Breaker ratings vs continuous loads

Strombea

Strombea

Senior Member
Location
Prescott, Arizona, USA
So I know that a standard breaker is rated for 80% of its rating and 100% rated breakers are rated with an approved assembly for 100% continuous load.

But my question is with Non Continuous loads on a standard breaker. Is it still only 80%? And if so does that mean that there are 2 different deratings for the same load?

20 amp breaker can be loaded to 16 amps ...

But continuous loads need to be calculated first.

So a 14 amp wall heater @ 125% = 17.5 amps (more than 16) Can NOT be on a 20 amp breaker even though the 125% has already been accounted for. (its only pulling 14 amps)


I think the answer to my question is that there are 2 different rules happening here and they both have to be done to comply.

And with a 100% rated breaker... do you still need to add the 125% to a continuous load first as it is a different rule than the breaker rule? (the heater is still seen as 17.5 amps... NOT 14 amps)


Side question: Is a 100% rated breaker the same thing as a fully rated breaker in a series/fully rated system, or are these different products, Meaning if you wanted to not series rate and it was a continous load, do they make that combination in a breaker?
 
Strombea said:
But my question is with Non Continuous loads on a standard breaker. Is it still only 80%? And if so does that mean that there are 2 different deratings for the same load?
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For a non-continuous load the circuit with a standard 80% breaker can be loaded to 100% of the breaker rating. So a non-continuous 20 amp load can be on a 20 amp individual branch circuit. For a continuous load the maximum rated current for the same 20 amp circuit would be 16 amps. (16*125%=20 amps)
 
I like to take it the other way, because TECHNICALLY, there is no such thing as an "80% rule" for circuit breakers. Here is the proper way to look at it:

  • CONDUCTORS for continuous loads must be sized at minimum 125% of the load.
  • For non-continuous loads, they do not, so you can size the conductors for 100% of the expected load.
  • BREAKERS must be sized to protect the conductors.
  • So if you size the breakers for the properly selected conductors, whether it is continuous or not, you are always correct.
So case in point: if you have a continuous load that is 100A, the conductors must be sized art 125A. So if you size the breaker at 125A, in EFFECT, the breaker is only be used at 80% of it's rating, because 80% is the inverse of 125%.

Conversely if you have a NON-continuous load of 100A, you can use conductors rated for 100A, ergo you can use a breaker rated 100A.
 
Ok thats what I have always thought and it makes sense. But I have inspectors and engineers telling me that even for non continuous loads you would never load a breaker to 100%, you would always load it to 16 amps. So in the case of 20 amp branch circuits say for a vending machine, if it pulls 19 amps then there is no problem using a 20 amp breaker.



Any thought on if you have ever seen a fully rated breaker for short circuit purposes crossed with a 100% rated breaker for continuous load purposes
 
Strombea said:
Ok thats what I have always thought and it makes sense. But I have inspectors and engineers telling me that even for non continuous loads you would never load a breaker to 100%, you would always load it to 16 amps. So in the case of 20 amp branch circuits say for a vending machine, if it pulls 19 amps then there is no problem using a 20 amp breaker.
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Whoever is saying that is incorrect. In your example the 19 amp load of the vending machine which is not a continuous load can go on a 20 amp individual branch circuit. {210.22}

Regarding 100% rated breakers for continuous loads you wouldn't see them in small ampere rated breakers.

210.22 Permissible Loads, Individual Branch Circuits.
An individual branch circuit shall be permitted to supply any load for which it is rated, but in no case shall the load exceed the branch-circuit ampere rating.
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Strombea said:
Any thought on if you have ever seen a fully rated breaker for short circuit purposes crossed with a 100% rated breaker for continuous load purposes
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I am not sure what you are getting at.
Short circuit ratings use but the term 'fully rated', while loading uses "100% rated". These two factors are not mutually exclusive.

It is rare to find 100% rated breakers in less than 400A frames although some manufacturers may offer them smaller. I don't believe anyone has ever made one in a non-electronic molded case version.
 
Strombea said:
Side question: Is a 100% rated breaker the same thing as a fully rated breaker in a series/fully rated system, or are these different products, Meaning if you wanted to not series rate and it was a continous load, do they make that combination in a breaker?
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This refers to the KAIC rating, and is unrelated from the continuous duty vs standard rating.

Fully rated means each breaker requires a KAIC rating that meets or exceeds the available fault current at its position in the distribution network, independently of other OCPD's in the system.

Series rated means an upstream breaker can protect a downstream breaker, so that the downstream breaker doesn't require the full KAIC rating for its location. Could be a main breaker and the branch breaker in its own panel, or it could be the breakers in a subpanel protected by a breaker in the main panel. This requires listed combinations of breakers, or listed combinations of breakers and fuses, to take credit for it. Breakers may be series rated with other breakers of the same manufacturer, or with a given fuse class as the upstream device. You are unlikely to find a combination between two different brands, that allows series ratings, if any even exist at all.
 
electrofelon said:
I have found that to be a pretty common belief in the industry, but it is incorrect.
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It’s one of those persistent myth issues.

It is partially perpetuated by the existence of “100% rated” breakers, the idea being that if they sell special version breakers that are 100% rated, it IMPLIES that those that are NOT must therefore be less than 100%, then the “80% rule” myth works its way in. They are separate issues.

