Terminal temp rating vs equipment temp rating

Status
Not open for further replies.
Twoskinsoneman

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
Ok I have seen several comments that conductor temps are limited to equipment temp limits even if the terminals on such equipment are allowed to be higher. (example 90 degree terminal on 75 degree device/equipment).

Why is that? My understanding is that the heat produced by 90 degree wire at full ampacity is ok for the conductor insulation but not necessarily ok for the terminal. The terminal can be adversly affected by the heat of the conductor... But why would that affect the equipment/device in general? Is the idea that the heat might tranfer from the terminal to the device and adversly affect it?

BTW I agree 110.14(C) supports this... I'm not asking about code but actual reasoning behind it.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
For items like over current devices I believe the conductors are used as a heat sink to pull excess heat from the over current device. If the entire conductor was running closer to 90 C than 75 C it would not pull nearly as much heat out of the device.

I know of some 3000 amp Square D breakers that require 90 C conductors but the ampacity must be calculated at 75 C.

Hopefully Jim D will see this thread, he has much more knowledge about than I.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Bob, has it correct.

The conductors (actually about 4ft of them) are included when the device's heat rise tests were conducted. The device termination almost always is intended to run extremely cooler than the conductor insulation. I believe most overcurrent device terminations are designed to operate at about 50?C even if the conductor insulation is 75?C. If the device termination runs hotter (like a 100% rated one would or the ones to a ballast) then the conductor insulation must be 90?C even though it is sized using the 60 or 75?C columns.
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
iwire said:
For items like over current devices I believe the conductors are used as a heat sink to pull excess heat from the over current device. If the entire conductor was running closer to 90 C than 75 C it would not pull nearly as much heat out of the device.
Click to expand...

The overall thermal design is more complex than this, but the heat conduction is a two way street. Ex. on electronic overloads there is no internal heating occurs, whilst mechanical OL's do utilize the heat rise to produce mechanical movement or softening of eutectic alloys to produce the trip. Most devices are designed to operating ambient temperature of 40C.

iwire said:
I know of some 3000 amp Square D breakers that require 90 C conductors but the ampacity must be calculated at 75 C.
Click to expand...

Could you send me or post a link where this can be found? There are breakers with bus extensions available where the connecting lugs can be rated to 90C without ever seeing a single word about how the wires should be sized.
 
StephenSDH

StephenSDH

Senior Member
Location
Allentown, PA
Excerpt from Square D Manual. Looks like IWire is off the hook.

http://www.google.com/url?sa=t&sour...dilcoc-VSmznG_zmQ&sig2=Tqzrf8P5ZMfGdLHC0o4w-A

"A final note about equipment? some equipment requires the conductors that
are terminated in the equipment to have an insulation rating of 90 ?C, but an
ampacity based on 75 ?C or 60 ?C. This type of equipment might include
100% rated circuit breakers, fluorescent lighting fixtures, etc., and is marked
to indicate such a requirement. Check with the manufacturer of the
equipment to see if you need to take into account any special considerations."
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
StephenSDH said:
Excerpt from Square D Manual. Looks like IWire is off the hook.

http://www.google.com/url?sa=t&sour...dilcoc-VSmznG_zmQ&sig2=Tqzrf8P5ZMfGdLHC0o4w-A

"A final note about equipment? some equipment requires the conductors that
are terminated in the equipment to have an insulation rating of 90 ?C, but an
ampacity based on 75 ?C or 60 ?C. This type of equipment might include
100% rated circuit breakers, fluorescent lighting fixtures, etc., and is marked
to indicate such a requirement. Check with the manufacturer of the
equipment to see if you need to take into account any special considerations."
Click to expand...

You probably just made that up. ;):grin:
 
Status
Not open for further replies.
Top