Although they are listed as required in the NEC book, the circumstances where they would be needed in any residential installation would be rare...if not non-existant. Only under extreme temperature changes and excessive lengths are they required, and not in burial situations.
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Its need is more common that you think. The code states an expansion joint “shall be provided” in straight runs between securely mounted items with an expected expansion of ¼”. A 20' straight run that’s mounted on the side of a house with a 40 degree temperature change would require an expansion joint.
Last edited by hpp58; 04-19-2011 at 04:22 AM.
Reason: Word Change