- Location
- Wisconsin
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- PE (Retired) - Power Systems
Can a bad breaker affect voltage?
- Thread starter ritelec
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All to many people make this more of an issue than it really is. They zero in on a breaker being 80% rated when that really isn't the issue at all.
You must look at it this way. All breakers are designed and tested to carry 100% if their ratings as Jim Dungar stated. It is how a "standard breaker" is allowed to be applied as compared to a 100% rated breaker. Please note that I have avoided the term 80% and used "standard."
In most cases forget about the 80% issue as breakers are intended to protect wire, wire is sized to carry the calculated load.
Wire is sized to carry 125% on the continuous load plus 100% of the noncontiguous load. That wire must carry that load. Since you would have to be luck to find a wire with the exact ampacity you must select a rated wire which can. Then you elect a breaker rated to protect that wire. If you are lucky to find a breaker that has the exact same rating as the wire (not the load but the wire ampacity) that would be the breaker to use. If that ampacity is not standard you are allowed to select the next larger breaker for breakers sizes up to 800A.
Now, let?s go back over this, there is nothing that would make it important to apply that 80%. The only remote way that the 80% may come into this would be should you have just a continuous load for which the wire mist be sized to carry the load, and you can find a wire with that exact amp rating, and be even luckier to find a breaker that has the exact same rating as the wire which happens to be the exactly the same as the load. Add, magically when the wire is size at 125%, the breaker is size the exact same ampacity of the wire, the breaker is applied at 80%. The only way that you could get yourself into trouble with this illustration is if you elected to purposely undersize the breaker. If you did there would be no safety issue other than the potential of nuisance tripping the breaker on thermal.
The 100% rated breaker is used in the same way except that the loads is calculated as 100%+100% 90degC rated wire must be used and sized to carry that load. The 100% breaker is simply sized to protect that wire and installed per the manufacturers instructions.
too funny.....ha ha ha,ha thanks! ha ha
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I agree. When you have too much resistance in the wire and it drops the voltage, the heat from that resistance is spread out over the entire length of the wire, so the likelihood of it causing noticeable problems is minimal. But when you have a significant voltage drop at a termination or a set of contacts, ALL of the heat from that resistance is expressed right there. The device fails in an obvious and quick manner. Bottom line if the CB was causing a 15% voltage drop, it would be smoking.
That was me... and I was straining my brain when writing that post to try to remember that exact saying.
The other one I like that finally came to me too late was "When you see a burned tree in a green field, think lightning, not alien invasions."
Taking it as a yes............thank you.
Another thing that can influence the failure of a breaker contact is a poor line end connection to the breaker. If that connection in compromised the heating that occurs there will conduct itself into the breaker to the stationary contact. The heating will continue to the moving contact and heat the contact spring that holds the contact closed. That heat often times has the capability of decreasing the spring?s tension thus less contact pressure which intern causes the contacts to heat themselves which leads to contact failure.
When I was a breaker applications engineer I had the opportunity to analyze breakers for a cause of failure as well as having a rare opportunity of sitting at the warranty return work station and evaluate breakers. It was extremely important to find a cause of failure because it would determine the difference between a possible warranty replacement issue or that should the customer purchase a replacement breaker it doesn?t fail again. It is extremely important to identify the cause of failure to prevent future failures. All too often the customer who had minimal knowledge other that the manufacturer's name on the product identifies the manufacturer as having a crappy product.
The objective never was to get out of replacing a product under warranty but to assure that the product did not fail because of a manufacturing defect and that it had been applied properly so that when replaced there would be no additional failures.
When I was a breaker applications engineer I had the opportunity to analyze breakers for a cause of failure as well as having a rare opportunity of sitting at the warranty return work station and evaluate breakers. It was extremely important to find a cause of failure because it would determine the difference between a possible warranty replacement issue or that should the customer purchase a replacement breaker it doesn?t fail again. It is extremely important to identify the cause of failure to prevent future failures. All too often the customer who had minimal knowledge other that the manufacturer's name on the product identifies the manufacturer as having a crappy product.
The objective never was to get out of replacing a product under warranty but to assure that the product did not fail because of a manufacturing defect and that it had been applied properly so that when replaced there would be no additional failures.
kwired
Electron manager
- Location
- NE Nebraska
I am not familiar with actually using 100% rated breakers, but do not believe you must use 90oC conductor unless the listing of the breaker requires it. Besides if the breaker has 90oC terminals whatever is at the other end of the conductor must have 90oC terminals or you will still need to size the conductor for 60oC or 75oC according to that device.
I was taught the reason we derate a conductor to 80% for continuous loads is because the conductor acts as a heat sink for the overcurrent device it is connected to. If the overcurrent device is 100% rated then we do not have to derate because the device does not sink any heat (or at least not like a standard breaker does) into the conductor.
Which if what I was taught is correct it would make no sense for the terminals to be rated 90oC as the terminals should not operate as hot as their counterparts on a standard breaker operate with the same load.
The fact that most if not all 100% rated breakers have electronic trip mechanisms instead of thermal trip mechanisms also gives this theory more credibility IMO. The thermal trip device by design should develop heat in normal operation where the electronic device may create some heat, but will be minimal in comparison.
JMO shoot holes in it if you want.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You never, ever, use the 90?C columns when you terminate at any non-'ANSI switchgear' overcurrent device.
