Proper wire Ampacity

Proper wire Ampacity, and choosing the correct wire size for a particular installation seems to be an area many of us get rusty on. calculating correct wire Ampacity is a critical step in order to protect and provide for proper branch circuit loads and feeder requirements.
Table 310.16 is too confusing! I hear this a lot. As an electrical instructor for Texas Electrician Exams, I teach many journeymen, masters, even city inspectors hot to study for, and successfully pass their Texas electrical PSI provided test. It is my opinion that wire sizing for correct amperage is the single largest areas of confusion. to be fair, the 2008 National Electrical Code added an additional step to the Ampacity de-rating requirements. I have even seen recent nationally publishes articles that have incorrectly applied these new de-rating factors. So don’t feel bad if you have some confusion, you’ve got lost of company!
There is some hope with the new edition of the 2011 NEC in it’s proposed format. A new layout for T310.16 is probably going to happen, and hopefully this new layout will make it an easier table to utilize.
I always instruct my students to mark step I through Step IV in their code books. Step I is the top chart on page 147. We begin (always) with what I like to call our “core” Ampacity. For example, a 350 THHN cu conductor under the 75 degree column is 310 Amperes. This is the core value that we will perform all subsequent steps on.
The biggest question I get, by far, is “What is the 90 degree column for, and how/when can I use it?” Well, it’s simple really, you must have three (3) separate items (rules) in order to utilize that column as your beginning core value AND your final calculation must answer TRUE to one additional item. The 3 caveats are:

Rule I. There must be de-rating factors to consider.
Rule II. The insulation of the conductor must be rated for the 90 degree column (and not limited by any specific code article to a lower temperature).
Rule III. The installation must be a dry or damp location for the conductors.

True or false: The final adjusted must NOT be larger than that Ampacity value under the 75 column.

Under Rule I, we must have an ambient temperature adjustment, roof-top temperature adjustment, or a number of current carrying conductor adjustment. If none of those apply then the 90 degree is off limit. Any one of these adjustments factors alone or together with others, would be enough to allow for the 90 degree as a starting point.
Under Rule II, this initially seems to be a given, however, a closer look at certain insulation ratings in their various respective sections in Chapter 3, and T310.13, restrict certain type and certain installations to a lower temperature rating. (Examples: 336.26 (NMS) restricts the cable assembly to the 60 degree column- even though it requires the individual conductors to be rated at 90 degrees; Types XHHW, XHH, THHW, and RHH(among others) are all limited by T310.13 to 75 degrees column when installed in a WET location.)
Under Rule III, we acknowledge the previous examples and use caution when installing in a wet location. Most 90 degrees rated insulation types are restricted to 75 degrees in wet locations. Further, new code rules for 2008 classify ALL interiors of raceways, where directly exposed to weather or buried below grade, are classified as wet locations.
Finally, our calculated core Ampacity-after adjustments for any applicable de-rating factors-must not exceed the value for the same size wire under the 75 degree column. So to answer the 90 degree column question: hardly ever used.
Back to our 350 kcmil cu conductor Ampacity calculation. Step I we found the core Ampacity to be 310A under the 75 degree column (let’s assume a roof top installation: therefore a “wet location”). Next we examine our two ambient temperature steps. Under the main amperage chart on page 147, we see C degrees and F degrees de-rating factors. Notice 26-30 degree C and 78-86 degree F are equal to 1.00 or (100%). This is because the table is based on 86 degrees F initially. Temperatures greater (hotter) than 86 degrees F or (30 degrees C) must be de-rated. (Heat causes resistance-resistance in turn causes more heat, with a potential of a thermal run-away effect). These values are multipliers. A 106 degrees F installation would have a factor of .82 (or 82%). Thus our 310 Amps would be de-rated at 310AX.82=254.2 Amps. Flipping back one page (146) we see the new rooftop temperature adder table. If exposed to sun-light on a roof top, we have to increase our ambient temperatures PRIOR to choosing our multiplier.
The table in listed by heights (in inches) above the roof top surface. (The closer to the surface- greater convection of heat transfer occurs due to the reflected heat). At 3” above we would increase our 106 degree F by an amount of 30 degree F. Therefore we actually have a 136 degree F installation. That ambient temperature has a factor of .58 (or 58%). Thus our 310A core Ampacity is=310AX.58=179.8A (or 180 Amps) A very significant Ampacity de-rating!
Finally we must count our current carrying conductors under T310.15(b)(2)(a) for more than 3 current carrying conductors. Always keep in mind that neutrals can sometimes be considered a C.C.C. and might push a 3 phase service into this de-rating table.
That’s it. Pretty simple-yes? If anyone has further questions, comments, suggestions, or wants more practice, contact us via email and we will be glad to help.

Thanks for Reading,

Mitchell S Tolbert
1-888-473-1826
Contact@ElectricianTesting.com
www.ElectricianTesting.com