three phase system in conduit

[boyles]

I need information on installing a 3 phase, 4 wire system in rigid conduit from the transformer secondary to a secondary bus. Specifically, is there an explanation out there regarding placing conductors for each phase and the neutral in conduits. More specifically, the pairing should be A-B-C-N in each conduit, not A-A-A-N, B-B-B-N, C-C-C-N in the conduits because of the induced neutral current in the second configuration?

 

[dpc]

Well, if you are in the US, this is a legal requirement per the NEC. The reason is to avoid induced current in the metal conduit. If only one phase is place within a steel conduit, a current can be induced in the conduit that will cause the conduit to get hot, causing the cable insulation to overheat and fail.

If all the conduit is non-metallic, this is not an issue.

Try searching this form or Google. Should be plenty of information out there.

BTW, this has nothing specifically to do with the neutral. The same problem exists even without a neutral.

David Castor

http://www.cvoes.com

 

[amptramp]

Follow this thread:

http://www.eng-tips.com/viewthread.cfm?qid=260050&page=1

 

[waross]

Even with PVC conduit I have seen severe heating of the steel panels. Keep your cables in similar A,B,C,N groups. You may often get away with installing a neutral in less than all the groups if there are less neutral conductors than phase conductors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[boyles]

Thank you all for the information. It may not have anything to do with the neutral, but we had a situation where the neutral in a A-A-A-N, B-B-B-N, C-C-C-N configuration was carrying excessive current and overheating and the jacket was literally melting. The conduit of course was also overheating.

 

[dpc]

It was probably not due to excessive current. The conduit can get hot even for normal load current. And again, it has nothing to do with the neutral.

David Castor

http://www.cvoes.com

 

[jghrist]

A-A-A-N, B-B-B-N, C-C-C-N grouping can also induce circulating currents in the neutrals.

 

[waross]

The last time I saw A-A-B-N B-C-C-N the heat from the conduits was conducted down the cables and destroyed a 400 amp switch with heat corrosion of the copper components. The current peaked at about 130 Amps. I was told that the switch had failed two or three times in the past from heat corrosion.
After correcting to A-B-C-N A-B-C-N everything cooled down and there have been no further issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter