Motor Calculation

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DAWGS

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300HP Air compressor unit. 480V 3-phase. FLA of unit is 342A. My calculation 342@125%=427.5A. I'm feeding it with parallel 4/0 tray cable. 75 degree column, 230A x 2= 460A. I'm getting questioned that one single 500MCM is sufficient. Am I missing something?
 
DAWGS said:
300HP Air compressor unit. 480V 3-phase. FLA of unit is 342A. My calculation 342@125%=427.5A. I'm feeding it with parallel 4/0 tray cable. 75 degree column, 230A x 2= 460A. I'm getting questioned that one single 500MCM is sufficient. Am I missing something?
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Presuming your 342 FLA is correct value you need a minimum conductor ampacity of 427.5.

One 500 MCM @ 75C is only good for 380A.

Now if the two 4/0 in parallel require any ampacity adjustments is possible those aren't large enough either, but at same time they likely can start adjustments from 90C value.
 
Have you seen this

430.6 Ampacity and Motor Rating Determination. The size of
conductors supplying equipment covered by Article 430 shall
be selected from the allowable ampacity tables in accordance
with 310.15(B) or shall be calculated in accordance with
310.15(C). Where flexible cord is used, the size of the conductor
shall be selected in accordance with 400.5. The required
ampacity and motor ratings shall be determined as specified in
430.6(A), (B), (C), and (D).
(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. Where a motor is marked in amperes, but not
horsepower, the horsepower rating shall be assumed to be that
corresponding to the value given in Table 430.247
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There is an exception

Exception No. 3: For a listed motor-operated appliance that is marked
with both motor horsepower and full-load current, the motor full-load
current marked on the nameplate of the appliance shall be used instead
of the horsepower rating on the appliance nameplate to determine the
ampacity or rating of the disconnecting means, the branch-circuit
conductors, the controller, the branch-circuit short-circuit and groundfault
protection, and any separate overload protection.
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361 x 1.25 = 451.25, 2 x 4/0s are rated for 460, but as kwired said, it may need de-rating. You could use the 90C table values for de-rating, assuming it is90C rated cable, so you would start with 520A for two, then apply your de-rate, etc. etc.

But a single 500kCMIL is a non-starter, it is only rated 380A. While that appears to fit the 342A (or even 361A) motor rating, it is NOT going to meet Code. So that was likely picked by some bozo that does not understand ALL of the rules.
 
don_resqcapt19 said:
The exception is for household type appliances like central vacuum units, garage door openers, and air compressors that use a very inflated horsepower for marketing reasons.
For this one, I would most likely use the 361 amps from Table 430.250.
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I'd possibly expect a 300 HP air compressor to potentially have more loads than just the compressor motor and possibly need a little more supply conductor ampacity than what just the compressor motor requires. Such unit will likely state rated amps and/or minimum supply amps as well as maximum overcurrent protection on nameplate.
 
kwired said:
I'd possibly expect a 300 HP air compressor to potentially have more loads than just the compressor motor and possibly need a little more supply conductor ampacity than what just the compressor motor requires. Such unit will likely state rated amps and/or minimum supply amps as well as maximum overcurrent protection on nameplate.
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Have never seen that marking on anything other than equipment covered by Article 440.
 
don_resqcapt19 said:
Have never seen that marking on anything other than equipment covered by Article 440.
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I will say maybe not as standardized as art 440 equipment is, pretty certain I've seen equipment that states what to supply it with whether on nameplate or at least in the instructions, both ampacity and overcurrent protection. Other HVAC items that are not art 440 equipment is pretty good about it most the time.
 
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