ABB TMax T5/T6 Termination Ratings

[JuliaTruchsess]

I've corresponded with ABB's tech support and they've advised me that "The terminal rating is the same as the breaker rating, 70 Deg. C."

I don't have a TMax on hand to look at, and I'm wondering if there are markings (there must be markings, right?) on the TMax conforming to the more standard ratings of 60 or 75C - if anyone has experience with these breakers and can tell me how they're marked, that'd be helpful.

Assuming they're marked 70C, what are my options? Can I use the 75C column of 310.15(B)(16)? Can I interpolate between the 60C and 75C or between the 60C and 90C columns to get my own 70C value? Do I have to use the 60C column?

 

[JuliaTruchsess]

I note, FWIW, that the UL Whitebook, under DHJR Circuit Breakers, says "A circuit breaker with a current rating of more than 125A is suitable for use with conductors rated 75C"

 

[Julius Right]

According to NEC 2017 Art.110.14 (C)
Informational Note: With respect to 110.14(C)(1) and (C)(2),
equipment markings or listing information may additionally
restrict the sizing and temperature ratings of connected conductors.
In this case 60oC is suitable if you'll use the tables. However:
310.15 Ampacities for Conductors Rated 0–2000 Volts. (C) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general equation:
See Neher and McGrath theory [ IEEE Std 835-1994]:
The Calculation of the Temperature Rise and Load Capability of Cable Systems by
J.H.Neher and M.H. McGrath.
The calculation is detailed also in IEEE Std 835-1994.

 

[Julius Right]

On the other hand if we try to use NEC recommended formula:
I=SQRT((Tc-Ta)/(Rdc*(1+Yc)*Rca))*10^3 then:
knowing for 70oC Yc will be less then Yc for 75oC but considered the same
I75/I70=sqrt((75-30)/(70-30)*(234.5+70)/(234.5+75))=1.052 [a bit conservative]
So you can divide by this the ampacity for 75oC column.:thumbsup:

 

[jim dungar]

I haven ever seen anyone go through this much effort to size conductors, using the 75C column, for a breaker listed under UL489.

According to ABB literature the normal ambient temperature of your breaker is 70C. Their tech support may have given you the operating temperature of their breaker terminal pads, and not the wire sizing temperature used during the UL testing process. Just because wire is sized using the 75C column does not mean that it will be operating at 75C at its terminations, there may be enough heat sink effect of the breaker terminal pad to maintain to offset any heat gain from a properly sized lug and conductor.

 

[JuliaTruchsess]

I haven ever seen anyone go through this much effort to size conductors, using the 75C column, for a breaker listed under UL489.

According to ABB literature the normal ambient temperature of your breaker is 70C. Their tech support may have given you the operating temperature of their breaker terminal pads, and not the wire sizing temperature used during the UL testing process. Just because wire is sized using the 75C column does not mean that it will be operating at 75C at its terminations, there may be enough heat sink effect of the breaker terminal pad to maintain to offset any heat gain from a properly sized lug and conductor.

70C is not the "normal ambient" of the breaker, it's the Maximum Operating Temperature of the breaker. If the terminals are at 70C, then so is at least part of its innards. If a conductor is running at its ampacity as listed in the 75C column, then it's at 75C. Not good.

 

[Julius Right]

If we shall use my above formula [I75/I70 for instance] for inverting the 75oC ampacity to 60oC the error is less than 1%.

 

[Julius Right]

For instance using this factor:
I75/I60=sqrt((75-30)/(60-30)*(234.5+60)/(234.5+75))=1.194697
If I75=255 A [250 mcm] I60=255/1.194697=213.4 A
and for 500 MCM I75=380 I60=380/1.194697=318
or for 1000 MCM I60=545/1.194697=456 A

 

[Jraef]

You are conflating the maximum rated ambient operating temperature of the device (breaker) vs the wire temperature rating of the conductor terminals. All breakers are calibrated for 40C and must be de-rated for operating temperatures over that, UP TO the maximum rated temperature, in this case 70C. The lugs themselves are rated for 75C rated conductors. Just because you can use 75C conductors doesnt mean the breaker can be used st 75C. Different issues.

 

[jim dungar]

70C is not the "normal ambient" of the breaker, it's the Maximum Operating Temperature of the breaker. If the terminals are at 70C, then so is at least part of its innards. If a conductor is running at its ampacity as listed in the 75C column, then it's at 75C. Not good.

I have a copy of the standard NEMA AB1-1986, yes it is old but that is all I have access to right now. For circuit breakers 150A and larger the only acceptable conductors are sized using the 75C column. The minimum length of the connected cable is 4 ft. Thermocouples are used to measure the temperature rise of various locations while the breaker is carrying 100% of its rated current in open air. In order to pass, the temperature rise is limited to 50C on wiring terminals (where the conductor insulation is). For 100% rated breakers, the testing is done in enclosures, not free air, and the manufacturer shall specify 90C conductors be installed (although they are not part of the test procedure), if the temperature rise is more than 50C and less than the fail point of 60C. The ambient air temperature is measured 12" from the breaker. Main operating parts of the breaker can run as high as 60C rise above a 25C ambient.

I believe UL489 is similar to these tests.

 

[JuliaTruchsess]

My switchboard shop had a look at the TMax breakers and advises they're marked 75C. Thanks to all who replied.