You've pretty much got it. The temperature ratings in Table 310.15(b)(16) are for different types of conductor insulation. The common ratings we see are 60, 75, and 90 C. Typically, we're limited to 75 degrees or less by the termination ratings of the breaker lugs, but when you're calculating the allowable ampacity of a conductor and taking ambient temperature and number of current-carrying conductors into account, you are allowed to use the actual temperature rating of the conductor.

For example, THHN is a common insulation type rated at 90 degrees, but we're usually limited to using the 75 degree column because of the termination points on breakers or equipment. Basically the circuit ampacity can't be higher than the weakest temperature rated point. However, when correcting for ambient temperature or more than 3 current-carrying conductors in a raceway, then for THHN you can do those calcs with the 90 degree number. At the end, you take the lesser of your calculated result vs. the normal 75 degree ampacity.

Also, if you're using NM cable, typically the outer jacket is only rated for 60 degrees, so when doing calculations with NM you're limited to using the 60 degree column. If you look at Table 310.104 you'll see a big list and description of all sorts of different insulation types and their characteristics. Some even have multiple temperature ratings depending on where they're installed (THHW for example). If you hang on a minute, I'll post a "derating procedure" guide I wrote up for my apprentices.

 

Conductor Derating Procedure​

1. Look up base ampacity for insulation type in appropriate column of Table 310.15(B)(16).
2. Apply correction factor for ambient temperature (if applicable) from Table 310.15(B)(2)(a). If installed on a rooftop exposed to sunlight, factor in appropriate temperature adder per Table 310.15(B)(3)(c).
3. Apply adjustment factor for more than 3 current-carrying conductors per Table 310.15(B)(3)(a).
4. Look up the conductor ampacity in the 75°C column (or 60°C where limited by NM sheath or 60°C insulation of other conductors in the same raceway, or other limitations).
5. Use the lower of your results from Step 3 or Step 4. This is your final allowable ampacity.

Notes​

• Small conductors are limited to OCPD sizes based on 240.4(D) for general applications.
• If conductors serve a continuous load, remember that your final ampacity must be 125% of load (or, inversely, load can not exceed 80% of circuit ampacity).

 

Example: Ten (10) current-carrying #12 Copper THWN conductors are installed in PVC raceway on a rooftop. They are mounted on 2x4 blocks, which keeps the conduit 1.5" away from the rooftop. Average high summertime temperatures in the area are 100 degrees fahrenheit. What is the allowable ampacity of each conductor?

 

OKAY... here goes... only been in apprentice classes for about 5 weeks so if I totally blow it, be gentle

#12 THWN cu has an ampacity of 25A and a temperature rating of 75 C at an ambient temperature of 30 C

There are ten of these in a PVC raceway- so first I am going to look at table 310.15(B)(3)(a) for adjustment factors for more than three current carrying conductors. It shows that the ampacity of the conductors will be 50% of what's shown in table 310.15(B)(16).

50% of 25 A is 12.5A

Then, the conductors are in PVC on a rooftop where the ambient temperature can get up to 100 F, so now I need to look at the ambient temperature correction factors in Table 310.15 (B)(2)(a) and apply that value to the conductors for the 100F ambient temp in the summer. The correction factor is .88 for the conductors, which have a temperature rating of 75 C.

So.... 12.5A x .88 = 11A for each conductor.

Did I get it right???

 

White PVC or grey?:jester:

Pete

 

Whatever your mom prefers.

 

Example: Ten (10) current-carrying #12 Copper THWN conductors are installed in PVC raceway on a rooftop. They are mounted on 2x4 blocks, which keeps the conduit 1.5" away from the rooftop. Average high summertime temperatures in the area are 100 degrees fahrenheit. What is the allowable ampacity of each conductor?

Awesome. Lemme go get my code book... No one else answer!!

 

Ha ha... wrong again! Starting ampacity should be based on the OCPD sizing... so that adjusts everything down a notch. My final answer is 5.8A per conductor.

 

No, your starting ampacity will be based on the insulation temperature rating column (60, 75, or 90) out of Table 310.15(B)(16). Don't worry about OCPD until after you figure out the ampacity you can even put on a wire. Furthermore, the size of OCPD will vary depending on what types of loads are being supplied, whether or not it is a continuous load, etc.

Derate first, then size OCPD.

In most circumstances, you can't put #12 copper on anything bigger than 20 amps. However, there is a list in 240.4(G) of circumstances in which you can put #12 on breakers over 20 amps. Similarly with the other small conductors.

If you follow the Procedure thing I posted above, the final step will determine the final allowable ampacity for your wire. If your final calculated allowable ampacity of some random wire supplying a general purpose receptacle circuit is 21 amps, then you are limited to a 20 amp breaker by 240.4(D). If, however, your final calculated allowable ampacity is 19 amps, then you can't put it on a 20 amp breaker anymore.

 

It is a common misconception that conductor temperature ratings are how much the insulation can take. It is actually the temperature that a conductor will heat up to when carrying a particular current, with an ambient of 30°C.

For example, a #6 carrying 55 A will heat up to 60°C. That same #6 carrying 75 A will heat up to 90°C.

