75*C conductor temperature

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mbrooke

mbrooke

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I'm doing some 240.92 (B) equations. Would 40*C be more reasonable vs 75*C? What is an actual reasonable operating temperature of current carrying conductors?
 
Ampacity calcs are designed to get the conductors up to their rated temperature in a 20C ambient...and when you consider the often needed 80%/125% rules one would expect that the resulting usage at 80% of ampacity would result in 64% of nominal temperature rise (meaning about 55C for nominal 75C conductors)

Yet in my experience it is rare for conductors to be more than slightly warm to the touch.

Jon
 
winnie said:
Ampacity calcs are designed to get the conductors up to their rated temperature in a 20C ambient...and when you consider the often needed 80%/125% rules one would expect that the resulting usage at 80% of ampacity would result in 64% of nominal temperature rise (meaning about 55C for nominal 75C conductors)

Yet in my experience it is rare for conductors to be more than slightly warm to the touch.

Jon
Click to expand...


Neat, thanks! :)

And yahh, I've seen the same- conductors only get warm to the touch. Nothing that can cause you to go "ouch"
 
My experience has been somewhat different. The things is that the heat is proportional to the square of the current, and most loads rarely run at 100%. The exception is process plants when the motor is the bottleneck and operating a can carefully adjust say a VFD or fan to run at maximum load. Under those conditions it runs pretty warm and ANY issue no matter how small such as worn starter contacts or elevated room temperatures will cause failures.

Even running 10% below rated current reduces the heat load 21%. And 75 C really isn’t that hot. It is enough to cause a burn but not even enough to boil water. And the 75 C limitation is usually at the terminals where there is a small resistance, enough to cause elevated temperatures, while most wiring is rated 90 C so the insulation won’t be affected,
 
Table 240.92(B) Q...>>>

t = time of short circuit in seconds (for times less than or equal to
10 seconds)
T 1 = initial conductor temperature in degrees Celsius
T 2 = final conductor temperature in degrees Celsius

Is 10 seconds the difference betwixt T1 & T2 ???

~RJ~
 
romex jockey said:
Table 240.92(B) Q...>>>

t = time of short circuit in seconds (for times less than or equal to
10 seconds)
T 1 = initial conductor temperature in degrees Celsius
T 2 = final conductor temperature in degrees Celsius

Is 10 seconds the difference betwixt T1 & T2 ???

~RJ~
Click to expand...


When would 10 seconds be reasonable? Generator hook up? :unsure:❓
 
paulengr said:
My experience has been somewhat different. The things is that the heat is proportional to the square of the current, and most loads rarely run at 100%. The exception is process plants when the motor is the bottleneck and operating a can carefully adjust say a VFD or fan to run at maximum load. Under those conditions it runs pretty warm and ANY issue no matter how small such as worn starter contacts or elevated room temperatures will cause failures.

Even running 10% below rated current reduces the heat load 21%. And 75 C really isn’t that hot. It is enough to cause a burn but not even enough to boil water. And the 75 C limitation is usually at the terminals where there is a small resistance, enough to cause elevated temperatures, while most wiring is rated 90 C so the insulation won’t be affected,
Click to expand...


I'm guessing 75*C is insulation or standing neutral to ground fault with conduit (ferrous) circuit? 75*C seems extreme imo, but, when it comes to steady state amapacity the code does have a really good reason.
 
mbrooke said:
When would 10 seconds be reasonable? Generator hook up? :unsure:
Click to expand...

75 C is the limit. From working in a foundry where melting and burning stuff is called “production” the limiting factor is the plastics and rubber used in most terminals. Even bus bar with lug connections is supported by say FR3 and electrically conductive materials are also massively thermally conductive. We found damage in the strangest places. Plus radiation (think standing by camp fire) is amazingly efficient and bridges any gap but a thin sheet of aluminum almost instantly shuts it down.

Ampacity tables are developed from Neher-McGrath formulas based on “continuous” duty (defined as 3 hours). However under short circuit conditions steady state heat transfer doesn’t happen the same way and different formulas are needed. That’s your 10 second rule although most switchgear is rated 3 or 30 cycles.

Sometimes it would be nice if NEC had less lawyer and more scientific (why) information.

Arc flash is different still. We only care about 2 seconds or less and as long as it doesn’t catch on fire a heavy winter jacket does the trick. 40 cal suits are available as winter overalls and jackets.
 
paulengr said:
Sometimes it would be nice if NEC had less lawyer and more scientific (why) information.
Click to expand...

Good one Paul....

, so a second is a lot of time ,a lot of damage can occur

So i'm confused as to why , within the same code article, we've gone from 10 seconds to .07
:oops:~RJ~:oops:
 
paulengr said:
75 C is the limit. From working in a foundry where melting and burning stuff is called “production” the limiting factor is the plastics and rubber used in most terminals. Even bus bar with lug connections is supported by say FR3 and electrically conductive materials are also massively thermally conductive. We found damage in the strangest places. Plus radiation (think standing by camp fire) is amazingly efficient and bridges any gap but a thin sheet of aluminum almost instantly shuts it down.

Ampacity tables are developed from Neher-McGrath formulas based on “continuous” duty (defined as 3 hours). However under short circuit conditions steady state heat transfer doesn’t happen the same way and different formulas are needed. That’s your 10 second rule although most switchgear is rated 3 or 30 cycles.

Sometimes it would be nice if NEC had less lawyer and more scientific (why) information.

Arc flash is different still. We only care about 2 seconds or less and as long as it doesn’t catch on fire a heavy winter jacket does the trick. 40 cal suits are available as winter overalls and jackets.
Click to expand...

At 10 seconds we can still assume no heat has escaped, correct? 150*C final temp?
 
240.67(B) (4) ,along with 240.87(B)(3) and info note seem to be asking for the same thing>>>
An energy-reducing active arc flash mitigation system

Can someone explain this?

~RJ~
 
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