2011 NEC Annex D8...

[eric dolphy]

please take a moment to clarify an example given in Annex D8...

the conductor ampacity calculation has a wound-rotor motor, 30hp, 460v, 3 phase, nameplate 38a (primary) and 65a (secondary)...

in determining the proper feeder conductor, the wound-rotor primary value appears to derive from 430.250 is 40 amps (i.e., 30hp @ 460v), but the secondary value appears to derive from the nameplate FLA of 65 amps.

both the 40a and 65 amp figures are then factored @ 125% : 50a and 81a respectively.

1) is all of the above a correct interpretation of D8?

2) are the resulting derived figures then simply added to determine the conductor size without further factoring (310.16)?

thank you

 

[ActionDave]

Wound rotor motors are a curious animal. I have only seen one in my life and it wasn't running. Here's a code reference for the secondary conductor sizing.

430.23 Wound-Rotor Secondary
(A) Continuous Duty. For continuous duty, the conductors
connecting the secondary of a wound-rotor ac motor to its
controller shall have an ampacity not less than 125 percent
of the full-load secondary current of the motor.

They are separate from the primary and go to rotor so I would say you size them separately. I really don't know.

Somebody wake Jraef up again.

 

[Jraef]

Wound rotor motors are a curious animal. I have only seen one in my life and it wasn't running. Here's a code reference for the secondary conductor sizing.



They are separate from the primary and go to rotor so I would say you size them separately. I really don't know.

Somebody wake Jraef up again.

They are separate. Only the primary goes TO the motor, so there is nothing different about those. The rotor circuit is created BY the stator, but at a different voltage. Think of it like a transformer, with the rotor circuit like the secondary. All it feeds is a bank of resistors though. Changing the amount of resistance changes the torque characteristics of the motor which generally results in a speed change.

Not all Wound Rotor motors are used this way however, sometimes the circuit is just used as a type of soft starter. In that case, the rotor circuit is not continuous and you don't need to add the 25%. The motor nameplate will say "Non-Continuous" if it is, so if you don't see that, you have to assume it is continuous.