Overcurrent Protection of Y-Delta Starter Wiring

[DaveAlford]

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

 

[rbulsara]

Compare the thermal damage curve of the #6 conductor against the 150A fuse curve and you shall see whether or not it is protected against short circuit.

 

[resqcapt19]

The NEC specifically permits the conductors between the wye/delta starter and the motor to be sized at 58% of the motor FLA.

430.22(C) Wye-Start, Delta-Run Motor. For a wye-start, delta-run connected motor, the selection of branch-circuit conductors on the line side of the controller shall be based on the motor full-load current. The selection of conductors between the controller and the motor shall be based on 58 percent of the motor full-load current.

 

[waross]

In delta run, there are six conductors supplying current to the motor rather than the normal 3 conductors.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

More specifically, those six conductors carry winding current and not line current, hence the premitted reduction in cross section.


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If we learn from our mistakes I'm getting a great education!

 

[DaveAlford]

Thanks for the feedback everyone...the Canadian Electrical Code is not as specific as the NEC when it comes to Y-Delta motor starting(unless I missed something). I suspected that the conductors were allowed to be downsized as I have seen many such applications with approval stickers, but I just wanted to make sure.

 

[rockman7892]

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.

 

[DaveAlford]

I believe the FLA on the motor nameplate would always be line current, at the voltage specified.

 

[jraef]

Nameplate would always be Line current. Winding current is only relevant to the motor mfr., with the two exceptions being Y-Delta and part Winding. But in both of those scenarios you have 6 motor leads to the controller to consider, not 3, so conductor sizing is based on the configuration of the winding connections.

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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[geoffqis]

RE the Canadian Electrical Code. I can't find a specific reference to the conductor sizing for wye delta starters, however rule 28-306 (2) states "Where a motor overload device is connected so that it does not carry the total current designated on the
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?