# 80% rule?



## TomSpano (Jan 26, 2014)

Ok so my understanding is an overcurrent device can only be loaded up to 80% of its rating. Can someone please shed some light on this as to what specific applications this rule applies? I know it can't be applicable when dealing with motors. How bout services? Thanks for the help


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## backstay (Feb 3, 2011)

TomSpano said:


> Ok so my understanding is an overcurrent device can only be loaded up to 80% of its rating. Can someone please shed some light on this as to what specific applications this rule applies? I know it can't be applicable when dealing with motors. How bout services? Thanks for the help


It's more like a guideline!:whistling2:


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## 1.21gigawatts (Jun 22, 2013)

80% rule applies to continious loads such as motors, lighting or any load expected to be on 3 hours or more. A breaker is rated for 100% of the noncontinious load which may include outlets or other small appliances. 


There are fully rated breakers / panels that are rated for continious use And will be listed for this application. 


It is a good practice to load branch circuits to a fraction of their rating.

For example a lighting circuit on a 20a braker should only be loaded to no more than 16a. Or 16a continious load x 125% = 20a.


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## LBC Jesse (Apr 26, 2012)

Another quick note, in the world of circuit breakers, the trip curves set by the manufacturerer also begin at 80%. In theory a 1000a breaker should hold 1000 amps indefintly, but in reality the trip curve began earlier at 800a...albeit a very slow curve, it bends quite rapidly after 1000a... (Google trip curve on SqD NA breaker to see a graph). Breakers can be purchased as 100% rated (at quite the adder cost)... As previous poster stated, an electrical engineer SHOULD have that all calculated ...inductive, continuous and resistive loads... Prior to the recommended breaker size given... Past that tidbit of info, I'm about as a smart as a popcorn fart in a hurricane, and troll these posts to further enlighten my self by the extraordinary wisdom you EC's have to offer.


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## Meadow (Jan 14, 2011)

LBC Jesse said:


> Another quick note, in the world of circuit breakers, the trip curves set by the manufacturerer also begin at 80%. In theory a 1000a breaker should hold 1000 amps indefintly, but in reality the trip curve began earlier at 800a...albeit a very slow curve, it bends quite rapidly after 1000a... (Google trip curve on SqD NA breaker to see a graph). Breakers can be purchased as 100% rated (at quite the adder cost)... As previous poster stated, an electrical engineer SHOULD have that all calculated ...inductive, continuous and resistive loads... Prior to the recommended breaker size given... Past that tidbit of info, I'm about as a smart as a popcorn fart in a hurricane, and troll these posts to further enlighten my self by the extraordinary wisdom you EC's have to offer.


Some but not all, some even start slightly after the handle rating.


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## dmxtothemax (Jun 15, 2010)

I too think it's more good practice rather than a rule !

I have seen plenty of circuits loaded to 100%.
But they tend to trip for no apparent reason sometimes !
So thats why I tend to use the 80% thing 
it's saves on nuisance tripps.


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## TomSpano (Jan 26, 2014)

Interesting. So there isn't a word of this in the nec? And what about an exam?


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## Shockdoc (Mar 4, 2010)

It depends on what , in the world of overpaid, underachieving afci breakers, I max + every circuit out to limit how many of those worthless breakers I need to buy. New house im wiring this week gets five.


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## Pharon (Jan 20, 2014)

TomSpano said:


> Interesting. So there isn't a word of this in the nec? And what about an exam?


The 3 hour continuous rule is in 210.19. (Everyone calls it the 80% rule, but it's really the 125% rule.) :thumbsup:

There is also 210.23 that says for fixed utilization equipment (i.e. dishwasher, etc.) on a circuit that also supplies lighting and receptacles, the equipment can't exceed 50% of the branch circuit rating.


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## JRaef (Mar 23, 2009)

Pharon said:


> The 3 hour continuous rule is in 210.19. (Everyone calls it the 80% rule, but it's really the 125% rule.) :thumbsup:
> ...


