NPT vs. NPTF Threads on Conduit entries

[JeanH]

The NEC, UL and CSA standards and ATEX standards require NPT threads per ANSI/ASME B1.20.1. However, we are also trying to meet IP (ingress protection) standards for European ATEX Zone 2 and want a conduit fitting that won't leak.

In looking at the differences between the NPT and NPTF (dryseal) threads, it appears that the dryseal threads are a tighter version of the NPT threads. In other words, NPTF threads will pass when checked with an NPT thread gauge. Is there any reason why we would not want to put NPTF threads in conduit entries?

 

[rbalex]

Technically, NPTF is compliant with the actual NEC requirements such as those stated in Section 344.28. ANSI/ASME B.1.20.1-1983 is only mentioned in Fine Print Notes.

The most likely general manufacturer’s and contractor’s objections would be the added cost to both of them for what would be seen essentially as an unnecessary level of precision in US domestic usage. This is especially true since it would require new tooling because both internal and external threads must meet the same Standard. A mixture of NPT and NPTF threads are not supposed to be used on the same joint even though they would seem to fit.

While we are on the subject, what IP are you attempting to achieve for ATEX Zone 2?

 

[JeanH]

It appears that CSA (CSA C22.2 No 0.5-1982 Clause 4.2) requires NPT for all threaded conduit entries. This may be an artifact of an old NEC Code since the standard hasn't been updated since 1982. Thanks very much for the NEC FPN information - I wasn't aware of that.

Most of the products that we build are used worldwide and our customers want to standardize on single item numbers for their installations around the world. We use a lot of NPTF threads for hydraulic connections and they are not more difficult for us to produce. If we use a NPT gauge, the threadform passes and it also passes with an NPTF gauge. The NPTF thread form is within the tolerances fo the NPT threadform. The conduit entries would be identified as NPT for installation purposes, so the customers don't need to use an NPTF threaded conduit. If I do the flame propagation testing with an NPTF thread and an NPT conduit, and it passes, shouldn't it be okay?

For ATEX Zone 2, we must have IP54, but again our customer want IP56 or better. It just seems like we win all around with making an NPTF threadform rather than NPT.

 

[rbalex]

The problem as I see it is that neither Standard ANSI/ASME B.1.20.1-1983 nor ANSI B1.20.3 – 1976 nor any other Standard I am aware of recognizes that the two systems may be mixed in any given application. This doesn’t mean it won’t work; simply that it isn’t directly recognized to my knowledge. And while both systems technically meet Section 344.28 for example, how would you be sure you are still actually getting a “dry seal” if you mixed them.

Except possibly in a few food processing applications, it is difficult to believe anyone would see a need for IP56 for conduit threads; even then it’s a stretch.