Multiple OCPDs on tap conductor

[kolyur]

Hello all,

This is my first post but this forum has been very helpful over the years so let me start by saying "thank you" to everyone here.

We are having some internal debate over the NEC compliance of a portion of the panel wiring shown in the photo. Incoming 208V 3ph from fused disconnect goes to power distribution blocks. Tap conductors from distribution blocks feed banks of (4) 3-pole fuses which are ganged with comb busbars.

The question is whether it is OK for one set of tap conductors to feed multiple fuse blocks. This could be considered a "tap from a tap" situation if you think of the comb busbar as an additional tap conductor (although it doesn't fit the NEC definition--I'd call it a manufacturer accessory).

Mike Holt's EC&M article on feeder taps says you can't tap a tap:

You can't tap a tap; that is, don't use a tapped conductor to supply another conductor. This rule isn't explicitly stated, but: You can infer this rule from the first paragraph of 240.21(B).

I'm not sure that I agree about the rule being inferred, at least not in the first paragraph of 240.21(B). What about in the subsequent rules given for specific tap lengths? Well, 240.21(B)(2) for taps under 25 ft requires that "the tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors." OK, that's pretty clear cut that multiple OCPDs aren't allowed. But if that was meant to be a universal restriction, wouldn't it be repeated under the section for taps 10 ft or less, 240.21(B)(1), or located outside of the length-specific sections altogether?

I am leaning towards thinking that the current setup is compliant, provided that the tap conductors are under 10 feet and are sized properly for the loads. (In this case they are motor feeders so ampacity is 125% of largest FLC plus the sum of the other three FLC's per 430.24.)

I'm interested in hearing other opinions on this. What have I missed?

 

[GoldDigger]

I think the wording that governs is the provision that the tap conductor (the conductor not protected at its allowed ampacity at the upstream end) must terminate on a single OCPD. The rest is not relevant.

 

[EC Dan]

I think the "can't tap a tap rule" is fairly explicitly stated in the wording of the first paragraph of 240.21:

Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device...

Your wires coming into the bus bar have been supplied under 240.21(A), therefore they can't supply other conductors (the bus bar). Should be easy enough to add an OCPD to those wires coming out of the PDB to make it compliant.

 

[kolyur]

I think the "can't tap a tap rule" is fairly explicitly stated in the wording of the first paragraph of 240.21:

Yikes, you are exactly right. Somehow I had skipped over that section and went right to Feeder Taps in (B). Should have known better than to question Mr. Holt!

 

[petersonra]

those bus bars are listed by UL as being suitable for tap conductor protection.

 

[GoldDigger]

those bus bars are listed by UL as being suitable for tap conductor protection.

Which means exactly what, since the busbars are not OCPD?
I imagine there are cases where a UL listed device is still prohibited by NEC.
Must the sum of the OCODs on the bus be less than the tap ampacity?

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