No 80% rule

[bphgravity]

Alot of my students and alot of users of the forum refer to the old 80% quite often. I believe this is misused terminology. Do we not size our OCD per our load and not the load to the OCD? I see people write over and over, "make sure not to load the breaker to more than 80%" etc, ect..... Thats not the idea, and I think thats why this language has been removed for the 2002. If a load is to be continuous, we must size our OCD to 125% anyway. This automatically covers the 80%. For example: The continuous load is 18-amp. 18 x 125% = 22.5 - 25-amp OCD. Assuming only one utilization device. 25 x 80% = 20-amp. This making any sense?

 

[iwire]

Re: No 80% rule

I really think it's all the same thing, some people like to state it like the code "add 25% for continuous loads" and some like myself say the "equipment is good for 80%" both ways work fine if all the loads are continuous, if there are not you end up with larger equipment than code requires not a bad thing.

I guess this comes from not really having to think about it, very few loads in commercial work are not continuous.

But I am sure your students want and should get it the way the code states it.

[ March 18, 2003, 10:41 PM: Message edited by: iwire ]

 

[pwhite]

Re: No 80% rule

bphgravity,

maybe this will help. i spoke to one of my electrical suppliers and asked him about the 80% rating of breakers. when he came back to me he stated that breakers are only rated for 80% of their amperage. a 100 amp breaker will handle 80 amps continuously without tripping. going over this amount will cause the breaker to trip due to its thermal overload characteristics and the amount of current past 80 amps.

i must admit that i don't understand why a 100 amp breaker isn't rated 100 amps continuous.

there are breakers thar have adjustible trip currents out there, but they cost more.

 

[ron]

Re: No 80% rule

There are some breakers now rated @ 100% continuous load. It is noted in the literature and on the breaker.

ie. Ge Spectra SGHH36BB0150

[ March 19, 2003, 11:55 AM: Message edited by: ron ]

 

[elec-student]

Re: No 80% rule

my teacher teaches us the 80% "rule" it does help out a lot when we were firstlearning and we keep using it to be safe when we wire when were out on the job.

 

[rdoan71]

Re: No 80% rule

A simple way that seems to work with my students, is to just give them different Ohm's law problems, some with the words "continuous duty" some without. They know that continuous duty means to size the OC device by taking the current of the circuit and dividing by .8 or multiplying by 125%. It makes them look at the probems more closely, because one wrong answer will invariably lead to a second.

Now, if I can just get them to realize that it effects only the OC device and NOT THE CURRENT OF THE CIRCUIT.

 

[toddb1963]

Re: No 80% rule

i think that people use the 80% rule to ensure that they do not overload the ocpd. a 15 amp breaker will hold a 15 amp load, but not a continuous load for a long period of time.

 

[frank]

Re: No 80% rule

IF I HAVE A 19 AMP LOAD NON CONTINUOUS BY CODE I CAN USE A 20 AMP BREAKER AND WHERE IN THE CODE CAN I FIND THIS

 

[frenchelectrican]

Re: No 80% rule

i will not get involed with this debate but for 80 % load and look up in nec codes what it say for continuous load and non continuons load is meaing and i will try to explain it here ::

80 % load continouns load mean the load stay appied more than THREE hours rating or the load never change the rate at all


non continous load it mean variable load it can go up to 100 % load but depend on the conducter and over current devices it can take

btw check on breaker manufacters it will listed on the load setting for 80 or 100% load it will make a big differnce with it

merci marc

 

[bphgravity]

Re: No 80% rule

Thanks for all of your replies. I still think it is important to understand that the 80% rule no longer exists in actual language of the code. It has been embeded with load calculations to cover this limited capacity of typical breakers at continuous loads. The proper way to design a circuit is to start with the load and size the OCD and not select an OCD and then determine how much load it can carry.

 

[james wuebker]

Re: No 80% rule

pwhite, I didn't know about 100a CB only rated at 80%. I know that most CB's will hold it's rating for some time. The one reason they have the 80% rule is because of the service box. If each breaker was drawing it's full rating the box would get very hot inside causing the main to trip and also the branch circuits. That"s one of the reasons for the 80% rule. Everyone has great anwsers on this topic.

 

[bphgravity]

Re: No 80% rule

As of 2002 NEC, there is no 80% rule!

 

[curt swartz]

Re: No 80% rule

According to the UL Green Book (DIVQ)

Unless otherwise marked, circuit breakers should not be loaded to exceed 80 percent of their current rating where in normal operation the load will continue for 3 hours or more.

 

[iwire]

Re: No 80% rule

Originally posted by bphgravity:
The proper way to design a circuit is to start with the load and size the OCD and not select an OCD and then determine how much load it can carry.

That is certainly true for a "fixed load" like one electric heater.

But if I have to lay out lighting circuits in a large office space (the panel is already there) with 100-.5 amp fixtures I do not start with the load and find a OCPD large enough to run it all, which in this case would be

100 x .5 = 50 amps x 1.25 = 62.5 amps I am not trying to give you a hard time we just look at it from different perspectives.

On the job I just find it easier to keep 80% in my mind the only mistake from this will be a larger circuit then code requires if applied to non continuous loads.

Add 25% or derate to 80% the result is the same, so aren't we both right?

[ March 23, 2003, 07:11 AM: Message edited by: iwire ]