Specifications of Rigid Metal Conduit (RMC) for use in class 1 div 1?

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jgat

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Hi,

I am a newbie here. We are connecting two explosion proof enclosures according to the (NEC 501.15 (A.3)) referred below. The question I have is - I can find elbows, unions and other fittings that are rated and certified for class 1 Div 1. I also need a male to male straight conduit nipple as part of the connection. However, there are no conduit nipples that are certified for class 1 div 1.

Why is this the case? Can I buy a pipe nipple from Lowes and use it with the other HazLoc certified fittings? If the threading on the conduit nipple is not made according to certain standard, how can it be guaranteed to withstand explosion? So, I doubt I can use a random metal conduit nipple. Can someone please clarify?

Best Regards,
Jgat


(501.15(A.3)) Two or More Explosionproof Enclosures. Where two or more explosionproof enclosures for which conduit seals are required under 501.15(A)(1) are connected by nipples or by runs of conduit not more than 900 mm (36 in.) long, a single conduit seal in each such nipple connection or run of conduit shall be considered sufficient if located not more than 450 mm (18 in.) from either enclosure.
 
  • RMC may not be specifically marked for Class I but it must be listed as required by Section 344.6 and, if properly listed, is permitted in classified locations under Section 344.10(A)(1).
  • A conduit coupling is a recognized ancillary component of standard RMC. [Section 344.130] That is, it is listed to the same standard as RMC.
  • A "nipple" is simply a field or factory made piece of shortened (listed) RMC as also permitted by Section 344.130.
  • All field or factory threading in Class I must be consistent with Section 500.8(E).

NOTE: If RMC were marked for every acceptable application you would need that full 10' mentioned in Section 344.130.
 
Last edited:
jgat said:
Hi,

I am a newbie here. We are connecting two explosion proof enclosures according to the (NEC 501.15 (A.3)) referred below. The question I have is - I can find elbows, unions and other fittings that are rated and certified for class 1 Div 1. I also need a male to male straight conduit nipple as part of the connection. However, there are no conduit nipples that are certified for class 1 div 1.

Why is this the case? Can I buy a pipe nipple from Lowes and use it with the other HazLoc certified fittings? If the threading on the conduit nipple is not made according to certain standard, how can it be guaranteed to withstand explosion? So, I doubt I can use a random metal conduit nipple. Can someone please clarify?

Best Regards,
Jgat


(501.15(A.3)) Two or More Explosionproof Enclosures. Where two or more explosionproof enclosures for which conduit seals are required under 501.15(A)(1) are connected by nipples or by runs of conduit not more than 900 mm (36 in.) long, a single conduit seal in each such nipple connection or run of conduit shall be considered sufficient if located not more than 450 mm (18 in.) from either enclosure.
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as long as it's a close nipple that has a standard pipe thread on it,
it's acceptable for cl1 div1, as was mentioned.... running thread
however, is another story. i don't believe it's acceptable, as the
reduced wall thickness and straight thread compromise the connection.

there are folks on here with a lot more classified experience than i have,
maybe they can clarify if running thread is ok, or not.

you need five turns of thread engagement for the connection to be
explosion proof. even if the fitting is not wrench tight, it will still be
explosion proof, as the explosion within the conduit will be contained,
and cool below the flash point of the surrounding atmosphere, before
it escapes.

however, you need a tapered thread.

classified junction boxes that have a threaded cover don't need the
taper, as the entire assembly is listed. the cover still has to be
spun on five turns, however.
 
Running threads aren't too much of another story. They're already prohibited at couplings in Section 344.42(B); it doesn't need to be repeated in Chapter 5. Any "special" threading requirements for classified locations are described in Section 500.8(E).
 
rbalex said:
  • RMC may not be specifically marked for Class I but it must be listed as required by Section 344.6 and, if properly listed, is permitted in classified locations under Section 344.10(A)(1).
  • A conduit coupling is a recognized ancillary component of standard RMC. [Section 344.130] That is, it is listed to the same standard as RMC.
  • A "nipple" is simply a field or factory made piece of shortened (listed) RMC as also permitted by Section 344.130.
  • All field or factory threading in Class I must be consistent with Section 500.8(E).

NOTE: If RMC were marked for every acceptable application you would need that full 10' mentioned in Section 344.130.
Click to expand...


Thank you very much everyone for the answers. The support and expertise on this forum is just great. I have much better understanding on this now. I have one follow up question.

I understand that it does not need any markings on the RMC. When NEC specifies "it must be listed", what does "listed" mean? Does it mean the manufacturer should specify that the RMC conforms to NEC 344.6?
 
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