80% rule misapplied so often. WHY?

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jschultz

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can you load a 80% breaker to 100% of its rating if it is 100% non-continous load?

can you load a 80% breaker to 100% of its rating if it is 100% continous load?

can you load a 80% breaker to 100% of its rating if (100% continous load + 125% of non-continous load) is equal to the breaker rating?
 
Re: 80% rule misapplied so often. WHY?

Is 3va/sf demand load for a residence considered continous or non-continous load?
 
Re: 80% rule misapplied so often. WHY?

That animal does not exist. All circuit breakers are tested at 100% in the open and listed for 100% load. You end up loading them at 80% of the continuous load and 100% of the non-continuous load since they are in an enclosed area and the heat can not escape.

Drop the idea of an 80% overcurrent device. Section 210.19(A)(1) states, "Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load." Since the rating of a branch circuit is the overcurrent size, the circuit breakers you are asking about are loaded the same.

"Is 3va/sf demand load for a residence considered continuous or non-continuous load?"

It is considered non-continuous load. If you calculate 20 amperes, you may put the load on a 20 ampere overcurrent device.
 
Re: 80% rule misapplied so often. WHY?

The NEC is not a design manual.

To determine the proper overcurrent protective device to meet the NEC:
1) Determine the current of the load.
2) Apply a factor of 125% if the load is continuous.
3) Choose the conductor taking into account all appropiate amperage deratings for number of conductors in a raceway, terminal temperature ratings, ambient temperature ratings.
4) Finally choose an OCPD that does not exceed the conductor ampacity as determined in step 3.

Why do most people want to start at the OCPD and then work towards the load?

>With credit to Scott Adams and Dilbert - If you combine con and insult you get "consult".<
 
Re: 80% rule misapplied so often. WHY?

Originally posted by charlie:
Drop the idea of an 80% overcurrent device.
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Can you talk to UL about that, they keep the idea alive.

Part of (DIRW) CIRCUIT BREAKERS, MOLDED-CASE AND CIRCUIT BREAKER ENCLOSURES (DIVQ) 2003 White Book
MAXIMUM LOAD
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.
Click to expand...
So now I am confused. :confused:

Bob
 
Re: 80% rule misapplied so often. WHY?

It is the attempt to use the name ?80% breaker? that is causing confusion. A 20 amp breaker should not be given the name ?16 amp breaker,? even though we may choose to load it no higher than 16 amps.
 
Re: 80% rule misapplied so often. WHY?

I agree with the Charlies. If you calculate the load, size the cable, then apply the breaker. The "80%" issue is a non-issue.
A 100% rated breaker is applied in a similar way starting with calculating the loads and cable as is allowed for 100% rated breaker applications and the applying a 100% rated breaker.
The breaker is the last thing in the food chain.
 
Re: 80% rule misapplied so often. WHY?

I guess it all depends if you are actually doing engineering or just planing some branch circuits. ;)

If I walk on to the job and I have to add a certain number of light fixture I do not start with the total load say 50 amps and design a circuit based on that.

The breaker is the last thing in the food chain.
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Not really for me, I know from the start I will be using 20 amp breakers and 12 (or 10 AWG for voltage drop).

I work in a commercial environment so I consider the lighting as continuous.

That said I can not exceed 16 amps on each 20 amp circuit, I will be shooting for a number between 10 and 12 amps with 16 being the top end.

It is just the way I do it and it works for me. :p

If I was designing the entire buildings electrical system I would have a lot to learn and I would have to look at it differently.
 
Re: 80% rule misapplied so often. WHY?

Bob,

As an installer, as we both are, we tend to look at circuit breakers from a different perspective.

2003 UL White Book
CIRCUIT BREAKERS, MOLDED-CASE AND CIRCUIT BREAKER ENCLOSURES (DIVQ)PRODUCT TYPE
MAXIMUM LOAD
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.
Click to expand...
We like to be sure we do not overload the circuit breaker and have a code violation or a trip situation. So we usually look at it from the point of view as the installer nornmally would. And we are right in doing so.
However, when looking at the NEC and the way we are to calculate circuit loads, the NEC says we are to calculate the non-continuous at 100% plus the continuous at 125% to arrive at our rating. There is no provision that I can find that specifically states "non-continuous are calculated....". The best I can see is when we calculate general circuit loads we use the V/A method.
The bottom line is we end up with a minimum of code compliance and usually better!
 
Re: 80% rule misapplied so often. WHY?

Dave we are on the same page. :)

My way works for me for my limited application and leaves room to spare on the circuits when the customer adds things while I am cleaning up to leave. :D
 
Re: 80% rule misapplied so often. WHY?

when designing lighting branch circuit I also try to stick to the 10-12 amp range for the same reason, lighting is usually added9or changed to higher wattages) during construction or afterwards when people move in and then a new branch circuit is not required.
 
Re: 80% rule misapplied so often. WHY?

Well the Chicago electrical inspectors seem to think residential loads are continous. And you can only load a device to 80% of its rating. Most residential electricians around here seem to think the same, probably because the code inspector told them that.
I hear things like you can only load the service to 80% all the time.
I believe i can load the thing to 100% of my calculated demand load. Because my calculated demand load has already taken continous and non-continous into consideration.

most of the breakers are 80% rated for continous loads and you only get 100% rated if you request it.
 
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