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?

 

[davidbeach]

It's a thermal issue. Breaker could over heat, lugs could over heat. Failure might take a fair amount of time, but the code violation would be as soon as the three hour mark was past.

 

[dpc]

The electronic trip adds an interesting twist, but as you probably know, the NEC does not make that distinction. Also, you cannot buy an individual circuit breaker that is 100% rated. It can only be 100% rated in an assembly that has been specifically listed for 100% rated breakers.

It is the long term overloading that would be a concern, as David mentioned.

 

[jraef]

All breakers are tested and rated at 100% of their rating. But that does not mean they can be USED at that rating, because the use is not specifically defined by the breaker, it is defined by the application. As dpc pointed out, the key issue here is that you said "A distribution panel contains..." Even if you bought a breaker with a l;abel that said it was 100% rated, it is still not going to be rated for 100% when put into a distribution panel, unless the ditribution panel WITH THAT SPECIFIC BREAKER is also rated for 100%.

And they never are.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[RLebel]

I am aware of the of code violation, however there is no way to prevent someone down the road from plugging in an additional 20A of loads in receptacles. And in this example, the breakers are rated for 80%, so the 100% does not come into the picture.

This leads to my second question

2) A 100A breaker rated at 80% has an electronic trip unit which can be set at 100A (100% of it rating). Does this mean that the manufacturer would allow an overload setting which could lead to heat induced failure on the breaker? Does this not defeat the whole purpose of having a breaker in the circuit?

For those interested, here are the specific components of the design:

The breakers in question are the Powerpact H Frame from SquareD (Schneider Electric) equipped with the Micrologic 3.2 trip unit.

The trip unit that comes with the breaker is dependent on the frame size of the breaker and can be adjusted up to 100% of the breaker frame size.

The breakers provided are standard 80% rated breakers as seen on page 26 of the link below. Although 100% rated breakers are available, the panel in which the breakers are being installed is not. Purchasing 100% rated breakers would not help in this case.

See the catalog here

On page 59 you will find the specifics of the trip unit.

So essentially an 80% breaker is being supplied with a 100% trip unit. If the trip is set at 100%, the breaker would generate excess heat potentially leading to a premature failure. The manufacturer has no restrictions on the breaker setting.

The easy answer is to reduce the trip setting to 80%, however my concern is that a manufacturer provides a breaker with a trip unit that would allow a breaker to be overloaded, leading to premature breaker failure.

On thermal-mag breakers, this was a non issue as the increase in temperature would lead to a faster trip, however electronic trip units are unaffected by temperature.

Someone following the code, installs a 100A breaker with a 100A trip setting for an 80A load as per code. An office admninistrator down the road plugs in 19A of additional continuous load via receptacles. The breaker is not rated to handle the additional load, however the trip unit would not trip as it is still below 100A.

Sorry if the post is a little long, I wanted to provide as much information as possible.

 

[rhatcher]

RLebel,

These breakers are offered with 80% rated and 100% rated electronic trip units. Check the catalog number of your breaker.

Electronic Trip Unit - 3 pole breakers only - page 28
HDL36100U31X 80% rated LI
HDL36100U33X 80% rated LSI
HDL36100CU31X 100% rated LI
HDL36100CU33X 100% rated LSI

Thermal Trip Unit - 3 pole breaker - page 26
HDL36100 80% rated (for 2 pole: HDL26100)
HDL36100C 100% rated (for 2 pole: HDL26100C)

Note: for different interrupting ratings, substitute G, J, or L place of the D (for example HGL36100U31X)

Whether using a thermal trip or an electronic trip, selection of the 80% rated breaker will meet the NEC requirement. If you select the 100% rated breaker, you are in violation of the NEC.

To answer your question: For a 100A setting, the 80% rated electronic trip unit will trip at 80A and the 100% rated electronic trip unit will trip at 100A. In either case, the trip unit will not allow the breaker continuous current to exceed the appropriate rating of 80% or 100%.

Finally, as pointed out by jraef, the breaker frames and contacts are the same for the 80% rating and the 100% rating. The only differences between the two ratings are the trip units and the lugs (80%=aluminum, 100%=copper).

 

[RLebel]

For a 100A setting, the 80% rated electronic trip unit will trip at 80A and the 100% rated electronic trip unit will trip at 100A.

Where did you get this information from?

The same trip unit, micrologic 3.2(LI) or 3.2S(LSI), is provided for both the 80% and 100% rated breakers. The units have a setting of 100A for the 100A frame regardless of whether it is a 80% or 100% rated breaker.

If your statement is true, then a setting of 100A on the breaker would actually be 80A?

 

[RLebel]

PS I should probably have specified in my previous post: the breakers provided are 80% rated, the trip unit provided by Schneider for the 80% rated breakers are the 100% rated electronic trip units. They do not have an 80% rated trip unit (or so I am told by their technical support)

RHatcher if you have and can provide additional information I would appreciate it!

Thanks to all the replies so far.

