LFMC instead of expansion fitting on long Rooftop EMT conduit Run

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My company is investigating using a short section of LFMC (flex) in place of an expansion fitting to compensate for expansion and contraction of long runs of EMT conduit on Rooftops. The idea is that this will save cost. I'm not entirely supportive, but we have a few folks pushing the idea.

There are a few concerns:

- the LFMC might sag onto the roof. possible solution is to bend the EMT up a bit at the ends so the curve of the LFMC goes away from the roof, rather than sagging onto the roof
- the LFMC is difficult to bend and so it will build up pressure in the conduits and not releive the strain on the conduit and support system that the expansion fittings are meant to mitigate.

We had one of our subs build a little mock up with a 1" and 3" section. we are thinking it might be more feasible with the 1" because it's easier to bend...

Does anyone out there have experience with this? Thoughts?

Thanks,

LFMC vs. expansion fitting_mock-up2.jpg LFMC vs. expansion fitting.JPG
 

luckylerado

Senior Member
...compensate for expansion and contraction of long runs of EMT conduit on Rooftops....

Is this a problem that you are having? Granted, I have not run many long runs on rooftops but I have never installed an expansion fitting for this scenario.

That said, I do not know why what you suggest will not work. May not be the most fun fishing a tape through it.
 

iwire

Moderator
Staff member
Location
Massachusetts
My company is investigating using a short section of LFMC (flex) in place of an expansion fitting to compensate for expansion and contraction of long runs of EMT conduit on Rooftops.

Unless you are passing over a building expansion joint there is no reason at all to install expansion fittings on these conduits.
 

iwire

Moderator
Staff member
Location
Massachusetts
I use conduit couplings like that often but there are some AHJs / inspectors that will fail that use as it is not listed for the combination.

We have used FMC when crossing buildings expansion joints inside the building.
 

roger

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Staff member
Location
Fl
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Retired Electrician
Using a short piece of a flexible wiring method in place of an expansion fitting as always been my preferred method.

Roger
 

Carultch

Senior Member
Location
Massachusetts
Unless you are passing over a building expansion joint there is no reason at all to install expansion fittings on these conduits.

So even if the change in length exceeds a lot more than 1/4", you still don't need an expansion fitting in steel conduit? Is that what you are telling me?
 

Carultch

Senior Member
Location
Massachusetts
My company is investigating using a short section of LFMC (flex) in place of an expansion fitting to compensate for expansion and contraction of long runs of EMT conduit on Rooftops. The idea is that this will save cost. I'm not entirely supportive, but we have a few folks pushing the idea.

There are a few concerns:

- the LFMC might sag onto the roof. possible solution is to bend the EMT up a bit at the ends so the curve of the LFMC goes away from the roof, rather than sagging onto the roof
- the LFMC is difficult to bend and so it will build up pressure in the conduits and not releive the strain on the conduit and support system that the expansion fittings are meant to mitigate.

We had one of our subs build a little mock up with a 1" and 3" section. we are thinking it might be more feasible with the 1" because it's easier to bend...

Does anyone out there have experience with this? Thoughts?

Thanks,

View attachment 14109 View attachment 14108


It has been my company's basis of design to specifically require IMC or RMC on rooftops to begin with, and not even permit EMT outdoors where exposed. From what I understand, it is because the rooftop is a place that could be interpreted as subject to physical damage, the steel finish isn't built for system longevity outdoors, and the couplings for EMT aren't as reliable as threaded RMC/IMC ones in corrosion resistance, strength and chance of installation error. But given that none of this is required by the NEC, there is no objective requirement that would disallow EMT on a rooftop.

Here's an example of a failed EMT coupling. Probably as a result of not anticipating thermal expansion. An RMC coupling would be more robust, but is still subjected to the expansion load.
http://solarprofessional.com/sites/...o.06.NFPA-Fire-Report.failed-emt-coupling.jpg

Even so, the underlying issue with building an expansion joint from LFMC could still apply. In your mock-up, the Durablok assemblies are connected with an axial piece of strut, and the staps constrain axial movement. This will negate the entire functionality of the expansion joint. The axial strut is a good idea, to prevent the joint from sagging too much, but I 'd recommend using straps that can allow for movement on this particular pair of sleepers.
 

iwire

Moderator
Staff member
Location
Massachusetts
Here's an example of a failed EMT coupling. Probably as a result of not anticipating thermal expansion.

I do not believe that came apart due to thermal cycling. That likely was never tightened sufficiently. These compression fittings are often poorly made and even if wrench tight come apart. I laugh when EEs specify them for better connections. :)

EMTs is allowed on roofs but here in the Northeast it does seem to rust out pretty quickly. In other areas of the country it is used successfully outdoors for decades.

For this reason many of the job specs do require IMC or RMC on roofs yet we can still couple them with compression fittings if we choose.
 

Carultch

Senior Member
Location
Massachusetts
I do not believe that came apart due to thermal cycling. That likely was never tightened sufficiently. These compression fittings are often poorly made and even if wrench tight come apart. I laugh when EEs specify them for better connections. :)

That's a good point too. Perhaps why it separated, but its twin conduit didn't separate.

For this reason many of the job specs do require IMC or RMC on roofs yet we can still couple them with compression fittings if we choose.

I think there is a difference in reliability between a compression fitting every 10 ft, and just a single compression fitting where a field-cut conduit may arise.

I've used RMC compression fittings in this environment, primarily because field threading is a major endeavor. Looking at the fine print, one issue I notice, is that a lot of RMC compression fitting datasheets omit indicating whether or not the fitting could be used in a wet or damp location. Many say concrete-tight, but few say raintight.
 

iwire

Moderator
Staff member
Location
Massachusetts
That's a good point too. Perhaps why it separated, but its twin conduit didn't separate.

