Tap Rule

[augie47]

In another thread ( https://forums.mikeholt.com/threads/splicing-tap.2569512/#post-2775506 ) there was a tap which appeared to be a #10 off what is assumed to be a 400 amp Over-current device.
Question: Would this meet the tap rule ? I applying the 10% rule are we allowed to use the 90° rating of the conductor. The downstream OCP device might limit the actual current to 20 or 30 amps but in order to meet the 10% restriction the 90° rating would need to be employeed,

 

[infinity]

IMO you cannot use the 90° C rating of a tap conductor because once it hits a 75° C termination it's ampacity is no longer that of the 90° rating.

#2 and #3 in that drawing are service conductors so no tap rules apply.

 

[augie47]

#2 and #3 in that drawing are service conductors so no tap rules apply.

I saw that later but was not clear originally... I started the thread to remove confusion as my question was somewhat generic.

 

[wwhitney]

IMO you cannot use the 90° C rating of a tap conductor because once it hits a 75° C termination it's ampacity is no longer that of the 90° rating.

I don't think the termination requirements change the ampacity of the conductor, it's just another limit that applies to the overall circuit.

Cheers, Wayne

 

[infinity]

I don't think the termination requirements change the ampacity of the conductor, it's just another limit that applies to the overall circuit.

Cheers, Wayne

I should have used the words usable ampacity. End result is the same but I agree it's still a 90 degree conductor.

 

[Dsg319]

In another thread ( https://forums.mikeholt.com/threads/splicing-tap.2569512/#post-2775506 ) there was a tap which appeared to be a #10 off what is assumed to be a 400 amp Over-current device.
Question: Would this meet the tap rule ? I applying the 10% rule are we allowed to use the 90° rating of the conductor. The downstream OCP device might limit the actual current to 20 or 30 amps but in order to meet the 10% restriction the 90° rating would need to be employeed,

Just from memory I think it’s pretty much free game if the tap conductors are outdoors.

Not sure if this is the case or not.

 

[bwat]

I'm confused where this landed.

I should have used the words usable ampacity. End result is the same but I agree it's still a 90 degree conductor.

So are you saying it's the usable ampacity that is referenced here in 240.21(B)(1)(4)? (my bold)

For field installations, if the tap conductors leave the
enclosure or vault in which the tap is made, the ampacity
of the tap conductors is not less than one-tenth of the
rating of the overcurrent device protecting the feeder
conductors.

I would think that this has significant impacts and makes the end result change in a lot of situations, so I'm confused by your saying that the end result is the same.

 

[wwhitney]

I should have used the words usable ampacity. End result is the same but I agree it's still a 90 degree conductor.

Right, so as bwat pointed out, since 240.21(B)(1)(4) refers to "ampacity of the tap conductors," not "usable ampacity" or "circuit ampacity" or something else, the example in the OP is fine.

Cheers, Wayne

 

[infinity]

I agree with Mike:

 

[wwhitney]

I agree with Mike:

The drawing doesn't actually say that the conductors shown have 90C insulation. So it sidesteps the question in the OP.

Cheers, Wayne

 

[infinity]

The drawing doesn't actually say that the conductors shown have 90C insulation. So it sidesteps the question in the OP.

Cheers, Wayne

It says "equipment rated 75 degrees C" which was my original point. Even if the conductor were 90 degrees C like THHN that is irrelevant.

 

[don_resqcapt19]

Right, so as bwat pointed out, since 240.21(B)(1)(4) refers to "ampacity of the tap conductors," not "usable ampacity" or "circuit ampacity" or something else, the example in the OP is fine.

Cheers, Wayne

"Ampacity" is a defined term and is the amount of current the conductor is permitted to carry under the conditions of use. That leaves the question of "is the termination temperature limit, a condition of use?"

 

[wwhitney]

It says "equipment rated 75 degrees C" which was my original point. Even if the conductor were 90 degrees C like THHN that is irrelevant.

The second sentence is not part of the graphic, and I don't see the graphic as taking that position. It would need to show a #10 Cu with 90C insulation going to the 30A disconnect, along with text saying that is a violation, in order to take that position.

Cheers, Wayne

 

[wwhitney]

"Ampacity" is a defined term and is the amount of current the conductor is permitted to carry under the conditions of use. That leaves the question of "is the termination temperature limit, a condition of use?"

Excellent question. "Conditions of use" is not defined anywhere in the NEC, and the phrase is only used 20 times in the 2017 NEC, not many of which are directly in regards to ampacity. But if we look at Annex D Example 3a, the section "Ungrounded Feeder Conductors" starts off:

"The conductors must independently meet requirements for (1) terminations, and (2) conditions of use throughout the raceway run."

So that sentence certainly sees termination requirements as independent of the "conditions of use." Accordingly, the ampacity of 90C #10 Cu, where no ampacity correction or adjustment is required, is 40A, regardless of the termination temperature limits.

Cheers, Wayne

 

[don_resqcapt19]

Excellent question. "Conditions of use" is not defined anywhere in the NEC, and the phrase is only used 20 times in the 2017 NEC, not many of which are directly in regards to ampacity. ...

Cheers, Wayne

The conditions of use for conductor ampacities are shown at the top of each of the ampacity tables in the 2017 and older codes and in the sections that introduce the ampacity tables in the 2020 code.

 

[wwhitney]

The conditions of use for conductor ampacities are shown at the top of each of the ampacity tables in the 2017 and older codes and in the sections that introduce the ampacity tables in the 2020 code.

Ah, thank you. In the 2017, I didn't see the phrase "conditions of use" associated with the tables. But in 2020, Table 310.16 Note 3 says "Section 310.16 shall be referenced for conditions of use." And Section 310.16 specifies the conductor voltage rating (0 to 2000V), the conductor operating temperature rating (60C, 75C, or 90C), the ambient temperature (30C), and the maximum number of CCCs (3).

So those are the conditions of use; termination limitations are not part of the conditions of use, and therefore do not affect conductor ampacity.

Cheers, Wayne

 

[david luchini]

So those are the conditions of use; termination limitations are not part of the conditions of use, and therefore do not affect conductor ampacity.

See 110.14(C)

 

[bwat]

See 110.14(C)

I don't see anything in there that says the temperature limitations for termination provisions impact the conductor's ampacity.

Also, I think it would be useful to point out that the definition of ampacity mentions the conditions of use without exceeding the conductor's ampacity. (my underline)

Ampacity. The maximum current, in amperes, that a conductor
can carry continuously under the conditions of use without
exceeding its temperature rating.

 

[infinity]

See 110.14(C)

I believe that is the basis for Mike's graphic on post #9.

 

[wwhitney]

See 110.14(C)

Yes, the first sentence of (2017) 110.14(C), "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device" does make it sound like landing a 90C conductor on a 75C terminations reduces the conductor ampacity.

However, the language in 2020 section/table 310.16, as well as in Annex D Example 3A, make it sound like the conductor ampacity is not reduced, and the termination limits imposed by 110.14(C) are separate (maybe a termination ampacity).

So the two conflicting points of view appear to be supported by separate sections of the NEC. I lean towards the latter point of view.

Cheers, Wayne