Tap Rule

[OldTiredEngineer]

I have a 800Amp CB for a 480VAC System. On the load side of the breaker, the only lug size they offer is 2-500MCM connections per pole. After the 800Amp CB, there are four 200Amp fuse blocks. So I took 1-500MCM to a distribution block rated for 760Amps. The load side i have 3/0 wire going to two of the 200amp fuse blocks. I took another 1-500MCM to a second distribution block and have 3/0 wire going to the other two 200amp fuse blocks.

I guess this is the first time I have ever had to split the load side of a circuit breaker into two separate distribution blocks.

The corporate engineer for the company we are working for is saying this is a violation of the tap rule. While I can kind of understand his point, I disagree. So I'm looking for another opinion.

 

[GoldDigger]

I have a 800Amp CB for a 480VAC System. On the load side of the breaker, the only lug size they offer is 2-500MCM connections per pole. After the 800Amp CB, there are four 200Amp fuse blocks. So I took 1-500MCM to a distribution block rated for 760Amps. The load side i have 3/0 wire going to two of the 200amp fuse blocks. I took another 1-500MCM to a second distribution block and have 3/0 wire going to the other two 200amp fuse blocks.

I guess this is the first time I have ever had to split the load side of a circuit breaker into two separate distribution blocks.

The corporate engineer for the company we are working for is saying this is a violation of the tap rule. While I can kind of understand his point, I disagree. So I'm looking for another opinion.

If this were field building wiring, subject to NEC, then the fact that each of the 500MCMs is a tap conductor and does not terminate directly on an overcurrent protective device is probably a violation of the tap rule. Instead of going through a distribution block you would instead have to go to a fuse block first.
If this is instead internal wiring of a UL-listed custom panel, then the NEC tap rule does not directly apply. Instead, I believe, the reference would have to be to UL standards. (Looking this alternative, since your profile mentions panel building.)

 

[tom baker]

I have seen similar in control panels. Check UL 508A

 

[jap]

I would think he flagged you because you didn't bring the full 800 amp feeder from the load side of the breaker to a common power distribution block before dividing the taps.


Jap>

 

[jap]

If the parallel 500mcm's had of landed on a common distributing block, then split from there to the 4 individual fuse blocks, I don't think there would have been an issue.

Jap>

 

[OldTiredEngineer]

If the parallel 500mcm's had of landed on a common distributing block, then split from there to the 4 individual fuse blocks, I don't think there would have been an issue.

Jap>

I know I'm responding to JAP here but I'm replying to all of you. Thank you all!

Yes this is within one enclosure we will be fabricating.

JAP is correct, how I would have normally done things was full amperage covered from the disconnecting means to the distribution block and then followed the 10% rule from the block as long as my wire length was less than 10ft. I was hoping I could get away with 500MCM. Yes I know that's only 380A. Math says I need a 800amp disconnect. The current system also has a 800 disconnect but the system is only supplied with a 400A buss plug. That's why I thought I could do this.

The system is a machining transfer line with 66 motors ranging from 1/2hp to 50hp. Since it is a transfer system all motors run simultaneously.

Thanks for the help guys.

 

[kwired]

You might be able to do that per listing requirements if you can apply those otherwise the old saying when it comes to NEC is "you can't tap a tap".

 

[OldTiredEngineer]

Thank you Guys

Thank you Guys

Thank you guys for your comments. UL508A allows such things. NEC, well correct, I can't tap a tap. So solution was come away from the 800Amp CB with DLO cable over sized to carry 400 amps to two 400 amp fuse blocks. From there I'm splitting off with 3/0 to two 200 amp fuse blocks per 400 amp fuse blocks. I've been blessed by both UL and our local NEC code enforcer.

Thank you all for your help.