What are the common nominal sizes for rigid steel conduits?

Common sizes include 16, 20, 25, 32, 40, 50, and 63 mm (Table 1).

What is the difference between Class A and Class B conduits?

Class B conduits have a thicker wall than Class A for the same nominal size, providing greater mechanical strength and protection (Table 1).

What is the standard length of a steel conduit?

The standard length is 3 metres with a tolerance of ±10 mm. Other lengths can be supplied by agreement (Clause 6.3).

How are conduits marked?

Conduits are marked with the manufacturer's name/trademark and may be color-coded for classification (e.g., Heavy Protection). The BIS certification mark may also be present (Clause 10).

IS 9537 Part 3

: 1983

Conduits for electrical installations - Part 3: Rigid steel conduits

CurrentFrequently UsedSpecificationBIMMEP · Electrical Installations

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This standard specifies the material, dimensional, constructional, and mechanical requirements for rigid steel conduits intended for the protection of electrical wiring. It covers two classes of conduits (Class A and Class B) and various levels of corrosion protection, ensuring safety and durability in electrical installations.

Specifies requirements for rigid steel conduits used for the protection of electrical wiring installations.

Quick Reference Values

Standard supplied length3.0 m (±10 mm)

External diameter of 25mm conduit25.0 mm (+0, -0.3 mm)

Minimum wall thickness for 25mm Class B conduit1.40 mm

Thread type for screwable conduitsMetric screw threads as per IS 1367 series

Minimum compression force resistance (Heavy Duty)1250 N over 200 mm

Minimum impact resistance at -5°C (Heavy Duty)2 kg from 100 mm height

Practical Notes

Always use a proper pipe bender to avoid kinking the conduit, which compromises its integrity and makes wire pulling difficult.

For outdoor or corrosive environments, specify conduits with Class 4 (Heavy) zinc coating for long-term rust prevention.

Ensure threads are cleanly cut and burrs are removed to maintain good electrical continuity for earthing through the conduit system.