240.21(B)(2) FRAME Rating of Single Circuit Breaker or Single Set of Fuses

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Natfuelbilll

Senior Member
What must be the frame rating must the the equipment supplied by the tap conductors?

For example consider tapping a 1200A feeder with a <25 ft long tap using 400A conductors. Say the load supplied by the tap is 30 amps.

Tap conductors are not less that 1/3 rating of device protecting the tapped feeder.
Tap conductors terminate in single set of fuses that limit the load to the ampacity of the tap conductors.
Tap conductors are physically protected.

What must be the FRAME rating of the fused disconnect switch 400 amps or 30 amps?

Thanks
 

Natfuelbilll

Senior Member
And a related question. That 1200amp feeder is 4-500kcmil per phase. Does the code say I need to tap each of the paralleled conductors on EACH phase?
 

Carultch

Senior Member
Location
Massachusetts
The frame rating has no impact on any of the rules in 240.21(C), other than for it to be possible to terminate the tap conductors in the device. As an example, suppose I needed 150A of tap conductor for a 100A load, and my 100A fused disconnect has terminal capacity up to 1/0, I can do it with 1/0Cu, and the 100A disconenct's "frame size" is good enough.

In your example, it's not going to be possible to terminate 400A conductors in a device built for just 30A. So in this example, you'd likely need a 400A frame size to have the capacity to terminate the wires, since no frame rating 100A and less can handle 400A of wiring. Equipment may have extra terminal capacity for a few sizes above the minimum size, but the specifics are product-specific.

What frame rating means, is the way the manufacturer categorizes ranges of OCPD ratings, so they can standardize on components. For instance, you might build 110A, 125A, 150A, and 200A all in the same physical size, same lugs, same housing, etc, which we'd call a 200A frame. The essential difference is the calibration of the trip mechanism, or what fuses you put in a fused disconnect.
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
One obvious thing... can you find a 30A fused disconnect frame with lugs which can handle at the least... 2 sets of 3/0 copper? If you're tapping onto the conductors themselves and not the line side of the bus, you should tap each conductor at the same point, so that would mean you would need 4 sets of #3... but that would violate 2020 NEC 310.10(G)(1).

That may not answer your question entirely, but I hope it points in some kind of a direction for thought.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
I agree, you need a frame with lugs large enough to accept your conductors, likely a 400 amp frame but you could check with manufacturer to see if there is a smaller frame to accept your conductor.
Any tap would require you connect to all phase conductors,
(A "polaris" connector is commonly used)
 

Carultch

Senior Member
Location
Massachusetts
And a related question. That 1200amp feeder is 4-500kcmil per phase. Does the code say I need to tap each of the paralleled conductors on EACH phase?

It doesn't strictly say it, but it is clear that the intent is to not do anything that would break the symmetry of parallel sets. And with electricity fundamentals, you can prove why it is a problem to only tap one set. Either you need to source your tap at a location where the conductors are already paralleled, like the otherwise-vacant terminal on a piece of equipment, or use a tap connector that has 2*F+T terminations. F is the number in parallel on the feeder and T is the number you need for the tap. Or use as many sets in parallel on the tap, so they can line-up 1-to-1 to tap each feeder conductor.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
And a related question. That 1200amp feeder is 4-500kcmil per phase. Does the code say I need to tap each of the paralleled conductors on EACH phase?
While it was always required to tap all of the conductors of the set, in the 2020 NEC language was added to make this clear in 300.3(B)(1).
 

Natfuelbilll

Senior Member
While it was always required to tap all of the conductors of the set, in the 2020 NEC language was added to make this clear in 300.3(B)(1).
A point to question: if we are tapping the cables of a paralled feeder then ampacity of each of the paralleled tap cables on each phase are added together to obtain the ampacity of the tap - right? For example, if the feeder overcurrent device was rated 1000A and consisted of 3 400kcmil per phase and using the 25' tap rule then the tap would need an ampacity of 333 amps and could be accomplished with 3 - 2 awg per phase - right?
 

david luchini

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Staff member
Location
Connecticut
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Engineer
For example, if the feeder overcurrent device was rated 1000A and consisted of 3 400kcmil per phase and using the 25' tap rule then the tap would need an ampacity of 333 amps and could be accomplished with 3 - 2 awg per phase - right?
Not quite right, as you can't parallel conductors smaller than 1/0. But you are correct, would need a tap conductor with an ampacity of 333A (such as a single 400kcmil.) A multi-port tap block would likely be your best bet.
 
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Natfuelbilll

Senior Member
My installation is a 1200A overcurrent with 4-1/C 500kcmil per phase. Using the 25 ft tap I need a tap conductors to have 400A ampacity and can use 2-1/C 4/0 awg per phase. The conduit will contain these 6 conductors. Do these conductors need to be derated to 80% per Table 310.15(B)(3)(a)? What gives my brain a squish is that the tap conductors terminate at 30A fuses.

edited: So the 310.15(B)(3) addresses this and I think then the taps need to be sized at 250 kcmil....
 
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infinity

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Location
New Jersey
Occupation
Journeyman Electrician
Is there a physical limitation that would prohibit you from using the 10' tap rule? At 10% you'll only need a 120 amp tap conductor.
 
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