What he means by this, in case it confuses you, is that you must size the CONDUCTORS to 125% of the FLC of the equipment, so the inverse of 125% is 80%. In other words since you cannot load the conductors to more than 80% of their rated ampacity anyway (for continuous loads as defined above), there is no point in having the breakers rated beyond that 80% level. 

Since breaker loading relates to heat, which then has everything to do with panelboard design, that is how breakers are rated when used in panelboards. If you want to use a "100% rated" breaker and follow the other rules associated with that, you will find that 100% rated breakers cannot be used in panelboards, they can only be used stand-alone.


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## Meadow (Jan 14, 2011)

Shockdoc said:


> It depends on what , in the world of overpaid, underachieving afci breakers, I max + every circuit out to limit how many of those worthless breakers I need to buy. New house im wiring this week gets five.


Not ranking on you, but your method is now done by nearly every contractor/electrician. The true result of AFCIs, residential wiring that is more dangerous then every. 

CMP is a joke.


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## fisstech (Feb 2, 2013)

1.21gigawatts said:


> For example a lighting circuit on a 20a braker should only be loaded to no more than 16a. Or 16a continious load x 125% = 20a.


this is what baked my noodle in school. just multiply 20 X .8 to get your 16 haha. Throwing that 125% in there complicated things in my opinion.


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## Pharon (Jan 20, 2014)

It's only complicated if you don't know division. 20 / 1.25 = 16

It's also important to point out that the whole point of this exercise is to size circuit breakers that will protect the wire from overheating and catching on fire. That's it. Everything else is secondary.


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## fisstech (Feb 2, 2013)

Pharon said:


> It's only complicated if you don't know division. 20 / 1.25 = 16
> 
> It's also important to point out that the whole point of this exercise is to size circuit breakers that will protect the wire from overheating and catching on fire. That's it. Everything else is secondary.


i understand how to do it 

im saying it needlessly complicates it. we're talking 80 percent 80 percent 80 percent and then this 125 comes out of nowhere.


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## Pharon (Jan 20, 2014)

Funny, to me it's the opposite. There's nothing in the NEC about 80%. That's just the street interpretation for people who prefer multiplication over division. :thumbup:


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## dave91 (Jan 8, 2015)

Branch circuits at 80 % service breaker at 100 %


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## Dennis Alwon (May 9, 2009)

dave91 said:


> Branch circuits at 80 % service breaker at 100 %


That is not correct. A branch circuit can be loaded to 100% as long as the load is not continuous- ie, on for 3 hours or more. I can have a calculated load of 20 amps on a 20 amp breaker as long as the load is not continuous.


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## Meadow (Jan 14, 2011)

LBC Jesse said:


> Another quick note, in the world of circuit breakers, the trip curves set by the manufacturerer also begin at 80%. In theory a 1000a breaker should hold 1000 amps indefintly, but in reality the trip curve began earlier at 800a...albeit a very slow curve, it bends quite rapidly after 1000a... (Google trip curve on SqD NA breaker to see a graph). Breakers can be purchased as 100% rated (at quite the adder cost)... As previous poster stated, an electrical engineer SHOULD have that all calculated ...inductive, continuous and resistive loads... Prior to the recommended breaker size given... Past that tidbit of info, I'm about as a smart as a popcorn fart in a hurricane, and troll these posts to further enlighten my self by the extraordinary wisdom you EC's have to offer.


Wait, I thought they began around 100 or more often at 125%?


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## Rollie73 (Sep 19, 2010)

dave91 said:


> Branch circuits at 80 % service breaker at 100 %





Dennis Alwon said:


> That is not correct. A branch circuit can be loaded to 100% as long as the load is not continuous- ie, on for 3 hours or more. I can have a calculated load of 20 amps on a 20 amp breaker as long as the load is not continuous.


 
Our rule for it reads like this......



> (3) The calculated load in a consumer’s service, feeder, or branch circuit shall be considered a continuous load
> unless it can be shown that in normal operation it will not persist for
> (a) a total of more than 1 h in any two-hour period if the load does not exceed 225 A; or​(b) a total of more than 3 h in any six-hour period if the load exceeds 225 A


.


