The Middle Of The Wire Isn't Its End

March 1, 1998
The temperature restrictions in the Code for terminations have nothing to do with how you figure ampacities over the length of the circuit. Here's a contractor-focused review of both.If we don't do anything else in this trade, we all select, install, and terminate conductors. We've seen many misconceptions over the years, and they usually involve confusing the middle of a wire and the end of the same

The temperature restrictions in the Code for terminations have nothing to do with how you figure ampacities over the length of the circuit. Here's a contractor-focused review of both.

If we don't do anything else in this trade, we all select, install, and terminate conductors. We've seen many misconceptions over the years, and they usually involve confusing the middle of a wire and the end of the same wire. Now, you probably think you know the difference, and you do. Just be sure you keep that difference in mind when you select your conductors.

Current vs. heat. Every conductor has some resistance. As you increase the current, you increase the amount of heat-all other things being equal. In fact, you increase the heat by the square of the current. That's what the old formula W x I(squared) X R really means.

The ampacity tables in the Code reflect heating in another way. Everyone knows they tell you how much current you can safely draw through a conductor under the prevailing conditions. This, after all, is how "ampacity" gets defined in Art. 100:

Ampacity: The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

The ampacity tables do much more than this, however. They show us the range of currents below which a wire runs at or below certain temperature limits, by implication. Remember, conductor heating comes from current flowing through metal arranged in a specified geometry (generally, a long cylinder of specified diameter and metallic content). This means for the purposes of these calculations, you can ignore the different insulation styles in trying to decide how hot a conductor is going to be running. Let's make this into a "rule" and then see how the Code makes use of it:

A conductor, regardless of its electrical insulation system, runs at or below the temperature limit in an ampacity column when, under the conditions of use, it's carrying equal or less current than the ampacity limit in that column.

For example, a 90 DegrC THHN No. 10 conductor has an ampacity in Table 310-16 of 40A. Using our "rule," this means when these conductors carry 40A under normal use conditions, they'll reach a worst-case steady-state temperature of 90 degrees C just below the insulation. Meanwhile, the ampacity definition shows this temperature won't damage the wire, regardless of duration. That's not true of the device, however. If a wire on a wiring device gets too hot for too long, it will cause problems over time with how well the metal parts inside keep their temper, as well as the stability of nonmetallic parts. In addition, you can have problems with overcurrent devices misbehaving because their calibration shifts.

For these reasons, manufacturers try to limit how hot the conductors can be that you put on their terminals. Remember, a metal-to-metal connection that is sound in the electrical sense is probably sound in the sense of heat conductivity as well. If you terminate a 90 degrees C conductor on a circuit breaker and it reaches 90 DegrC, the inside of the breaker won't be much below that temperature. When you think about it, asking the breaker to perform reliably with even a 75 DegrC heat source bolted to it is asking a lot.

Testing laboratories know this, and there have been listing restrictions for many, many years designed to prevent wire usage that would cause device overheating. Smaller devices (generally, 100A and lower, or with termination provisions for No. 1 or smaller wire) weren't assumed to operate with over 60 DegrC wires, such as TW. Higher rated equipment assumed 75 DegrC conductors, but generally no higher for 600V equipment and below. Sec. 110-40 sets 90 DegrC as a default termination temperature for medium-voltage equipment, but the remainder of this article focuses on 600V and below.

Although enforceable through Sec. 110-3(b), those rules went largely ignored, however, until the Code incorporated them into Sec. 110-14(c). Now everyone's heard about them. Let's take a closer look at the elements that make up this subsection:

Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be so selected and coordinated as to not exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

Termination provisions of equipment for circuits rated 100 amperes or less, or marked for Nos. 14 through 1 conductors, shall be used only for conductors rated 60 DegrC (140 DegrF).

Exception No. 1: Conductors with higher temperature ratings shall be permitted to be used, provided the ampacity of such conductors is determined based on the 60 DegrC (140 DegrF) ampacity of the conductor size used.

