ECN Forum Forums Code Related Discussion NEC & other Code issues flexible metal conduit

Can someone please tell me which article in the code covers the use of flex...sometimes called greenfield.I use the 3/8 size for hooking up pumps and relays on residential boilers. Am I required to use anti-shorts sometimes called redheads with flex and where does it state this in the code if so required.Also is there a restriction in the length that can be used.And is a grounding conductor required to be installed in lengths over 6 feet???An associate said if we had to use anti-shorts they would come with the roll of flex when purchased like with BX and MC cables.I always use redheads anyway but couldn't find it in the code book.

To my knowledge, anti shorts are not required for anything other than Type AC (BX) cable.

Article 348 covers type FMC or "flex". There is no requirement to use the anti-short bushings. It is a matter of choice.

My company requires them to be used. We build HVAC&R equipment that can have high vibration levels. There isn't a specification to use them other than the fact that it enhances protection and can save alot of money as opposed to leaving them off and shorting out the equipment. Case in point a unit cooling an office in Arizona in August. Greenfield not tightened enough, conduit slips out and vibrates a hole right through the wire. Trips the compressor out and no air-conditioning. Alot of griping and litigation threats for what the cost of a simple cheap protector could have saved.

Look at Article 350 for Flexible Metal Conduit (Greenfield). Read the Grounding section carefully. Like Liquidtight you have to read through the exceptions and the requirements ten times to get the jist of it.

The reference to 250-118 will nock out the use of conduit for grounding in a heartbeat if you don't meet the requirements. We have recently been arguing this same problem with Liquidtight and I told the guys on the floor to run a ground wire and put that dog to bed. Without the wire there is the need to have fittings and conduit rated for grounding and that if the flex is needed for flexibility you need the ground wire anyway. That way if you didn't put in enough to flex and the conduit gets pulled out you will still have an effective ground. (And if you use redheads you probably will save a short circuit problem.)

You can search for grounding in the forum and have plenty to read for the rest of the week. Always remember that the care you take to make the product safe is just as valuable as making the product work.

Robert

The Code uses the phrase "insulating bushing or its equivalent protection shall be provided between the conductors and the armor". For type AC (as previously mentioned)See section 320.40

3/8" FMC is addressed in Article 348 of the NEC.Conductor fill is covered in Table 348.22 make note as to whether you are using connectors that are internal or external.

Grounding requirements are covered in 348.60 Where used to connect equipment where flexibility is required, an equipment grounding conductor shall be installed. If you install one it counts as conductor fill.

Also Article 250.118 recognizes FMC as an equipment grounding means if the conduit and the fittings are listed as such. Or you could be useing FMC that is listed, but not listed as a grounding means(and it’s probably not) under the following conditions. The total length (return path) is not greater than 6 feet, the fittings are listed for the purpose (few check), and the conductors are protected at 20 A or less. Of course it can not be installed to provide flexibility, and function as a EGC.

Note that section 348.42 does not mention insulating bushings. But then again the NEC is only a minimum standard!

[This message has been edited by cpal (edited 08-06-2004).]

[This message has been edited by cpal (edited 08-06-2004).]