CABLE VS. CONDUIT TO CONNECT MOTOR

[polareng]

I need to connect a 10HP, 3ph motor to contoller box. Do I have to use conduit or can I use cable? The run is less than 10ft. Am I bound by NEC 400.7 Uses Permitted? because it is not specifically Not Permitted per 400.8. I just find it hard to believe that this would not be allowed.

 

[wrsharper]

With a 10HP three phase motor I would definitely use conduit to connect the motor if the motor is permanently installed. For one thing you have a chance that the cable could be cut.

If it is say a conveyor motor that has to be moved around frequently then where I work at and other plants a vertical cord drop with a disconnect rated recpt./plug is used i.e. Meltrec. We only use this at locations where we move portable equipment around alot. I think that the code frowns on this practice and we do have problems with an occasional fork lift catching the cord drop and a subsequent "sparks a flying". So if possible use conduit.

 

[VxA]

I don't think it's a code issue so much as a best practice issue. I know lots of facilities that would use an armoured cable (type MC or Teck) and route it in some sort of channel to about 3 feet from the motor and then let the cable loop into the motor connection box.

If you're using tray cable I think you'd be best off to route it in conduit, or at least in some sort of raceway until you get very close to where you need to be (say 2 or 3 feet max). It's pretty common to see rigid conduit up to a local motor disconnect switch and then a flexible conduit from the disconnect to the motor connection box (providing the disconnect is 3' or less from the motor).

As wrsharper stated, you should consider the likelihood of mechanical damage occurring.

 

[itsmoked]

Locally in the last month a worker was killed because he pitchforked a 480V cable lying in the material he was moving.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

In NEC land you can use armored cable, or cable in flex conduit, but not just unprotected insulated cord.

Actually, it is specifically prohibited in 2005 NEC 400.8 "(7) Where subject to physical damage"

 

[wrsharper]

in re itsmoked, anybody who puts a pitch fork into a cable will be out of the gene pool. A pitch fork should go into a pile of dodo.

 

[itsmoked]

So you're saying the cable shouldn't have been in the pile of doodoo? T'was a mushroom compost(straw - horse manure) shredder.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

Cables end up in strange places and are attacked by even stranger beings. That's why God made conduit.

 

[jraef]

400.7.A.6 cuts a wide swath if you ask me. In the food processing, brewing and dairy industries where equipment is taken apart to be cleaned or moved around (transfer pumps, sorters, augers etc.), motors are connected with watertight flex cables and twist-lock or pin-and-sleeve cap and plug sets all the time. The same is true for the Aggregate industry because crushing plants are often moved as the quarry is mined out. You can make a case for anything, just keep in mind that ultimately, interpretation will be up to the AHJ on inspection and it's difficult to appeal the "ruling on the field". For all the effort it would take to try to get away with it, it would probably be easier to just use conduit. A lot of people just use seal-tight flex conduit within the last few feet.

JRaef.com
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[wrsharper]

I am not going to make any coment.

 

[itsmoked]

What about a comment ?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[wrsharper]

I meant to use the word commit. I stutter and get my "i's and e's mixed up.

 

[VxA]

I think the thing to remember here is that someone lost their life. It's ok to discuss the shortcomings in design, implementation, operations, etc. that may have lead to this accident, but ridiculing the victim is something we as engineers should avoid.

 

[panelman]

my thoughts exactly

 

[HCBFlash]

polareng,

You sound (to me) like you're a little out of your element in making this decision. Your term "controller box" is kind of what gives you away. "conduit" and "cable" are only descriptive of 2 categories of typical wiring methods. You may in fact use both!

Let me say (for everyone here) that you should almost certainly NOT use any non-flexible conduit to connect to a motor.

There are countless factors that go into any such installation. NFPA 70 (a.k.a. "The N.E.C.", a.k.a. "the national exception catalog") only places limits on how specific things may be done. It does not determine what is the most appropriate, reliable, durable, flexible, affordable, easily constructed, or aesthetically pleasing method of conducting any electrical installation.

NFPA70 is not applicable to all electrically related stuff in the western hemisphere.


It in fact may not be at all applicable to your installation. Is this all a part of a "machine" or other unit? Is this onboard a ship or any other vehicle?


Please let us all know how this works out for you! and don't forget faq731-376

 

[wrsharper]

I nor anyone else meant to ridicule the fact that someone lost his life. There are situations where common sense over rules.

 

[polareng]

Thank you all for your advise on the matter. A little more background, the motor is a fan motor and the controller is a VFD. The reason I was questioning the conduit vice cable is because the cable is rated for all weather use and would be out of the way of any harm. I'm just trying to understand the justification for why conduit would be absolutely required--what's its purpose other than to protect conductors and provide some shielding, that is, that would make it absolutely required. For this application I would be using shielded cable, which is weather resistant, provides shielding and not susceptible to damage so why would I need to provide conduit in addition to all weather type cable?

I admit, I am a little out of my element. Much of my background experience is marine application. Facilities is new to me and, I am finding, falls under much different requirements.

 

[dpc]

The conduit is for physical protection of the cable and protection of people nearby. In other parts of the world, you will see 600V exposed cable leaving a tray and running a few feet exposed connecting directly to a motor.

In the US, that is generally not allowed per NEC. Standard practice is to make the final connection to the motor using liquid-tight flexible metal conduit (a.k.a. "Sealtight"), to allow for vibration.

 

[HCBFlash]

You certainly don't need to use a conduit instead of a cable to satisfy the requirements of the NEC, or more importantly the AHJ (authority having jurisdiction). I'm finding myself to be considered more and more of an "old timer" even at the ripe old age of 48. That's sometimes because my obvious prefferance for metallic wiring methods and use of conduits and raceways (I like using the Chicago Electrical Code),,,, but even I am changing a bit. It is in fact becoming more and more common to use cables instead of conduits (raceways) because the continual improvements of the material used in cables, as well as the (percieved) istallation labor costs. If your blower is rigidly attached in a fixed location, "sealtight" or other "flex) is quite typical.

"S.O. cord" is more common for fractional and small integral horsepower motors between the local disconnecting means and the motor itself, when the motor vibrates and moves a bit, such as a variable sheave or similar belt drive. If your installation is in a location which is exposed to damp conditions at times, and condensation may form, a cabled connection is far prefferable.

I've replace more than a few "sealtights" in HVAC and refrigeration, outdoor, or wash-down installations which had rusted to nothing inside and didn't provide any useful shield or ground continuity (or frequently mechanical protection). Why use "flex" at all? Construction; "pull 'em long and use the cuts for the whip". Just the same THHN wire as the rest of the circuit keeps it simple. Heck I've even seen some really lousy foremen have stuff trimmed-in with the same solid #12 they like to use throughout a job. (I can really get going on that kind of stupid stuff!)

Cable is not always sized the same as conductors for general wiring, as it has it's own heating tolerance and characteristics. Actual SO cord is obsolete. There's some very good cable available as far as durability, oil, moisture, chemical resistance, all-temperature flexibility, limitation or elimination of moisture migration, etc, etc. By all means use a good quality, properly sized connector for terminations at the motor terminal box and the other end. Some amazingly good non-metallic ones of these are now available also.

Please let us all know how this works out for you! and don't forget faq731-376

 

[electricpete]

Those are good and relevant comments itsmoked.

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