14-17 Log #481 NEC-P14 Final Action: Accept in Principle( 500.8(D) )
Submitter: Vic Gournas, ISA-The Instrumentation, Systems and Automation Society
Comment on Proposal No: 14-24
Recommendation: 500.8(D) Threading. All threaded conduit or fittings referred to herein shall be threaded with a National (American) Standard Pipe Taper (NPT) standard conduit cutting die that provides a taper of 1 in 16 (3/4-in. taper per foot). Such conduit shall be made wrenchtight to prevent sparking when fault current flows through the conduit system and to ensure the explosionproof or flameproof integrity of the conduit system where applicable. Equipment provided with threaded entries for field wiring connections shall be installed in accordance with 500.8(D)(1) or (D)(2). Threaded joints with field threaded NPT entries shall be made up with at least five threads full engaged for entries into explosionproof equipment. Threaded joints with factory threaded NPT entries shall be made up with at least 4 1/2 threads fully engaged for entries into explosionproof equipment. Threaded joints with metric entries shall be made up with at least five threads fully engaged for entries into explosionproof
equipment.
Substantiation: ISA disagrees with the Code-Making Panel 14 rejection of this proposal based on the following:
The reasons for rejecting the proposal involve issues that can be effectively addressed by the revisions proposed. As the IEC standards are definitely being revised to reflect the ?US? NPT gauging practices included in ANSI B 1.20.1, manufacturers will be very negatively impacted if a compromise position cannot be found.
Justification for changes to NEC 500.8(D), 501.4(A) and 505.9(E):
Reducing the 5 thread engagement requirement specified within the NEC text for factory cut female NPT threads to 4 1/2 thread engagement, allows a more manufacturable product for worldwide use, and better alignment with the IEC product standard gauging practices for NPT threaded joints. This change allows manufacturers to gauge female NPT entries to a gauging practice of (0 to +1 turns of L1) for international use that than the currently restrictive gauging practice (+1/2 to + 1 turns of L1). This proposal will not affect field cut NPT threads, male or female.
Panel Meeting Action: Accept in Principle Revise the proposed text to read:500.8(D) Threading. All NPT threaded conduit and fittings referred to herein shall be threaded with a National (American) Standard Pipe Taper (NPT) thread that provides a taper of 1 in 16 (3/4-in. taper per foot). Conduit and fittings shall be made wrenchtight to prevent sparking when fault current flows through the conduit system, and to ensure the explosionproof integrity of the conduit system where applicable. Equipment provided with threaded entries for field wiring connections shall be installed in accordance with 500.8(D)(1) or (D)(2). Threaded entries into explosionproof equipment shall be made up with at least five threads fully engaged.
Exception: For listed explosionproof equipment, factory threaded NPT entries shall be made up with at least 4 1/2 threads fully engaged.
Panel Statement: The panel agrees with the submitter's substantiation but has limited its application to listed equipment with NPT threads only. The panel is concerned that there is less control over field installed-threads and thus has not extended the 4 1/2 thread provision to those installations. In addition, metric entries have not been included in this exception, as they are not tapered threads. These changes do not modify the requirement that threaded joints be made up wrenchtight.
Number Eligible to Vote: 15
Ballot Results: Affirmative: 12 Negative: 3
Explanation of Negative: BRIESCH: The panel action should be to reject this comment and retain the requirement for 5 full thread engagement. This proposal is in direct conflict with ANSI requirements for both threading and gauging of NPT threads for not only hazardous locations electrical fittings, but general purpose electrical fittings, boxes and enclosures.
NPT electrical conduit and fitting threading and gauging practice is harmonized in ANSI/NEMA FB-1, ANSI/UL 1203, ANSI/UL 514A, ANSI/UL 50, ANSI/UL 6 and CSA C22.2 No. 0.5. A correctly formed NPT male thread will end a distance of L4 from the end of the conduit or fitting plus or minus the 1 turn gauging tolerance allowed by the standard. Obtaining 5 thread engagement of the female and male tapered NPT threads depends upon the correct thickness of the female wall for the thread pitch involved, the length of the threaded male fitting and forming the threads correctly to achieve the desired engagement. North American electrical fitting NPT thread forms are required by the above standards to be formed in a manner such that not less than 5 full threads can be achieved upon wrench-tight engagement. In the worst case, the female entry gauged at +1/2 and a male part gauged at -1 and made wrench-tight will engage 5 threads. It should be noted that there are requirements for general purpose boxes and enclosures that require 5 threads in the female entry in UL 514A, UL 50 and NEMA FB-1. It should also be noted that installation documents are currently globally harmonized at 5 full threads engagement for explosionproof and flameproof tapered entry threads.
COOK: Based on the substantiation provided in Comment 14-23, it appears that this action would allow a product to be installed in a hazardous (classified) location that would not meet the product standards for ordinary locations.
WIRFS: Mr. Briesch and Mr. Cook have pointed out compelling substantiation to maintain the full five-thread engagement requirements and consistency with a large base of manufacturing standards. Until there is further substantiation to modify this requirement in a manner consistent with those standards, it should not be changed in the NEC.
