Is there an electrical engineering calculation why the code does not allow more than 1.8m or 6 ft of flexible conduit in installations such as taps for lighting fixtures or motor terminations?

I don’t have a code book in front of me but I think it has to do with the conduit being used as an equipment grounding conductor, most flexible metal conduit is not listed for use as an equipment grounding conductor so the lengths are limited to 6 feet or less. Most of my experience is with Liquid tight flexible metal conduit which can be run in unlimited lengths if it has an equipment grounding conductor run with the circuit conductors. I think its article 350 of the 2005 NEC. Someone please correct me if I am wrong.

Contrivers is exactly correct. It has o do with the felex acting as a grounding conductor. Check site below.
http://www.ecmweb.com/mag/electric_nec_limit_flexible/index.html

Thanks very much Contrivers & wareagle. To summarize your replies and tips, the code does not limit the length of any flexible conduit as long as there is a separate grounding conductor run inside it and the conduit run is properly supported as required by code. Is this correct?

NEC 348.60 covers you question.

Thanks for the tip wareagle. The edition of NEC I have is 2002 and 348.60 is about grounding and bonding. So it has all to do with grounding - flexible conduits are not recognized as a ground path. Can I get a yes or no to my previous question then?

I'd add NEC 250.118 as well -- that lists all the permissible equipment grounding conductors.  Liquid-tight is listed in 250.118 as being an acceptable grounding conductor in lengths not to exceed 6 feet.

Yes, you understand correctly -- if you run an equipment grounding conductor inside the flex, you are essentially unlimited regarding the length of the flex.  Beware that some local authorities might not agree with that statement, though, so I'd recommend checking with them first.

According to my book, if you need the flexibility that either FMT or LFMC can provide, you need run that ground wire anyway. Even with short runs.

PeeBee said "Yes, you understand correctly -- if you run an equipment grounding conductor inside the flex, you are essentially unlimited regarding the length of the flex."
PeeBee
I think your intent is correct but I believe there is a rule of thumb 100 ft limit to the distance of the EGC listed in table 250.122. Beyond that 100 ft there should be some checking to ensure that the fault current will trip the OC device in a reasonable length of time. A reasonable length of time is determined by the engineer.

Wareagle -- you're right, to say "unlimited" is an overstatement.

The EGC needs to comply with 250.4(A)(5) or 250.5(B)(4), which both require the EGC to provide a "permanent, low-impedance circuit" to ensure that fault current is properly carried and/or tripped.  That requirement applies to any installation -- even if you're using rigid conduit or thinwall as the EGC.  That could well require a larger-sized EGC than those listed in Table 250.122.

NEC does not dictate any 100-ft rule of thumb on that, it applies to all installations....  so be careful with the rules of thumb....