For those that don’t understand:
The fact that normally, breakers are sized for the conductors, and CONDUCTORS must be sized for 125% of any continuous load, results in breakers never SEEING more than 80% of their rating continuously. So panel manufacturers take that into consideration when designing the heat density of the panel and breaker arrangement. The back-in result is that breakers in a panel are not going to be used at more than 80% of their rating., ergo the panel mfrs don’t allow for them to be in the panel at more than that 80% value (continuously). They are technically still actually RATED for 100% of what they say they are, you just can’t actually use them that way.

But if you WANT to design from the other direction and run a continuous load at 100% of the breaker’s rating, then that becomes a problem for a breaker that is stuffed into a panel with other breakers around it, all of them potentially giving off heat. So a “100% rated” breaker is typically** used in a stand-alone enclosure, or as part of a switchgear CELL where it is by itself, or maybe as a Main breaker where it has lots of empty space around it. So to attain that “100%” use rating, there are specific conditions that must be met, hence the breaker LISTING is different in order for the manufacturer to ensure how that is accomplished. This is why you rarely see 100% rated breakers smaller than 400A, because the only REASON to use a “100% rated” breaker for something is to avoid having to increase the physical SIZE of it, and below 400%, that’s just not a factor. So as an example of I have a 550A continuous load, I would need a breaker rated at least 700A, so an 800A frame (800AF / 700AT). But if I don’t have room for an 800A frame breaker, I can use a 600AF / 600AT breaker that is 100% rated.

**There are a few exceptions, such as I-Line panels and others with certain types of bus arrangements that can act as heat sinks, but those are exceptions to the rule.
 
Jraef said:
The fact that normally, breakers are sized for the conductors, and CONDUCTORS must be sized for 125% of any continuous load
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While that's true in practice, the logic is a bit reversed.

A 100A ampacity wire is rated for 100A continuously; looking just at the wire there's no need to use 125A wire for a 100A continuous load. However, a normal 100A breaker is not rated for 100A continuously, so the 100A continuous load requires a 125A breaker. And the wire that terminates on that breaker must be rated 125A (using the termination temperature for ampacity) to provide the heatsinking for the breaker that exists during the standard UL testing. So the wire size is increased because of the limitations of normal breakers. But elsewhere in the circuit, the wire need only be 111A, the minimum ampacity that may be protected at 125A per 240.4(B).

So for example, take a 100A continuous load with aluminum conductors and a normal 125A breaker. The 75C/90C ampacities of the relevant sizes are #2 = 90A/100A ; #1 = 100A/115A ; #1/0 = 120A/135A; and #2/0 = 135A/150A. At the breaker we need a 75C ampacity of at least 125A, so #2/0 is the smallest usable. Same at the load end if the load has no special labeling and 75C connections. [I believe some listing standards may permit the load to be tested with #1/0 Al, and if the temperature rises are within spec, be labeled as such.] And elsewhere in the circuit, a wire segment with 90C rated connections at both ends need only be #1 Al.

Cheers, Wayne
 
Thanks everyone…

My question on the fully rated breaker was vague… but it all makes sense.

I was getting at the fact that you might need a fully rated breaker (22kaic for example) at the same time needing it to be 100 % rated for a continuous load. I know they are 2 separate issues but I was just throwing it out there that that may be harder to find… probably just easier to use fused disco or something. I’m realizing you couldn’t get that combo in smaller amperages
 
Strombea said:
harder to find… probably just easier to use fused disco or something. I’m realizing you
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100% rated breakers are electronic, so they should be available fully rated to 65kAIC and higher.

While fuses are often marketed as 100%, the manufacturers tend to gloss over the issue that, they must be mounted into switches and enclosures and so are subject to the same ventilation issues as breakers mounted in enclosures.

100% rated fuses are usually only available in Class L sizes and switches.

The use of 100% rated equipment is not common except in larger ampacities, such as 800A and larger, mounted in switchgear.
 
Strombea said:
Thanks everyone…

My question on the fully rated breaker was vague… but it all makes sense.

I was getting at the fact that you might need a fully rated breaker (22kaic for example) at the same time needing it to be 100 % rated for a continuous load. I know they are 2 separate issues but I was just throwing it out there that that may be harder to find… probably just easier to use fused disco or something. I’m realizing you couldn’t get that combo in smaller amperages
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There aren't two separate issues, and 'fully rated' isn't an accurate phrase.

If your maximum load is not continuous*, there is no issue. The only issue is whether your load is continuous.*

If your maximum load is continuous, you address that by upsizing the conductors and OCPD by 25% (most common and often the only option), or by using an OCPD rated for continuous use at 100% of its rating (rare, I've never had the opportunity).

*Some code sections define certain loads or sources as continuous, on top of the article 100 definition.
 
Wile not a code requirement a common design requirement is a minimum headroom for future expansion and that minimum is often 20-25%.
 
jim dungar said:
On each branch?
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Not each branch but some circuits yes, they do for transformers feeders and panels very often yes, some places have kinda vague language its usually "DIVISION 26".

DIVISION 26 – ELECTRICAL
3.1.E. Include provisions for 20 percent future capacity at building completion.
This includes spare devices and space requirements. Use of spares for
building controls, etc. is not acceptable. This includes switchboards,
panelboards, fire alarm panels, lighting control, and other similar equipment

1.3. Transformers shall be sized to support all building loads and allow for an additional 20
percent future growth.
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