NEC 110.14(C) & Temperature limitations of terminati

[klm0824]

An existing 480V system has 500kcmil THWN/THHN cable fed from an existing 400A circuit breaker with 75 degree terminations. Routing is overhead RGS conduit. At first glance, the 400A breaker does not protect the 500kcmil.

Using table 310.16, 75 degree column, at 40 degree ambient, the ampacity is 334 amps - which would require a 350 amp breaker. The way I understand article 110.14(C) is that I can use the 90 degree ampacity (430A) and its derating factor (.91) to calculate the cable ampacity (391A), compare this ampacity to the 75 degree ampacity (380A) and use the smaller of the two (380A). This would allow the 400A circuit breaker to protect the 500kcmil.

Problem is that I don't understand how you can put 380 amps (vs 334 amps) on the 75 degree wire with a 40 degree ambient and not exceed 75 degrees. In this case, the insulation is the same for both ratings, and the copper has the same losses. And, what exactly does putting more that 75 degrees on a 75 degree termination do to the termination?

 

[charlie b]

Re: NEC 110.14(C) & Temperature limitations of terminati

Very good question! I think I can answer it, but I?ll invite anyone else to let me know if they disagree with this interpretation.

If you had a 500 MCM cable that was only good for 75C (e.g., type RHW), and if it were in a 40C ambient environment, any current over 334 amps (i.e., 88% of 380) would risk damage to the insulation of the cable itself. In other words, the 334 is a cable limit, not a termination limit.

If you had a 500 MCM cable that was good for 90C (e.g., type THHN), and if it were in a 40C ambient environment, any current over 391 amps (i.e., 91% of 430) would risk damage to the insulation of the cable itself.

The two types of cables described above have the same metal within different insulation systems. It is only the metal that comes into contact with the terminations. Running more than 380 amps through terminations built to accept a 500 MCM conductor would risk damage to the insulation of the terminations. Would the termination melt? Probably not. Would it fail? Eventually, and most likely it would be sooner than it would have been, had the current been kept below the 380 amp limit. That is true without regard to the ability of the cables? insulation systems to withstand high currents or high temperatures. So the type of cable does not influence the 380 amp limit for the terminations. That is why the limit for the 500 MCM THHN is 380 amps, and not 391.

 

[itasca_mn]

Re: NEC 110.14(C) & Temperature limitations of terminati

The size of the conductors and overcurrent protection in this case are based on the actual load being served, not on each other.

For the sake of discussion, lets assume that on this circuit we have 200 amps of continuous load, and 105 amps of non-continuous load.

For wire sizing we must first take 125% of the continuous load, plus 100% of the non-continuous load.

200 amps X 125% = 250 amps
250 amps plus 105 amps = 355 amps.

According to article 110.14C, our equipment temp rating is 75 degrees C.

The 75 degree column of table 310.16 requires a 500 kcmil conductor for 355 amps. If no further derating was needed, this would be our conductor size. But, we are using 90 degree rated wire, and we have an ambient temperature correction of 40 degrees C. The 500kcmil selected in this step will be compared to the one selected in the next step after derating.

The ambient temperature correction factor in the 90C column is .91. This adjustment can be calculated at the actual load (not at 125% of continuous plus non-continuous). Our total load is 305 amps(200 + 105). 305 divided by .91 = 335 amps. Looking in the 90 degree column of 310.16 we see that a 300kcmil conductor is required for 335 amps. We now compare this conductor to the one that we selected earlier from the 75 degree column. The larger of the two wins. A 500 kcmil THHN is required as per article 110.14C1(b)2.

If we were using 75 degree rated wire, we would calculate the 305 amps divided by .88 for a total of 346.5 amps. Looking in the 75 degree column of 310.16 we see that a 500kcmil conductor is required. 500kcmil was also selected for comparison earlier. A 500kcmil 75 degree rated conductor is required.

I assume that this is a feeder(you didn't say). Overcurrent protection is selected at not less than 125% of continuous, plus 100% of non-continuous loads as per article 215.3. Again this totals 355 amps. The next higher standard sized circuit breaker is 400 amps as per article 240.6(A).

 

[klm0824]

Re: NEC 110.14(C) & Temperature limitations of terminati

This is the reply of my coworker:


If you review the history of NEC article 110-14(c) "Temperature Limitations", you will notice that this section did not exist in the NEC before the 1993 edition. The NEC Handbook of 1993 stated that "maximum load at terminations based on the device terminal temperature rating, has been misunderstood by many Code users, ..." and was added to increase restrictions on the requirements pertaining to load current limitations.

(* In other words, too many people were getting it wrong and causing fires at the terminations! *)

In the 1996 & 1999 editions of the NEC, article 110-14(c), it stated "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."

The 2002 edition added additional wording to remind the Code user that they need to use derating factors when determining the termination provisions. It reads "... conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(1) through (6)". In other words, 310.15(B) says you need to derate based on your application.

Also, the NEC paraphrases itself in the same section with additional cautions. In terms of the specific example below, the answer can be found in the NEC at 110-14(C)(b)(2) which states: "Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75?C ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors." The answer is in the last phrase "IF THE EQUIPMENT IS LISTED AND IDENTIFIED FOR USE WITH SUCH CONDUCTORS".

The reply given did not take into account ambient temperature nor time frame when determining maximum load through the termination. In order to maintain the 75?C temperature that the termination has been rated and labeled to function by the manufacturer, the maximum load through the termination must be reduced for higher ambient temperatures and for length of time it is subjected to 40?C ambient. The laws of thermodynamics have not changed just because the cable is at an equipment termination.

In conclusion, if the manufacturer of the termination does not label the termination as suitable for higher temperature rated cables, then you MUST derate based on temperature. In other words, the termination should not exceed 334 amperes in a 40?C ambient environment on a continuous basis unless the manufacturer says it's o.k. The NEC's attempt in the last ten years to simplify the laws of thermodynamics into a simple "chart" for electrical folks still needs some work!