Always add a safety margin. For continuous loads (running >3 hours), multiply the load current by 1.25 (125%). Step 2: The "Big Three" Correction Factors Cable tables (like NEC Table 310.16 or IEC 60364-5-52) assume perfect conditions: 30°C air, free airflow, and pure copper. Your site isn't perfect. You must derate.
When in doubt, go one size larger. The cost of copper today is trivial compared to the cost of a trench re-dig, a fire investigation, or a day of factory downtime. cable rating calculation
Disclaimer: This post is for informational purposes. Always refer to your local electrical code (NEC, IEC, BS) and consult a licensed electrical engineer for critical installations. Always add a safety margin