 

[rhatcher]

RLebel,
When I first checked the catalog, I had expected to find that the breakers equipped with electronic trip units were 100% rated. The prospect of a 80% rated breaker being equipped with a trip unit allowing 100% continuous current flow didn't make sense. No offense but, I thought that you must be mistaken.

However, when I saw that they listed UL rated electronic trip unit breakers with different catalog numbers for 80% or 100% rating, I made the presumably safe assumption that the trip units were rated differently. Specifically, the long time pick-up points would be 80% of the rating for 80% breakers and 100% of the rating for 100% rated breakers. I would think that it would have to be this way. If the breaker is 80% rated, the trip unit should (must?) limit the continuous current to 80% of the breaker rating. Otherwise, the 80% rating is meaningless.

However, now that I have checked the catalog in detail, I cannot find anything to support my assumption. The only thing that I can see related to this question is the statement: "Circuit breakers with 100% rating can also be used in applications requiring only standard (80%) continuous loading." (bottom of page 23)

Of course, now I am really confused. How can they state that a 100% rated breaker can be used in a 80% rated (NEC?) application? How can the 80% rated breaker be in compliance with the NEC if the trip unit allows 100% continuous current? How can the breaker be UL rated as a 80% rated breaker if it allows 100% continuous current? And, again, what is the meaning of the 80% rating if the breaker can actually carry 100% of rated current???

If I were you, I would be asking all of the above questions to the technical support staff because if what they say is true, the 80% rating makes no sense.

Finally, I will apologize. I thought that I was helping but it would appear that I did not. The lesson here (for me) is that even safe assumptions are still assumptions. I should have checked in detail before answering.

 

[RLebel]

rhatcher, no offense taken.

Your posts help me, if nothing else, it reassures me that I am not the only one who has a hard time understanding their definition of an 80% rated breaker.

Logically I think we are on the same page. Logic would dictate that a breaker manufacturer would not allow a trip setting higher than what the breaker can handle. Unfortunately after I read their catalogs, I came to the conclusion that a trip setting of 100% is achievable on an 80% rated breaker.

I did forward my questions to the manufacturer and I am waiting to hear back from them. In the meantime I posted on here in the hope that someone had dealt with an 80% rated breaker equipped with a electronic trip unit. Like I said before, this would likely be a non-issue if the breaker was thermal-mag. The increased heat load would cause the thermal element in thebreaker to trip faster, therefore protecting the breaker. This would not be the case with an electronic trip unit.

My questions to the manufacturer were the following.
1) Can an 80% rated breaker be operated indefinitely at 100%? If not what are the consequences?
2)if it can operate at 100%, then why rate it at 80%?
3)if it cannot operate at 100%, then does a setting of 100A actually mean 100A which is the frame rating or is it 80A which would be 80% of the frame rating? (if the latter, adjusting the breaker setting would be extremely confusing, a setting of 100A on an 80% breaker would be 80A?)
4)if a setting of 100A is actually 100A, then the thermal protection on the breaker can be set higher than what the breaker can actually handle. Couldn't this lead to a breaker failure?

 

[davidbeach]

Absolutely no difference in the trip curve between the 80% and 100% version of the same trip unit. Even 80% devices can safely go to 100% for short periods of time (requirement applies at 3 hours). The long time characteristic of an ANSI/UL breaker won't trip at 100%, ever.

 

[rhatcher]

RLebel,

Thanks for your understanding. I do have a lot of breaker experience and did not pull my assumptions out of thin air. The 80% rating is already a point of confusion for many people. However, with this twist, it is now a point of confusion for me with regard to these breakers and this manufacturer.

Anyway, my concern is not damage to the breaker. In most cases, the breaker frames and contacts (current carrying components) are the same for both 80% rated and 100% rated versions of the same breaker. Normally, the only difference is the trip unit and perhaps the lugs. As such, the breaker frame and contacts are usually capable of carrying 100% rated current continuously without damage.

The purpose of the 80% rating is the application based on NEC requirements, not a limitation in the breaker design. My concern here is what affect the 100% continuous current capability will have relative to the breaker providing proper, 80% rated, protection to the conductors and other devices downstream. Once again, if the breaker trip unit will allow 100% continuous current flow, then what is the basis for the 80% rating??

As a note, you can always set the breaker trip unit for 80% long time pick-up. However, the fact that it is capable of 100% continuous current with the twist of a dial does not eliminate the chance that someone in the future will change the setting.

 

[living2learn]

I always thought that the breaker is tested in free air and would have a rating of 100%. The secondy you put it in an enclosure the heat build up requires it to be derated at 80%. If the circuit breaker (and switchboard) is 100% rated it must be tested in a UL listed enclosure.

This says it is not just the circuit breaker that must be 100% rated; rather, the entire installation must be suitable for 100% operation. Switchboard manufacturers, do not label their enclosures as either 100%- or 80%-rated. Instead, a switchboard is suitable to have 100%-rated circuit breakers installed and applied at 100% if:

1. the minimum enclosure size and ventilation requirements set forth on the circuit breaker are met, or
2. the switchboard has been specifically UL tested for 100% operation of the circuit breaker.

So UL 489 deals with this.

