This can be a confusing topic. The issue usually arises in nuclear power plants because most cable is rated at 90 degrees C, but equipment terminals are rated for 60 or 75 degrees C. Getting this wrong leads to costly rework down the line.
The NEC doesn’t really help matters much because it is written in legalese, which rarely clarifies confusing technical topics. For example, here is what the 2014 edition of the NEC says in Article 110.14(C). Can you identify the key phrase?
(C) Temperature Limitations. 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. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
The key phrase is this: Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Does that help your understanding? Probably not if you are normal.
I think it’s helpful to break this problem into smaller sub-problems. You should think of these two cases:
- In the case when your cable insulation temperature rating matches the terminal rating, you are concerned about protecting the conductor insulation.
- In the case when your cable insulation temperature rating exceeds the terminal rating, you are concerned about protecting the terminal.
Consider the first case. Based on NEC 2014 Table 310.15(B)(16), a #2 AWG copper conductor rated at 75C has an ampacity of 115 amps in a 30-degree C ambient temperature. So, in a 30-degree environment and connected to equipment with 75C terminals, the cable’s ampacity is 115 amps.
When ambient temperature increases, we derate the cable ampacity to compensate. If we didn’t, then the 115 amps would raise the cable temperature to above 75 degrees. This would start damaging the insulation. If the situation became worse — more than 3 current-carrying conductors in a conduit, cables in crowded cable trays — then the derating needs to become more severe to compensate for the extra heat created by those conditions.
CLOSE ENOUGH FOR ILLUSTRATION PURPOSES
This isn’t exactly right, but I’d say it’s close. Approximately speaking, this #2 cable carrying 115 amps in a room at 30 degrees C heats up to what a #2 cable carrying 86.25 amps in a room at 50 degrees C would.
In both cases, the 75-degree terminal rating is protected, as is the cable insulation.
Now, consider the second case. Based on Table 310.15(B)(16), a #2 AWG copper conductor rated at 90C has an ampacity of 130 amps in a 30-degree C ambient temperature. However, in a 30-degree environment and connected to equipment with 75C terminals, the cable’s ampacity is limited to just 115 amps. In this case, we are losing about 12% of the cable’s ampacity.
If we operated the cable at 130 amps, the cable insulation would be fine indefinitely; the equipment terminations, however, would sustain damage because they aren’t capable of dissipating the extra heat effectively. They would heat up to higher than 75C. Given enough time, they would start to degrade.
But, what we can do is use 90C-rated cable to our advantage when heavy derating is needed. The benefit is that we can use a smaller cable rated at 90C than if we were using only a cable rated for 75C.
This is where the confusion sets in.
If you select a 90C cable, but start your derating at its 75C ampacity — in other words, you assume the baseline ampacity of a 90C, #2 cable is 115 amps instead of 130 amps — then you are being ultra conservative. The relationship between a 75C cable and a 90C cable would be one-for-one. You would gain no advantage for using the 90C cable, so why not just use a cheaper 75C cable and save some money?
The idea is that, by using a 90C cable, we might be able to get away with installing a smaller cable than if we were using 75C cable. Smaller cables are easier to install because they are easier to work with. They are more flexible and have a smaller bend radius. A small cable bend radius might mean you can install smaller-diameter conduit.
You can see how the cost savings might start piling up.
The cable selection process, when using 90C cables with 75C equipment terminations, can be boiled down to the following: after derating the 90C cable, is the derated ampacity less than the base ampacity (at the base ambient temperature) of the same size cable rated for 75C (at the same base ambient temperature)? If so, then you can use it.
A SIMPLE EXAMPLE CLEARS IT UP
An example might clear it up. Let’s assume multiple derating factors are needed: ambient (50C) and more than three current-carrying conductors (13).
We have a noncontinuous load of 175 amps with equipment terminations rated for 75C. Using Table 310.15(B)(16), and selecting 75C cable, the derating factors are 0.75 (ambient) and 0.5 (13 conductors). We need a 750 MCM cable:
475 amps x 0.75 x 0.5 = 178.125 amps, derated.
That’s good enough to handle our 175-amp noncontinuous load.
Now, let’s assume we’re using a cable rated at 90C. The derating factors are 0.82 (ambient) and 0.5 (13 conductors). We need a 500 MCM cable:
430 amps x 0.82 x 0.5 = 176.3 amps, derated.
It’s close, but still good enough to handle our 175-amp noncontinuous load. But can we use it with the 75C terminals?
The derated ampacity is 176.3. The base ampacity of a 500 MCM cable at 30-degrees-C rated for 75C is 380 amps. Since 176.3 is less than 380, then yes, we can use this cable.
Essentially what we are saying is that a 90C, 500 MCM cable carrying 176.3 amps in a 50-degree ambient environment in a cable tray with 13 other fully-loaded conductors generates less internal heat than a 75C, 500 MCM cable carrying 380 amps in a 30-degree ambient environment (with no other conductors nearby to generate heat).
You can find this procedure explained similarly across the web. Here’s an example:
Mike Holt has at least one article on it, too: