code j motor

[jim gage]

I installed a 25 hp code j- blower motor at 480v 3 phase fed with #8 thhn copper and I am getting a rattle in the conduit on start up. As I see it #8 at 55 amps is large enough for this installation but does anyone see any down side to the rattle in the 16' vertical run of conduit. Would #6 thhn copper eliminate this problem?

 

[Jraef]

I installed a 25 hp code j- blower motor at 480v 3 phase fed with #8 thhn copper and I am getting a rattle in the conduit on start up. As I see it #8 at 55 amps is large enough for this installation but does anyone see any down side to the rattle in the 16' vertical run of conduit. Would #6 thhn copper eliminate this problem?

The rattle is from the magnetic repulsion going on between the unsecured conductors. Increasing the size only stops it when the size gets so big that the natural stiffness of the conductors overcomes the mechanical force exerted by the magnetic repulsion, and there's no what to know when that will happen. Triplexing (twisting) them will stop it though. If you can't pull them out, triplex them and re-pull them, then live with the rattle or maybe buy a soft starter.