Spacer (Type IGS) cables. By adhering to these rules, electricians and engineers can ensure reliable underground power distribution while minimizing risks. Understanding the specific installation and operational requirements of these cables is critical for ensuring long-term system performance.