80% rated breakers 2

[RLebel]

Most manufacturers advertise their breakers at 80% and in some cases 100%.

I understand the NEC rules and its intent. My question is what happens to a 80% breaker which is loaded at 100%?

Example:
A distribution panel contains a mixture of lights and receptacles. The panel load is 80A. The cable and breaker feeding the panel are rated at 100A (125% of the estimated load)

Someone plugs in an additional 20A of continuous loads in the receptacles.

The load on the breaker is now 100A.

1) Assuming the breaker is an electronic trip breaker, its characteristics curve would be unaffected by the increased heat load, meaning the 100A of loads could run continuously without tripping the breaker. Could the added heat load cause a breaker failure?

 

[jraef]

... what is to stop a plant operator to increase the setting down the road?

Again, common sense and his duty to provide a safe working environment. The EXACT SAME THING that prevents him from installing a fixed trip breaker that is incorrectly sized.

Besides, if a manufacturer provides a trip unit which can be adjusted to unsafe levels, I would ASSUME that the restriction on the trip setting would be provided by the manufacturer, however that doesn't seem to be the case.

So then should every mfr provide breakers that are keyed in some why so as to avoid ANYONE from EVER putting in a breaker that is the wrong size for the conductors attached to it or the enclosure it is used in?

You don't seem to be getting the point. THE MANUFACTURER cannot be held responsible for someone misapplying a product. I know lawyers want us to believe otherwise, but it is just plain impossible.

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[RLebel]

We agree the manufacturer is not responsible for the application.

My original question was could a breaker shipped directly from the manufacturer be set at a value which could damage itself? Based on your posts the answer is yes, but it is the designer's responsibility to adjust it to a safe setting? Essentially the 100% setting on the trip unit is just for show on 80% rated breakers and should NEVER be used? One trip unit for both 80% and 100% rated breakers simplifies production and the expectation is for the designer to never raise the setting above 80%. I would've expected/liked the manufacturer to state that restriction though.

In my previous experience, trip units were based on the rating of the breaker. Maybe this is all changing and they are more of a one size fits all solution when it comes to electronic trip units (same trip unit used on both 80% and 100% rated breakers).

PS. I am not a lawyer and do not plan on suing anyone over this.

 

[davidbeach]

Remember, there is no prohibition on using an "80%" breaker at 100% for periods not exceeding 3 hours.

 

[LionelHutz]

RLebel - a similar arguement could be made about motor overloads. The breaker is usually 1.5X to 2.5X the motor FLA so it doesn't protect the motor and many overloads can set much higher then the motor FLA. At the end of the day, the manufacturer is responsible to provide equipment which will work in the intended application and the personel installing and operating the system are responsibile to ensure the components are installed and set-up correctly. The manufacturer isn't required to provide "idiot proofed" components.

 

[urded]

80% rating only applies to continuous loads. Not non continuous loads.
you can load a standard breaker (80% rated) up to 100% of its rating.
You multiply the continuous load x 1.25%
You then add the noncontinuous load at 100%.
The rating of the breaker has to be more than the sum of those two values.

1999 NEC Handbook on page 98 has an example
125A non continous load
200A continous load
200A x 125% = 250A computed continous load
125A non continous + 250A continous = 375A
OCPD size selection
The minimum standard size OCPD, according to section 240-6 is 400A
(no mention of 100% rated device needed)
400A x 80% would be 320A. So the NEC handbook would be wrong if you could only load it to 80% like most people think.

I would say 80% of electricians and engineers misapply the "80% rule".

hope that helps.