80% Breaker Rating Rule?

[eastcreek]

List,

I've just discovered that the 80% loading of a breaker rule was eliminated in the 2002 code because calculations required to properly size wire and devices properly compensate and make the 80% rule no longer needed.

What amazes me is that it possibly should still be in place for older installations.

Mike

[ September 22, 2004, 02:08 PM: Message edited by: eastcreek ]

 

[charlie b]

Re: 80% Breaker Rating Rule?

If you have a branch circuit supplying nothing but ?continuous loads,? then the minimum conductor size must have an ampacity of 125% of the load ? 210.19(A)(1) ? and the overcurrent protection must be 125% of the load ? 210.20(A). Therefore, the breaker must be loaded to no more than 80% of its rating. These were not changed in the 2002 code.

What ?rule? do you believe had been eliminated?

 

[templdl]

Re: 80% Breaker Rating Rule?

This is the way I have looked at and explained the breaker applications: Please correct me if I'm wrong but some misunderstand the concept of a breaker that is "80%" rated and the ones that are "100%" rated.
As you have noted Charlie if the cable is sized correctly based on the computed load a standard breaker is selected to protect the cable based upon that 125% is automatically applied at 80%.
To apply a 100% rated breaker one can omit that extra 25% that was added to the continuous load where 90degC rated cable must be used but its ampacity still applied per the 75degC column. The 100% rated breaker must be installed in an enclosure listed for the 100% application otherwise it defaults back to a standard breaker and the cable then must be applied as such.
So sizing cable comes first and the breaker sized to protect the cable case upon the ampacity rating of the cable.

 

[iwire]

Re: 80% Breaker Rating Rule?

In addition to the NEC there is the UL listing to consider.

From the 2003 UL white book

CIRCUIT BREAKERS, MOLDED-CASE AND CIRCUIT BREAKER ENCLOSURES (DIVQ)

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.

 

[eastcreek]

Re: 80% Breaker Rating Rule?

List,

I was thinking more along the lines of a feeder breaker. But I see it's blatently obvious at 215.2 A 1. Seek and ye shall find!

[ September 23, 2004, 09:57 AM: Message edited by: eastcreek ]

 

[612278]

Re: 80% Breaker Rating Rule?

How about NEC 384-16(d) and also what about the Celsius rating of breaker. (some zinsco breaker as low as 40* c)

Steve

 

[templdl]

Re: 80% Breaker Rating Rule?

Personally, and please correct me if I'm wrong, I don't know why this 80% thing is an issue. Think about it, you have continuous and noncontinuous loads, 125% of the continuous load + 100% on the non continuous loads, art. 210.19. The cable most be sized to carry that computed load.
The breaker is sized to protect that cable, that is the rated ampacity of the cable and not the computed load. If there isn't a standard breaker size, art. 240.6, which is the same and the cables rated ampacity then you are allowed to apply the next standard breaker rating for ratings up to 800a arts 240.4 and 240.4(B) and (C).
If cable and breaker are applied correctly why would there be a concern about this 80% breaker rating thing unless one gets the cart before the horse? Or am I missing something?

 

[iwire]

Re: 80% Breaker Rating Rule?

I do not think your wrong for most circuits.

Originally posted by templdl:
If cable and breaker are applied correctly why would there be a concern about this 80% breaker rating thing unless one gets the cart before the horse? Or am I missing something?

Think of 15, 20 & 30 amp circuits, we must (most times) use 14, 12, & 10 AWG copper for these circuits.

A 12 AWG copper is rated 25 amps minimum.

So we could load a 12 AWG copper 20 amps continuous, 20 x 1.25 = 25 amps. But the standard 20 amp breaker can not be loaded 20 amps continuous.

 

[templdl]

Re: 80% Breaker Rating Rule?

Great discussion.
Yes, I agree Ref. 240.4(d) Small conductors
unless permitted by 240.4(E)through (G).
Remember that there are 25at breakers available.

 

[a.wayne3@verizon.net]

Re: 80% Breaker Rating Rule?

is it NM or thhn/wn in pipe 90/75 degree select 1

 

[paul32]

Re: 80% Breaker Rating Rule?

Originally posted by iwire:
Think of 15, 20 & 30 amp circuits, we must (most times) use 14, 12, & 10 AWG copper for these circuits.

A 12 AWG copper is rated 25 amps minimum.

So we could load a 12 AWG copper 20 amps continuous, 20 x 1.25 = 25 amps. But the standard 20 amp breaker can not be loaded 20 amps continuous.

Wouldn't you really say the 12AWG could be loaded to 25 amps continuous? The definition of ampacity is the current it can handle continuously. I know the breaker is still limited to 20, but if you had a continuous 25 amp load that met an exception to the 20A breaker limit, couldn't you use 12AWG?