Need help with NEC wire sizing question

[Smart $]

You didn't receive the post, the post was put up on a forum. When you reply without quoting we don't know what you are replying to, since other posts could have been put up between the one you are responding to and your response. Also, just saying "wrong!" isn't very informative.

Wrong.

 

[jeremy.zinkofsky]

Daniel, I'm not asking how to do it. Obviously you follow the nameplate. I was trying to figure out WHY it is done the way that it is so I could better understand.

Rob, thank you for the knowledge. I appreciate it.

Good on you for trying to understand something rather than blindly follow what info you are given.

A motor circuit needs to be protected from two events:

1. Motor Overload/Motor Inrush
2. Short Circuit Current/Motor Short Circuit Contribution

For event No. 1:

As you may very well know, an induction motors stator is made up of multiple turns of copper wire around an iron core. The rotor is typically bars of copper around an iron core. Same as with a transformer, when current is placed on one set of windings a current is induced in the second set of windings. The resultant induction generates a rotating magnetic field or emf. Such emf has a resistance or impedance that limits the current flow. Keeping Ohms Law in mind, when resistance is low current is high and vice versa.

What happens when you start a motor is that initially there is no emf present so there is little to no resistance (only the resistance of the copper windings). A high current is produced as a result of this minimal resistance. Once the motor starts spinning an emf is generated between the stator and rotor which in turn creates a resistance to current flow.

If you put an amp probe on the circuit you will see the motor current jump up to it's maximum nameplate rated amperage for less than a second. Once the rotor starts spinning the current drops down to the running amperage which is the minimum value on the nameplate.

The NEC requires that the conductors feeding a motor shall be capable of handling the brief starting amperage of a motor and whatever heat (created by current) the motor is capable of producing.

For event No. 2:

During a short circuit the voltage collapses wherever the fault occurs. The emf in the motor does not instantaneously collapse with the voltage because the rotor is still spinning and still generating current. During a fault, the motor acts like a generator and adds to the fault. This only lasts a few electrical cycles until the emf dissipates, however a few cycles is plenty of time to cause damage. It is during these first few cycles that the OCPD needs to interrupt the fault and open the circuit.

So, the OCPD needs to be sized to handle the short circuit current that can be generated by a motor.

That is why you see a 60A breaker with #10AWG wire. The breaker is sized to handle the short circuit current while the cable is only needed to handle heat generated during initial motor startup or seizure of the rotor.

Savvy? :lol: