Wire Rating based on terminal lugs

P

pwhite

Senior Member
Location
Middle Tennessee
Occupation
Electrical Engineer
Hi all,

it's been a long time since i posted i had to reregister. i'm now back into the electrical design rather than controls programming.
i cannot remember the code, but i seem to remember that electrical conductors must be rated based on the temperature rating
of the terminal lugs of the circuit breaker box / disconnect.

for example, if the disconnect lugs are rated at 60 degrees c, then you must use the 60 degree column for determining the size of the current conductor.
you would not use the 75 or 90 degree column.
if i am correct, please point me to the code, i have a concern about the wire my coworker says he is going to use.
if i am incorrect, please let me know.
Thanks in advance,
Philip
 
Yes. You usually have two calculations to do. One based on the temp rating of the wire and then modified due to bundling and ambient temperatures. Then, you do one based on terminal limits. You use which ever calculation gives the the largest wire requirement.

There is a default 60C and 75C rating in the NEC based on amp rating. However, many smaller things are marked 60/75C or 75C and it is OK to use them at 75C if so marked. Where many go wrong is using 90C wire and ampacity tables and then putting those into 75C lugs. You can't exceed the 75C ampacity limit if you have 75C rated terminations.
 
You use the temperature rating of the conductor for any required ampacity adjustment and/or correction. If the termination is rated for 60°C, then the operating current of the conductor is not permitted to exceed the value for that size conductor in the 60° column.
 
Suemarkp,
i knew about the ambient temperature and the bundling, that was no issue.
we have a controlled temp building and it never gets above 75 degrees F. my coworker forgot about the derating due to bundling in a conduit.
I am searching for the code for the wire lugs on the disconnect. they are rated at 60 degrees c and i was needing the code number to show
him. He is also trying to get his masters electrician license.
I have been questioning him on various things and he is very up to date on the code, but this one is one i need an answer to also for an upcoming
project.
thanks,
philip
 
I dont know if it explicitly states it in code book but i does say that you have to follow manufacturers instruction. If you look at the cut sheet for the product, it will give the temp rating. so yeah it does say in a round about way to use the 75C column.

BTW, the derating of wire is based off the 90C column for wire like THHN, not the 75C column for terminations. with this downgrade of ampacity at the start usually allows for a little derate of ampacity due to wire bundling without a need to upsize wiring. usually........
 
Infinity,
no, this is a new install. i will double check the manufacturers info. i may have misread it.

The lugs on another part of the project have no markings and no info in the box.
Does this mean that i have to assume 60 degree c rating?
philip
 
pwhite said:
Infinity,
no, this is a new install. i will double check the manufacturers info. i may have misread it.

The lugs on another part of the project have no markings and no info in the box.
Does this mean that i have to assume 60 degree c rating?
philip
Click to expand...
It depends on the wire size or the lug amp rating. What kind of equipment are you dealing with, and what is the amp rating? Is it a 40A HVAC circuit on a 60A disconnect, or a 150A motor circuit going into a 200A disconnect?
 
i am going from a main breaker panel to feed a 75kva transformer.
main panel is 480 volt and will feed a disconnect before the transformer. the lugs are 60 degree c.
the transformer will be 480 / 240 volts. the main panel breaker lugs are 75 degree c. and that is what my coworker wants to use.
i am saying that we need to use the 60 degree c wire column because that's what the disconnect lugs at the transformer are rated at. (this is a new disconnect). i will start looking at the code you referenced.
thanks,
philip
 
So what is the amp rating if the disconnect? If it is over 100A, it must be rated for 75C and no label needs to say that. If 100 or under, they must be 60C minimum, but can be rated higher. I would be surprised to find a modern 100A disconnect that did not have 75C rated lugs. But if you cant find documentation of that, then you assume 60C. It may say 60/75 which means you can use it at 75C.

Do you have the make/model of the disconnect or a photo of the labeling inside it?
 
Don't misconstrue the AMBIENT temperature rating of the enclosed device for the TERMINAL temperature rating, they are usually not the same. For example, a lot of enclosed disconnects have a maximum AMBIENT rating of 40, 50 or 60C, but the terminals are actually rated for use with 75C conductors, which is what 110.14(C) is concerned with. I just checked Square D, Siemens, Eaton and GE disconnects, all of them use terminals rated for 75C conductors.
 
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