National Electrical Installation Standards

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Friday, December 31, 2021

Question:

Good Day Students of the Code,

My question is about the metallic tape shielding in medium voltage cables. Can the metallic tape insulation shield be used as the required required equipment grounding conductor for the circuit? Some say it can't be used in that manner and a separate equipment grounding conductor must be installed. We hope you can provide some guidance.

Al Hopper 

A

Answer:

Hello Al,

This comes up quite often. The default position of the NEC is no for metallic tape insulation shields. Section 250.190(B)(2) indicates that the metallic insulation shield encircling the current carrying conductors shall be permitted to be used as an equipment grounding conductor, if it is rated for clearing time of ground-fault current protective device operation without damaging the metallic shield. The metallic tape insulation shield and drain wire insulation shield shall NOT be used as an equipment grounding conductor for solidly grounded systems. Section 250.190 (B) (3) addresses EGC sizing and requires equipment grounding conductors be sized in accordance with Table 250.122 based on the current rating of the fuse or the overcurrent setting of the protective relay. Additional information and requirements about conductor shielding is provided in Section 310.10(E) of the 2017 NEC and Section 311.44 (2020 NEC).

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