Temp selection of wire

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cloudibew

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Have a question pertaining to the temp column in the conductor amp rating tables. Now i know that thhn falls under the 90c column and have seen several references to using that temp. However i have always been told that you have to base it on the 75c column because thats the temp rating on most lugs. Now is this just having a complicated set of rules simplified to be safer than code for men in the field?

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cloudibew said:
Have a question pertaining to the temp column in the conductor amp rating tables. Now i know that thhn falls under the 90c column and have seen several references to using that temp. However i have always been told that you have to base it on the 75c column because thats the temp rating on most lugs. Now is this just having a complicated set of rules simplified to be safer than code for men in the field?

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The key is the temperature rating of the terminal to which is to be terminated on. Breakers for example are either 60/75degC or 75degC.
The advantage of the 90deC tasted wire is the additional temperature latitude that you have to work with when required to derate the wire such as for ambient temperasture, etc as well as light fixtures that require 90degC wire to be used.
How often have you been requested to replace a fixture where 60degC rated wire had been used and when you attempt to remove the connections from the old fixture the insulation on the wire crumbles away.
It is to be noted that not only is copper an excellent conductor f electricity but heat also. Hearing from a light fixture often extends along the conducted which feed the fixture outside the box.
 
cloudibew said:
Now is this just having a complicated set of rules simplified to be safer than code for men in the field?
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I'd say it's about the limitations created by common materials used together in the field.

The common lugs are NRTL tested to 75 degrees C, but not beyond. If you terminate a conductor that has been evaluated and approved to get hotter, then, if the conductor indeed gets hotter, it is heating the lugs beyond their 75 degree limit.

It's about saying the conductor is not isolated. The heat of the conductor is in the presence of the things around it, and connected to it. The heat moves.

When a group of things are connected together, and they have different heat tolerating abilities, you are instructed to pay attention to the most sensitive thing, and not overheat it.
 
cloudibew said:
Have a question pertaining to the temp column in the conductor amp rating tables. Now i know that thhn falls under the 90c column and have seen several references to using that temp. However i have always been told that you have to base it on the 75c column because thats the temp rating on most lugs. Now is this just having a complicated set of rules simplified to be safer than code for men in the field?

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I agree that this is kind of confusing. But you have to remember that your components work together as a system, and you have to make sure you do not overheat the weakest link. Terminations on both sides of the wire, and then the wire itself, all in series, all subject to the same amperes, and all must be able to carry it safely.

In most practical situations, equipment is rated for 75C terminations. And this is true, even when the lugs are marked for AL9CU, or something similar that might indicate a 90C lug rating. The terminals on manufactured product are tested as a system with that manufactured product. And most of the time, manufactured product is only tested to allow for the 75C termination ratings. It is very rare that you have a situation where you can take advantage of a 90C lug rating.

For 100A and less, the official rule is that terminations are by default rated for 60C. This is mostly academic today, as most new equipment is listed and labeled otherwise for 75C.

This does make you wonder, what is the value of 90C wire, if I can't use the 90C column of the NEC?

Well, when you read the final sentence of NEC110.14(C), you'll see the most common reason why 90C wire has an advantage. The 90C column can be used for your conditions of use, temperature corrections and bundling ampacity adjustments. As long as the final ampacity is limited to the 75C value (without adjustments/corrections) when you terminate to equipment.

As an example, consider 4 current-carrying #6 conductors in a conduit 4" above the rooftop, in a place where the ambient is 33C.

4 CCC's in a raceway = 0.8 ampacity adjustment
4" above rooftop in direct sunlight = +17C temperature adder
Add that to 33C, and get 50C
50C for 90C rated wire = 0.82 temperature correction

Total correction factor = 0.82*0.8 = 0.656
Using 90C rated #6 Cu wire: 75A * 0.656 = 49.2A

Terminations at 75C are rated for 65A
Wire at conditions of use is rated for 49.2A
The ampacity of this circuit is thus rated for the smaller of the two, which is 49.2A
 
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cloudibew said:
... Now is this just having a complicated set of rules simplified to be safer than code for men in the field?
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Yes and/or no, depending on your understanding of the rules and how you are looking at it based on that knowledge...

The reality is there are several requirements for choosing the smallest permitted circuit conductor. The first two are minimum size and minimum ampacity. Here you should especially note that a conductor's [insulation] temperature rating has no bearing on the permitted minimum SIZE. Why? Because conductor terminations are made in enclosures, boxes, etc., i.e. not under the conditions of use in raceway, cable, free air, cable tray, etc. for which the ampacities tables are developed along with ampacity adjustment and correction to those values for conditions of use beyond those stipulated in the table titles.

The preceding statement seems contradictory because we use an ampacity table to establish the minimum size. What that amounts to is just not having implement an individual table for minimum size determination. What we are establishing when we use the 60?C or 75?C column to size the conductor is the self-heating effect of current through the conductor, and making certain the maximum nominal current does not heat the conductor to a temperature above the rating of the termination.

The rest of the rules fall into proper perspective once the preceding is understood.
 
Regarding wirig ceiling mounted light fixtures, specifically the flush mounted incandencent type, are the fixures marked or tagged that the termination are to be made to 90degC wire?
 
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