80% or 100% rated

[Mike01]

maybe someone can better explain this a MCCB (UL489) circuit breaker with an 800AF standard breaker apply solid state electronic trip device now with a 800AF / 800AS / 600AT now the normal breaker rating at 800A for continuous operation is 640A with a 600AT setting the 80% continuous rating is 480A why is the trip derated 80% also?

 

[Mr. Bill]

This is a bit confusing to read. Can you restate the question? Add punctuation.

 

[templdl]

Since you question I'll try to shed some light on it for you.

It all starts with a 100% rated OCPD such as a molded case circuit breaker that is UL listed for application at 100% of its rating. These breaker will have a solid state trip unit and usually will be at least a 400A frame if not larger.
100% rated breakers aren't any better than the its common counterpart. The only difference if that they have been UL list for 100% application only when used in an enclosure of installation that is also listed with it for 100%.

When you are so inclined to apply a 100% breaker and read the instructions carefully you will find that it must be applied with 90degC rated cable, the cable of which is applied at 75degC. Remember that cable must be sized to carry the load and breakers then protect the cable.

Normally, the cable is sized 100% of the non-continuos load plus 125% of the continuous load. The cable size selected must carry that load. The breaker is sized to protect the cables rated ampacity that is often more than the calculated load because the breaker rating is based upon the cable rating and not the load.

When applying 100% rated breakers it is 100% plus 100% of the continuous and non-continuous loads. You select the cable size based upon 75degC and you will note that it gives you the opportunity to select a smaller cable.

Smaller copper cable means lower cost. BUT, that cable MUST BE RATED 90degC and not 75degC. This may also give you the opportunity to step down a breaker frame size depending upon what cable size that you end up with and you get lucky. The next smaller frame size can also reduce cost.
So applying cable at 100% is a bit more than that.
I trust that this sheds some light on the application.

You may also not that 100% rated breaker have electronic trip units. The standard breaker will have mechanically thermal magnetic as well as the available identical electronic trip unit. The electronic trip unit does derate with ambient temperature that makes it ideal for 100% rated breakers. Electronic trip units don't make a better breaker but they do provide the ability to apply the LSIG features, digital read outs, communication, etc. Electronic trip units provide a lot more capability to coordinate.

To be honest, we in OEM sales are very intrigued by those who think they are getting a better breaker because it's 100% rated. We love to collect that extra money. I was a Sales and Application engineer for 18 years and am now a Sales Engineer for a dry type transformer manufacturer and we still supply 100% breaker because they are specified knowing that they can't be applied per their 100% UL listing. They can only be applied, as would be a standard breaker.

 

[Mike01]

molded case

molded case

thanks for all the information definately sheds some light however this is my dilema question really. if I have a 400A molded case circuit breaker with a 250A sensor and a 250A trip the trip unit is rated 100% continuously correct? your would not apply the 80% twice and double dip? the way i have seen it is some manufactureres catalogs the continuos rating of the trip is dependant on a couple of things continuous power = frame x sensor% x long time setting. with the understanding that a 400A frame with a 400AS and 400 AT sensor is rated at 80% unless it is a 100% rated frame breaker. does this sound correct?

 

[kroken]

I though that advantage of 100% rated breaker is that it allows you to utilize 100% of the breaker rating. For example, if I have 360A continous load, I cannot specify 400A CB with 80% rating since it would trip the breaker. I though that if I use 100% rated breaker, 125% of continous load is not apply (see 210.20). with the same example of having 360A continous load, I use 600A CB with 80% rated, I still need to run feeder at least 600 Amp (2 set of 350 mcm) as to match OCPD.

 

[jim dungar]

thanks for all the information definately sheds some light however this is my dilema question really. if I have a 400A molded case circuit breaker with a 250A sensor and a 250A trip the trip unit is rated 100% continuously correct? your would not apply the 80% twice and double dip? the way i have seen it is some manufactureres catalogs the continuos rating of the trip is dependant on a couple of things continuous power = frame x sensor% x long time setting. with the understanding that a 400A frame with a 400AS and 400 AT sensor is rated at 80% unless it is a 100% rated frame breaker. does this sound correct?

