Time/Current Curves

[Mel Coben]

Molded Case circuit breaker manufacturers provide curves specifying minimum and maximum trip times vs % of rated current, but these curves vary substantially from manufacturer to manufacturer. For example, Company A says their 15 Amp breaker @ 300% load, 40 degrees C, should trip in 14 to 45 seconds; Company B says 4.5 to 18 seconds; Company C says 5 to 30 seconds; Company D says 15 to 50 seconds.

Since these breakers are to protect the same #14 wire, it would behoove the wire manufacturers to provide TCC's for their products.

 

[brian john]

While I do not have the standards before me, I believe they do. The CB manufacturers are designing to the minimum and maximum currents and delay.

 

[jim dungar]

Molded Case circuit breaker manufacturers provide curves specifying minimum and maximum trip times vs % of rated current, but these curves vary substantially from manufacturer to manufacturer. For example, Company A says their 15 Amp breaker @ 300% load, 40 degrees C, should trip in 14 to 45 seconds; Company B says 4.5 to 18 seconds; Company C says 5 to 30 seconds; Company D says 15 to 50 seconds.

Since these breakers are to protect the same #14 wire, it would behoove the wire manufacturers to provide TCC's for their products.

A UL 489 listed overcurrent device will protect a <600V conductor applied per the NEC.

FYI, fuses are not necessarily better than breakers, at 45A a Bussmann FRS-R15 will take about 60 seconds to 'open'.

My cable damage curves do not show how long a #14 THHN wire can carry 45A. My curves cut off at 10Sec.

 

[don_resqcapt19]

Jim,
Can you work the I^2t ratings backwards to come up with how long a #14 can carry 45 amps without damage? If so, using the values from Table 1 in this document, I come up with 23.4 seconds. This would be the time it takes for the #14 to reach 150?C with a current of 45 amps. That seems like a short time to me, so maybe you can't work the numbers in that table backwards for times that exceed the 5 seconds that the table is based on, or maybe I didn't do the math correctly.

 

[brian john]

Jim,
Can you work the I^2t ratings backwards to come up with how long a #14 can carry 45 amps without damage? If so, using the values from Table 1 in this document, I come up with 23.4 seconds. This would be the time it takes for the #14 to reach 150?C with a current of 45 amps. That seems like a short time to me, so maybe you can't work the numbers in that table backwards for times that exceed the 5 seconds that the table is based on, or maybe I didn't do the math correctly.

That does not seem right.

 

[jim dungar]

Jim,
Can you work the I^2t ratings backwards to come up with how long a #14 can carry 45 amps without damage? If so, using the values from Table 1 in this document, I come up with 23.4 seconds. This would be the time it takes for the #14 to reach 150?C with a current of 45 amps. That seems like a short time to me, so maybe you can't work the numbers in that table backwards for times that exceed the 5 seconds that the table is based on, or maybe I didn't do the math correctly.

I do not believe that you can use that table to work backwards.
That table is based on short circuit currents not 'overloads'. Also it is based on the conductor starting at 75?C. As I mentioned previously it appears a Bussmann FRS-R15 fuse is actually slower than some breakers at 3x FLA (45A). I am sure Bussmann feels their fuse will protect a #14AWG THHN conductor.

 

[mivey]

Jim,
Can you work the I^2t ratings backwards to come up with how long a #14 can carry 45 amps without damage? If so, using the values from Table 1 in this document, I come up with 23.4 seconds. This would be the time it takes for the #14 to reach 150?C with a current of 45 amps. That seems like a short time to me, so maybe you can't work the numbers in that table backwards for times that exceed the 5 seconds that the table is based on, or maybe I didn't do the math correctly.

Using a fusing calculation and going from 20 deg C to 90 deg C, I get 26.72 sec.

for 20 to 150 deg C I get 45.4 sec
for 75 to 150 deg C I get 23.9 sec

 

[steve66]

Jim,
Can you work the I^2t ratings backwards to come up with how long a #14 can carry 45 amps without damage? If so, using the values from Table 1 in this document, I come up with 23.4 seconds. This would be the time it takes for the #14 to reach 150?C with a current of 45 amps. That seems like a short time to me, so maybe you can't work the numbers in that table backwards for times that exceed the 5 seconds that the table is based on, or maybe I didn't do the math correctly.

Don:

It looks like your method is exactly how they got the 5 second rating numbers.

But for lower currents, its possible there is more time for the heat to dissipate, so you might get a little extra "safety margin" by working backwards.

Steve

 

[zog]

Molded Case circuit breaker manufacturers provide curves specifying minimum and maximum trip times vs % of rated current, but these curves vary substantially from manufacturer to manufacturer. For example, Company A says their 15 Amp breaker @ 300% load, 40 degrees C, should trip in 14 to 45 seconds; Company B says 4.5 to 18 seconds; Company C says 5 to 30 seconds; Company D says 15 to 50 seconds.

Since these breakers are to protect the same #14 wire, it would behoove the wire manufacturers to provide TCC's for their products.

ANSI/NETA has a max time spec for all MCCB's by frame size, this table comes from NEMA Standard AB 4-1996. For <250V 0-30A MCCB the max trip time at 300% is 50 seconds, so as long as it trips before that it meets the NEMA, ANSI, and NETA spec.