Can I add another tap to 500A feeder

[Malywr]

I have feeder 480V, 3ph, 500A cb to air compressor 200HP with 400A disconnect non fusable one tap 6’ #4, 60A fusable disconnect with 20A fuses for air dryer it takes 12A. My question is can I add another tap for condensation evaporator 43A it is mostly resistive load or that would be to much for this feeder


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[Malywr]

this is drawing


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[kwired]

I have feeder 480V, 3ph, 500A cb to air compressor 200HP with 400A disconnect non fusable one tap 6’ #4, 60A fusable disconnect with 20A fuses for air dryer it takes 12A. My question is can I add another tap for condensation evaporator 43A it is mostly resistive load or that would be to much for this feeder


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200 Hp FLA is 240.

240 x 1.25 = 300
+ 12
+ 43

= 355 needed minimum conductor ampacity. You didn't say what size feeder conductor is.

CB can be more than 500 if it won't hold for starting, in fact can be more than 500 even before you added your 43 amp load.

 

[Malywr]

. Compressor manufacture recommend feeder vs170 460V. Feeder 2x300 mcm I calculate compressor package (there is 3 small motors, Controll and one 200 HP motor I measured and it is not using more than 265A) package 305Ax1.25 = 381A other loads 100%. 12+43 =55A + 381A=436A. So I should be within limit of 500A. Please tell if I am correct if not help me understand why and correct me pls.


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[kwired]

I wouldn't have any problems with adding what you want to add, with what information I have seen so far.

 

[Malywr]

Thank you ....
I have another question not related to tap.
Why manufacture would recommend 2x300 MCM if 2x3/0 is sufficient for 400A?


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[Adamjamma]

Possibly requiring larger cables due to inrush current? not what I am learning so far, but would be what I would look at.

 

[tw1156]

Thank you ....
I have another question not related to tap.
Why manufacture would recommend 2x300 MCM if 2x3/0 is sufficient for 400A?


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Their note for VS-170 @ 460V notes that the parallel conductors are installed in the same conduit, instead of separate conduits. Derating may be factored into their comment.

 

[retirede]

Their note for VS-170 @ 460V notes that the parallel conductors are installed in the same conduit, instead of separate conduits. Derating may be factored into their comment.

This....

This compressor is driven with a VFD. It’s not about inrush.

 

[kwired]

Their note for VS-170 @ 460V notes that the parallel conductors are installed in the same conduit, instead of separate conduits. Derating may be factored into their comment.

Yes.

More than 4 current carrying conductors will require ampacity adjustment - and they factored that in their table already.

 

[Malywr]

Yes.

More than 4 current carrying conductors will require ampacity adjustment - and they factored that in their table already.

I did not think about that by now 310.15.B.16 for derating MCM 300 is good for 500A x2 1000A. 4-6 conductors in same raceway 310.15.B.3.a. 80% gives me 800A. If I terminate at 75* lugs I am still above 570A. If I use 4/0 conductor after derating I would be at 416A.
Unless it is about voltage drop so one size up would be good too.
VS170 is driven with VFD so there is not in rush current.
Thank you everyone for input.


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[kwired]

I did not think about that by now 310.15.B.16 for derating MCM 300 is good for 500A x2 1000A. 4-6 conductors in same raceway 310.15.B.3.a. 80% gives me 800A. If I terminate at 75* lugs I am still above 570A. If I use 4/0 conductor after derating I would be at 416A.
Unless it is about voltage drop so one size up would be good too.
VS170 is driven with VFD so there is not in rush current.
Thank you everyone for input.


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300 kcmil is only good for 285 A (75C) 320 A (90C)

But take closer look at the table you posted a picture of - they are also adjusting for 40C ambient.

I didn't run their numbers vs NEC, but seems they must be about right per NEC with quick guesstimates.

If they included voltage drop, they about need to specify maximum circuit length to go along with it.

 

[Malywr]

300 kcmil is only good for 285 A (75C) 320 A (90C)

But take closer look at the table you posted a picture of - they are also adjusting for 40C ambient.

I didn't run their numbers vs NEC, but seems they must be about right per NEC with quick guesstimates.

If they included voltage drop, they about need to specify maximum circuit length to go along with it.

Thank you for explaining now I can see that. I guess when I look at the table I missed a lot of stuff. Looks like I have to learn a lot ....


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