Tap Ampacity

[Smart $]

... You don't know what a conductor's ampacity is, until you take all the conditions of use into consideration.

Amen Brother

I'll go along with that for the sake of amiable blink discussion...


You guys are saying 110.14(C) affects and usually modifies the allowable conductor "ampacity". As such why can we not, i.e. where does the NEC state that we cannot invoke 310.15(A)(2) Exception :?

 

[Smart $]

So, in your opinion, we can not use the 90? rating to meet the 1/3 ampacity requirement even if we are protected at the 75? rating ?
Seems it would be no different from using the 90? ampacity for derating for conduit fill. The conductor is not overloaded and the insulation is rated for the installation.

Exactly.

 

[Volta]

...


You guys are saying 110.14(C) affects and usually modifies the allowable conductor "ampacity". As such why can we not, i.e. where does the NEC state that we cannot invoke 310.15(A)(2) Exception :?

As in, that would let us use an inch of the circuit (75 deg termination) at the higher (90 deg conductor) length of wire?

Don't you think the CMP saw that one coming, and wrote the FPN for it?

 

[rbalex]

A conductor usually has two termini. In the case of the tap conductor under discussion, one is at the tap and the second is at the breaker.

Let?s assume the tap termination is suitable for 90C and the breaker termination is 75C. Let?s further assume ambient and other ?conditions of use? do not enter into the analysis.

At the tap, the "conditions of use" permit the tap conductor its 90C rating. At the breaker, the "conditions of use" limit the tap conductor to the 75C rating. Since the current can?t exceed 65A, what is its ampacity as defined by Article 100? Well IMO it?s 75A at the tap and 65A at the breaker. (BETWEEN the tap and the breaker termination the ampacity is 75A under that ?condition of use?)

Since the breaker limits the tap?s current (NOT ampacity) to 60A, it is adequately protected.

 

[Smart $]

As in, that would let us use an inch of the circuit (75 deg termination) at the higher (90 deg conductor) length of wire?

Don't you think the CMP saw that one coming, and wrote the FPN for it?

So we are to surmise that an FPN overrides an exception :blink::blink::blink:

 

[Volta]

So we are to surmise that an FPN overrides an exception :blink::blink::blink:

No, but we might that 110.14(C) does. Else, nearly every circuit temp/ampacity could be considered from the long length of wire, and we could ignore the terminations.

 

[rbalex]

...
Don't you think the CMP saw that one coming, and wrote the FPN for it?

Well, no they didn't (and, in my opinion, still don't see the problem. But that's another story.)

You and Smart$ got your posts in before I dropped my explanation above

 

[Smart $]

No, but we might that 110.14(C) does. Else, nearly every circuit temp/ampacity could be considered from the long length of wire, and we could ignore the terminations.

Well when 110.14(C) isn't a conductor ampacity limitation it works... because 110.14 only alters the circuit rating... i.e. 310.15(A)(2) Exception cannot apply to a circuit current limitation; it can only apply to adjacent conductor ampacities.

But if 110.14(C) changes conductor ampacity, it is open to application of the Exception.

Logical, eh? :roll:

 

[Volta]

Well when 110.14(C) isn't a conductor ampacity limitation it works... because 110.14 only alters the circuit rating... i.e. 310.15(A)(2) Exception cannot apply to a circuit current limitation; it can only apply to adjacent conductor ampacities.

But if 110.14(C) changes conductor ampacity, it is open to application of the Exception.

Logical, eh? :roll:

My head's spinning as fast as your avatar...

 

[infinity]

No one has mentioned how the 1/3 ampacity actually changes anything. Replace it with 1/4 and everything is still the same, meaning the tap conductor is protected by the OCPD at the end of the tap. Seems to me that the 1/3 number is somewhat arbitrary.

 

[Volta]

No one has mentioned how the 1/3 ampacity actually changes anything. Replace it with 1/4 and everything is still the same, meaning the tap conductor is protected by the OCPD at the end of the tap. Seems to me that the 1/3 number is somewhat arbitrary.

Maybe. That is kind of what I was asking in post 40.

What do you think, can it be short-circuit?

 

[elohr46]

Mike Holt explains the 25' tap rule for EC & M magazine. He uses the 75 deg column in his example.

ecmweb.com/nec/code_qa/code_qa_070711/index.html

 

[charlie b]

Did you mean not adequately protected? Less than 1/3. . .

