Special permission?

[wwhitney]

What numbers do you all come up with?

Nice example, although I believe you've avoided the need to figure out the intricacies of 430.63. I'm newish to motor computations, so here goes, I may have some oversights:

From Table 430.249, 4-wire 2-phase, FLC 50 HP = 56A, 75 HP = 83A. On the lights, 2A on each of the 4 wires would run 4 lights, so I get 16A for 30 lights (rounding up to a multiple of 4).

Then 430.24 tells us the feeder conductors need an ampacity of 1.25 * 83 + 83 + 15 * 56 + 1.25 * 16 = 1047A.

On the OCPD, I think it depends on the fuse type of the 1200A fuses. If time delay fuses, then 430.63 tells us it shall be not less than 1.75 * 83 + 83 + 15 * 56 + 1.25 * 16 = 1088 A. While if non-time delay, it would be 3 * 83 + . . . = 1192A.

No problem with 1200A MCC, 1200A feeder, and 1200A OCPD, for either fuse type.

Now suppose you want to add one more 50 HP motor. The 430.24 calculation is now 1103A, still fine. The 430.63 calculation would be 1144A for time delay fuses, or 1248A for non-time delay fuses.

Does that mean that it would only be allowed if the feeder fuses are time delay fuses? Or does the 430.63(3) language "the rating permitted by 430.62" not mean the maximum rating permitted by 430.62, but any rating permitted by 430.62? In which case the 300% factor for non-time delay fuses per 430.52 isn't required, and it would be OK to use a smaller value, and 1200A non-time delay fuses?

Cheers, Wayne

 

[tortuga]

Nice example, although I believe you've avoided the need to figure out the intricacies of 430.63. I'm newish to motor computations, so here goes, I may have some oversights:

From Table 430.249, 4-wire 2-phase, FLC 50 HP = 56A, 75 HP = 83A. On the lights, 2A on each of the 4 wires would run 4 lights, so I get 16A for 30 lights (rounding up to a multiple of 4).

Then 430.24 tells us the feeder conductors need an ampacity of 1.25 * 83 + 83 + 15 * 56 + 1.25 * 16 = 1047A.

On the OCPD, I think it depends on the fuse type of the 1200A fuses. If time delay fuses, then 430.63 tells us it shall be not less than 1.75 * 83 + 83 + 15 * 56 + 1.25 * 16 = 1088 A. While if non-time delay, it would be 3 * 83 + . . . = 1192A.

No problem with 1200A MCC, 1200A feeder, and 1200A OCPD, for either fuse type.

Now suppose you want to add one more 50 HP motor. The 430.24 calculation is now 1103A, still fine. The 430.63 calculation would be 1144A for time delay fuses, or 1248A for non-time delay fuses.

Does that mean that it would only be allowed if the feeder fuses are time delay fuses? Or does the 430.63(3) language "the rating permitted by 430.62" not mean the maximum rating permitted by 430.62, but any rating permitted by 430.62? In which case the 300% factor for non-time delay fuses per 430.52 isn't required, and it would be OK to use a smaller value, and 1200A non-time delay fuses?

Cheers, Wayne

Nice! thats what I came up with also for the 430.24, I had not considered the non-time delay fuses part.

Art 430.24​	480V L-L 240 L-N 5 Wire Two phase​
​	​	Amps each​	qty​	Amps​
​	​ 50 HP pump​	56​	15​	840​
​	​ 75 HP Pump​	83​	2​	166​
​	Add 25% of highest rated motor​	20.75​	1​	20.75​
​	Max Tunnel Lights 240V between a phase​	2​	8​	16​
​	Add 25 % of continuous load​	4​	1​	4​
​	​ Total​	​	​	1047​

 

[don_resqcapt19]

You mean it’s not ‘permitted’ - but you could still do it. And, the bus would not catch on fire or blow up.


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And neither would the 1200 amps of conductor that feeds the MCC.