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

Introduction

NEC Article 326 provides comprehensive guidelines for the use, installation, and construction of Integrated Gas Spacer (Type IGS) cables. Type IGS cable is a specialized system designed for underground applications and features a robust construction that includes solid aluminum conductors, kraft paper insulation, and pressurized sulfur hexafluoride (SF6) gas within a polyethylene conduit. This design ensures reliability, high performance, and safety in demanding environments.

Scope

This article applies to integrated gas spacer cables used as service-entrance conductors, feeder conductors, or branch-circuit conductors. It defines specific construction standards, installation practices, and restrictions to maintain system integrity and safety.

Key Features of Type IGS Cable

1. Construction:
   • Solid aluminum rod conductors ranging from 250 kcmil to 4750 kcmil in size.
   • Insulated with dry kraft paper tapes and housed in a medium-density polyethylene conduit filled with SF6 gas at a pressure of 20 psi.
2. Marking:
   • Each cable is marked with essential information, including voltage ratings, temperature ratings, and conductor size, in accordance with NEC 310.8.

Permitted and Prohibited Uses

Permitted Uses:

Type IGS cable is allowed for:

1. Service-Entrance Conductors: Used for delivering power to buildings or facilities.
2. Feeder and Branch-Circuit Conductors: Installed underground, including direct burial.
3. Underground Service Conductors: Provides safe and reliable underground power distribution.

Prohibited Uses:

1. Type IGS cables cannot be used as interior wiring.
2. Not allowed in exposed installations where the cable comes into direct contact with buildings.

Installation Requirements

1. Bending Radius:
   • Bends must adhere to minimum radii as specified in Table 326.24 to avoid damage to the conduit or cable:
     • Trade size 2: Minimum radius 24 inches (600 mm).
     • Trade size 3: Minimum radius 35 inches (900 mm).
     • Trade size 4: Minimum radius 45 inches (1150 mm).
2. Bend Limits:
   • A cable run must not exceed four 90-degree bends (360 degrees total) between pull boxes or terminations.
3. Fittings:
   • Terminations and splices must be designed to maintain the SF6 gas pressure and ensure system reliability.
   • Valves and caps are required for pressure monitoring and gas injection.
4. Conduit Requirements:
   • Medium-density polyethylene conduits identified for natural gas piping are used.
   • Conduits are rated for specific sizes, with percent fill limits specified in Table 326.116.

Ampacity and Conductor Specifications

1. Ampacity:
   • The ampacity of Type IGS cable is determined based on conductor size and installation conditions, as listed in Table 326.80:
     • 250 kcmil: 119 amperes.
     • 4750 kcmil: 519 amperes.
2. Insulation:
   • Kraft paper insulation with a thickness of 1.02 mm (0.040 inches) for 250–1000 kcmil conductors.
   • 1.52 mm (0.060 inches) for 1250–4750 kcmil conductors.

Applications

1. Utility Power Systems:
   • Enables safe underground transmission of electricity over long distances.
2. Industrial Facilities:
   • Supports high-capacity power needs in large-scale operations.
3. Renewable Energy:
   • Facilitates connections between renewable energy sources and the main grid.

Compliance and Maintenance Tips

1. Inspection and Testing:
   • Regularly inspect the system for pressure loss or mechanical damage.
   • Monitor gas pressure and ensure valves are operational.
2. Proper Bending Practices:
   • Avoid exceeding specified bending radii to maintain cable integrity.
3. Approved Materials:
   • Use only listed fittings and components for Type IGS cable systems.

Conclusion

NEC Article 326 offers detailed guidelines for the installation and maintenance of Integrated Gas Spacer Cable (Type IGS). This high-performance system is critical for underground power distribution in utility, industrial, and renewable energy applications. By adhering to the specifications in this article, professionals can ensure safety, reliability, and long-term performance.