I say this is not correct either. If you have an 225 amp OCPD you are permitted to tap by 1/3 which is 75 amps or higher. If you are terminating the tap conductor in a 75 deg lug then you will need a #4 copper conductor.

My bad. I was paying attention to another aspect of the question, and missed the fact that 225/3 = 75. :ashamed:

 

[charlie b]

. . . where does the NEC state that we cannot invoke 310.15(A)(2) Exception

Well, I can't invoke it because I never understood it. :? If a single wire has two different ampacities, one applying for part of its length, and the other applying for the remainder of its length, you still can't put more current through it than the smaller ampacity value. So what difference does it make to declare that part of the wire has a higher ampacity, and that you can use that higher ampacity for some distance beyond the transition point?

 

[rbalex]

I thought I explained how a conductor may have several ampacities in Post 44although no one seems to have paid much attention to it. If you carefully read only the text of 310.15 (B)(2) and don't let the title confuse the issue, I think it clears it up.

 

[Smart $]

Well, I can't invoke it because I never understood it. :? If a single wire has two different ampacities, one applying for part of its length, and the other applying for the remainder of its length, you still can't put more current through it than the smaller ampacity value. So what difference does it make to declare that part of the wire has a higher ampacity, and that you can use that higher ampacity for some distance beyond the transition point?

Well I can see that you don't understand it :happysad:

The exception says we can use the higher ampacity for x distance beyond the point of transition. If you have a 10" circuit (one-way length) and the middle 9" (wire) has an ampacity of 100A and half an inch on each end (termination) has an ampacity (and I'm using this term loosely) of 85A, the 85A ampacity portion can be ignored because it is only 0.5 ? 9 = 5.6%, i.e. <10%, of the higher ampacity distance. Also keep in mind the Exception uses the word "circuit" and not "wire" or "conductor".

 

[jumper]

Well I can see that you don't understand it :happysad:

The exception says we can use the higher ampacity for x distance beyond the point of transition. If you have a 10" circuit (one-way length) and the middle 9" (wire) has an ampacity of 100A and half an inch on each end (termination) has an ampacity (and I'm using this term loosely) of 85A, the 85A ampacity portion can be ignored because it is only 0.5 ? 9 = 5.6%, i.e. <10%, of the higher ampacity distance. Also keep in mind the Exception uses the word "circuit" and not "wire" or "conductor".

I can understand what you are saying, but can you give me an example of another actual install. I am asking to try and learn.

 

[Smart $]

I can understand what you are saying, but can you give me an example of another actual install. I am asking to try and learn.

Say you have a 100' circuit of #12 THHN Cu and between the supplying panel and a junction box 4' away you have 10 current-carrying-conductors, including the 2 circuit conductors of issue. Beyond this junction box, the 2 circuit conductors are run in a separate conduit by themselves. So you have 4' of wire that gets derated to 50% of 30A which is 15A. However, 96' of the circuit does not get derated, having an ampacity of 30A. Since 96' is less than 100' (i.e. 10% of 100' is 10', the max distance this Exception can apply), we calculate for the lesser which has to be not more than 10%... 4/96 = 4.2%. This means we can use the 30A ampacity for the 4' section that is otherwise derated to 15A.

 

[RUWired]

View attachment Tap Conductors 240.21(B)2.pdf


I thought it would be interesting to put together a spread sheet using the two ideas of a tap conductor rated at 90 and a tap conductor rated at 75 to see what differences there would be.
With using conductors rated for 90, there are 13 examples where the tap conductor is good for 1/3 but the OCPD is not. There are 10 examples where the same 90 degree rated wire falls short of the 1/3 requirement at the 75 rating.

When using conductors rated for 75, there are 6 examples where the tap conductor is rated for 1/3, but the ocpd is not.
In both cases the conductors are protected by the OCPD.

 

[Smart $]

Now let's get a little more technical and apply this to the termination issue...

First, we also have to account for the wire in the panel, let's say there's 2' before it enters to conduit with 10 CCC's. The termination is only but say 1/2" while the other 23-1/2" has an ampacity of 30A. The termination is obviously less than 10% of the adjacent higher ampacity length of the circuit...