Help understanding tap rule?

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t_ray7

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Hello all, forgive my ignorance on this topic but I can't seem to fully wrap my head around the tap rules. I am a control panel designer and have a special instance where we need to put in two distribution blocks to have enough connections to all of our branch circuit protection devices. I've attached a sample image as well - the sizes listed and number of devices don't really apply to this design, it's just an example. Each PDB will have at least 6 connections on each load side terminal.

NEC-UL508-A-QUESTION.png


I have a fused disconnect that has 2-port lugs for each phase where we are planning on running a 4/0 wire to each distribution block. From each distribution block will be multiple wires to each individual breakers or fuses depending on the circuit. The wires will be no smaller than 14AWG.

From what I've read, it seems like the 4/0 needs to be sized to handle the total 400A (600mcm) to each distribution block and also I can't have the 'load side' conductors of the distribution blocks without the need for an overcurrent device ahead of the distribution block. Am I correct in that thinking? I'm pretty new on this side of things and want to get this right since a lot of our old designs follow this same setup.

This will be a UL 508A panel. I think the above is a violation of the NEC but I'm not sure if it's a violation of 508A.
 
First off were just talking ' Internal wiring' inside a single piece of 508A equipment, no 'Feild wiring ' branch circuits or feeders constructed by electricians? And I am no expert but, for starters you cannot tap a tap.
Each tap needs to end in a OCPD.
UL 508A said:
32.3 Sizing of overcurrent protection
32.3.1 The size of the overcurrent protection shall not exceed:
a) The rating of the largest branch circuit protective device in the circuit plus the full-load
currents of all other motors or other loads in the group; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.
Click to expand...
So per 32.3.1(b) my guess is you would need to protect that 4/0
What type of insulation are we working with MTW? THWN?
 
tortuga said:
First off were just talking ' Internal wiring' inside a single piece of 508A equipment, no 'Feild wiring ' branch circuits or feeders constructed by electricians? And I am no expert but, for starters you cannot tap a tap.
Each tap needs to end in a OCPD.

So per 32.3.1(b) my guess is you would need to protect that 4/0
What type of insulation are we working with MTW? THWN?
Click to expand...

Correct, this is all internal wiring. Our disconnect is being fed from the customer and it's usually never given information about what device it's being supplied from.

The 4/0 wiring is Class K Cobra X-Flex.
steve66 said:
This looks good:


One of the first sentences talks about "tapping a tap".
Click to expand...

I've read that article, but I'm question if that applies to only disconnects that have multi-tap lugs associated with it?

The way I take that is if I have, for example, an A-B 194R-J60-1753 without distribution lugs attached to it and wire to a UL 508A feeder-spacing compliant distribution block like a Marathon 1343580, I can wire to either circuit breakers or fuses from that block.

Now if I have that same 194R-J60-1753 with the 3-port lug kit installed ( 194R-60-MTL3 ) then I CAN'T use a distribution block without first having some kind of protection ahead of it. Correct?
 
t_ray7 said:
Correct, this is all internal wiring. Our disconnect is being fed from the customer and it's usually never given information about what device it's being supplied from.

The 4/0 wiring is Class K Cobra X-Flex.


I've read that article, but I'm question if that applies to only disconnects that have multi-tap lugs associated with it?

The way I take that is if I have, for example, an A-B 194R-J60-1753 without distribution lugs attached to it and wire to a UL 508A feeder-spacing compliant distribution block like a Marathon 1343580, I can wire to either circuit breakers or fuses from that block.

Now if I have that same 194R-J60-1753 with the 3-port lug kit installed ( 194R-60-MTL3 ) then I CAN'T use a distribution block without first having some kind of protection ahead of it. Correct?
Click to expand...
Don't know what 508 allows either, but per NEC you have 400 amp feeder at the load side of the fuse, you then tap that with 4/0 to two different distribution blocks - those are feeder taps themselves, you can not make another tap to a tap, you must end the tap at an overcurrent device that protects that conductor. Had you run 400 amp of conductor to each terminal block you non longer have a tap for that segment and can then make your other taps.
 
tortuga said:
I see, now the plot thickens, Cobra flex is a dual rated conductor NEC/UL 105C rated and type THW or MTW conductor.
The mfr lists 4/0 as 454 amps at 105C:
So first you need to determine what the limiting factors on your conductor ampacity are if any.
Click to expand...

