NEC Annex D questions: D8 Motors

[TheWantonElectrician]

2017 NEC: Annex example D8

Question 1: Under the Branch-Circuit Short-circuit and Ground-fault Protection section, is there a reason to not work out the selection for the 30-hp motors? I calculated them to need a 125A nontime-delay or 70A time-delay protective device. Can anyone confirm this selection?

Question 2: This might get answered from Q1's answer, but why does the Feeder Short-circuit and Ground-fault Protection section use the 25-hp motor's protective device rating rather than the protective device for one of the 30-hp motors (which I assume to have a largest BC protective device)? I calculated the feeder to need a 175A nontime-delay protective device. Can anyone confirm this selection?

Question 3: The nameplate secondary full load current is given for the 30-hp motors. How can I use this information for motor calculations?

I have read through the following thread and think my questions may be related, but I still don't really understand it: https://forums.mikeholt.com/threads...nd-rotor-load-left-out-of-feeder-calc.103192/

 

[don_resqcapt19]

Q2 See Table 430.52. The percentages for the maximum permitted protection for the wound-rotor motor are smaller than those for the squirrel cage motor. The largest device will be the one for the 25 hp motor.

Q3 The nameplate current is used for the calculation of the rating of the overload protection device.

 

[TheWantonElectrician]

Q1: Given what don wrote, here are my new selections: 60A nontime-delay and 60A time-delay.
Q2: Great, thanks, I missed that completely!
Q3: In the example the primary full load current is 38A, while the secondary is 65A, does that mean it'll need two overload protective devices? Or does it only need one in accordance with 430.32(E)? If so, is the "motor-overload device" based on the primary, the secondary, or the higher of the two?

 

[don_resqcapt19]

See 430.225(B)(2)

 

[TheWantonElectrician]

That's under Part XI. Over 1000 Volts, Nominal. Should I assume it applies to the secondaries?

So, the overload protection which is based on the primary nameplate rating will protect the secondary?

 

[don_resqcapt19]

That's under Part XI. Over 1000 Volts, Nominal. Should I assume it applies to the secondaries?

So, the overload protection which is based on the primary nameplate rating will protect the secondary?

Sorry, I did not look at the part and I do know better...try 430.32(E). Yes, that says that the only overload protection needed is for the primary.