Flexible Conduit

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timm333

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Minneapolis, MN
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Electrical Design Engineer
If we want to avoid the transfer of vibration from one building to an adjacent building, we can use flexible conduit between the two buildings. Does NEC say what is the maximum allowed length of this flexible conduit?
 
As long as it's properly supported, length limits in general have to do with size {3/8" no greater than 6'} or if it's used as an EGC {250.118(5)}.
 
infinity said:
As long as it's properly supported, length limits in general have to do with size {3/8" no greater than 6'} or if it's used as an EGC {250.118(5)}.
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Remember that if there is thermal expansion and contraction involved, you will need to allow enough extra length in the run when installed to allow for the contraction of the lowest expected winter temperature to avoid the conduit pulling itself apart when the temperature goes low. Or put in expansion joints, of course.
 
Actually the issue is not the length limitation of flexible conduit run on wall of floor; the issue is length limitation of flexible conduit run in air. When we run cables in between two buildings, we cannot put the cables in rigid conduits because rigid conduit will transfer the vibration (of machines) from one building to the other building. So we need to use the flexible conduit. And this flexible conduit cannot be supported on the wall because there is no wall between the two buildings. So this flexible conduit will hang in free space without any support in between the two buildings. One end of this flexible conduit will be supported on one building and the other end will be supported on the other building . How much would be the maximum allowed distance of flexible conduit in between the two buildings.
 
I would run an wire rope messenger cable between the two buildings attached at each end to an Eye bolt. Then your flexible conduit could be supported by the wire rope. Vibration should not be an issue. As to your original question "How long between support for flexible conduit" I do not know. 10feet ?
 
John120/240 said:
I would run an wire rope messenger cable between the two buildings attached at each end to an Eye bolt. Then your flexible conduit could be supported by the wire rope. Vibration should not be an issue. As to your original question "How long between support for flexible conduit" I do not know. 10feet ?
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A taut messenger cable (rather than one dangling in an arc) can transmit vibration just as well as a rigid raceway.
Do not let a short distance tempt you to pull the messenger wire or cable straight.
 
Yes messenger wire can transmit vibration; also when the size of cables is big, there is not much room for making a dangling arc; this is the problem.
 
What about:

348.30 Securing and Supporting. FMC shall be securely
fastened in place and supported in accordance with 348.30(A)
and (B).
(A) Securely Fastened. FMC shall be securely fastened in
place by an approved means within 300 mm (12 in.) of
each box, cabinet, conduit body, or other conduit termina-
tion and shall be supported and secured at intervals not to
exceed 1.4 m (4 1 ? 2 ft).
Exception No. 1: Where FMC is fished between access
points through concealed spaces in finished buildings or
structures and supporting is impracticable.
Exception No. 2: Where flexibility is necessary after in-
stallation, lengths from the last point where the raceway is
securely fastened shall not exceed the following:
(1) 900 mm (3 ft) for metric designators 16 through 35
(trade sizes
1 ? 2 through 1 1 ? 4 )
(2) 1200 mm (4 ft) for metric designators 41 through 53
(trade sizes 1 1 ? 2 through 2)
(3) 1500 mm (5 ft) for metric designators 63 (trade size
2 1 ? 2 ) and larger
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