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

Wire Ampacity and Conduit Fill Calculation

Wire Ampacity and Conduit Fill Calculation

This week I want to take a minute to review our wire ampacity and conduit fill (wire fill) calculation.

There seems to be a lot of common confusion out there when it comes to wire fill. Annex "C" in the back of the 2208 NEC © is a good tool, but it can be very dangerous to use it without considering the impact of de-rating factors found in Chapter 3.

We normally use a 20 amp circuit breaker to protect #12 AWG cu wire. Annex C tables show that we can safely place a total of 16 of this size (Type THHN, THWN, or THWN-2) in a 3/4" EMT conduit. this is only for the protection of the wire from a physical stand point. Annex C is only concerned with the protection of the wire during installation. It does NOT take the ampacity of the wire into consideration!

When de-rating wire, we begin with table 310.16 (page 147, 2008 NEC ©) and de-rate for our ambient temperature AS WELL AS the number of current carrying conductors. The de-rating table T310.15(b)(2)(a) on page 145 shows us:

"Adjustment Factors for more than Three Current carrying conductors in a Raceway or Cable"

Number of C.C.C.	% of 310.16 Values
4-6 Conductors	80%
7-9 Conductors	70%
10-20 Conductors	50%
21-30 Conductors	45%
31-40 Conductors	40%
41+ Conductors	35%

You can quickly see that in our earlier example, if we put 16, #12 THHN conductors in that 3/4" conduit, we would have to drop the circuit breaker to a 15 ampere size! Now seriously, how many electricians in the field really do that? You understand now how dangerous the casual use of Annex "C" can be.

For all practical purposes, only 9 current carrying conductors could be placed in that conduit without having to drop the OCPD (circuit breaker) size: [T310.16 value for #12 THHW=25 Amps. 25 Amps X 70% (the de-rating for 7-9 conductors) would equal 17.5 Amps, and due to 240.3(B) we are allowed to round up to a 20A breaker.]

The grounded conductor (neutral) can also cause confusion sometimes. It counts as a "current carrying conductor" in some cases, but not in others. If it is shared between two different phases in a multi-wire circuit, it usually does not count as a current carrier , as it only carries the unbalanced load in the circuit. [For example, a "full boat with circuits #3, #5, #7 in a 30 panel, would not usually have their neutral counted.]

Where non-linear loads are supplied however the neutrals DO count. If you have electronic fluorescent ballasts or heavy computer loads (etc...) on a shared branch circuit neutral, it will have harmonic distortions that will disrupt the cancellation of loads between phases and can actually cause larger loads on a neutral that are found on any single ungrounded phase conductor in the circuit.

If the (neutral) grounded conductor is NOT shared between phases, i.e. a single 120V single phase branch circuit, such as a general purpose receptacle outlet circuit conductor, it counts as a current carrying conductor. In this installation it carries the full load on it back to the distribution point. Two other common issues with Annex C are considerations for different sized EGC/GEC conductors, and differences between Annex C values and manual fill calculations using Chapter 9, tables 4 and 5.

The value of Annex C is a "staff it full" value. Therefore you would have to keep in mind your additional ground wire, which is usually a smaller size than that of the phase conductors. The only way to use Annex C in this case is to always consider your EGC or GEC as a full size conductor count. A manual calculation is more accurate than Annex C and it has been this author's observation that Annex C values sometimes vary quite a bit from a manually calculated fill based on Chapter 9 tables.

All of that considered, going back to our 3/4" raceway, we could put 3 "full boats" in this raceway (3 neutrals not counted, 9 phase conductors, and 1 equipment ground) for a total of 13 conductors. This results in 3 less than our "fill amount" according to our Annex C tables. Thus, our advice is to always carefully consider your ampacity de-rating factors as required by 310.15.

(Author's Note: As evidenced by the above, an electrician would have to refer to several different tables in order to correctly and safely calculate the correct fill and ampacity values for watch and every pipe run. we are excited to provide and industry first for all our members. Electrician Testing has painstakingly prepared a custom table; available only through us, that considers ALL required factors. Everything is brought together in one quick and easy to follow chart. Contact us today to get yours, and look for our upcoming publication that will include this chart along with many, many more custom time saving tables!)