Understanding NEC Article 337: Type P Cable

Introduction

NEC Article 337 outlines the use, installation, and construction standards for Type P cable, a versatile wiring method designed for industrial settings and hazardous environments. Type P cable is available in both armored and unarmored configurations and supports systems up to 2000 volts. Its robust design and compliance with stringent safety standards make it an excellent choice for demanding applications.

Scope

This article applies to Type P cable used in power, control, and instrumentation circuits. It includes provisions for installations under strict engineering supervision and in hazardous locations where safety and reliability are paramount.

Key Features of Type P Cable

1. Voltage Rating:
   • Supports circuits up to 2000 volts.
2. Construction:
   • Conductors: Tinned copper with flexible stranding.
   • Insulation: Thermoset material suitable for wet locations, with a minimum thickness of 0.76 mm (30 mils).
   • Jacket: Nonmetallic, impervious to moisture, corrosion-resistant, and sunlight-resistant.
   • Armor (optional): Braided basket weave made of commercial bronze, tinned copper, stainless steel, or aluminum.
3. Grounding:
   • Multiconductor cables include an equipment grounding conductor per 250.122.
4. Marking:
   • Complies with NEC 310.8, indicating voltage rating, armor presence, and other relevant details.

Permitted and Prohibited Uses

Permitted Uses:

Type P cable may be used:

1. Under Engineering Supervision:
   • In industrial installations where qualified personnel maintain and service the system.
2. Hazardous Locations:
   • As permitted by other NEC articles for classified areas.

Prohibited Uses:

Type P cable cannot be used:

1. Where exposed to physical damage.
2. In locations not specifically permitted by other NEC articles.

Installation Requirements

1. Securing and Supporting:
   • Must be supported using listed cable ties, straps, hangers, or similar fittings that do not damage the cable.
2. Bending Radius:
   • Follow manufacturer guidelines to prevent damage during handling and installation.
3. Single Conductors:
   • Installations must comply with NEC 300.20 for single-conductor cables.
4. Ampacity:
   • Determined using NEC 310.14(A) or (B) for 14 AWG and larger conductors, and Table 402.5 or 310.14(B) for 18 AWG and 16 AWG conductors.
   • For installations in cable trays, ampacity can be calculated under NEC 392.80.

Construction Standards

1. Armor:
   • Provides mechanical protection but cannot serve as a current-carrying conductor or equipment grounding conductor.
2. Insulation and Jacket:
   • Designed for durability in wet, corrosive, and sunlit environments.
   • Single-conductor cables rated 2000 volts with conductor sizes ≥ 4/0 AWG may use increased insulation thickness instead of a separate jacket.
3. Shielding:
   • Metallic shields are allowed over individual conductors or groups of conductors for added protection.

Applications

1. Industrial Facilities:
   • Used in factories, refineries, and chemical plants where robust and reliable wiring is essential.
2. Hazardous Locations:
   • Suitable for installations in areas with flammable gases, vapors, or dust.
3. Power and Control Systems:
   • Supports power distribution and instrumentation in controlled environments.

Best Practices

1. Use Approved Components:
   • Ensure all fittings and accessories are listed and compatible with Type P cable.
2. Protect from Damage:
   • Avoid installing cables in areas with potential physical damage unless armored versions are used.
3. Routine Inspections:
   • Inspect cables for wear, corrosion, and compliance with installation guidelines.

Conclusion

NEC Article 337 provides clear guidelines for the safe and effective use of Type P cable. Its durability and versatility make it a preferred choice for industrial and hazardous applications. By adhering to these standards, electricians and engineers can ensure reliable and safe electrical systems.