Exception No. 2: Equipment termination provisions shall be permitted to be used with higher rated conductors at the ampacity of the higher rated conductors, provided the equipment is listed and identified for use with the higher rated conductors.

The basic rule is to pick a wire whose ampacity isn't higher than the temperature rating of the device to which you're connecting. Paragraph (1) says that would be 60 DegrC for small equipment, such as would be involved with the No. 10 we've been discussing. Put those two together, and we get TW wire. But we only had THHN; Are we out of luck? No. Ex. 1 allows us to use THHN provided we pretend it's a 60 DegrC conductor when we think about how hot it might get. This exception validates the "rule" we just called out. Under standard conditions of use (not over three current-carrying conductors, not over 30 DegrC ambient), No. 10 THHN copper runs below 60 DegrC when it carries less than 30A, the ampacity limit in the 60 DegrC column.

What about larger conductors and equipment? This brings us to the next paragraph in the rule: Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than No. 1, shall be used only with conductors rated 75 DegrC (167 DegrF).

Exception No. 1: Conductors with higher temperature ratings shall be permitted to be used, provided the ampacity of such conductors is determined based on the 75 DegrC (167 DegrF) ampacity of the conductor size used.

Exception No. 2: Equipment termination provisions shall be permitted to be used with the higher rated conductors at the ampacity of the higher rated conductors, provided the equipment is listed and identified for use with the higher rated conductors.

As you can see, this is exactly the same as the other rule and exceptions, except it uses a 75 DegrC limit instead of 60 DegrC. This brings us to heat limitations when a conductor doesn't terminate except at another conductor, such as in the middle of a run. We do this whenever we splice conductors end-to-end, or when we make a field connection to a busbar. Here you only have to worry about the temperature rating of the compression connectors or other splicing means involved. Watch for a mark, such as "AL9CU." This one means you can use it on either aluminum or copper conductors, at up to 90 DegrC. Here's the actual Code rule, which follows in the same section:

Separately installed pressure connectors shall be used with conductors at the ampacities not exceeding the ampacity at the listed and identified temperature rating of the connector.

Watch the phrase "separately installed." Many contactors, panelboards, etc., have termination lugs marked to indicate a 90 DegrC acceptance. Ignore those markings because the lugs aren't "separately installed." Instead, apply the normal termination rules for this kind of equipment. What's happening here is the equipment manufacturer is buying lugs from another manufacturer, who doesn't want to run two production lines for the same product. The lug you field install on a busbar and use safely at 90 DegrC, also works when furnished by your contactor's OEM. But on a contactor, you don't want the lug running that hot. The lug won't be damaged at 90 DegrC, but the equipment it's bolted to won't work properly.

What if you see listing limitations or allowances? Observe them. Note the fine print note that concludes this part of the Code: (FPN): With respect to Sections 110-14(c)(1), (2), and (3), equipment markings or listing information may additionally restrict the sizing and temperature ratings of connected conductors.

Many testing limitations are less restrictive, as covered in Exception No. 2 to both paragraphs in the rule. If small equipment, for example, is marked for 75 DegrC terminations, then you can use it that way. Two general cautions apply here, however. A higher temperature rating on a circuit breaker, for example, applies to that breaker as a stand-alone use, or in its own enclosure. If the breaker will be one of many in a panelboard, for example, then the panelboard also needs a similar marking in order for you to use the higher temperatures. Secondly, never forget a wire has two ends. Make sure both the supply-side and the load-side equipment markings coordinate in order to do this.

What about continuous loads? We're all familiar with the restriction that keeps the continuous portion of a load to 80% of the ratings on most devices. The reciprocal of 80% is 125%, and you'll see it both ways. That is, restricting the continuous portion of a load to 80% of the device rating means the same thing as saying the device has to be rated 125% of the continuous portion of the load. Here's one version of this rule, in Sec. 220-10(b):

Continuous and Noncontinuous Loads. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. The minimum feeder circuit conductor size, without the application of any adjustment or correction factors, shall have an allowable ampacity equal to or greater than the noncontinuous load plus 125 percent of the continuous load.

