# Continuous Load on Breaker



## telsa (May 22, 2015)

Spark Master said:


> Got a call today about a hot breaker. 24 amps running continuously on a 20amp breaker. All day, all night. Doesn't normally trip, until they turn on the coffer machine; which obviously puts it well over 35 amps.
> 
> How can a 20 amp breaker handle 24 amps on a continuous basis ?? I've seen this before. They trip immediately on a dead short, but if it is gently overloaded on a continual basis, you can run them very hot. And 10-20% over loaded.
> 
> Do they design them with some sort of thermal curve ??


1) Original claims from wide-eyed non-electricians usually prove inaccurate exaggerations if not out right false.

2) There is a phenomena known as the defective breaker. It's buddy is the marginal breaker. 

Indeed, some breaker designs are notorious for NOT reacting to overloads. The result: fires, lawsuits, bankruptcy.

3) Few manufactures last forever. Just how old is this magic C/B and how nasty is the environment ?

4) Manufacturing tolerances are never so tight for the common branch circuit C/B.

It's one of the key reasons why the Code derates all of the common branch circuit conductors. (#14-#10) 

The derating provides design margin// 'insurance' against fuses and C/Bs that are a tad too permissive.

&&&

The solution is to re-jigger the loads -- this consumer needs another branch circuit in a major way.

Otherwise, how will they heat the coffer ?


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## Dennis Alwon (May 9, 2009)

Yes there is a trip curve but I would think it would trip after sometime. It is certainly doing damage to the breaker.

I believe a 20 amp breaker has an allowable trip time of 50 sec with 60 amps on it


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## Spark Master (Jul 3, 2012)

While I was running 100' of THHN, 3 phase circuit, to resolve the problem; and redesign their manufacturing area. I had my amp meter on this 20 amp breaker. *2 hour's at least, it was drawing 24 amps. Very hot, couldn't keep my finger on it; but it would not trip. *I've seen this before, and I know breakers will run over their rated amp limit for a very long time. Just curious.........


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## Bad Electrician (May 20, 2014)

I do not know what TCC you are lo0king at but the TCC for a QO molded case circuit breaker at 100% load is between 500-10000 seconds and that is where the curve stops.

I grabbed the second curve because????

Additionally is your amp clamp calibrated? So there is a plus or minus accuracy and than what was the ambient, with all that CB may never trip.

http://static.schneider-electric.us...rs/QO-QOB Circuit Breakers/0730CT9801R108.pdf


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## Big John (May 23, 2010)

Spark Master said:


> ...Do they design them with some sort of thermal curve ??


 Absolutely. The number on the handle of a thermal-magnetic molded case breaker just indicates it should never trip _below_ that value: It doesn't mean it will immediately trip when you exceed it. Take a look at the curve that _Bad Electrician _posted.

A 20 amp molded-case breaker listed to UL489 is only required to trip within 1 hour when run at a 135% overload. So if that breaker carried 27A for 59 minutes it would still be working completely properly.

The only feature that operates immediately is the instantaneous magnetic trip. For molded case the pickup is somewhere in the neighborhood of 10x the handle rating and the breaker will trip within 3 cycles or less.

The only exception to this rule is that there are breakers that provide no overload protection, like motor-circuit protectors. They operate only on short-circuit conditions and only have the instantaneous magnetic pickup. Those would carry an overload indefinitely because they are designed to operate in conjunction with separate downstream OL protection.


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## JRaef (Mar 23, 2009)

Big John is right.

The UL489 thermal current trip calibration test requirement for breakers 50A and under is 12 minutes at 200% of rating, 1 hour at 135% of rating. Nothing else.

Over 50A, the long time is *2* hours at 135% of rating.

At 24A (120% rated), it could have done that forever...


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## Spark Master (Jul 3, 2012)

JRaef said:


> Big John is right.
> At 24A (120% rated), it could have done that forever...


 That's exactly what's happening. 24amps for ever. The breaker is hot, the wire is hot. The conduit is certainly warm. But nothing is melted. And I know I've seen this before.

It's just fascinating because one would think 20amps is 20 amps max; and 21 amps would trip the breaker. 

Very interesting discussion. 

Thank you.


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## ELECTRICK2 (Feb 21, 2015)

Spark Master said:


> That's exactly what's happening. 24amps for ever. The breaker is hot, the wire is hot. The conduit is certainly warm. But nothing is melted. And I know I've seen this before.
> 
> It's just fascinating because one would think 20amps is 20 amps max; and 21 amps would trip the breaker.
> 
> ...


If the breaker and wire are "hot" there is a problem. If you have a new breaker on hand, swap it out and see if it trips. If you don't have a new breaker, put the 24A load on a different breaker in the same panel. Be interesting to see how much more load you could put on the bad breaker. Maybe not safe but interesting.


