FMC Supporting

[infinity]

I have no direct experience with how much transformers vibrate, but if somebody when to the trouble to use pads/springs, obviously it's a concern.

I think that more of the concern is the transfer of vibration (noise) to the surrounding building structure. If that isn't a concern then they can be bolted directly to the floor.

 

[wwhitney]

I think that more of the concern is the transfer of vibration (noise) to the surrounding building structure.

OK, but EMT or other rigid wiring method strapped to the transformer case and strapped to the building structure will also transfer vibration. So when someone has specified pads/spring for the enclosure mounting to avoid transferring vibration to the structure, then the use of a flexible wiring method is called for to comply with that goal.

Cheers, Wayne

 

[infinity]

OK, but EMT or other rigid wiring method strapped to the transformer case and strapped to the building structure will also transfer vibration. So when someone has specified pads/spring for the enclosure mounting to avoid transferring vibration to the structure, then the use of a flexible wiring method is called for to comply with that goal.

Cheers, Wayne

I agree, when there is an external vibration method employed it's likely that the designer wants a flexible raceway also.

 

[jaggedben]

Once the raceway is longer than about a foot I question whether vibration is still a valid reason to invoke the exception. So for example in the bottom right photo in post #4, the short section on the left side of the transformer seems fine without a strap. But the 3-4ft long section on the right side, not so much.

 

[kwired]

OK, but EMT or other rigid wiring method strapped to the transformer case and strapped to the building structure will also transfer vibration. So when someone has specified pads/spring for the enclosure mounting to avoid transferring vibration to the structure, then the use of a flexible wiring method is called for to comply with that goal.

Cheers, Wayne

A piece of flexible conduit that is secured per NEC requirements will still have similar vibration transmission characteristics I would think as one secured soley by it's terminations.

Many transformers have pads inside the enclosure and you may have to loosen or even remove fasteners to let the inside portion float on the pads. There can be other equipment that has similar vibration control methods as well.

 

[kwired]

Once the raceway is longer than about a foot I question whether vibration is still a valid reason to invoke the exception. So for example in the bottom right photo in post #4, the short section on the left side of the transformer seems fine without a strap. But the 3-4ft long section on the right side, not so much.

Kind of what I was thinking in my previous post

 

[wwhitney]

A piece of flexible conduit that is secured per NEC requirements will still have similar vibration transmission characteristics I would think as one secured soley by it's terminations.

Maybe. But either way it will behave differently, vibrationally, from EMT secured per NEC requirements. Which is why flexible conduit is appropriate when the enclosure is being vibration isolated from the building structure.

Many transformers have pads inside the enclosure and you may have to loosen or even remove fasteners to let the inside portion float on the pads. There can be other equipment that has similar vibration control methods as well.

When the transformer itself is vibration isolated from the enclosure, then there's no need to use flexible conduit to the enclosure (at least vibration-wise). The conductors themselves inside the enclosure will provide the vibration isolation; depending on their stiffness, a large loop may be appropriate.

Cheers, Wayne

 

[kwired]

Maybe. But either way it will behave differently, vibrationally, from EMT secured per NEC requirements. Which is why flexible conduit is appropriate when the enclosure is being vibration isolated from the building structure.


When the transformer itself is vibration isolated from the enclosure, then there's no vibration isolation requirement to use flexible conduit to the enclosure. The conductors themselves inside the enclosure will provide the vibration isolation; depending on their stiffness, a large loop may be appropriate.

Cheers, Wayne

Agree with both parts. Just having a foot of flex on something like this is what I had in my mind. No need to have 5 feet and unsecured to have some vibration isolation effects. Longer lengths being unsecured would be for things that normally do have more movement in regular use than just vibration.

 

[wwhitney]

Just having a foot of flex on something like this is what I had in my mind. No need to have 5 feet and unsecured to have some vibration isolation effects.

Maybe. Show me the calculation or testing that says that 1 foot of FMC of the size used has a fundamental frequency well below the lowest frequency to be isolated (presumably 60 Hz in the US), and I'll agree with you. But if the fundamental frequency for 1 foot works out to be near or above 60 Hz, you'll need a longer length (which will reduce the fundamental frequency) for proper vibration isolation.

One comment, though: 348.30 Exception 2 is primarily about increasing the maximum distance from the termination to the first strap from 1' to 3', 4', or 5', depending on trade size (although in trade sizes 2-1/2" and larger, maybe you could use it to increase the distance between straps from 4.5' to 5.0'). Strapping the FMC to the transformer enclosure is not necessarily going to impede vibration isolation of the enclosure to the structure, and the 4-1/2' between straps is likely to be enough for vibration isolation. So while the wording of the exception often permits you to omit strapping to the enclosure (if the first strap to the building structure is within the specified limit), it may not be particularly useful to do so.

Cheers, Wayne

 

[kwired]

Maybe. Show me the calculation or testing that says that 1 foot of FMC of the size used has a fundamental frequency well below the lowest frequency to be isolated (presumably 60 Hz in the US), and I'll agree with you. But if the fundamental frequency for 1 foot works out to be near or above 60 Hz, you'll need a longer length (which will reduce the fundamental frequency) for proper vibration isolation.

One comment, though: 348.30 Exception 2 is primarily about increasing the maximum distance from the termination to the first strap from 1' to 3', 4', or 5', depending on trade size (although in trade sizes 2-1/2" and larger, maybe you could use it to increase the distance between straps from 4.5' to 5.0'). Strapping the FMC to the transformer enclosure is not necessarily going to impede vibration isolation of the enclosure to the structure, and the 4-1/2' between straps is likely to be enough for vibration isolation. So while the wording of the exception often permits you to omit strapping to the enclosure (if the first strap to the building structure is within the specified limit), it may not be particularly useful to do so.

Cheers, Wayne

Liquid tight flex likely transmits more vibration than same length of FMC. Might even be difference between FMC aluminum and FMC steel. So yes there is more to it. But one foot of FMC is most certainly going to reduce vibration more than running something more rigid the entire way.