5 KV motor circuits

[Part 77]

When spec-ing out conductors (to be installed in rigid metallic conduit) for a 4160 volt 3 phase AC motor with normal torque characteristics, when does one need shielded conductor? What are the ramifications of shielded vs. non-shielded at this voltage? Also, the NEC does not have full load currents for 3 phase AC motors over 2300 volts in Table 430.250. What is a good reference for amperages of higher voltage motors? TIA.
Brian

 

[don_resqcapt19]

Brian,
Look at 310.6 for the shielded cable requirements. There was a change in this section for the 2005 code and if the system voltage is above 2400 volts, shielded cable is required.
Don

 

[Part 77]

Don, that answers that question. Thanks. Actually, it is conductors above 2KV, now that I checked. Now, all I need is a is a good source for amperages of motors above 2300 volts?

 

[Nick]

The motor manufacturer is probably your best source. I am on a job right now where we will be hooking up 4160V motors ranging from 1000HP to 2250HP. The engineered drawings circuit schedule stated TBD. When an RFI was written the engineer deferred us to the VFD/motor manufacturer. They came back with an amperage the cable should be rated. Now, the RFI has to go back for the engineer to size. It's alot of extra work chasing these things down especially when the motors were pre selected and this could have been done by the engineer in the first place.

 

[don_resqcapt19]

Brian,
It was really a change in the exception. In both the 2002 and 2005 codes the main rule required shieded above 2kv, but in 2002 using the exception you could use non-shielded up to 8kv, now in the 2005 code the exception stops at 2.4 kv. I agree with Nick that the best source for load currents of these motors would be the manufacturer.
Don