Tap Rule

[bwilder12]

I'm trying to figure out the power distrubition tap on a job I'm working is correct. Main service is a 1200 amp main. Its actually a fusable disconnect with taps on bottom side. One tap feeds a 400 amp disconnect that has 3 taps off of it. One tap feeds a 150a main in a vfd cabinet. The other two taps feed two 30a disconnects that feed motor starters. My biggest concern is the tap off the main that feeds a soft start for a 600a compressor. The tap off the main to soft star is 25 foot. The main in the soft start is 1200 amp. The way I understood the tap rule the breaker upstream had to be of the ampacity of the tap. Which is 600 mcm. Am I missing something because I feel that the tap to the soft start should be 1200a wire.

 

[iwire]

It may not really be a tap, it might just be a motor circuit.

 

[bwilder12]

Tap Rule

Well off the main 1200 disconnect there are two taps. One tap feeds a four motor branch circuit. Then the other tap feeds a single motor soft start. The branch circuit tap is sized correctly. The other tap is feed with the same size wire as the secondary out of the soft start. Is the single tap OK because it doesn't feed but one motor?

 

[GoldDigger]

Well off the main 1200 disconnect there are two taps. One tap feeds a four motor branch circuit. Then the other tap feeds a single motor soft start. The branch circuit tap is sized correctly. The other tap is feed with the same size wire as the secondary out of the soft start. Is the single tap OK because it doesn't feed but one motor?

Can we straighten out one thing here and try to get the terminology straight?:

You have a 1200 amp main disconnect, and then on the load side of that disconnect, without any individual OCPD, you have two sets of wires going other places, and neither set of wires has a rated ampacity of 1200 amps or more?
If that is the case, then you probably have two taps, yes.

If the single motor and its soft start together have some sort of internal OCPD which properly protects the wire going to it, then depending on length and actual ampacity of the wire you may have a legal tap. The 1200A breaker in the soft start alone will not do this if the wire is rated for only 600A. But if the motor itself has an internal overload at 600A, then you may be OK.
If the four motor "branch" circuit (probably a feeder) has a single OCPD (which is what makes it a branch circuit) then, again depending on the length and the wire ampacity compares to the rating of that OCPD, then you may have a legal tap.
Whether the conductors, disconnects and OCPD within that multi-motor branch circuit (if that is what it is, rather than a feeder and several branches!) are really correct or not gets complicated.
You have not told us the size of any of the conductors (except the 600mcm??).

 

[iwire]

I am still not convinced that the circuit feeding the large motor is a tap.


Consider this example.

460 volt, 3 phase, 400 HP motor.

Table 430.250 tells me it draws 477 amps.

The conductor size has to include 125% so we need conductors sized at least 596 amps. (477*1.25=596)

Now the for the overcurrent device, I will use a non-time delay fuse.

Table 430.52 tells us that the fuse can be 300%, so 477*3=1431

So we end up with this

Source>>1,400 amp fuse >> 596 amp conductors >> motor starter >> motor overload unit >> 596 amp conductors >> motor.

Are there any tap conductors in that arrangement?