Overcurrent Protection of Y-Delta Starter Wiring
Overcurrent Protection of Y-Delta Starter Wiring
(OP)
I would like to clear up some confusion I have with the proper sizing of fuses/wires for a simple Y-Delta motor starter configuration. I understand the concept of being able to use contactors rated at less than the motor horsepower/FLA and also about setting the overload relay to approx. 0.58% of FLA ( provided the overload relay is connected "inside the delta"). What I'm not sure about is if I can downsize the conductors on the load side of the contactors. The reason I ask, is that when I use a downsized contactor, the terminals often won't accept a conductor that was sized based on the full load motor current. However, by downsizing the conductors, it appears that they will no longer be adequately short circuit protected by the line fuses. I have attached a simplified connection diagram to help explain my confusion.
I would appreciate any input on this...it seems that I'm missing something obvious.
Dave
I would appreciate any input on this...it seems that I'm missing something obvious.
Dave





RE: Overcurrent Protection of Y-Delta Starter Wiring
RE: Overcurrent Protection of Y-Delta Starter Wiring
RE: Overcurrent Protection of Y-Delta Starter Wiring
Bill
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RE: Overcurrent Protection of Y-Delta Starter Wiring
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RE: Overcurrent Protection of Y-Delta Starter Wiring
RE: Overcurrent Protection of Y-Delta Starter Wiring
Good point guys about the conductors carrying winding current and not line current.
This leads me to the question: In a 3 lead delta connected motor is the FLA for the motor given in terms of the winding current, or the line current. I have always thought it to mean the line current.
RE: Overcurrent Protection of Y-Delta Starter Wiring
RE: Overcurrent Protection of Y-Delta Starter Wiring
Don't get the two scenarios mixed. From an installation engineering standpoint, internal motor winding current is typically irrelevant. It was only mentioned here as an explanation as to why the rules are slightly different for Y-Delta starters.
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RE: Overcurrent Protection of Y-Delta Starter Wiring
motor nameplate, such as for wye-delta starting, the percentage of motor nameplate current applying to
the selection or setting of the overload device shall be clearly marked on the motor starter shown in the
motor starter manufacturer's overload selection table."
Other rules indicate the conductor sizing is based on the overload protection, recognizing that the fuses are installed as overcurrent protection in short circuit conditions. Does this help?