I need some help interpreting this exception to 310.15(A)(2). The way I'm reading it just doesn't make sense.
Here's the bit I'm concerned with: "Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0m (10ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."
So, I'm thinking, if I have a circuit that passes horizontally through a high temperature area, and then extends away from that high temp area, the "away" portion should be able to be rated at a higher ampacity. Suppose that "away" portion was only 10 ft long. The above quoted section seems to be saying that after 10 ft, I can go ahead and use the higher ampacity figure. BUT, it also seems to be saying, since 10% of 10 ft (where 10 ft is the circuit length figured at the higher ampacity) is just 1 ft, that after 1 ft I can use the higher ampacity rating.
I'm thinking that they should have stipulated that you can go ahead with the higher ampacity rating so long as you extend beyond the transition point at least 10% of the LOWER ampacity portion of the circuit. So, using the above example, if the high ambient temperature portion (read as "lower allowable ampacity")was 100 ft long, you would have to extend at least 10% or 10ft beyond that hot spot before you could consider the conductor cooled down enough to allow a higher ampacity. What am I missing?
From the handbook; Example Three 500-kcmil THW conductors in a rigid conduit are run from a motor control center for 12 ft past a heat-treating furnace to a pump motor located 150 ft from the motor control center. Where run in a 78°F to 86°F ambient, the conductors have an ampacity of 380 amperes, per Table 310.16. The ambient temperature near the furnace, where the conduit is run, is found to be 113°F, and the length of this particular part of the run is greater than 10 ft and more than 10 percent of the total length of the run at the 78°F to 86°F ambient. Determine the ampacity of total run in accordance with 310.15(A)(2). Solution In accordance with the correction factors for temperature at the bottom of Table 310.16, the ampacity is 0.82 × 380 amperes, or 311.6 amperes. This, therefore, is the ampacity of the total run, in accordance with 310.15(A)(2). Had the run near the furnace at the 113°F ambient been 10 ft or less in length, the ampacity of the entire run would have been 380 amperes, in accordance with the exception to 310.15(A)(2). The heat-sinking effect of the run at the lower ambient temperature would have been sufficient to reduce the temperature of the conductor near the furnace.
This section of the code has always been kinda difficult for me to understand and explain but here's my understanding based on an example provided by Square D. If we had a circuit breaker rated at 100a. 600v., 75° terminals in a 86° ambient feeding a load 150 feet away, we could use #3 AWG conductors (THWN) for a distance of not less that 10 feet, make a splice to #1 AWG, run through an area of 100° F ambient then splice to #3 when we are back in the 86° ambient and terminate on the 75° terminals of the load. Now I need you guys to tell me if I'm wrong
Geesch! I think I get it, as Gfretwell described it. The point of transition must be the point at which the wiring coming from the MCC first encounters the furnace area. So, if the furnace is 10' wide or less, it would be less than 10% of a long run of 100'. The relatively long run would act as a heat sink. Then you could apply the higher allowable ampacity value to the portion of the circuit beyond that transition point..or, is it the next transition point where the wiring leaves the furnace area? It wouldn't make sense to allow a wire to cook behind a furnace and still be fully loaded per 75 degree table!
I guess I'm still confused as to where that transition point should be.
George, your example seems to be allowing someone to immediately go back to the #3, without any cool-down distance consideration! I think the whole point of this article is to keep the current/resistance induced heat from adding to the ambient induced heat, thereby cooking the insulation. If you allow the wire to cool down for a sufficient length after being exposed to a high temp area, then you won't run into trouble. Your example also ignores the usual branch circuit loading of only 80%. You were describing the load on that #3 as 100 Amp, and that would be putting a 100% load on that #3. But, I'm sure you knew that, and were just simplifying the example.