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## Spark Master (Jul 3, 2012)

ELECTRICK2 said:


> If the breaker and wire are "hot" there is a problem. If you have a new breaker on hand, swap it out and see if it trips. If you don't have a new breaker, put the 24A load on a different breaker in the same panel. Be interesting to see how much more load you could put on the bad breaker. Maybe not safe but interesting.


 Kinda too late now. I split it up to 3 phases. I'm sure we'll come across it again.


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## Big John (May 23, 2010)

Spark Master said:


> ...The breaker is hot, the wire is hot...


 The thing to remember is that there's Human-Hot and then there's Equipment-Hot, and those are two very different scales.

For humans, putting our hand on something that's 130°F is gonna be pretty painful, but that's only 54°C which isn't even the lowest termination temperature of most equipment.

When you get into high-temperature windings, Class F or H insulation is designed to run at temperatures that would boil water.

We see that a lot with those little encapsulated transformers: We'll come in and the customer will have all kinds of fans aimed at the thing desperately trying to cool down this transformer that they think is gonna burn down their building because it's way too hot to touch:

Well, the stupid thing is *designed *to run with a 65°C rise over a 40°C ambient: That's 221°F. Yeah, it's gonna be hot!


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## Bad Electrician (May 20, 2014)

Spark Master said:


> That's exactly what's happening. 24amps for ever. The breaker is hot, the wire is hot. The conduit is certainly warm. But nothing is melted. And I know I've seen this before.
> 
> It's just fascinating because one would think 20amps is 20 amps max; and 21 amps would trip the breaker.
> 
> ...


You are discussing a molded case thermal magnetic circuit breaker...Not going to happen


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## RIVETER (Sep 26, 2009)

Spark Master said:


> Got a call today about a hot breaker. 24 amps running continuously on a 20amp breaker. All day, all night. Doesn't normally trip, until they turn on the coffer machine; which obviously puts it well over 35 amps.
> 
> How can a 20 amp breaker handle 24 amps on a continuous basis ?? I've seen this before. They trip immediately on a dead short, but if it is gently overloaded on a continual basis, you can run them very hot. And 10-20% over loaded.
> 
> Do they design them with some sort of thermal curve ??


No, they are designed to open on overload....quicker if a shorted event.


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## Bad Electrician (May 20, 2014)

ELECTRICK2 said:


> If the breaker and wire are "hot" there is a problem. If you have a new breaker on hand, swap it out and see if it trips. If you don't have a new breaker, put the 24A load on a different breaker in the same panel. Be interesting to see how much more load you could put on the bad breaker. Maybe not safe but interesting.


Have you read what was posted above?

The only solution as I see it is to do as the OP did split the load up.


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## ELECTRICK2 (Feb 21, 2015)

Bad Electrician said:


> Have you read what was posted above?
> 
> The only solution as I see it is to do as the OP did split the load up.


Mea culpa, another case of read your post before you hit submit


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## Lone Crapshooter (Nov 8, 2008)

This phenomenon is easier to understand when you setup solid state trip units on draw out circuit breakers. It is called X1 PICK UP. It is a fancy way of saying 100% of breaker ampacity. A 20 amp breaker should carry 20 amps forever. 21 amps almost forever and as the current gets higher and higher the time gets shorter and shorter.
When I test Digitrips and Amptectors is 6X rating of the breaker and I have a number in the coordination book that tells me where that breaker should trip for that application on our system. 
LC


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## AK_sparky (Aug 13, 2013)

Spark Master said:


> It's just fascinating because one would think 20amps is 20 amps max; and 21 amps would trip the breaker.


This is covered in our first block of school. Trip curves and how to read them and why they matter in troubleshooting. In all honesty (an no insult intended) every electrician should at minimum know about the curves existence.


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## Spark Master (Jul 3, 2012)

AK_sparky said:


> This is covered in our first block of school. Trip curves and how to read them and why they matter in troubleshooting. In all honesty (an no insult intended) every electrician should at minimum know about the curves existence.


Maybe in Canada. Not here !!!!!!!!!!!!!!!!!


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## AK_sparky (Aug 13, 2013)

Spark Master said:


> Maybe in Canada. Not here !!!!!!!!!!!!!!!!!


It may not even be everywhere in Canada. To some degree it depends on the instructors and directors at the school you take your course at. I've had friends do their courses at other schools and they learned things in different orders than I did.


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## Bad Electrician (May 20, 2014)

At some point I would be the manufactures make a totally electric circuit breaker AFCI, GFCI and the works, with settings like over 125% for 10 seconds it trips. 

The good news is they will be mandatory and only cost 500.00 a CB.


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