You mean the conduit that subject to the same temperatures?:)

I have too many years doing repairs to believe expansion and contraction of steel conduit runs is an issue that needs to be considered unless passing over / across a building expansion joint.
 

SolarPro

Senior Member
Location
Austin, TX
Here's a recommendation from someone who has experience analyzing field failure modes in PV systems. See bullet #3 in particular:

REDUCING GROUND FAULTS

The following are some key design and installation tips—largely intended to minimize opportunities for conductor damage—that will reduce the occurrence of PV system ground faults.

Use shorter circuit lengths. Both copper and aluminum conductors expand and contract at different rates than steel raceways. Long-distance circuit runs magnify this difference and can cause significant problems at turns and terminations.

Avoid conduit bodies for 90° turns.
Where possible, use alternatives to LB-, LL- or LR-type conduit bodies for making turns in conduit. When coupled with dissimilar expansion and contraction rates, the tight turns associated with these fittings are a common cause of conductor damage.

Avoid the need for expansion fittings. Whenever circuit runs exceed a distance of 100 feet, consider using cable tray rather than conduit to eliminate the need for expansion fittings.

Use aluminum for large circuits.
To eliminate dissimilar expansion rates, use aluminum cable trays or raceways with aluminum conductors, which are also much lighter and cheaper than copper.

Terminate aluminum with care. Extra attention is warranted when terminating aluminum to ensure quality, long-lasting circuits. Verify that the terminals are rated for use with aluminum conductors and use antioxidant on all terminations.

Source: The Heat Is On: Fault Detection and Fire Prevention

Note that Section 300.7(B) applies to all raceways, and requires expansion fitting "where necessary to compensate for thermal expansion and contraction" where raceways are exposed to different temperatures. Obviously, different AHJ will apply this "where necessary" clause differently.
 
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iwire

Moderator
Staff member
Location
Massachusetts
Note that Section 300.7(B) applies to all raceways, and requires expansion fitting "where necessary to compensate for thermal expansion and contraction" where raceways are exposed to different temperatures. Obviously, different AHJ will apply this "where necessary" clause differently.

The rate of expansion and contraction between steel conduits and the steel or aluminum structures they are secured to are so similar it makes it unnecessary unless the goal is to overbuild the job for ones own reasons.

You PV guys do understand that we have been running long lengths of steel conduit on roofs long before PV was a thing right? :D
 

Carultch

Senior Member
Location
Massachusetts
Here's a recommendation from someone who has experience analyzing field failure modes in PV systems. See bullet #3 in particular:

How does a cable tray help you eliminate the need for thermal expansion compensation?

I would think that a cable tray would be a long linear piece of metal that is mechanically coupled in a rigid manner, just like conduit is. Unless the cable tray has expansion couplings as well.
 

Carultch

Senior Member
Location
Massachusetts
You mean the conduit that subject to the same temperatures?:)

I have too many years doing repairs to believe expansion and contraction of steel conduit runs is an issue that needs to be considered unless passing over / across a building expansion joint.

Yeah, I guess I realize that it is conduits subject to the same temperature situation. The one thing you can't see in that photo, is that the intact raceway could be a much shorter length, with therefore less of an issue of thermal expansion.

Can you rephrase your second sentence? It is unclear if you are saying something for or against expansion fittings.
 

Carultch

Senior Member
Location
Massachusetts
The rate of expansion and contraction between steel conduits and the steel or aluminum structures they are secured to are so similar it makes it unnecessary unless the goal is to overbuild the job for ones own reasons.

I thought about that as well. That the building itself would move at the same rate, provided that it is uniformly subjected to the same temperature changes. However, there is a difference in expected temperatures between a raceway and roofing in direct sunlight, and a building structure in thermal contact with the interior conditioned space.

The other issue I thought about is PVC underground. I know temperature swings are a lot less underground, but doesn't mean that the problem ceases to exist. Assume that temperature swings are 1/5th underground what they are in ambient air. That just means that 100 ft is "the new 20 ft". If you'd put in a PVC expansion fitting every 20 ft outdoors, then you'd logically have to put one in every 100 ft underground. And I don't see this as a common practice.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
...I notice, is that a lot of RMC compression fitting datasheets omit indicating whether or not the fitting could be used in a wet or damp location. Many say concrete-tight, but few say raintight.
A standard threaded conduit coupling doesn't say rain tight either.
 

don_resqcapt19

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Staff member
Location
Illinois
Occupation
retired electrician
Here's a recommendation from someone who has experience analyzing field failure modes in PV systems. See bullet #3 in particular:



Source: The Heat Is On: Fault Detection and Fire Prevention

Note that Section 300.7(B) applies to all raceways, and requires expansion fitting "where necessary to compensate for thermal expansion and contraction" where raceways are exposed to different temperatures. Obviously, different AHJ will apply this "where necessary" clause differently.
In general that sounds like it was written by someone who has no real world experience with installing conduit and wire or someone who has only inspected installations that were made by someone who had no real world experience with those types of installations. The only one that I would agree with is the last one...aluminum terminations need a bit more care in the installation that do copper ones.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
How does a cable tray help you eliminate the need for thermal expansion compensation?

I would think that a cable tray would be a long linear piece of metal that is mechanically coupled in a rigid manner, just like conduit is. Unless the cable tray has expansion couplings as well.
Typically a splice plate with slotted holes on one end, shoulder bolts for the slotted end connection and a bonding jumper.
 
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