What size ground wire is needed for 400 amp service entrance?

For a 400 amp service entrance, a grounding conductor of at least 3/0 AWG (American Wire Gauge) copper or 250 kcal (thousand circular mils) aluminum is required, in compliance with electrical codes and regulations. However, specific requirements may vary based on local codes and the specific conditions of the installation, so it's important to consult with a qualified electrician or refer to the applicable building codes for precise guidance.