Sizing Terminal Blocks by Conductor Cross-Section and Current
Terminal blocks are sized by conductor cross-section, rated current and permissible temperature rise. This guide explains the rated values in IEC 60947-7-1, how cross-section and current relate, and the common mistakes with grouping and temperature.
View terminal blocksWhich rated values drive the sizing?
Terminal blocks are described in IEC 60947‑7‑1 by three rated values: the rated cross-section, the rated current IN and the rated voltage. These figures apply under defined test conditions and form the basis of every compliant sizing decision.
The rated current is the current the block can carry continuously with the largest permitted conductor without exceeding the allowed temperature rise. It is determined by a test that measures the temperature increase at the connection points.
- Rated cross-section: the largest fixed conductor (e.g. 2.5 / 4 / 6 / 10 / 16 / 35 mm²).
- Rated current IN: continuous current at rated cross-section and 45 K rise.
- Rated voltage and impulse voltage: set clearance and creepage distances.
- Rated connectable range: smallest and largest permitted cross-section.
How do cross-section and rated current relate?
The rated current of a terminal block grows with cross-section, but not linearly. A larger copper cross-section carries more current at a lower rise, because both the conductor resistance drops and the heat-dissipating surface increases.
Important: the rated current of the block must not be confused with the current-carrying capacity of the cable. The block must always carry at least the current the upstream fuse allows, and the connected cross-section must fall within the block's range.
How do you account for heating and grouping?
In practice many blocks sit tightly grouped inside a closed enclosure. Both raise the operating temperature, so the datasheet value must be derated.
- Ambient temperature: above 20 °C the permitted current falls, often by 20‑30 % at 55 °C.
- Loading factor: apply the grouping factor when all blocks carry full load at once.
- Tightening torque: only the datasheet torque secures a low contact resistance.
- Conductor material: aluminium needs approved blocks and regular re-checking.
For high currents or dense grouping, keep a thermal reserve: choose one size larger, or run the operating current at about 70‑80 % of the rated current. The temperature rise then stays safely below the limit even in summer operation.
Frequently asked questions
Which standard governs terminal block sizing?
IEC 60947‑7‑1 (EN 60947‑7‑1 in Europe) is the reference. It defines the rated cross-section, rated current and the 45 K permissible temperature rise under test conditions.
Must the block rating match the cable current?
The block's rated current must be at least as high as the nominal current of the upstream fuse. In addition, the cable cross-section must fit within the block's connectable range.
Why derate the rated current when it is warm?
The rating assumes a 20 °C ambient and at most 45 K rise. Higher ambient temperature or dense grouping reaches the limit temperature sooner, so a derating factor applies.
How important is tightening torque?
Very important. Only the datasheet torque keeps the contact resistance low. Too little torque generates heat and hotspots, too much can damage the conductor or the block.
Sizing terminal blocks correctly?
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