3 phase 480v conductor sizing for equipment

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Bburdette

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Aberdeen, SD
We are changing out an existing Chiller that has 2 480v feeders running to it. One is 500a breaker w/ parallel 350kcm xhhw-2 and the other is 250a breaker w/ 350kcm xhhw-2. The new unit calls for a 500a circuit breaker at 416a load and a 350a circuit breaker at 224a load. Distance is 316ft to the disconnecting means. We need to change out the cable on the new 350a circuit, I had calculated it out at 500kcm xhhw-2. My struggle is calculating the 500a circuit. The HVAC contractor assumes that because it is already 500a breaker the cable should be adiquit but when I run the caluculations for the existing parallel 350kcm and its not coming out large enough to handle the new load. Thoughts? What am i missing?
 
Bburdette said:
We are changing out an existing Chiller that has 2 480v feeders running to it. One is 500a breaker w/ parallel 350kcm xhhw-2 and the other is 250a breaker w/ 350kcm xhhw-2. The new unit calls for a 500a circuit breaker at 416a load and a 350a circuit breaker at 224a load. Distance is 316ft to the disconnecting means. We need to change out the cable on the new 350a circuit, I had calculated it out at 500kcm xhhw-2. My struggle is calculating the 500a circuit. The HVAC contractor assumes that because it is already 500a breaker the cable should be adiquit but when I run the caluculations for the existing parallel 350kcm and its not coming out large enough to handle the new load. Thoughts? What am i missing?
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Capacity for parallel 350's is 620 Amps (Cu), 500 Amps (Al). Are you derating for temperature or more than 6 CCC's in one conduit? If so, derate according to the insulation rating (XHHW-2 90 C).

Am I the only one thinking that 500A breaker should be 100% rated or else it's going to trip after a few hours at full load?
 
Yes I'm adjusting for 6 ccc in one conduit. Cable is AL so that would be rated at 448a. Where I'm getting caught up is the continuous load of the unit and the conductor being rated for that. At 416a x 1.25 = 520a that needs to be covered by the cable. Correct?
 
Bburdette said:
Where I'm getting caught up is the continuous load of the unit and the conductor being rated for that. At 416a x 1.25 = 520a that needs to be covered by the cable. Correct?
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No, the cable doesn't need to be rated for 520A. Is the 416A the MCA or the RLA?
 
david luchini said:
448A is greater than the MCA of 416A. The existing feeder is adequate for the load.
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I agree, with an MCA of 416 amps all that you need after you derate for the 6 CCC's in a single raceway is a 416 amps or greater parallel conductors. The size of the OCPD (500 amps) is irrelevant compared to the conductor size.
 
infinity said:
The size of the OCPD (500 amps) is irrelevant compared to the conductor size.
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That would apply to the smaller circuit as well. The existing 350kcmil would be adequate for the 224A MCA.

But a wire type EGC would likely be undersized when the circuit breaker changes from 250A to 350A.
 
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