By what percentage must cable ratings be reduced when paralleling grounding cables for greater fault current capacity if unrestrained?

When paralleling grounding cables for increased fault current capacity in unrestrained conditions, it's essential to account for the fact that each cable will have to share the total fault current. However, due to the thermal and electrical limitations of the cables, their ratings must be de-rated to prevent overheating or damage.

A reduction of 20% is appropriate because when grounding cables are used in parallel, the effective ratings must consider factors such as bonding, resistance, and heat dissipation. Unrestrained cables do not have the benefit of external support or limiting mechanisms that can help manage the heat generated. Thus, reducing their ratings by 20% ensures that the cables operate safely within their thermal limits, fostering reliable and effective grounding. This percentage check helps maintain operational integrity during fault conditions while providing sufficient fault current capacity.

The other percentages, while they might make sense in different contexts or specifics, do not align with the standard requirements for unrestrained parallel grounding cables, particularly emphasizing the delicate balance needed in handling fault currents safely.