Tap Ampacity

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augie47

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I working with another thread I was reviewing the "tap rules" and something came to mind that I had not previously considered.
When the tap rule limits the ampacity of the tap such as the "1/3" rule of 240.21(B)(2), which ampacity are you allowed to use ?
In other words, if I had a 50 amp 20 ft tap on a 200 amp feeder could i use #6 with a 90? ampacity of 75 amps, greater than 1/3 of the OCP, or would I have to use a #4 with it's 75? rating ?
 

infinity

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I working with another thread I was reviewing the "tap rules" and something came to mind that I had not previously considered.
When the tap rule limits the ampacity of the tap such as the "1/3" rule of 240.21(B)(2), which ampacity are you allowed to use ?
In other words, if I had a 50 amp 20 ft tap on a 200 amp feeder could i use #6 with a 90? ampacity of 75 amps, greater than 1/3 of the OCP, or would I have to use a #4 with it's 75? rating ?

I say that you would use the 75? C ampacity for 75? C terminations. If your tap conductors terminated on a 60? C termination I would use the 60? C ampacity. Not sure why I'm saying that. :blink:
 

Smart $

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Ohio
I working with another thread I was reviewing the "tap rules" and something came to mind that I had not previously considered.
When the tap rule limits the ampacity of the tap such as the "1/3" rule of 240.21(B)(2), which ampacity are you allowed to use ?
In other words, if I had a 50 amp 20 ft tap on a 200 amp feeder could i use #6 with a 90? ampacity of 75 amps, greater than 1/3 of the OCP, or would I have to use a #4 with it's 75? rating ?
240.4 says...

Protection of Conductors. Conductors, other than
flexible cords, flexible cables, and fixture wires, shall be
protected against overcurrent in accordance with their ampacities
specified in 310.15, unless otherwise permitted or
required in 240.4(A) through (G).

240.4(E) is what allows conductor protection per 240.21. So 110.14(C) does not apply to the ampacity determination, but 310.15 derating does.
 

Smart $

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Ohio
... So 110.14(C) does not apply to the ampacity determination, but 310.15 derating does.
Let me clarify that...

The ampacity of the tap conductors, as determined by 310.15 requirements, must be equal or greater than 1/3 of the feeder ocpd rating.

110.14(C) will apply to the terminations, but have no bearing on the "1/3" determination.
 

infinity

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Let me clarify that...

The ampacity of the tap conductors, as determined by 310.15 requirements, must be equal or greater than 1/3 of the feeder ocpd rating.

110.14(C) will apply to the terminations, but have no bearing on the "1/3" determination.

So a 90? C conductor with 75? C terminations will allow you to use the 90? C ampacity of the tap condcutors?
 

Smart $

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So a 90? C conductor with 75? C terminations will allow you to use the 90? C ampacity of the tap condcutors?
For the 1/3 or better determination, yes (provided there are no 310.15 adjustments to the tabled values). As for the allowable ampacity of the conductor, termination temperature limitations may or may not have a role in what size is actually used.

EDIT to add: Take OP scenario for example. The minimum ampacity of the tap conductors must be 66.67A under 310.15... but it is only a 50A tap circuit. You can use #6 90?C wire. At 50A, the termination temperature limitation has no effect.
 
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infinity

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For the 1/3 or better determination, yes (provided there are no 310.15 adjustments to the tabled values). As for the allowable ampacity of the conductor, termination temperature limitations may or may not have a role in what size is actually used.

Interesting, the way that section is worded I would agree.
 

charlie b

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I am going to disagree with Smart $ on this one, though I concede that my reasoning might not be as watertight as I could wish. The tap rule requires an ampacity not less than one third the upstream OCPD. I look at the definition of “ampacity,” and I see the phrase “under the conditions of use.” The rating of the terminations is a factor in the “conditions of use” under which we are installing the tap. So I would not agree that 110.14(C) is not a player. To be specific, if the terminations are rated for 75C, then the ampacity of #6 THHN is 65 amps, not 75, and that is less than one third the rating of the OCPD.
 
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Smart $

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I am going to disagree with Smart $ on this one, though I concede that my reasoning might not be as watertight as I could wish. The tap rule requires an ampacity not less than one third the upstream OCPD. I look at the definition of “ampacity,” and I see the phrase “under the conditions of use.” The rating of the terminations is a factor in the “conditions of use” under which we are installing the tap. So I would not agree that 110.14(C) is not a player. To be specific, if the terminations are rated for 75C, then the ampacity of #6 THHN is 65 amps, not 75, and that is less than one third the rating of the OCPD.
I definitely see how it is easy to arrive at that conclusion. However, in actuality, 110.14(C) does not actually change the ampacity of a conductor. It rather limits "The temperature rating associated with the ampacity of a conductor..." A good way to look at it is "conductor ampacity" vs. "circuit ampacity". 110.14(C) only affects the latter. Conductor ampacity can be greater than circuit ampacity, but circuit ampacity can never be greater than conductor ampacity.
 

charlie b

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I definitely see how it is easy to arrive at that conclusion.
Thanks. I'll stick with that conclusion.

However, in actuality, 110.14(C) does not actually change the ampacity of a conductor.
It does change the ampacity value that we are allowed to assign to a conductor.

