National Electrical Installation Standards

Standards as High as Your Own

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Monday, May 28, 2018

Question:

Under table 300.50 - it does not clearly specify (in column 2) if those conduit depth requirements are for direct buried or concrete encased. We have a situation at JFK airport where we are struggling to meet the 18" depth for concrete encasement for runway and taxiway lighting installation (voltage between 1kv and 22kv). Also, in lieu of RGS (for 6" depth), is it compliant to use LFMC - concrete encase?

Al Volpe

A

Answer:

Hey Al thanks for your question. There is a superscript "c" at the end of the fourth subheading heading (types of conduits) in Column 2 of Table 300.50. The wording in the superscript "c" Specific Footnote indicates that if the conduit listed for direct burial without concrete encasement then concrete encasement is not required.

General Note 2, to table 300.50 allows 2 inches of concrete cover extending down to the rock for metal or nonmetallic raceways permitted for direct burial, if solid rock prevents meeting the cover requirements.

LFMC is not a type specifically included in the table so the local AHJ would need to make that determination.

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