Motor wiring using rigged metal conduit

[rickr911]

We have a 40 HP coolant pump motor attached to its base with a vibration dampening system (rubber mounts). The motor wiring is run to the motor enclosure with RMC. Is there an NEC or NFPA code that would prohibit this?

Is this type of wiring just fundamentally wrong? My boss would like some documentation stating that it is an incorrect method of wiring before changing the conduit to a flexible conduit.

Anyone have any suggestions?

 

[electricpete]

R.M.C. is certainly an allowed raceway under the code. Most of our motors are fed with rigid galvanized steel conduit that transition to flex conduit the last few feet before the motor term box.

What aspect are you questioning?

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[dpc]

The NEC permits use of a short length of flex to allow for vibration and movement,and this is done about 100% of the time, but I don't believe the NEC requires it if you don't feel vibration is an issue.

David Castor

http://www.cvoes.com

 

[rickr911]

The problem that I see is that the R.M.C. is run all the way to the motor enclosure with no flexible conduit in between.

I am proposing that there should be a short lenth of flexible conduit and I am trying to find some supporting documentation.

 

[dpc]

It is almost always done that way. It also greatly simplifies removing the motor. But AFAIK, it is not required by NEC or other codes. Falls under "good workmanship", I guess. We specify that it be done for all motors and small transformers in our construction specifications.

David Castor

http://www.cvoes.com

 

[jraef]

The only time I see RMC directly connected to a motor is when there is NO vibration suppression in the mounting system for the motor, i.e. rigid base, bolted to a plate or concrete pier etc. and the conduit is actually helping to keep the motor in place. I've always wondered if that was appropriate, but I don't think there is anything saying it's against code, unless there is an "appropriate use" clause somewhere that I have never seen (I've not done a lot of conduit specifications).


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[jraef]

From a local project, look at A-4:

http://www.cp.berkeley.edu/cds_ucb/Div-16-2.html

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[electricpete]

I never realized vibration issues were the reason for including flex conduit at the end, but it makes sense (along with the comment about ease of removal).

A quick review of NEC section 400 confirms dpc is right it is not a code requirement.

Even though not a code requirement seems prudent to prevent vibration of conductors can be a concern for chafing wires (even when protected by bosses at entrance to boxes) and other vibration-related problems.

Personally I would think it would be more important for equipment on isolators than typical rigid-mounted equipment... because equipment on isolators tends to shake more. Most of our fans have 0.1 - 0.2 ips bearing housing vibration. We have some fans on isolators that get up to 0.6 ips.

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[electricpete]

I should say vibration is not a function of one variable only (mounting). But the idea that flexibly mounted machines tend to vibrate more was not invented by me.... look at NEMA MG-1 and you'll see higher vibration limits allowed for machines tested on flexible resilient mount than on rigid mount. (Which paradoxically is the opposite of what it should be if you are looking at foot-induced 120hz vibration on NEMA frame 2-pole motors).

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[waross]

The only two applications I have encountered where rigid conduit was run directly to a motor have been;
1> Gasoline dispensing pumps where everything must be explosion proof and even there we often used explosion proof flexible sections. With the advent of premade mineral insulated gas pump connection whips, no one can aford the labour cost of fitting rigid conduit.
2> The other was an interesting individual. He called for a machine to be connected by rigid stainless steel conduit rising vertically through the ceiling without bend or offset.
No description of the machine, no size, no conduit or cable size, no location drawing to locate the conduit entry and the pressure was on the electricians to pre-install the conduit.
The designer showed up and introduced himself by denigrating the local work force,
"We build these plants all over the place with no trouble, what seems to be the problem here."
The electrical foreman showed him the print in question and asked for some further information.
"Wait, now I remember. You know, I remember when I was drawing this, wondering just what the hell that was!"
Then he left the site.
My boss knew my short tolerance for idiots and moved my to another project the next day.
But, back to the issue at hand. There may be something in the mechanical codes that says something like;
"The resilience of the resilient mounts shall not be compromised by bridging them with rigid members such as conduit."



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I agree with your last statement and was thinking the same thing also. Rigid conduit works against the objective of a flexible mount which is to lower natural frequency far far below running speed. Any stiffening from conduit would tend to increase the natural frequency of the mounted item.

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[MikeHalloran]

NEC and NFPA are only concerned with whether a particular installation would be likely to cause a fire.

They don't care if the machinery functions, now or ever, or if it produces or transmits noise or vibration.

You probably should....




Mike Halloran
Pembroke Pines, FL, USA

 

[whippee2]

I wouldnt say that using rigid metal would be an issue. I have seen this alot in here in australia. (I know the standards are different though). I would definetly earth/ground the metal conduit though, in case an active cable shorts to conduit.

Whippee2

 

[waross]

This a motor on resilient mounts, whippee2.

Bill
--------------------
"Why not the best?"
Jimmy Carter