Tap Rule

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OldTiredEngineer

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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.
 
OldTiredEngineer said:
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.
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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.)
 
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>
 
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>
 
jap said:
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>
Click to expand...

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.
 
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".
 
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.
 
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