Fine-Wire Bunch-Stranded and Rope-Lay Bunch-Stranded Copper Conductors for Use as Electrical Conductors

B738: Standard Specification for Fine-Wire Bunch-Stranded and Rope-Lay Bunch-Stranded Copper Conductors for Use as Electrical Conductors

In the realm of precision electrical engineering, the integrity and performance of conductors are paramount. ASTM B738 provides an essential, industry-leading standard that defines the requirements for fine-wire bunch-stranded and rope-lay bunch-stranded copper conductors, ensuring reliability in critical applications. This authoritative specification is indispensable for engineers and designers working with miniature or high-density wiring systems where standard conductors fall short.

What is B738?

ASTM B738 is a comprehensive standard that establishes specifications for bare bunch-stranded and rope-lay bunch-stranded copper conductors. These conductors are specifically designed for applications requiring extremely fine wires, often coated with tin or silver. Its primary purpose is to ensure consistent quality, performance, and interchangeability of these specialized conductors, facilitating their reliable use in demanding electrical systems.

Scope & Purpose

This standard governs the material, construction, and electrical properties of copper conductors made from round wires finer than No. 40 AWG (diameter less than 0.0031 in.). It covers both bunch-stranded (Type B) and rope-lay bunch-stranded (Type R) configurations. The specification details requirements for coated (tin or silver) and uncoated conductors, including temper and acceptable manufacturing practices for joints and lay length. It does not cover insulated conductors or conductors made from other materials.

Who Must Comply?

• • Primary audience: Electrical Engineers, Materials Engineers, Manufacturing Engineers, Product Designers, Cable Assemblers, and Cable Technicians.
• • Industries affected: Electronics Manufacturing, Telecommunications, Aerospace, Automotive, Medical Devices, and any industry utilizing fine-gauge, high-flexibility wiring.
• • Compliance nature: Voluntary, but adoption is highly recommended for quality assurance, interoperability, and meeting contractual obligations.
• • Enforcement: Typically enforced through contractual agreements, quality control processes within manufacturing facilities, and third-party inspections if specified in procurement documents.

Key Requirements Overview

• • The standard mandates specific classifications for conductor types (Bunch-Stranded and Rope-Lay Bunch-Stranded) and classes based on the AWG size of the individual fine wires used.
• • Professionals must ensure that the copper material meets the purity and quality standards necessary for the intended electrical performance.
• • Key provisions include detailed requirements for wire coating (tin or silver), including adherence to specified thickness and quality standards for coated wires.
• • The specification outlines acceptable methods for joining wires and defines parameters for the lay length and direction to maintain conductor integrity and flexibility.
• • Manufacturers must verify that the finished conductors meet specified electrical resistance limits and mechanical properties, such as tensile strength and elongation, through rigorous testing.

Related Standards & References

ASTM B738 works in conjunction with several other ASTM standards to provide a complete framework for conductor specification and testing. Complementary standards include:

• • Specification B33 (Tin-Coated Wire): Defines the requirements for tin coating on copper wires, which are often used in conjunction with B738.
• • Specification B193 (Resistivity Test Method): Provides the authoritative method for testing the electrical resistance of conductors, a critical parameter specified in B73