Purpose

Use current-carrying-capacity tables as one thermal stage in a complete cable design.

Learning objectives

• Distinguish Ib, In, It and Iz
• Select the applicable table
• Apply factors on the correct basis
• Identify remaining design checks

A table value is conditional: it belongs to a stated cable, number of loaded conductors, reference method and reference environment. It is not a free-standing amp rating.

Core theory

The usual coordination is Ib ≤ In ≤ Iz: load design current does not exceed device rating, and device rating does not exceed the cable's corrected current-carrying capacity. Overload-protection rules can change the precise design expression, so use the current clause.

It is the tabulated capacity at the table's reference conditions. Iz is the capacity for the actual declared conditions after only the applicable corrections are used on the basis required by that table.

Choose cable type, insulation temperature, conductor material, loaded conductors and physical installation before reading a row. Then separately verify voltage drop, fault protection/disconnection, short-circuit withstand, terminals, harmonics, neutral, mechanical/environmental suitability and regulations.

Assumptions: The table and factors are explicitly supplied; No other correction applies.

1. Product: Ca × Cg = 0.94 × 0.80 = 0.752.
2. Correct: Iz = 37 × 0.752 = 27.824 A.
3. Decision: This would not support a 32 A device under the basic relation.

Illustrative Iz is 27.8 A; select/recalculate rather than rounding it into compliance.

Reasonableness check: Both factors are below one, so Iz must be lower than It.

Knowledge check

Answer: Heat escape and reference conditions can differ. Construction, loading, environment and installation geometry all matter.

Practical exercise

Build a traceable selection record from a supplied table extract; do not use remembered values.

Summary

• A table value is conditional
• Ib, In, It and Iz are different
• Thermal capacity is only one design gate