There is no guarantee that an electronic trip breaker is 100% rated.

The trip unit is calibrated so, that at a 1.0 setting, it will perform like an 80% rated breaker unless you have a specific one calibrated to perform like a 100% rated one. The fact it is a 250A trip unit in a 400A frame with 400A sensors is immaterial.

 

[Pierre C Belarge]

this if from the '99 NEC, but it is still applicable.

"

As explained in Section 210-20(a) of the 1999 National Electrical Code
(NEC):
?Where a branch circuit supplies continuous loads or any combination of
continuous and noncontinuous loads, the rating of the overcurrent device
shall not be less than the noncontinuous load plus 125 percent of the
continuous load.​

​

​

​

?Exception: Where the assembly, including the overcurrent devices
protecting the branch circuit(s), is listed for operation at 100 percent of its
rating, the ampere rating of the overcurrent device shall be permitted to be
not less than the sum of the continuous load plus the noncontinuous load.?​

​

This exception allows for 100%-rated circuit breakers used in equipment
tested and listed for 100% of rating and has led to the phrases ?100%-rated
circuit breaker? and ?standard-rated (80%-rated) circuit breaker.?
Underwriters Laboratories Inc. (UL) specifies tests that must be conducted to obtain listings for continuous operation at 100% of rated current."​

Circuit 1
1. Using a standard-rated circuit breaker:
? Minimum required ampacity = (1000 x 1.25) = 1250 amperes
? Because 1250 A is not an available rating, a 1400-ampere circuit
breaker would be required.​

2. Using a 100%-rated circuit breaker:
? Minimum required ampacity = 1000 amperes
? A 1000-ampere circuit breaker would be required.
? In this case, the 100%-rated circuit breaker offers two-fold savings:
smaller frame size circuit breakers, which may reduce the size of the end-use equipment and the space needed in the electrical room; and smaller conductors.​

 

[Mike01]

hmm.

hmm.

The trip unit is calibrated so, that at a 1.0 setting, it will perform like an 80% rated breaker unless you have a specific one calibrated to perform like a 100% rated one. The fact it is a 250A trip unit in a 400A frame with 400A sensors is immaterial.

So i guess than the ansewer is that the trip is calibrated with a long time rating at 1.0 to act like a standard breaker with an 80% continuous rating of the sensor size. this just seems a little off you would think if the breaker is 400A (standard 80% rated) and you utilize a 250AS with a 250AT setting that since 250 is less than 80% of 400 (320) that the trip would be rated for continuous rated at 250A as it turns out unless you have a trip unit calibrated for 100% rating the 80% is applied to the trip setting also unless defined as a 100% rated trip or trip / breaker combination. I guess you could always specify a standard 80% continuous circuit breaker with a 100% rated trip unit as long as the trip unit does not exceed 320A.

 

[templdl]

Calibration is not the issue as the trip unit is solid state and depends upon CTs to provide it with current values.
Unlike its "physically" thermal magnetic counterparts that depend upon a bimetal thermal element that is sensitive to heat and a magnetic element that responds to magnetic attraction based upon current intensity the solid state trip unit is insensitive to both and depends upon the CTs for current flow information.
The 80% rated thermal elements react to ambient temperatures and to any heat generated by the cable. When a breaker is tested for UL489 it is tested with a specific length of rated cable when the breaker is tested. The cable helps act as a heat sink in helping the breaker stay within its temperature rise an not add heat to the breaker. If the cable added heat to the breaker it would influence the thermal element and change the trip characteristics.
Also, there are the 80% rated solid state trip unit counter parts that are tested and applied the same.
The manufacture takes the solid state trip unit breaker one step farther by being able to test and label it as 100% rated but the instruction that come with the breaker must be complied with in order to apply it at 100%. This basically involved the use of 90deg C cable and either and enclosures listed for such use or a minimum size enclose.
In addition, since breakers with solid state trip units are insensitive to heat they are susceptible to being overheated because of ambient temperatures or other issue that cause heating which prevents them form tripping thermally as they derate. Thus the solid state trip unites that I'm familiar with do use a 90deg C thermistor that will tell the trip unit to trip should the internal temperature of the breaker reach 90deg C or greater.