Motor feed calculations

[jatherton]

Chiller consists of 480v 3phase with an MCA of 214 amps.

Subfeeder feeding chiller consists of parallel 4/0, two per phase, with over current protection of 450 amps where subfeeder originates (at MDP).

With the current configuration, does this application meet N.E.C.?

 

[dirkhats]

Re: Motor feed calculations

Tables 310-16 & 310-15 may answer part of your questions.

 

[redfish]

Re: Motor feed calculations

If all of your conductors are in the same raceway,

Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable
Number of Current-Carrying Conductors Percent of Values in Tables 310.16 through 310.19 as Adjusted for Ambient Temperature if Necessary
4?6 80
7?9 70
10?20 50
21?30 45
31?40 40
41 and above 35

Then 250 kcmil is needed, if there are neutrals present,then 310.15(B)(4)(b) or (c) may apply.

(4) Neutral Conductor
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.

Then 350kcmil would be needed.

[ August 10, 2005, 10:44 PM: Message edited by: redfish ]

 

[infinity]

Re: Motor feed calculations

Let's look at 310.16:

2 sets of 4/0 in separate raceways
2 X 230 amps = 460 amps

2 sets 4/0 in same raceway
2 X 260 = 520 X 80% =416 amps

If the chiller falls under Article 440 than the MCA can be used to size the conductors. Therefore the feeder could be one set of 4/0 and the maximum fuse or CB size would be part of the listing of the unit. 450 amps with a 216 amp MCA sounds somewhat high for this application.

If this installation falls under Article 430 and not Article 440, than more information is needed to answer your question.

[ August 11, 2005, 05:13 AM: Message edited by: infinity ]