Technical Tip

AFC'S CONNECTORS PERMITTED FOR USE OVER 250 VOLTS TO GROUND

BACKGROUND

Research and tests conducted by Underwriters Laboratories in the '70's confirmed that connector designs which used set screws or bar clamps for securement to the cable or conduit and a locknut for attachment to the box provided a ground path equivalent to an equipment ground wire.

In the early '90's unique connector constructions and designs featuring new alloys and securement means were developed. The new fittings departed from traditional designs and prompted UL to add a revised resistance test, (to determine if the effects of the vibration and bending tests were detrimental to the fitting/cable assembly) plus a new current test to study the capability of all connectors to carry ground-fault current while still maintaining the integrity of the metallic grounding system during the fault.

AFC supported the revisions to UL 514B and participated in the test program. The connector designs which passed the more rigorous industry requirements, also satisfied NEC 250-51 providing an effective grounding path: "The path to ground from circuits, equipment, and metal enclosures for conductors shall (1) be permanent and electrically continuous, (2) have capacity to conduct safely any fault current likely to be imposed on it, and (3) have sufficiently low impedance to limit the voltage to ground and to facilitate the operation of the circuit protective devices."

CODE PROPOSAL

A proposed change to Section 250-76 of the 1996 NEC adding a new exception d to permit "listed fittings" based upon the new UL test requirements was approved by Code Making Panel 5. The submittor successfully argued that UL added high current tests as a basic requirement for fittings and "this test in itself should be sufficient for product approval for bonding circuits over 250 volts to ground."

SUMMARY

By adding the new exception d to 250-76 of the 1996 NEC, CMP 5 acknowledged the old '93 edition of this section was design restrictive. Now all listed fittings , the snap-in, tilt-in and tighten connectors and the conventional fittings with locknuts are permitted for circuits over 250 volts to ground to ensure "electrical continuity of metal raceways and cables with metallic sheaths."

Test Currents and Times

Trade Size Inch*	Test Time Seconds	Current Amperes	Minimum size of copper leads used to connect sample assembly to current source, AWG(mm2)
3/8	4	470	12(3.3)
1/2	4	1180	8(8.4)
3/4, 1	6	1530	6(13.3)
1-1/4, 1-1/2	6	2450	4(21.2)
2	6	3900	2(33.6)
2-1/2	6	4900	1(42.4)
3, 3-1/2, 4	9	5050	1/0(53.5)
4-1/2	9	6400	2/0(67.4)
5, 6	9	8030	3/0(85.0)

* Trade size for metal-clad cable fittings refers to the associated knockout.