Due to the extra heat of a 100% device, UL listing requires 90?C conductors sized used the 75?C columns.
kwired
Electron manager
- Location
- NE Nebraska
Why is there extra heat there? This concept kind of goes against what I thought I knew about this - the fact that a standard device uses the conductor as a heat sink but a 100% device does not.
Yes, I overlooked the statement that 90degC must be used but applied at 75degC. Standard breakers depend on the wire acting like a heat sink taking heat away from the breaker. With 100% rated breakers and the associated wire sizing requirements you loose that heat sink advantage and the wire is more apt to add heating of some degree to the breaker which leads to the fact that 100% rated breakers have SS trip units.
Then I go back to the original intent of the OP and wonder how we started this rabbet trail which I believe stareted with reference to 100% rated breakers.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
100% rated device produce tons of heat.
The normal heat sink effect is what requires the larger 75?C conductor size. The extra heat is what causes the need for the 90?C insulation (similar to the 90?C insulation requirement for fixtures).
kwired
Electron manager
- Location
- NE Nebraska
The fact that it produces more heat would make sense for requiring 90?C insulation, but I still can't get around the fact that if it produces more heat why the NEC would allow smaller sized conductor. A 3/0 copper conductor w/ cont load can only be loaded to 160 amps on standard breaker. Same conductor can be continuously loaded to 200 amps and landed on 100% rated breaker and is expected to operate at higher temperature, doesn't quite make sense to me, an increase in conductor ampacity because it is not expected to operate as hot does make sense, please enlighten me if I am wrong.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
It doesn't.
The conductor must be sized using the 75?C columns @ 125% loading, just like with any other protective device.
Remember, up to three hours the loading and the conductors can be the same for both a 100% and a standard (i.e. 80%) breaker.
So the difference is with the 25% adder to the protective device rating for 'continuous' loads. This is where the higher temp insulation comes into play as well as the 100% rated device being mounted in an enclosure that has greatly increased airflow to help manage the extra heat.
I wish that I still had access to the breaker design engineers that I once had years ago so that I could define this "extra heat" issue in order to identify what heat rise one would expect when applying a 100% rated breaker as compared to a standard breaker. As I recall it wasn't that big of an issue other that they couldn't rely of the cable being a heat sink. As I recall the test require 3' of rated line and load cable when doing the tests.
It may be that the 90degC requirement was a belt and suspenders thing but without having knowledge of the actual temperature rise we can only speculate. The mere fact that SS trip are used indicates that should the common TM trips be used they could derate causing nuisance tripping because of any additional heating that may occur. A SS trip is insensitive to ambient temperature either in a standard or 100% rated breaker) so for protection if I can recall they use an internal 90degC (this is only from memory and I can not confirm it) thermistor which senses if the breaker is getting too hot physically. And 90degC is a quite toasty (194degF). But I have no way to confirm that now.
And it is a common belief the 100% rated breakers are magically superior to the standard breakers which is not true as I have taken them appart to see what makes then different, that is the Eaton products. It is strictly in how the 100% breaker is allowed to be applied and it is imperative that the mounting and enclosure requirements are applied as Jim has suggested.
It may be that the 90degC requirement was a belt and suspenders thing but without having knowledge of the actual temperature rise we can only speculate. The mere fact that SS trip are used indicates that should the common TM trips be used they could derate causing nuisance tripping because of any additional heating that may occur. A SS trip is insensitive to ambient temperature either in a standard or 100% rated breaker) so for protection if I can recall they use an internal 90degC (this is only from memory and I can not confirm it) thermistor which senses if the breaker is getting too hot physically. And 90degC is a quite toasty (194degF). But I have no way to confirm that now.
And it is a common belief the 100% rated breakers are magically superior to the standard breakers which is not true as I have taken them appart to see what makes then different, that is the Eaton products. It is strictly in how the 100% breaker is allowed to be applied and it is imperative that the mounting and enclosure requirements are applied as Jim has suggested.
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kwired
Electron manager
- Location
- NE Nebraska
If minimum conductor size must be 125% of load how can you say a larger conductor is not required. Then you say the 100% rated device creates more heat, but it does not require the conductor to have an ampacity of 125% of load.
I'm not saying you are wrong, just trying to make sense of it.
Many have agreed that with standard devices the device depends on the conductor to sink some heat and that is reason for conductors needing to be rated 125% of continuous load.
If a 100% device creates more heat why are we allowed a smaller conductor (100% instead of 125% of load)? Maybe there is more heat but it is not intentionally sunk into the conductor? That is about only thing I can think of that would make any sense. In that case the 90?C conductor is probably required for ambient insulation temperature more so than for termination temp reasons. As has been said they usually want 90?C conductors but sized to 75?C rating. That to me means the termination rating is still 75?C. Am I going the right direction with this thought process?
Unless specified by instructions, I don't see the NEC requiring that. For branch circuits, 210.19(A)(1) permits sizing before the application of adjustment or correction factors, while the Exception thereto permits ampacity sizing at 100% value (i.e. both continuous and non-continuous at 100%). For feeders, 215.2(A)(1) mimics the preceding.
Termination temperature limitations limit smallest size permitted per 75?C column ampacity values, but that determination is based on the calculated load current value... which does not include 125% factoring of continuous loads.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
This article may be of interest to yall...
http://static.schneider-electric.us...nsulated Case Circuit Breakers/0613DB9902.pdf
http://static.schneider-electric.us...nsulated Case Circuit Breakers/0613DB9902.pdf
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Regardless what I may have posted, in a sleep deprived state, the above is correct.:ashamed1:
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