Is there a universal expansion gap for solar mounting rails?

Not by a universal rule. Long, continuously spliced rail runs may need thermal-expansion breaks, but the required gap and maximum run length are system-specific. Unirac's NXT UMount guide varies maximum continuous lengths with attachment configuration and temperature difference; SnapNrack's Ultra Rail manual specifies its own splice gaps. Use the current instructions and project engineering documents for the exact rail and attachment system, including the required splice detail and whether modules may cross a break. If these details are unavailable, obtain confirmation from the system supplier or project engineer before installation. Do not transfer another manufacturer's dimensions to an unverified system.

Sources: Unirac, 'NXT UMount Design & Engineering Guide': https://unirac.com/document/nxt-umount-design-engineering-guide/ SnapNrack, 'Ultra Rail Installation Manual', version 4.6.1: https://res.cloudinary.com/summitpoint/image/upload/v1772487360/SNR_Ultra_Rail_Installation_Manual_v_4_6_1_b751987632.pdf