Motor load calculation, commercial electrical inspector exam

[wlancaster]

Hello, trying to get some help on a question from the nec exam I failed this past monday. I don't remember the exact numbers but it was close to this:

5hp squirrel cage motor, 3phase, 208V. Calculate the branch circuit conductor size, use THHN wire.

my questions are should I assume 115% of the FLC since no info is given(nothing said about the duty of the motor), and since the answer will be less than 100A, I must use the 60 deg C column on the ampacity table, even though the questions says use THHN wire. right??
Thanks for your help!

 

[Dennis Alwon]

Look at the table 430.250 for the ampacity

also 430.6

(A) General Motor Applications. For general motor applications,
current ratings shall be determined based on (A)(1) and
(A)(2).
(1) Table Values. Other than for motors built for low speeds
(less than 1200 RPM) or high torques, and for multispeed
motors, the values given in Table 430.247, Table 430.248,
Table 430.249, and Table 430.250 shall be used to determine
the ampacity of conductors or ampere ratings of switches,
branch-circuit short-circuit and ground-fault protection,
instead of the actual current rating marked on the motor
nameplate.

The ampacity of the conductor would be 6o amps since we don't know if the equipment is rated higher. If we knew the equipment was rated 75C then we would use 75C col. The 90c column is used for de-rating 110.14

 

[infinity]

my questions are should I assume 115% of the FLC since no info is given(nothing said about the duty of the motor), and since the answer will be less than 100A, I must use the 60 deg C column on the ampacity table, even though the questions says use THHN wire

The conductor minimum size formula is 125%* the FLC in T430.250. Let us know what you come up with.

 

[wlancaster]

Hello, trying to get some help on a question from the nec exam I failed this past monday. I don't remember the exact numbers but it was close to this:

5hp squirrel cage motor, 3phase, 208V. Calculate the branch circuit conductor size, use THHN wire.

my questions are should I assume 115% of the FLC since no info is given(nothing said about the duty of the motor), and since the answer will be less than 100A, I must use the 60 deg C column on the ampacity table, even though the questions says use THHN wire. right??
Thanks for your help!

Hello, trying to get some help on a question from the nec exam I failed this past monday. I don't remember the exact numbers but it was close to this:

5hp squirrel cage motor, 3phase, 208V. Calculate the branch circuit conductor size, use THHN wire.

my questions are should I assume 115% of the FLC since no info is given(nothing said about the duty of the motor), and since the answer will be less than 100A, I must use the 60 deg C column on the ampacity table, even though the questions says use THHN wire. right??
Thanks for your help!

Ok sorry I'm just now getting back and thank you for your response. Here's how I would approach now that I understand use 125% FLC for branch circuit.
FLC = 16.7A from table 430.250 16.7*1.25= 20.875 or 21A Conductor from table 310.15B(16) 2017 NEC: must use 60 deg C column because less than 100A so conductor will be a Copper 10AWG........right??

 

[infinity]

Here's how I would approach now that I understand use 125% FLC for branch circuit.
FLC = 16.7A from table 430.250 16.7*1.25= 20.875 or 21A Conductor from table 310.15B(16) 2017 NEC: must use 60 deg C column because less than 100A so conductor will be a Copper 10AWG........right??

That's correct since the question does not say that you have 75° C terminations you're stuck with the 60° C ampacity. #12 THHN is rated for 20 amps @60° C so that's too small therefore you need #10 THHN. If the question told you that the terminations were 75° C then #12 THHN (25 amps @75° C) would be the minimum size.

 

[wlancaster]

Great! thank you so much for your time!