Can wire or cable be located above a transformers core and coil?
HPS recommends side or bottom entry for conduit into a type 3R, 12, 4 and 4X enclosure. Neither U.L. or NEC addresses this specifically with transformers. C.S.A. C22.2 No. 47 does address this and disallowed cable entry from the top of the enclosure. The issue with insulated cables is a transformer cools by convection with air moving from the bottom to the top of the coils. The air exiting from the top of the coils can be heated significantly higher than ambient. Cables which enter through the top of the enclosure will experience this heated air which causes three issues:
- Cable insulation damage within the enclosure.
- Hot air entering the conduit can cause insulation damage with the conduit.
- Hubs must be used to prevent water intrusion
Because of these issues, HPS does not recommend that cable enter through the top of a transformer’s enclosure in any application. Note that some of these issues can be mitigated if using non-jacketed hard bussing instead of cable.