Please Explain This!!!!!

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mcclary's electrical

Senior Member
Kinda of at an impasse here,somebody agree with me or convince me I'm dead wrong,maybe I'm dead and don't know it.:grin:


Dick, the way I read it is:

The temperature rating "asociated with the ampacity of a conductor"

shall be selected and coordinated as to not EXCEED (meaning to not go over) the lowest temperature rating of ANY CONNECTED TERMINATION, CONDUCTOR OR DEVICE.

So take those hightlighted parts and put them in other words,,,,the LOWEST rating you have ANYWHERE in your system,,,,,,,,whether that is a 1) wire, 2)a terminal, 3) or device,,,,,,you take that number,,,,,,,,,and you CANNOT EXCEED THAT NUMBER. I'm not sure how to make it any more clear than that.
Last edited:

charlie b

Staff member
Seattle, WA
Electrical Engineer
It is part of our job to figure out how much current a particular wire is likely to encounter, and then to select a wire that can handle that much current. A wire?s temperature rating (60C, 75C, and 90C are the common values) is based on the ability of its insulation system to tolerate that temperature, without melting or otherwise letting current leak to ground. A wire?s ampacity is the highest current that you can pass through it, under the specific conditions of its installation, without exceeding its temperature limit. That is the relationship between those two concepts.

The question then becomes, how do we determine the ampacity of a given wire, under the conditions in which we intend to install that wire? Table 310.16 has a column that applies to wires with insulation systems capable of handling a temperature rise of 90C. Given that we are assumed to start with an ambient temperature of 30C, that means the temperature of the wire could get as high as 120C, without damaging the insulation. If we keep the current below the value shown in the 90C column, then the heat generated by that current will not raise the temperature more than the 90C that the insulation is rated to handle.

But the wire?s insulation is not the only component at risk from excessive temperatures. We also have to consider the components to which the wires are connected. If we attach a 90C rated wire to a terminal block that has a 75C rating, then we are no longer allowed to use the ampacity values shown in the 90C column. That is because if we ran the 90C rated current through the wire, and through the termination point, the heat generated would not be enough to damage the wire?s insulation, but it might be enough to damage the termination. So we limit the ampacity of the wire to the value shown in the 75C column. That amount of current will be low enough such that the heat it generates will not damage the termination.


Senior Member
100708-1030 EST


I will try.

The ampacity of a cable is dependent upon the temperature rating of the insulation. Standard tables define this capacity for different temperature ratings. For the same current capacity a 90 deg wire will be smaller in diameter than a 60 deg wire. The 90 degree wire chosen from the table for a given current will run hotter than the 60 deg wire chosen for that current.

The 90 deg wire is expected to be able to operate at 90 deg with some reasonable life. The 60 deg would not have good life at 90 deg.

The weakest link in the system is going to determine the maximum allowed temperature. In the case being discussed the wire is not the weakest link. If the lug is rated at 60 deg, then the wire temperature should not exceed 60 deg.

Therefore you use the 60 deg wire table to choose wire size because this implies the wire temperature at the rated current. The wire's own insulation rating would allow 90 deg, but you do not want to operate at 90 deg because that would overheat the lug and thus you use a larger diameter wire as defined by the 60 deg table.

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..............
"lowest temperature rating " means weakest link and you do not want to exceed this weakest point. The tables are simply a cookie cutter approach to provide information based on broad imperical expeience of expected life.

Normally you would use the tables to select wire size for a given current based upon its insulation. But here the tables are indirectly used to provide information to meet a temperature limit unrelated to the insulation of the wire.



Senior Member
Yea but look at all the entertainment it provided and I'll bet a lot of people have a better understanding of 110.14 than they had yesterday.:)

Thanks guys

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