Exception: Where the assembly including the overcurrent devices protecting the feeder(s) are listed for operation at 100 percent of their rating, neither the ampere rating of the overcurrent device nor the ampacity of the feeder conductors shall be less than the sum of the continuous load plus the noncontinuous load.

This is a very complicated rule, often criticized for its complexity. Faced with a choice between simple and technically correct, the Code-Making Panel has paid us a compliment. They've said we can handle technically correct. Let's not disappoint them. Here's how it works sentence by sentence:

Everyone knows about the first sentence. Take the noncontinuous load and add 125% of the continuous portion of the load, and then compare the result with the device rating. Suppose, for example, a load consists of 52A of noncontinuous load and 68A of continuous load.

Step 1: 52A x 1.00 = 52A Step 2: 68A x 1.25 = 85A Step 3: Minimum = 137A

The result is a device, such as a circuit breaker, that will carry this load profile must be rated no less than 137A. In the case of overcurrent protective devices, the next higher standard size would be 150A.

The last sentence of the rule, which is new in the 1996 NEC, is the controversial one. The best way to understand it is to look at a continuous load through the eyes of a device manufacturer.

We handle conductors and worry about conductors getting overheated. Device manufacturers don't worry about conductors in this sense; they worry about their devices getting overheated and not performing properly. Continuous loads pose real challenges in terms of heat dissipation from the inside of mechanical equipment. Remember, when you bolt a conductor to a device, the two become one in the mechanical as well as the electrical sense. It turns out device manufacturers rely on those conductors as a heat sink, particularly under continuous loading.

Our No. 10 THHN conductor, for example, will carry 40A for a month at a time without damage to itself. It's just that under those conditions the conductor would represent a continuous 90 DegrC heat source. Now watch what happens when we (1) size the conductor, for termination purposes, at 125% of the continuous portion of the load; and (2) use the 75 DegrC column for the analysis. This assumes the termination is rated for 75 DegrC instead of the default value of 60 DegrC:

Step 1: 1.25 x 40A = 50A Step 2: Table 310-16 @ 75 DegrC = No. 8

We go from a No. 10 conductor to a No. 8 conductor (No. 6 if the equipment doesn't have the allowance for 75 DegrC terminations). Ho-hum, that's just one wire size, but look at it from the device manufacturer's perspective. No. 10 carrying 40A continuously is a continuous 90 DegrC heating load. What about the No. 8? Use the ampacity table in reverse, as we did before (40A happens to be the ampacity of a No. 8 60 DegrC conductor). Therefore, any No. 8 wire (THHN or otherwise) won't exceed 60 DegrC when its load doesn't exceed 40A. By going up just one wire size, the termination temperature dropped from 90 degrees C to 60 DegrC. By making the Code change, manufacturers and product standards can count on this headroom.

In the feeder example, including the 125% on the continuous portion of the load brings us to a 137A conductor, and the next larger one in the 75 DegrC column is a 1/0. Remember to use the 75 DegrC column here because the device carries a 150A rating. Sec. 110-14(c)(2) sets 75 DegrC as the default temperature rating for this size equipment. The extra 17A (the difference between 120A and 137A) is phantom load. You only include it so your final conductor selection is certain to run cool enough to allow it to operate in accordance with the assumptions made in the various device product standards.

Nothing in this article, so far, has anything to do with preventing a conductor from overheating.

That's right. All we've done is to be sure the device works as the manufacturer and test lab anticipate, in terms of the terminations. Now we have to be sure the conductor doesn't overheat. Again, ampacity is, by definition, a continuous capability. The heating characteristics of a device at the end of the run don't have any bearing on what happens in the bowels of a raceway or cable assembly.

You have to compartmentalize your thinking at this point. Analyze the terminations to determine the minimum conductor size at the terminations and nowhere else. This is where many contractors have almost a mental problem. They want to take the termination results and carry them forward as the basis for derating calculations for multiple conductors in a raceway, ambient temperature, or something else. Don't do it. If you have to, make the termination calculations on a separate piece of paper, and lock it away somewhere until you've done the next step. Only then should you go back and see which result represents the worst case, and therefore governs your conductor choices.