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## Bad Electrician (May 20, 2014)

meadow said:


> Wait, I thought they began around 100 or more often at 125%?


It is a circuit breaker not an exact device at those percentages, ambient and connections play into it for long time, you are close to the truth at somewhere above 100%


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## Meadow (Jan 14, 2011)

Bad Electrician said:


> It is a circuit breaker not an exact device at those percentages, ambient and connections play into it for long time, you are close to the truth at somewhere above 100%


I agree. Most of the manufacturer published trip curves I see start around 110 to 125% of the handle rating, but ambient shifts that curve around. UL actually allows breakers to hold 120% indefinitely.


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## Dennis Alwon (May 9, 2009)

Rollie73 said:


> Our rule for it reads like this......
> 
> .


Yes but the op is USA so I am not sure why we got into CEC


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## 51360 (Jun 9, 2014)

Dennis Alwon said:


> Yes but the op is USA so I am not sure why we got into CEC


In our code, the CEC, Rollie is correct, and I am sure your code is similar.

But, the breaker must be marked for continuous operation at 100%. That is NOT a standard breaker.

Borgi


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## Rollie73 (Sep 19, 2010)

Dennis Alwon said:


> Yes but the op is USA so I am not sure why we got into CEC


 
Yeah.....how did we get there? 

I remember......it was because dave91 mentioned something and you corrected him, and seeing as how I know Dave personally and I consider him to be a friend (even though he doesn't realize who I really am, even though he loans out his 38' bucket truck to me when I need it for jobs and the poor guy will be as confused as hell when he reads this) I felt I had to stick up for him:laughing::laughing:


Long winded as hell but I want to screw with Dave's head


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## Meadow (Jan 14, 2011)

LBC Jesse said:


> Another quick note, in the world of circuit breakers, the trip curves set by the manufacturerer also begin at 80%. In theory a 1000a breaker should hold 1000 amps indefintly, but in reality the trip curve began earlier at 800a...albeit a very slow curve, it bends quite rapidly after 1000a... (Google trip curve on SqD NA breaker to see a graph). Breakers can be purchased as 100% rated (at quite the adder cost)... As previous poster stated, an electrical engineer SHOULD have that all calculated ...inductive, continuous and resistive loads... Prior to the recommended breaker size given... Past that tidbit of info, I'm about as a smart as a popcorn fart in a hurricane, and troll these posts to further enlighten my self by the extraordinary wisdom you EC's have to offer.


Im sorry I cant look at this post anymore whith 2 likes being attached to it. You do know that is incorrect...?


Most trip curves start at 125%, not 80% for the listed temperature and handle rating.

The only reason why we have 80% and 100% breaker has nothing to do with the trip curve being shifted from 80% to 100%. Rather is has to do with heat build which in turn effects the trip curve and the panel board. The difference between 80% and 100% breakers comes form their ability to dissipate heat. 

This thread is a good read:

http://www.electriciansforums.co.uk...es-electrical-regulations/100841-80-rule.html


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## don_resqcapt19 (Jul 18, 2010)

LBC Jesse said:


> Another quick note, in the world of circuit breakers, the trip curves set by the manufacturerer also begin at 80%. In theory a 1000a breaker should hold 1000 amps indefintly, but in reality the trip curve began earlier at 800a...albeit a very slow curve, it bends quite rapidly after 1000a... (Google trip curve on SqD NA breaker to see a graph). Breakers can be purchased as 100% rated (at quite the adder cost)... As previous poster stated, an electrical engineer SHOULD have that all calculated ...inductive, continuous and resistive loads... Prior to the recommended breaker size given... Past that tidbit of info, I'm about as a smart as a popcorn fart in a hurricane, and troll these posts to further enlighten my self by the extraordinary wisdom you EC's have to offer.


Can you link one of those trip curves? I have never seen a trip curve that started at a multiple of less than 1x the breaker rating.


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## Meadow (Jan 14, 2011)

don_resqcapt19 said:


> Can you link one of those trip curves? I have never seen a trip curve that started at a multiple of less than 1x the breaker rating.


Same, Im not finding anything.


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