MV cable question

[cjhut]

We wanted to move an existing conduit for a 2300volt 250hp motor. The original conductors were 5kv nonshielded EPR insulation, in steel conduit in free air. We now want to move it underground in steel conduit. I called the local wire and cable distributor and it seems I have two options. There is a 2400V nonshieded EPR, or a 5kv sheilded EPR. Is there anything wrong with using the 2400v insulation? I guess that they don't make 5kv nonshielded anymore. Our utility can sometimes run our voltage a little on the high side. You might see 2500volts, but that would be when the plant is not running. Would it be OK to run right at the insulation level or should I go with the 5KV and stress cones and possibly have to go up a conduit size.

 

[jtester]

If you are governed by the NEC, refer to 310.6 In general, shielding is now required over 2000 volts, however there are some exceptions.

Jim T

 

[7anoter4]

In my opinion, only 5 kV cable has to be shielded according to NEC from 2005 edition. 2.4 kV non shielded is still permitted.
The problem is the required insulation level. If the grounding fault will be cleared in less than 1 minute then 2.4 kv insulated cable[100%] will be suitable. If the clearing will take more but less than 1 hour the cable shall have 133% minimum insulation level, that means 1.33*2.4=3.2 kv. There are [per IEC 60502] 3.3/1.9 KV cables. Per IPCEA or UL only 5 KV.
But, if the grounding fault clearing time is more than 1 hour a 173% insulation level is required.[2.4*1.73=4.15 KV.] So 5KV cable will be required any way.

 

[alehman]

In the 2011 NEC, the shielding requirement is for conductors operated over 2000 volts (phase-to-phase). Use of increased insulation level is common for non-solidly grounded systems, where ground faults can persist.


Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.