Circuit Breaker Trip Ratings

[kyoungberg]

Description:

Design indicates a 1200 Amp, 480 Volt, 3-phase, 3-wire Motor Control Center being fed from a Switchboard Circuit Breaker that utilizes adjustable solid state trip units. (1600 Amp Frame, 1200 Amp Trip)

Is it acceptable to route 3 sets of 500Kcmil, Cu, THHN, between this SWBD and the MCC?

If the trip unit is set at .95 then the overcurrent device becomes 1,140 Amps, which then meets the 75 degree column of 310.16.

My concern is this setting could be deliberately or inadvertantly modified in the future causing the installation at that time to be non-compliant.

It would be reasonable that this industrial facility could comply with 240.90, Supervised Industrial Installations.

Comments, Suggestions, Common Practices, Other applicable code?

Thanks, in advance!

 

[charlie b]

I think the setup you described is acceptable. You are not responsible for assuring that the next electrician does not create a code violation. Is it the best design for that particular facility? That I cannot answer without more information. But it is at least code compliant.

 

[bsh]

Look at NEC 240.6 (C). If access is restricted to the breaker adjustment mecahanism then you can adjust the trip rating to something less than the max. and provide appropriate cable. Someone can still modify the setting but they would have to know what they are doing.

 

[kingpb]

Keeping in mind that the Code is minimum requirements for safety and does not mean its good engineering or practical for the operation. As Charlie indicated, "it is at least code compliant." Least as in minimum.

 

[mayanees]

... common practice

... common practice

Common practice for an industrial giant that I used to work for was to post the settings on the breaker. That way it's readily apparent if they get changed.
JM

 

[mull982]

Description:

Design indicates a 1200 Amp, 480 Volt, 3-phase, 3-wire Motor Control Center being fed from a Switchboard Circuit Breaker that utilizes adjustable solid state trip units. (1600 Amp Frame, 1200 Amp Trip)

Is it acceptable to route 3 sets of 500Kcmil, Cu, THHN, between this SWBD and the MCC?

The three sets of 500kcmil will give you an ampacity of 1140A (3*380A=1140A) Is this enough to cover the 1200A bus on the MCC. Wouldn't the ampacity of your feeders have to be equal to or greater than the ampacity of the MCC bus which in this case is 1200A. In our industrial facility our 480V 1200A MCC's have 4 sets of 500Kcmil with the trip setting on the breaker set to 1200A.

 

[jim dungar]

The three sets of 500kcmil will give you an ampacity of 1140A (3*380A=1140A) Is this enough to cover the 1200A bus on the MCC. Wouldn't the ampacity of your feeders have to be equal to or greater than the ampacity of the MCC bus which in this case is 1200A.

No, the feeders need to be equal to or greater than the LOAD on the MCC not the bus bars.

 

[zog]

"If the trip unit is set at .95 then the overcurrent device becomes 1,140 Amps,"

Not necessarliy true, depends on the type of trip unit. Some have rating plugs, some have multiple tap CT's, some 1200A breakers have smaller CT's, some have a current setting or tap on the trip unit, any of those may cause your assumption to be false.

Now some trip units have security features that would require a password or a key to change the settings, if you are concerned about someone changing the settings, that would be your best option.