90 degree column

C

Choice_Gorilla

Senior Member
Location
New England
Occupation
Journeyman Electrician
Maybe a stupid question but is there ever a situation where wire can be sized using the values in this column? I was always under the impression I could use it for derating purposes but when it came to actually sizing wire I was to use the 75 or 60 degree column. I’ve got an engineer telling me I can use the 90 degree column because I’m terminating directly to bus bar with 90 degree rated lugs or crimps. Never sized wire this way so it seems wrong. I’m not one to argue unless I can back it up with code references. If it matters, one side is a MCC and the other is a large motor. I can’t find anything on either that has a temperature rating.
 
There is little to no equipment rated for 90°C. Just because the terminal may be listed for 90° C it would need to be connected to a piece of equipment that has the same rating which for all intents and purposes does not exist.
 
See this post on this topic from 4 years ago:

Using the 90C column in the Ampacity Table

Greetings, Do we use the cable ampacity in the 90C column only when the cable would be terminated at the lugs rated for 90C on both ends? Thanks, EE
forums.mikeholt.com

If your _assembly_ is listed for 90C terminations, then you can use 90C conductors.

It is plausible to me that in the situation you describe, both sides (MCC and motor) would work just fine with 90C conductors but neither side has been evaluated for 90C conductors.

Consider: A 90C lug can handle the temperature. The bus bar can handle the temperature. But the bus bar will conduct heat to other components. Has the _system_ been evaluated so that conducted heat from a 90C termination doesn't cause problems? What is the temperature rating of the bus bar support insulators? What about other terminals or wires on the bust bar? Similarly the motor lead wires are certainly rated for more than 90C, and most splicing devices are rated for 90C or more. What else is in the motor junction box? Any 75C RTD wires?

If you can get good documentation of 90C capability on both sides, then you can use 90C conductor ratings.

-Jonathan
 
Overcurrent devices, particularly in the 600 volt and less class, are almost never rated for 90C connections.

Individual lugs possibly can have 90C rating, but still can only be used at the rating of whatever they are attached to.

A conductor needs sized per the weakest link. Should you have 90C rated termination at both ends then the conductor can be used at 90C ampacity. Overcurrent device is almost always one of those weakest links. Then we have the small conductor rule of 240.4(D) that comes into play with a lot of general use and lighting circuits.
 
If this engineer can provide documentation that the equipment is listed for 90°C then you're good. Not likely so you're stuck with the 75° C conductor ampacity.
 
Choice_Gorilla said:
Maybe a stupid question but is there ever a situation where wire can be sized using the values in this column? I was always under the impression I could use it for derating purposes but when it came to actually sizing wire I was to use the 75 or 60 degree column. I’ve got an engineer telling me I can use the 90 degree column because I’m terminating directly to bus bar with 90 degree rated lugs or crimps. Never sized wire this way so it seems wrong. I’m not one to argue unless I can back it up with code references. If it matters, one side is a MCC and the other is a large motor. I can’t find anything on either that has a temperature rating.
Click to expand...
If you are indeed connecting both sides of the conductor to bus bar rated for 90 C with terminations rated for 90 C, he is right.
 
If the run is long you can use 90°C conductors as shown below. The question is, is it cost effective after the extra components are purchased and installed?

​


1725455974415.png
 
My understanding is there is no <600V equipment that has 90 degree terminations. It doesn't matter if the termination is on a bus bar seemingly far away from any OCPD in that equipment. If it lands on the equipment, it will have a 75 degree limitation.
 
electrofelon said:
My understanding is there is no <600V equipment that has 90 degree terminations. It doesn't matter if the termination is on a bus bar seemingly far away from any OCPD in that equipment. If it lands on the equipment, it will have a 75 degree limitation.
Click to expand...
I never looked real close but I thought it was under 600 Volts for connections rated 100 Amps or less.
 
electrofelon said:
My understanding is there is no <600V equipment that has 90 degree terminations. It doesn't matter if the termination is on a bus bar seemingly far away from any OCPD in that equipment. If it lands on the equipment, it will have a 75 degree limitation.
Click to expand...
True Switchgear (draw out breakers, separate rear connection compartments) terminations may be rated for 90°C.
I am not sure about transformers.
 
electrofelon said:
My understanding is there is no <600V equipment that has 90 degree terminations. It doesn't matter if the termination is on a bus bar seemingly far away from any OCPD in that equipment. If it lands on the equipment, it will have a 75 degree limitation.
Click to expand...
My understanding as well. I've never seen any equipment under 600 volts rated for 90° C.
 
roger said:
If the run is long you can use 90°C conductors as shown below. The question is, is it cost effective after the extra components are purchased and installed?

​


View attachment 2573298
Click to expand...
Not only the cost of the original installation, but the long term I²R losses that will be paid for by the owner if this is on the load side of the meter. Not something I would ever permit as a person paying the utility bill.
 
don_resqcapt19 said:
Not only the cost of the original installation, but the long term I²R losses that will be paid for by the owner if this is on the load side of the meter. Not something I would ever permit as a person paying the utility bill.
Click to expand...
Likely since the real load will be 1/2 or less of the calculated load the losses will be negligible. In most cases the wires won't even be warm to the touch.
 
tom baker said:
And you must consider both ends, what is the temp rating of the motor leads and lugs?
Click to expand...
Motor peckerhead is pretty big, basically a 36”x36” j box. Wires terminate directly to bus bar. Lugs or crimps would be provided by us. Motor has factory installed leads from bus bar to windings with no visible marking on them to indicate anything.
 
Thanks for all the feedback everyone. To sum things up, if I understand everyone correctly, every piece of equipment needs the 90 degree rating not just the termination points. So in my case, every piece of the MCC and every piece of the motor would have to be 90 degree rated. Seems like it would be nearly impossible to prove that, and from what you all are saying it would be highly unlikely as well.
 
Choice_Gorilla said:
Thanks for all the feedback everyone. To sum things up, if I understand everyone correctly, every piece of equipment needs the 90 degree rating not just the termination points. So in my case, every piece of the MCC and every piece of the motor would have to be 90 degree rated. Seems like it would be nearly impossible to prove that, and from what you all are saying it would be highly unlikely as well.
Click to expand...

I think it is more that the entire MCC would need to be listed as allowing the 90C termination, not that everything in the MCC would need a 90C rating. (So if the terminals heat up to 90C because you use 90C wire at full current, then every part either handles that 90C _or_ is by design and listing protected so that it doesn't exceed its allowed working temperature.) But that is a small nuance on the fact that the documentation is likely not available anyway.

-Jonathan
 
I didn’t read the above…but read NEC 110.14 (C)1/2. But yes in most cases terminals are rated 75 degrees. There is a case you can size the conductors using the amps in the 90 degree column of T 310.16 ( 110.14C2). In that case you could use a splicing or etc device rated 90 degrees. To use the 75 degree column your terminals, lugs, or other connection points must be rated 75 degrees. Obviously your conductor insulation must be rated. If any one terminal is rated for a lower temp you have to use that temp..
 
You must log in or register to reply here.
Top