- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
Gus, you done it again!:lol:
But who won?:?
But who won?:?
Tap Ampacity
- Thread starter augie47
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Not so obvious as black and white, and the FPN just makes it more a medium shade of gray :roll:
And look where the FPN is, right after an exception which technically allows us to use the 90?C ampacity, because the termination is defiinitely less than 10 ft and most likely less than 10% of the circuit length at the higher ampacity. So if 110.14(C) is actually a conductor ampacity adjustment, then we can legitimately use 310.15(A)(2) Exception to get around it. So which is it???
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
I Happen to agree with you; "obvious" was essentially the CMP's statement. I proposed a more definitive statement. The FPN came a few cycles later. FPNs aren't enforceable anyway.
The laws of men are subject to change;the laws of physics aren't. Basically, it would depend on the termination. Listed termination devices have temperature limitations. Those limitations are imposed by the size of the conductor they are connected to and not the conductor's temperature rating.
Temperature Limitations 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. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Note the restriction is at the termination not for the rest of the conductor's other "conditions of use."
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elohr46
Senior Member
- Location
- square one
But if you would be terminating that 90 deg conductor into a 75 deg rated lug the ampacity would be limited to the 75 deg column.
For others, exactly what are we doing here under 240.21(B)(2)? Are we not doing an ampacity adjustment where "The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors."?
110.14(C) must be "coordinated" into the minimum conductor size that will actually be used. Consider that the tap connection may likely be made using a 90?C or better rated method. So the only place 75?C termination temperature limitation comes into play for the tap conductor is at the load end. If 110.14(C) is an ampacity adjustment, then invoke 310.15(A)(2) Exception. If 110.14(C) is not an ampacity adjustment, then as long as the ocpd is not rated greater than the 75?C ampacity for the wire size used, the tap conductors meet the requirement. Lacking clarity in the NEC on the issue, install to the safer latter method.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
We are to determine the ampacity of the conductor at the tap based on the termination's temperature rating. At the tap, only the conductor's size is relevant, not its temperature rating - unless it is lower than the termination's.
That's not too likely at all.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
I need to back peddle. If the termination has a temperature rating equal or greater than the conductor's, it has no effective difference.You can stress the point however you want to no avail. I would agree with you totally if the wording explicitly supported your perspective. However, current NEC wording does not. I can see nowhere in the NEC that states 110.14(C) stipulations actually change the ampacity of an insulated wire conductor. In the meantime, I have no problem with operating under the assumed intent. But as long as the wording stays the same, I will continue to argue the point.
So termination temperature rating at the tap equal or better than conductor temperature rating means, from the perspective of conductor protection, the full ampacity value (e.g. 90?C column value adjusted per 310.15) can be used for the 1/3 ampacity determination, right?
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
NEC wording in both 110.14(C) and by reference via 110.3(B) require it. (I already cited the UL reference.) SO- If you want to say 310.15 doesn't say it explicitly you are right. If you want to say the NEC doesn't require it - you are wrong; it just doesn't require it in 310.15. {That was what I tried to get the CMP to see - eventually they added the FPN to "remind" folks about 110.14(C)}
That being said, see my next response
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
YES
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I'll second Bob's "YES," but I will point out that this situation would require 90C rated terminations. To be explicit, if the termination temperature rating at the tap (i.e., 90C) is equal to or better than (which it would be) the conductor temperature rating (THHN, rated for 90C), then by all means, use the 90C column for the 1/3 ampacity determination.
Doesn't work that way for 75C terminations. See my next post.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
They don't. That is the point that I think you are missing. They don't change the ampacity, they provide a rule that is relevant to the process through which we discover what the ampacity is. As Bob said recently, and as I said earlier, the temperature rating of the terminations is one of the "conditions of use" that comes into play, when determining how many amps a conductor can handle, without exceeding its temperature rating. Other conditions of use include the number of current carrying conductors within a raceway, the ambient temperature, and (for underground ductbanks) the burial depth and soil conditions. You don't know what a conductor's ampacity is, until you take all the conditions of use into consideration.
Often times the actual load and OCP would be less than the 1/3 rating. In those situations there should be no problem using the 90? ampacity to meet the 1/3 rule with the OCP at the termination being within the 75? rating, correct ?
( for example you have a 225 amp feeder and you tap with a #6 THHN based on the 90? rating of 75? and your OCP at the end of the tap is rated 60 amps)
( for example you have a 225 amp feeder and you tap with a #6 THHN based on the 90? rating of 75? and your OCP at the end of the tap is rated 60 amps)
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Not correct. What you have is a 225 amp feeder that you tap with a #6 THHN based on the 75? rating of 65 amps, and since your OCP at the end of the tap is rated 60 amps the conductor is adequately protected.
Volta
Senior Member
- Location
- Columbus, Ohio
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Amen Brother
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.
elohr46
Senior Member
- Location
- square one
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.
Volta
Senior Member
- Location
- Columbus, Ohio
I guess my question is whether the acceptable tap rule fractions / lengths / ampacities are in part coordinated with the equipment's and conductor's short-circuit withstand time in mind.
I assume that they are, and if so, it may be that a 25' conductor that has less than the ampacity of 1/3 the OCPD could be expected to have the insulation compromised before the OCPD opens.
If the same diameter conductor is rated at a higher temperature / ampacity (including termination provisions), might it then be considered able to retain its integrity after such a fault?
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