/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgUL, NEC Move Toward 2,400V ShieldingULNECMoveToward2400VShielding.htmRecent changes in NEC requirements for non-shielded cable are moving through the UL process. To shield, or not to shield? Above 2,400V, that is no longer a question.
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UL, NEC Move Toward 2,400V Shielding

Recent changes in NEC requirements for non-shielded cable are moving through the UL process.

To shield, or not to shield? Above 2,400V, that is no longer a question.

“The 2005 NEC clearly calls for shielded cable in applications operating above 2,400V,” says Paul White, Southwire applications engineer. "There has been some confusion about the new NEC requirement, because UL has not yet changed their listing criteria for unshielded cables. But UL is on the way to harmonizing with the NEC.”

In the 2005 NEC, paragraph 310.6 says, “solid-dielectric insulated conductors operated above 2,000V in permanent installations shall have an ozone-resistant insulation and shall be shielded.” Following that wording, there is an exception. “Non-shielded insulated conductors listed by a qualified testing laboratory shall be permitted for use up to 2,400V.” UL is one such qualified testing laboratory.

Shielding distributes insulation stresses

What is the rational behind the 2005 NEC revisions to Article 310? It’s a response to a long-standing application question. In most medium-voltage cable designs, the insulation is surrounded by a semiconducting layer at ground potential. This layer distributes electrostatic stress evenly around the conductor, and drains surface charges to ground. (See illustration.)

Semiconducting insulation shields drain surface charges and extend insulation life by distributing electrical stress more evenly

Without the insulation shield, at operating voltages above about 2,400V, normal insulation leakage currents create potential shock hazards to personnel.

In addition, contamination on non-shielded conductors can be the starting point for surface discharge currents that create carbon tracks on the insulation. The tracks can lead to cable failure.

The ends of non-shielded conductors are the most exposed to contamination, and the most prone to tracking. The problem increases in areas prone to contamination and where cables are installed close to grounded surfaces, as in electrical switchgear and cabinets.

UL and NEC are temporarily disconnected

Earlier UL practice was to list unshielded cables for use up to 5,000. Now, UL is in the process of changing their standards so that any cable listed for use above 2,400V will need to be shielded. “UL’s goal is to complete the listing change by mid-2006, but it may be later,” White says. In the meantime, there is a temporary disconnect between NEC requirements and UL listing standards.

So, if you have a 4,160V motor that you want to connect with unshielded 5kV cables, if you’re being inspected to NEC standards, it won’t pass, even though it’s still technically OK with UL. If you need to replace a portion of a currently non-shielded 4,160V circuit, NEC-oriented inspectors will require the use of shielded design in the replacement cable.

Southwire’s unshielded MV cables are manufactured to 5kV specifications, but won’t be listed above 2,400V. “Even prior to the 2005 NEC requirements, Southwire recommended limiting 5kV non-shielded cables to operation at 2,400 Volts or less,” says White. “Southwire is already printing, ‘5kV, but limited to 2,400V’ on UL-listed 5kV non-shielded cable.”