[img]/forums/images/icons/tongue.gif[/img] This question falls into the "construction electrical" category so it isn't something that most electricians would run into; but it is a good study nonetheless. [img]/forums/images/icons/tongue.gif[/img] Lem N. Squezins of Lem's Electric installs a big 3-Phase panel that has a metering section and a big main switch in it. He also installs a big DISTRIBUTION panel and sets them side by side...wall to wall. Lem cuts three holes that go through both panel walls and installs a nice bushing in each hole. So now he has a way of getting the wires from the meter-main to the big distribution circuit breaker section. There is a lot of current so Lem uses multiple conductors on each leg....three on leg "A", three on leg "B", and three on leg "C".Lem stuffs the three "A" conductors through the first bushing, the three "B" conductors through the second bushing, and the three "C" conductors through the third bushing, and hooks them up to the bus in the distribution panel. When I check it under load, the wires are about 90 degrees F....no problem....the PANEL WALLS, however, are 185 degrees F. What's going on here? [img]/forums/images/icons/crazy.gif[/img]
Inductance heating.............cut a slot between the holes and Lem's panel will no longer heat up. Or run an A,B and C through each of the 3 bushings.
[img]/forums/images/icons/cool.gif[/img] The In-Spec-Tor has got it.....it was circulating current caused by inductance that was not cancelled out by grouping wires of each phase in each hole. To fix the problem, Lem did exactly as the inspector said....he divided up the wires and put an A-B-C group in EACH hole......the temp returned to normal. Lem made the mistake of thinking that circulating current was only a problem in CONDUIT runs and that the walls of the panels would not be a source, but in reality all it takes is a circle of ferrous material around the wire and you have a problem with circulating current. If the panels were STAINLESS they would not have done that. [img]/forums/images/icons/crazy.gif[/img]
Posting Permissions
Reply With Quote