80% breaker rating

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charlie b

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What does that mean? It?s a matter of some debate in my office. Here is an example:

  • A dwelling unit has a main service panel rated for 225 amps.
  • It has a main breaker rated for 225 amps, but it is an 80% rated breaker.
  • 80% of 225 is 180 amps.
  • Suppose the total connected load is 200 amps. But after applying the appropriate demand factors (e.g., 125% for continuous loads, 35% for lighting over 3KVA, 80% for 4 kitchen appliances, or whatever else might apply), the ?demand load? is 185 amps.

Question 1: Since the connected load exceeds 80% of the 225 amp main breaker rating, would I need to call for a 100% rated breaker?

Question 2 (assumes the answer to Question 1 is ?no?): Since the demand load exceeds 80% of the 225 amp main breaker rating, would I need to call for a 100% rated breaker?

Question 3: Does the notion of ?80% rated breaker? mean,

  1. The total connected load cannot exceed 80% of the breaker?s current rating,
    • or
  2. The breaker can handle a load of 80%, all of it being continuous, so that the calculated load works out to be the same as the breaker?s current rating,
    • or
  3. The breaker can handle a load of 100% of its current rating, as long as none of the load is continuous,
    • or
  4. Both B and C above,
    • or
  5. None of the above?
 

Npstewart

Senior Member
I just went through this same thing with a plans examiner. I think the article & podcast does a good job clearing it up. Essentially the plans examiner was saying a 320 amp meter can cannot handle more then 320 amps. My argument was that a 320A meter can handle 400 amps of non-continuous, or 320 amps of continuous. 320A would only be your maximum allowed if the whole load is continuous (400 * 0.8 = 320) or more correctly, the inverse, 320 * 1.25 = 400A

1: I think the answer is no.

2 & 3: I think these statements are true.

I think you mis-labled the answers with numbers instead of letters.
 

d0nut

Senior Member
Location
Omaha, NE

Question 1: Since the connected load exceeds 80% of the 225 amp main breaker rating, would I need to call for a 100% rated breaker?

No.
Question 2 (assumes the answer to Question 1 is ?no?): Since the demand load exceeds 80% of the 225 amp main breaker rating, would I need to call for a 100% rated breaker?

No.

Question 3: Does the notion of ?80% rated breaker? mean,
  1. The total connected load cannot exceed 80% of the breaker?s current rating,
    • or
  2. The breaker can handle a load of 80%, all of it being continuous, so that the calculated load works out to be the same as the breaker?s current rating,
    • or
  3. The breaker can handle a load of 100% of its current rating, as long as none of the load is continuous,
    • or
  4. Both B and C above,
    • or
  5. None of the above?
None of the above. There isn't really such thing as an 80% breaker. There are circuit breakers, and there are 100% rated circuit breakers.

Talking about 80% versus 100% at the breaker level only leads to confusion. The concept really has nothing to do with the breaker; it only applies to how the load is calculated. A 100A circuit breaker can have a calculated load of 100A or less on it, whether it is a normal breaker or a 100% rated breaker. The only difference is how the load is calculated. I am oversimplifying a bit, but for a normal breaker to calculate the load, you use 125% of the continuous load plus 100% of the noncontinuous load. For a 100% rated breaker, you calculate the load by adding 100% of the continuous load and 100% of the noncontinuous load.

If you don't want to calculate your load using 125% of the continuous load, you must use a 100% rated breaker. If you calculate the load "normally", a normal breaker will be sufficient provided its rating is greater or equal to your calculated load.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
My rough understanding is that normal breakers are treated as having a continuous current handling capacity (the 'ampacity' of the breaker, if you will) that is only 80% of their trip rating.

Thus to protect the _breaker_ the design continuous load must be less than 80% of the trip rating.

Or put another way, if you loaded a normal breaker to 100% of its trip rating, it is not supposed to trip, and thus might not be able to protect itself.

An alternate explanation that I have also heard is that the 80%/125% requirements are made to prevent nuisance tripping of breakers.

But to allow the breaker to properly protect the circuit conductors, the ampacity of the conductors must be match to the trip rating of the breaker.