In enclosures that are supplied with standard-rated circuit breakers, it is not necessarily possible to replace a standard-rated circuit breaker with a 100%- rated circuit breaker and obtain a 100% rating. The enclosure must meet the
minimum enclosure and ventilation requirements of the circuit breaker and be marked as such.

 

[dpc]

UL has specific wire sizing requirements for molded case circuit breakers - these must be followed even if the NEC says a smaller size can be used.

Also, I believe that if a 100% breaker is to be applied at its 100% rating, there are special wiring requirements: 90 deg C wiring must be used, but applied only up to the 75 deg C ampacity. This is how it used to be, at least.

I suspect a lot of this apparent confusion is due to the long-standing requirements of the NEC to size breakers for 125% of the continuous load plus 100% of the non-continuous load. This is the inverse side of the 80% breaker rating. We have generations of electricians trained based on using 125% of the continuous load. From a Code-Making Panel perspective, what is to be gained by making this more complicated than it already is?

It is reasonable to assume that the breaker, whether solid-state or thermal trip will be self-protecting against long-term overloads and overheating. I'm not a breaker manufacturer, or UL, but I have to believe this issue has been addressed, since these static trip units have been around for a long time.

For low voltage systems falling under the NEC, you will get frustrated if you try to out-think the Code or try to apply logic.

Whenever I have tried to get around a design issue by using to 100% rated breakers, it almost never worked out economically. I spec 100% rated for large main breakers in switchboards, but other than that, I don't bother. Low voltage power circuit breakers are all 100% rated, by standard, BTW.

 

[RLebel]

davidbeach - I understand that the breaker can safely operate at 100% for <3 hours, but what is to prevent it from running at 100% indefinitely? Could it be damaged by the additional heat being generated?

rhatcher - The 80% breakers are tested at 100% in free air, however when they are installed in an enclosure, a panel in my case, their ability to dissipiate heat is diminished. So although they are rated to carry 100% of the current in free air, my concern is they may not be able to do so in an enclosure. A similar situation would be the current carrying capacity of a cable in free air vs in a loaded cable tray.

That is my question, if the breaker can operate safely at 100%, why is it rated at 80%?

As for adjusting it to 80%, that is a valid path, but my concern is that an inexperienced person down the road may raise the overload setting to what is potentially an unsafe setting.

living2learn - My understanding of the 100% rating is similar to yours. My question is what prevents someone from installing an 80% rated breaker in an non-100% rated enclosure and setting the overload at 100%?

dpc - The cable in my installation is rated at 90 deg C and it is derated beyond the 75 deg C ampacity.

Although it may be reasonnable to assume the breaker will be self protected, no one has been able to confirm it. If the breaker can operate at 100% indefinitely and there is no difference between the two breakers, then why does the manufacturer have an 80% rated breaker and a 100% rated breaker?

I'm not trying to out-think the code, the breakers/panel assembly is rated at 80% and is not going to change. My concern again is simply to ensure that there isn't a setting on the breaker which is unsafe.

I try to make designs as idiot proof as possible, because everyday the world keeps coming up with better idiots. So to have a breaker with an adjustable setting beyond the current it can safely carry sets off warning bells.

Again sorry for the long post

 

[davidbeach]

There are some things you just have to accept. The rules as they are have been found to work in the vast majority of cases, but don't necessarily make total and complete sense.

 

[resqcapt19]

I really doubt that running the 80% rated breaker at 100% would cause any damage since the UL standard for breaker trip time permits them to operate at 134% of their rating forevern. The breaker must trip at 135% of rating in one hour or less.

 

[jraef]

But UL is not the National Electric Code. It is a testing standard.

UL (and the NEC for that matter) is also not there to protect the world against misapplication. Sure, a breaker with an adjustable trip makes it EASIER for someone to adjust it to where it no longer is safely applied, but that does not excuse the person who did it from responsibilioty for doing so, just as if he removed the breaker and installed one of the same frame size but with a different trip unit. Remember, there is no difference in price between a 400AF/250AT breaker and a 400AF/400AT breaker, so that possibility exisist no matter what.

That said though, Electronic Trip breakers are not sold to the DIY marketplace for that exact reason, it is ASSUMED that people who would spend the extra money to get one would do so KNOWING that they still must be properly applied. In the DIY and Residential Construction market, electric inspectors are there to help guard against misapplication, but that still does not stop the countless fires started by people plugging in a 30A breaker on 12ga wire because "The 20A breaker kept tripping!"

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[RLebel]

I don't challenge the rules, I challenge the fact that a breaker manufacturer would provide a breaker rated at 80% with a trip unit rated at 100%. If the trip unit should not be set at the maximum level, as a minimum, the manufacturer should provide restrictions with the breaker.

Is the 134% rating based in free air? Inside an enclosure that value may be lower...

Assumptions can be a dangerous thing, I know the NEC rules and will apply them properly, however what is to stop a plant operator to increase the setting down the road? 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.

I may looking at this too closely, but I would rather be safe than sorry.

 

[resqcapt19]

I believe all of the UL testing is in free air or a single breaker in an enclosure. It is interesting that 134% of 80% is over 100%