What are the requirements for expansion joints on rigid conduit in an underground installation?

In underground installations of rigid conduit, expansion joints are required to accommodate thermal expansion and contraction due to temperature changes. The National Electrical Code (NEC) typically mandates the use of expansion joints when the run of conduit exceeds specific lengths, generally around 100 feet, or when the temperature variation could cause significant movement. Additionally, they should be installed at locations where the conduit is subject to significant temperature fluctuations or where it crosses different thermal environments. Proper spacing and installation are essential to ensure the conduit system remains intact and functional over time.