Bringing up a dwelling example rolls in a huge amount of additional complexity to the discussion, because of the various exceptions permitting smaller than normal conductors for a given breaker rating...I'd leave out dwellings if you want to discuss the difference between 80% and 100% breakers :)

I don't understand why, given modern manufacturing techniques and modern materials, that breaker trip curve tolerances could not be tightened and all breakers made as 100% rated devices.

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
None of the above. There isn't really such thing as an 80% breaker. There are circuit breakers, and there are 100% rated circuit breakers.

Talking about 80% versus 100% at the breaker level only leads to confusion. The concept really has nothing to do with the breaker; it only applies to how the load is calculated.

For purpose of applying code, I agree with you. Nowhere does code say anything about 80% loading of breakers, only about a 125% factor applied to the load calculations.

For purpose of understanding the reason behind the code, however, the 80% factor comes into play.

If you have a continuous load, and follow code calculations with a normal breaker, then your load will be limited to 80% (1.0/125%) of the breaker trip rating, and 80% of the conductor ampacity. It is totally reasonably to ask why the code effectively mandates, for a normal breaker, that only 80% of the breaker trip rating be used. But again, you don't need (and probably shouldn't want) to use an 80% number to _apply_ the code.

-Jon
 

jim dungar

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I don't understand why, given modern manufacturing techniques and modern materials, that breaker trip curve tolerances could not be tightened and all breakers made as 100% rated devices.

All breakers are 100% rated based on their testing, in open air, for UL489 standards. The problem shows up when the breaker is mounted into an enclosure. The restricted air flow requires the breaker to effectively be rerated. The 'special' 100% rated breakers have had additional testing, in enclosures with sufficient ventilation so that they approach open air.
 

iceworm

Curmudgeon still using printed IEEE Color Books
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North of the 65 parallel
Occupation
EE (Field - as little design as possible)
80% breaker rating
What does that mean? It?s a matter of some debate in my office.
Charlie -
Here is my take. It is not even a professional opinion - cause I didn't stamp it and it's roughly worth what you paid for it.
Note: following is not discussing 15A, 20A, 30A household/receptacle circuits.
Note 2: No news here. This is all stuff everyone knows.

A 100% CB rating is a marketing issue. Consider:

Circuit loading is limited by design - not the CB. The CB is there to keep the structure from catching fire if something goes wrong and the wire overloads to where the insulation overheats.

The wire ampacity (excellent word with a strange meaning) is chosen be equal/greater than the Load = non-continuous X 100% + continuous X 125%. The CB is then chosen such that the number on the handle is at least equal to the load, but not more than the conductor ampacity (Yes there is some forgiveness for next size up and lack of forgiveness >800A - doesn't matter for this discussion). So, with a 100% continuous load, the wire ampacity and the CB are both only rated to 80%

Now look at the CB:
Using UL489 listed CBs for an example: Per the UL spec, the CB is required to never trip at 100% rated load, but must trip within 1 minute at 130% (I don't have the reference available today, but the numbers, if not correct, are close)
Both of the following meet UL spec:​
CB holds forever at 100%. Trips instantly at 101%
CB holds forever at 129%. Trips instantly at 130%​

One might say that the UL rates all CBs at 100% continuous load.

Put that CB in a panel, and the NEC now controls. And one can only load (continuous) to 80% of the number on the CB handle. What changed?

The code panel is well aware of the tolerance on the CB trip. So they set the conductor ampacity and CB rating such that if the CB trip tolerance is at the outer end (130%), the wire insulation is not overheated.

So, what does a 100% CB do for you.

Well, if the max CB the panel will is take 225A, and the NEC calculated load is >225A (or continuous load is >180A), but <225A continuous, then one can purchase a 100% rated CB and be just fine. Of course, the wire ampacity must meet the NEC calculated load.

So, what did the manufacturer do? They sold you a 300A CB in a 225A frame, with 225 stamped on the handle and "100% rated" written on the sticker on the side (Okay, not a 300A CB, but a 281.25A CB) Maybe they put an extra $0.50 more in the contacts to help with the heat dissipation.

So,
No
Yes
Yes
Yes
No

ice
 
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don_resqcapt19

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Illinois
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retired electrician
...
Using UL489 listed CBs for an example: Per the UL spec, the CB is required to never trip at 100% rated load, but must trip within 1 minute at 130% (I don't have the reference available today, but the numbers, if not correct, are close)...
ice
I think UL 489 say the breaker must trip at 135% in 60 minutes or less.
 

d0nut

Senior Member
Location
Omaha, NE
For purpose of applying code, I agree with you. Nowhere does code say anything about 80% loading of breakers, only about a 125% factor applied to the load calculations.