 

You are correct, but as soon as it heats up past that point the insualtion begins to bread down.

 

It is a common misconception that conductor temperature ratings are how much the insulation can take. It is actually the temperature that a conductor will heat up to when carrying a particular current, with an ambient of 30°C.

For example, a #6 carrying 55 A will heat up to 60°C. That same #6 carrying 75 A will heat up to 90°C.

Aaah ok... That is helpful. And that is where the temperature ratings come into play then. I would guess its uncommon to load conductors to their full ampacity?

 

Um I don't know about that. I've seen #6 conductors carrying more than 55 amps for substantial periods of time and they were barely any more than warm to the touch. 60 degrees centigrade will burn you.

....

Sorry but I think InPhase is InCorrect. If conductor temperature went up to 60 or 75 or 90 degrees centigrade when loaded to full rated ampacity then the inside of your electrical panel would be like the inside of an oven. 60 degrees C is like 140 F, 90 degrees C is nearly the boiling point of water. That just doesn't happen unless something is wrong or a conductor is WAY overloaded or if a conductor is packed with a ton of other current-carrying conductors. Hence the derating procedures.

 

Informational Note No. 1: The temperature rating of a
conductor [see Table 310.104(A) and Table 310.104(C)] is
the maximum temperature, at any location along its length,
that the conductor can withstand over a prolonged time
period without serious degradation. The allowable ampacity
tables, the ampacity tables of Article 310 and the ampacity
tables of Informative Annex B, the ambient temperature
correction factors in 310.15(B)(2), and the notes to the
tables provide guidance for coordinating conductor sizes,
types, allowable ampacities, ampacities, ambient temperatures,
and number of associated conductors. The principal
determinants of operating temperature are as follows:

(1) Ambient temperature — ambient temperature may vary
along the conductor length as well as from time to
time.

(2) Heat generated internally in the conductor as the result
of load current flow, including fundamental and harmonic
currents.

(3) The rate at which generated heat dissipates into the
ambient medium. Thermal insulation that covers or surrounds
conductors affects the rate of heat dissipation.

(4) Adjacent load-carrying conductors — adjacent conductors
have the dual effect of raising the ambient temperature
and impeding heat dissipation.

InPhase, check out that last one.... if you had a conduit with, say, 5 current-carrying #12 copper THHN conductors running 16 continuous amps each (which is fine and legal), then you're saying that those conductors would heat up to 60 or 75 or 90 degrees C. That would make the temperature inside the conduit at or above that temperature. Correcting for that would make for an extremely low allowable ampacity and render those conductors pretty much worthless and non-compliant.

I'm not trying to come across like a d**k, I just think that your interpretation of temperature ratings of insulation types is off. :thumbup:

 

I'm not trying to come across like a d**k, I just think that your interpretation of temperature ratings of insulation types is off. :thumbup:

It's how Tom Henry explained it to me, as well as (I think) the American electrician's Handbook. But I'm not great with references, so one of these other fellas I'm sure will have one soon.

But as far as a #6 not reaching 60°C when carrying 55 A, I believe that is because of the fact that it is usually not 30°C ambient, coupled with the large conservative wiggle room built into the NEC.

 

Eric, I know that 30°C is just 86°F, but that is a pretty warm day. Like I said the code is quite conservative with ampacities. I can't quite peg the search terms that let Google shed some light on it, but I did find this at Holt's http://forums.mikeholt.com/showthread.php?t=91137

I know it isn't textbook facts, but at least it is some corraboration.:thumbup:

 

Okay, I see what you're saying now. However, as the posts on Page 2 of that thread indicate, it takes an extreme circumstance for conductors to attain that temperature. If you ran three CCCs in a blanket of thermal insulation and ran their maximum allowable ampacity continuously at ambient temperature then there's an outside chance you will reach the temperature limit of those conductors. Under normal circumstances, and under properly sized and calculated conductors, you won't get anywhere near that temperature. If you ran a piece of EMT above a suspended ceiling in an office building and pulled three current-carrying #8 THHN conductors in it and put 50 amps through them continuously, you'd be lucky to even hit half of the temperature limit. Which is what I thought you were trying to say. Very rare is the circumstance when loading up a conductor to ampacity will result in it hitting its temperature limit.

 

30° C is only 86° F. There are plenty of places with ambient temperatures at or around that level. The NEC spells it out pretty clearly as I posted: the conductor temperature limitations are based on the maximum temperature the insulation can withstand continuously at a single point on its length without failing prematurely. It does not indicate the temperature rise of a conductor given a certain load.

As an example, MTW type insulation is rated for 60°C in wet locations, but 90°C in dry locations. If you run #12 copper MTW in a conduit inside an office pulling 16 amps, will it heat up a different amount than a #12 copper MTW pulling 16 amps in a conduit in a washdown area with a similar ambient temperature? No, of course not. The temperature rating of the insulation is the maximum allowable limit of temperature, not the temperature rise of the conductor (although that is a significant factor).

Thanks for that explanation too. You guys rock- When I can't call my instructor- there's always the forum .