NEC 2017_Annex D_Example D8_Motors

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pcsailor

pcsailor

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Hi,

I may be missing something very basic but can someone explain something to me about NEC 2017_Annex D_Example D8_Motors_Page 800?

The example motor specs are given in the beginning (a) and (b) sections.
(a) is one 25-hp motor with a nameplate rating of 32 amps
(b) is two 30-hp motors with nameplate ratings of 38 amps (primary)

When you look through the example answers, in the 'motor overload protection' section, they're using the given 32 & 38 amps to calculate the answer.

With every other solution, they're using 34 and 40 amps, respectively, to determine the answer.

Why? Why have they up'ped the amps by 2 for the motor calculations?

Thank you,
Phil
 
I think your answer is here.

430.32(A)(1) Separate Overload Device. A separate overload device
that is responsive to motor current. This device shall be selec‐
ted to trip or shall be rated at no more than the following
percent of the motor nameplate full-load current rating:


Motors with a marked service factor
1.15 or greater 125%
Motors with a marked temperature rise 40°C or less 125%
All other motors 115%
Click to expand...
 
Well it does explain it but I assumed you knew that the other calculation are based on the tables at the end of art. 430. Table 430.250 has the motors rated 34 and 40. For some calculations like conductor size you use the Table while the other calculations use the nameplate.

25 hp at 480 is 34 amps
30 hp at 480 is 40 amps

Use the 460 col for 480v
 
From 430.22
430.22 Single Motor. Conductors that supply a single motor
used in a continuous duty application shall have an ampacity of
not less than 125 percent of the motor full-load current rating,
as determined by 430.6(A)(1), or not less than specified in
430.22(A) through (G).
Click to expand...

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.
Click to expand...
 
Yes, that does answer the question Dennis.
I assumed, incorrectly, the answer would go through this step, of finding the conductor amps, and I forgot this step in my haste.
With motors:
Table amps for conductors
Nameplate amps for overloads
Need to get that through my head.
Thank you <:
Phil
 
pcsailor said:
Yes, that does answer the question Dennis.
I assumed, incorrectly, the answer would go through this step, of finding the conductor amps, and I forgot this step in my haste.
With motors:
Table amps for conductors
Nameplate amps for overloads
Need to get that through my head.
Thank you <:
Phil
Click to expand...
With motors the table amps give worst case efficiency/worst case power factor you would find on a particular standard motor size. I think the idea is should you replace motor with a less efficient one but same HP you will still have sufficient conductor size.

Then with overload protection you really need to protect according to actual motor rating, so nameplate applies for overload selection.
 
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