High Temp Wire - Downsize?

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aNerdyEngineer

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I've had some coworkers suggest using a high temp wire (SRML) to mitigate the need to upsize wire in higher temp situations - essentially eliminating the need to use the temp multipliers. I don't think I agree with that adjustment of ampacity based on the jacketing (at least as far as NEC is concerned). Anyone have any code sections that allow such a thing?
 
The likely problem is with 110.14. The equipment at termination may require using the 60° or 75° ampacities depending on the rating of the equipmemt/device.
 
The NEC certainly allows this for wire types that it recognizes.

For example, if you have an elevated temperature situation, 90C conductors have different temperature correction factors than 60C conductors, and will be able to carry more current in a given condition.

Consider a situation where the conductors would be in a 55C environment. If the conductor had a 60C insulation rating, then you would need to adust its ampacity to 41% of the normal value, but a 90C insulated conductor would retain 82% of its ampacity. If you could use a 150C rated conductor with its 150C rating, then that conductor would retain 90% of its normally rated ampacity.

But you can't simply use any old wire type; it needs to be in the NEC. Additionally if you look at the table of different insulation types, you will see the term 'special application' mentioned whenever a temperature rating > 90C is used. I don't know what the requirements are for 'special application', but I don't think you can normally make use of insulation temperature rating > 90C.

-Jon
 
Both very good points. But does 110.14 force me into sizing per the 75C if that's what my terminations are, regardless of my wire rating possibly being 90C? I think 110 would take the precedence over these higher order columns in 310...
 
Think of the system as a chain, only as strong as the weakest link.

At the _terminals_ you are forced to use the 75C ampacity, but presumably the terminals are in a cool location. Then the wire goes from the terminals through conduit, possibly passing through a hot location. In the hot location you calculate the ampacity based on the allowed wire rating and the ambient temperature at that location. The wire continues to another terminal with its temperature rating and ambient temperature, and you calculate the ampacity at that location. You can only use the lowest ampacity of the chain.

-Jon
 
winnie said:
At the _terminals_ you are forced to use the 75C ampacity, but presumably the terminals are in a cool location.
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What happens when the terminals aren't in a cool location?

For example, suppose a piece of equipment with 75C terminals needs a 50A (non-continuous) circuit and is located where the ambient temperature is 33C, as is all the circuit wiring. At 90C (wiring insulation), #8 copper has an ampacity of 55A, and the temperature correction factor is 0.96, so the corrected ampacity is still above 50A. At 75C (terminals), #8 copper has an ampacity of 50A, and the temperature correction factor is 0.94. Do we need to apply the 0.94 factor at the terminals and upsize to #6 copper? Or is it ignored, and #8 copper is fine? The language in 110.14(C) is not a model of clarity.

If the NEC does not require the 0.94 factor at the terminals, is there an engineering justification for that? Or is that the equipment listing/instructions should deal with the (possible) issue?

Cheers, Wayne
 
To the best of my knowledge, the temperature adjustment factors only apply to the conductor insulation.
If your device is rated for 75°,. a #8 conductor is fine as long as it's adjusted ampacity is 50 amps or higher.
 
wwhitney said:
What happens when the terminals aren't in a cool location?

For example, suppose a piece of equipment with 75C terminals needs a 50A (non-continuous) circuit and is located where the ambient temperature is 33C, as is all the circuit wiring. At 90C (wiring insulation), #8 copper has an ampacity of 55A, and the temperature correction factor is 0.96, so the corrected ampacity is still above 50A. At 75C (terminals), #8 copper has an ampacity of 50A, and the temperature correction factor is 0.94. Do we need to apply the 0.94 factor at the terminals and upsize to #6 copper? Or is it ignored, and #8 copper is fine? The language in 110.14(C) is not a model of clarity.

If the NEC does not require the 0.94 factor at the terminals, is there an engineering justification for that? Or is that the equipment listing/instructions should deal with the (possible) issue?

Cheers, Wayne
Click to expand...


I had the same question.

For bundling derate, it makes sense to me to not require it to apply at the terminals. The wire isn't bundled at the terminals, so no bundling adjustment factor should apply there, just because it applies in the nearby raceway. However, ambient temperature I question. On the surface it seems like >30C ambient corrections to temperature would apply for both wire and terminals, but this isn't the case.

110.14(C) states that "temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both". This tells me that the raw (usually 75C) rating for terminations, doesn't need to get recalculated for conditions of use. Products carry an ambient temperature rating (suppose 40C), and it would be tested with its intended size wire in a sample environment of its ambient temperature rating. This is the most common value to having 90C wire, when you don't get to take credit for its rating directly. I.e. it gives you headroom to use it for your derate calculations.
 
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