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Old 04-01-2014, 09:56 PM   #1
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Default 80% rule?

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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Old 04-01-2014, 10:03 PM   #2
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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!
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Old 04-01-2014, 10:13 PM   #3
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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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Old 04-01-2014, 10:30 PM   #4
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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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Old 04-01-2014, 11:26 PM   #5
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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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Old 04-02-2014, 06:15 AM   #6
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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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Old 04-02-2014, 06:30 AM   #7
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Interesting. So there isn't a word of this in the nec? And what about an exam?

Last edited by TomSpano; 04-02-2014 at 06:39 AM.
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Old 04-02-2014, 06:30 AM   #8
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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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Old 04-02-2014, 08:43 AM   #9
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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.)

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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Old 04-02-2014, 11:35 AM   #10
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The 3 hour continuous rule is in 210.19. (Everyone calls it the 80% rule, but it's really the 125% rule.)
...
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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Old 04-02-2014, 11:39 AM   #11
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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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Old 04-02-2014, 11:44 AM   #12
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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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Old 04-02-2014, 11:50 AM   #13
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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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Old 04-02-2014, 11:53 AM   #14
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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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Old 04-02-2014, 12:01 PM   #15
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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.
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Old 03-23-2015, 03:54 PM   #16
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Branch circuits at 80 % service breaker at 100 %
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Old 03-23-2015, 04:01 PM   #17
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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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Old 03-23-2015, 06:24 PM   #18
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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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Old 03-23-2015, 06:36 PM   #19
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Originally Posted by dave91 View Post
Branch circuits at 80 % service breaker at 100 %
Quote:
Originally Posted by Dennis Alwon View Post
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......

Quote:

(3) The calculated load in a consumerís 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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Old 03-23-2015, 07:05 PM   #20
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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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