It rather limits "The temperature rating associated with the ampacity of a conductor..."
And if the terminations are rated for 75C, then we don?t get to use the temperature rating of 90C that a THHN would otherwise have, when we use the table to look up the ampacity of a conductor. That brings me back to the ampacity of a #6 THHN is 65 amps. The fact that there is a column that might have assigned an ampacity of 75 is not relevant to the installation.

A good way to look at it is "conductor ampacity" vs. "circuit ampacity". 110.14(C) only affects the latter.
The NEC only defines ?ampacity? in connection with a conductor, not a circuit. So making a distinction between the two does not clarify the situation.

 

elohr46

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I am going to disagree with Smart $ on this one, though I concede that my reasoning might not be as watertight as I could wish. The tap rule requires an ampacity not less than one third the upstream OCPD. I look at the definition of ?ampacity,? and I see the phrase ?under the conditions of use.? The rating of the terminations is a factor in the ?conditions of use? under which we are installing the tap. So I would not agree that 110.14(C) is not a player. To be specific, if the terminations are rated for 75C, then the ampacity of #6 THHN is 65 amps, not 75, and that is less than one third the rating of the OCPD.

I agree here, using the "weakest link method" the conductor to be tapped is most times connected to a 75 deg termination, that is your starting point.
 

petersonra

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I think the ampacity of the conductor comes from the table that lists the ampacity of various types of conductors. The ampacity of the conductor can be adjusted downwards for various reasons, but no where in the tap rules does it say that the ampacity mentioned in the tap rules is the adjusted ampacity.

I vote for 90 deg C column for THHN.
 

Smart $

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Ohio
... but no where in the tap rules does it say that the ampacity mentioned in the tap rules is the adjusted ampacity.

...
That is correct. However, precursory Sections mandate adjustments and corrections (see post #4 in this thread) per 310.15... but where does it actually say the ampacity of a conductor is "adjusted" per 110.14, anywhere???

110.14 says there is (or may be) a temperature limitation resulting from terminations. It in essence says we have to use a certain size conductor to prevent the termination temperature from exceeding its limit...

...but nowhere does it say the ampacity of the conductor itself is different because of this limitation. This limitation is being construed as a change in conductor ampacity.
 
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Smart $

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... A good way to look at it is "conductor ampacity" vs. "circuit ampacity". 110.14(C) only affects the latter. ...
... The NEC only defines “ampacity” in connection with a conductor, not a circuit. So making a distinction between the two does not clarify the situation.
Okay...

...then let me restate it as:
A good way to look at it is "conductor ampacity" vs. "circuit current rating". 110.14(C) only affects the latter.

Also note the only mention of 110.14(C) in 310.15 is in an FPN.
 

charlie b

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110.14 says there is (or may be) a temperature limitation resulting from terminations.
I think this is the essence of our disagreement, the thing that we are seeing differently. I submit that 110.14(C) is not giving us any temperature limit, but rather a limit on the temperature rating that we are permitted to use, when determining ampacity. Table 310.16 has three temperature columns, and for any given conductor there is a temperature rating associated with an ampacity value. What 110.14(C) is telling us is that we have to select a temperature rating for the conductor that does not exceed the temperature rating of the terminations. When we do that, the higher temperature ratings become irrelevant.

It in essence says we have to use a certain size conductor to prevent the termination temperature from exceeding its limit…
I agree that the purpose of this requirement is to prevent overheating the terminations. But the requirement itself does not directly speak to the size of the conductor. Rather, it speaks to the manner of determining the ampacity of any given conductor you happen to be considering using for the application.

...but nowhere does it say the ampacity of the conductor itself is different because of this limitation.
I am not saying that either. What I am saying is that we don’t get to learn what the ampacity of the conductor is, until we look in the table, and when we do look in the table, we have to disregard the 90C column.
 

infinity

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I only see these words in the tap rule:

(1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.

I'm not seeing where the termination temperature is mentioned here. :?
 

rbalex

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We have discussed this before. A termination or connection is simply one of the ?conditions of use? that must be considered when determining a given conductor?s overall ampacity. See the Art 100 definition of ampacity and 310.15(A)(2) and it?s FPN.

For a lengthy discussion, see the UL White Book, Category Code AALZ. Check its various Sections on Terminations. If you also carefully track most of the other Category Codes, you find virtually all of them will ultimately reference their way back to AALZ. So this makes termination temperature considerations a 110.3(B) issue as well.
 

infinity

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We have discussed this before. A termination or connection is simply one of the ?conditions of use? that must be considered when determining a given conductor?s overall ampacity. See the Art 100 definition of ampacity and 310.15(A)(2) and it?s FPN.

This is sort of like when asked the question what's the ampacity of a #12 THHN conductor. If I look in 310.16 (2008) it says 30 amps, but 30 amps isn't really usable because of the termination limitations. That does not change the fact that the conductor has an ampacity is 30 amps based on the definition of ampacity.

Ampacity. The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
The tap rule says: " (1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors", IMO that makes the temperature rating of the termination is irrelevant.
 

rbalex

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Actually Table 310.16 is a very specific set of ?condition of use? for ??Insulated Conductors Rated 0 Through 2000Volts, 60?C Through 90?C (140?F Through 194?F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30?C (86?F).? Section 310.15 is a set of rules for appropriately adjusting the Table. As 310.15(A)(2) FPN mentions, 110.14(C) is part of the set.

Side note: When I first Proposed what is essentially the FPN, the CMP rejected it with a statement that was essentially, "Well that's just obvious."
 
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