Grouping Factor
When there are several loaded strands inside the same conduit, trunking or tube, each traps the heat of the others and the ampacity falls. The factor applied gets smaller as the number of strands rises; in a bundle of six loaded strands roughly half the current of a single cable can be carried.
Heat is given off from the cable surface to the surroundings. A strand in the middle of a bundle is surrounded by other cables heating up just like it; the heat cannot escape and the conductor temperature rises. That is why a grouping factor is applied in addition to the ampacity tables.
- The strand counted is the loaded one: signal strands and idle spare strands do not enter this calculation.
- Simultaneity matters: for circuits that do not run at full load at the same time the factor can be applied more leniently.
- It is multiplied by the temperature factor: a bundle of six in a 45 °C compartment can bring the capacity below half the table value.
The practical conclusion is this: gathering all the power and control lines into a single thick conduit may look tidy but is electrically the worst option. Run high-current motor and valve supplies on a separate route. Since this separation also moves the power and signal lines away from each other, it brings a gain in terms of interference as well.
Context in which this term is used: Installation, redundancy and field continuity