For purpose of understanding the reason behind the code, however, the 80% factor comes into play.

If you have a continuous load, and follow code calculations with a normal breaker, then your load will be limited to 80% (1.0/125%) of the breaker trip rating, and 80% of the conductor ampacity. It is totally reasonably to ask why the code effectively mandates, for a normal breaker, that only 80% of the breaker trip rating be used. But again, you don't need (and probably shouldn't want) to use an 80% number to _apply_ the code.

-Jon

I completely agree that the "why" behind the code language is as important, and sometimes more important, than simply following the code because you have to. That being said, the application of this particular 80%, 100%, 125% concept seems to get many people wrapped around the axle. Given 10 minutes, I am sure I could find at least that many posts on this forum of people confused by the application of 80% or 100% rated breakers because they are looking at the problem backwards. The confusion seems to almost always stem from something along the lines of "I have this breaker rated XXX amps. How much load can I put on it?"

That is the exact backwards approach to sizing conductors and circuit breakers. I think that if people would take a step back, most of the confusion would go away. The procedure for sizing conductors and breakers is not terribly complicated. I have a certain load. I have to apply these adjustment factors. I can apply these demand factors. My final result is X. I need conductors rated for that value, and a breaker rated for that value. The end.

I know I am oversimplifying, but approaching the design from the correct direction (size everything based on the calculated load) should make the confusion disappear.

It may be a bit of a rant, but so much metaphorical ink has seemingly been spilled on this topic for what I see as a trivial piece of electrical equipment with limited application. I would think it would always be cheaper and easier to calculate the continuous loads at 125% and use a normal breaker than to buy a special circuit breaker just so you can calculate the continuous loads at 100%. I am sure there are some good applications for a 100% rated circuit breaker, I just haven't run into any of them yet in my career.
 

mivey

Senior Member
Question 1: Since the connected load exceeds 80% of the 225 amp main breaker rating, would I need to call for a 100% rated breaker?

Question 2 (assumes the answer to Question 1 is ?no?): Since the demand load exceeds 80% of the 225 amp main breaker rating, would I need to call for a 100% rated breaker?
No x2. It would not work in the panel. You need a bigger panel and breaker.

Question 3: Does the notion of ?80% rated breaker? mean...
Both B and C above
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Interesting enough is that equipment that meets IEC standards are all "100%" , regardless of the amp rating.

If it says 20A, it can handle 20A continuously and it doesn't matter what panel its installed in.

What a concept.

In the US, MHO is it's a safety margin to avoid nuisance tripping. Lets face it, when it comes to electricity, most people are stupid. How many service calls are there to find out the GFI receptacle needs to be reset. DOH!!!
 

mivey

Senior Member
How many service calls are there to find out the GFI receptacle needs to be reset. DOH!!!
Or that they were using the wrong switch for the light.

"I know we used to use that switch". Ummm, OK, I guess the wires are jumping around in the wall.
 

iwire

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Location
Massachusetts
Given 10 minutes, I am sure I could find at least that many posts on this forum of people confused by the application of 80% or 100% rated breakers because they are looking at the problem backwards.

Hmmm. That would be your opinion of it (backward) but it is just that. If everyone arrives at the same conclusion all paths are valid.

I run into this with my wife, her way of doing any mathematical problem blows my mind. Many times I don' know how she does it but she gets a correct answer. I have learned to stop trying to force my way of doing things on her.
 

iwire

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Staff member
Location
Massachusetts
Interesting enough is that equipment that meets IEC standards are all "100%" , regardless of the amp rating.

If it says 20A, it can handle 20A continuously and it doesn't matter what panel its installed in.

What a concept.

It would be great if NEMA stuff was like that.

However I find IEC stuff is generally on the edge of self destruction regardless of 80 or 100 % loading.
 

don_resqcapt19

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Illinois
Occupation
retired electrician
... However I find IEC stuff is generally on the edge of self destruction regardless of 80 or 100 % loading.
I think that is just because we know that we can "abuse" the NEMA equipment and get away with it, and expect the IEC equipment to be the same, but it isn't.
 
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