I think the limiting factors are the 75 deg C terminals, which means I would need to derate the wire to the values in UL 508A Table 28.1.
 
kwired said:
Don't know what 508 allows either, but per NEC you have 400 amp feeder at the load side of the fuse, you then tap that with 4/0 to two different distribution blocks - those are feeder taps themselves, you can not make another tap to a tap, you must end the tap at an overcurrent device that protects that conductor. Had you run 400 amp of conductor to each terminal block you non longer have a tap for that segment and can then make your other taps.
Click to expand...

Thanks for the response. I think I'm understanding what a 'tap' is now.
 
Got it! I have no idea why that was such a hard concept to grasp. Reading too many different codes and interpretations can really muddy the waters. Thanks for the help everyone!
 
IMHO people try to explain tap rules in a convoluted manner. It is this simple. A smaller conductor attached to a larger conductor that has an overcurrent device above the ampacity of the smaller conductor. Next, add in the length rules. 10 ft., 25 ft., over 25 ft., ect.
 
xformer said:
IMHO people try to explain tap rules in a convoluted manner. It is this simple. A smaller conductor attached to a larger conductor that has an overcurrent device above the ampacity of the smaller conductor. Next, add in the length rules. 10 ft., 25 ft., over 25 ft., ect.
Click to expand...
Connecting a "smaller" conductor directly to a feed thru lug or even pretty much directly to an OCPD can seem to confuse a few, it does not have to be a distribution block or polaris tap. Still comes down to overcurrent device above the ampacity of the conductor, with exception of some situations where higher than usual setting is permitted - like motor circuits those still have a second protective device for overload protection though.
 
The 4/0 wiring is Class K Cobra X-Flex.
Click to expand...
Side issue: make sure your distribution blocks are rated for Class K stranding, many are not. Those that are will also typically need SPECIFIC fuses ahead of them in order to have any decent value of SCCR, otherwise they are limited to 5kA or 10kA, which in an industrial environment is nearly impossible to meet in the field.
 
Jraef said:
Side issue: make sure your distribution blocks are rated for Class K stranding, many are not. Those that are will also typically need SPECIFIC fuses ahead of them in order to have any decent value of SCCR, otherwise they are limited to 5kA or 10kA, which in an industrial environment is nearly impossible to meet in the field.
Click to expand...

You're absolutely right. I've reached out to Marathon in the past about this and was told that they used to rate their blocks for Class K but many years ago revoked it because the strands would get caught in the screws and give false torque which led to overheating. They advised that many of their customers opt to use a ferrule with an appropriate crimp, but of course they can't say that it is then acceptable.

The hardest part about panels with such large current ratings always seems to be finding the room to fit all of your components (with the appropriate clearances), wire and such while fitting everything in a box that meets the customer demands. Sometimes we can talk them into a bigger enclosure, but other times they'll go to another manufacturer who will do whatever they want.
 
There are distribution blocks out there that have terminals rated for Class K stranding. Most of them are IEC style, not the big bulky ones like Marathon, and like I said, you will need fuses ahead of them to get a decent SCCR.
 
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kwired said:
Connecting a "smaller" conductor directly to a feed thru lug or even pretty much directly to an OCPD can seem to confuse a few, it does not have to be a distribution block or polaris tap. Still comes down to overcurrent device above the ampacity of the conductor, with exception of some situations where higher than usual setting is permitted - like motor circuits those still have a second protective device for overload protection though.
Click to expand...
Yes. Motors have overload protection. I haven't done all the research myself, but I am guessing the the reason for the shortened length on taps is so that they draw enough current to trip the OCPD ahead of them if there is a fault. After all, #12 cu conductor will hold 5300 amps of fault current. But, not for very long. :)
 
xformer said:
Yes. Motors have overload protection. I haven't done all the research myself, but I am guessing the the reason for the shortened length on taps is so that they draw enough current to trip the OCPD ahead of them if there is a fault. After all, #12 cu conductor will hold 5300 amps of fault current. But, not for very long. :)
Click to expand...
No, it's more like the fact that INSIDE of the same enclosure, how would you get a short or overload on that wire? If the overload is on the end device, there is going to be a Branch OCPD protecting it that will trip. If it is a motor circuit, it will be the OL relay for an over current event and the OCPD for a short circuit. So the only risk is that the short piece of wire inside of the panel from the tap point to that branch device is going to go to ground or another wire, and they consider that a very low risk scenario.
 
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