12awg cable won't coordinate with 20A CB

designer82

Member
Location
Boston
For the life of me I can't understand why this #12awg cable overlaps the 20A breaker curve on the TCC and I can't get it to coordinate.

If I change it to a 30A cable, it will not overlap anymore or if I make the breaker 15A. None of that should be necessary with a #12awg cable and
20A CB though.

Any help is appreciated


picture
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
You really have not provided much context, but when I look in the upper left of your graph it has a box that says Cable HV193 Ampacity with 3 red dots on the 15A line.

So my first question is: what about the installation conditions or software assumptions makes the system think the ampacity of a 12 ga cable is 15A?

Jon
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
All molded case breakers listed to UL 489 will always protect UL listed conductors, properly applied per the NEC, even though the TCC does not show it. This is partly why 4ft of conductor is included as part of the breaker testing procesure and set up.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
You might check the cable details - it is set for copper right? THHN or something similar in conduit? Is there an ambient temp setting?

I agree with Jim, you shouldn't have to worry about #12 coordinating with a 20A breaker, but the plot of the wire does look like it goes farther left than it should.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
You might check the cable details - it is set for copper right? THHN or something similar in conduit? Is there an ambient temp setting?

I agree with Jim, you shouldn't have to worry about #12 coordinating with a 20A breaker, but the plot of the wire does look like it goes farther left than it should.
The plots will never show small size conductors as being protected. It is a problem with the way conductor and breaker plots are created.

Follow the results from the actual testing.
 

designer82

Member
Location
Boston
Thanks guys much appreciated.

Yes the cable is set properly. #12 THHN, cu in conduit.

I understand what Jim is saying that it doesn't coordinate correctly on the software.


Bummer the customer will get the Etap auto-coordination print out that shows red on this branch. Oh well
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Bummer the customer will get the Etap auto-coordination print out that shows red on this branch. Oh well
Why even show the conductors on the TCC? Make sure your write up includes the explanation that conductor protection is demonstrated by the UL procedures.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The plots will never show small size conductors as being protected. It is a problem with the way conductor and breaker plots are created.

Follow the results from the actual testing.
EasyPower shows the entire #12 wire plot completely to the right of a 20A breaker curve from 0.2 seconds to 10 seconds. And the vertical long term wire rating is on the far right side of the breakers long time rating.

So I still think there is something wrong with the plot posted - the wire curve crosses into the breaker curve at 0.2 seconds, and stays there. And the long time plot is on the left side of the breaker curve.
 

designer82

Member
Location
Boston
EasyPower shows the entire #12 wire plot completely to the right of a 20A breaker curve from 0.2 seconds to 10 seconds. And the vertical long term wire rating is on the far right side of the breakers long time rating.
Do you have the utility at 480V-3PH and similarly rated breaker?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I used a 208 volt utility and a QO breaker, but the voltage shouldn’t make any difference in this case.

If you have a system with multiple voltages, say 208V and 480 volts, if the 208v components are plotted on a 480 v scale, they will be shifted by a factor of about 2. But it doesn’t look like that’s what is happening in your case.
 
Top