Figuring conductor sizes to prevent overheating shouldn't be new, since we've been doing it the same way for many years. Simply take the load, and adjust for the conditions of use. In general, you always want to look at two issues: mutual heating and ambient temperature. Both of these factors reduce ampacities from the table numbers. Here's a review.

The Code addresses mutual heating in Note 8(a) to the ampacity tables. The more current-carrying conductors are in the same raceway or cable assembly, the less they can radiate their heat out into the ambient. In turn, the note limits the permissible load by giving derating factors that apply to table ampacities. However, going from Table to Note 8(a) to load is for inspectors to check your work. You want to go the other way: Load to Note 8(a) to Table. Here's an example, using the feeder with 52A of noncontinuous load and 68A of continuous load.

Suppose you have two of those feeders supplying identical load profiles, and run in the same conduit. That would be (assuming insignificant neutral load) six current-carrying conductors in the raceway. For this part of the analysis, ignore continuous loading and termination problems. Remember, you should be using a fresh sheet of paper.

Start with 120A of actual load (52A + 68A), and divide by 0.8, to get 150A in this case. In other words, any conductor with a table ampacity that equals or exceeds 150A will be mathematically guaranteed to carry the 120A load safely. A No. 1 THHN conductor, with an ampacity of 150A, will carry this load safely under the conditions of use. It might appear to work, but read on.

Ambient temperature derating works the same way. If these conductors go through a 45 DegrC ambient, their ampacity goes down (for 90 DegrC conductors) to 87% of what it used to be. Here again, start with 120A, and divide by the 0.87 to get 138A. Any 90 DegrC conductor with an ampacity equal to or higher than 138A would carry this load safely. What happens if you have both a high ambient temperature and mutual heating? Divide twice, once by each factor. In this case we would have: 120A / 30.8 / 0.87 = 172A

A 2/0 THHN conductor (Ampacity = 195A) would carry this load without damaging itself. Again, this would be true whether or not the load was continuous, and whether or not the devices were allowed for use with 90 DegrC terminations. Don't cheat; the termination calculations are still supposed to be locked up in another drawer.

Now let's pick a conductor. Put both sheets of paper in front of you, and design for the worst case by installing the largest conductor that results from those two independent calculations. The termination calculation came out needing conductors sized, under the 75 DegrC column, no less than 137A even though the load was only 120A. You could use 1/0, either THHN or THW.

Suppose you put two feeders (6 conductors) in a conduit. The termination calculation still comes out 1/0, but as we've seen, the raceway derating calculation comes out No. 1 THHN. Again, the termination rules are the worst case and govern the conductor selection process. If the same raceway also goes through the area with high ambient temperature, however, you'll need 2/0 THHN. Now the raceway conditions are limiting, so size accordingly.

Remember, here again the terminations and conductor ampacity are two entirely separate issues. Just because you need to use the 75 DegrC column for the terminations doesn't mean you start in that column to determine the overall conductor ampacity. Go ahead and make full use of the 90 DegrC temperature limits on THHN, and its resulting ampacity, to solve derating problems. That's why the last sentence was added to Sec. 110-14(c).

Don't get confused by the fact that the derating factor for continuous loads (0.8) is the same for four to six conductors in a raceway (0.8). This is only a coincidence. One applies to devices and terminal heating at the end of a conductor, and the other applies over the interior of the raceway. They never apply at the same point in a circuit, because the technical basis for each is entirely different. The middle is never the end, so never apply the rules for one to the other.

One final issue. Don't lose sight of the fact that the overcurrent device must always protect the conductor. For 800A and smaller circuits, Sec. 240-3(b) allows the next higher standard size overcurrent device to protect conductors. Above that point, Sec. 240-3(c) makes you cable no less than the rating of the overcurrent device. As a final check, be sure the size of the overcurrent device selected to accommodate continuous loads protects the conductors in accordance with these rules; if it doesn't, you'll need to increase the conductor size accordingly.

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