Why does it exist?

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mbrooke

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Why does the 80% and 125% rule exist in the code? Why bother when the breakers can hold 100% for every by design?




One possible explanation I have heard is ohms law and voltage boundaries... but then why not for all loads?
 

iwire

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My understanding it is simply a matter of getting rid of the heat that thermomagnetic breakers produce. When you get a 100% rated breaker it goes in its own individual enclosure and not sandwiched in between other breakers.

It would be like operating them in a high ambient temperature area which might result in tripping at current levels below their specifications.
 
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mbrooke

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My understanding it is simply a matter of getting rid of the heat that thermomagnetic breakers produce. When you get a 100% rated breaker it goes in its own individual enclosure and not sandwiched in between other breakers.

It would be like operating them in a high ambient temperature area which might result in tripping at current levels below their specifications.


Makes sense.


Out of curiosity, why are some loads like electric heat forced to be 80%?
 

Jraef

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It's more of an "end around" situation than one might think.
1) code says conductors must be rated for 125% of maximum continuous load.
2) inverse of 125% is 80% (1/1.25)
3) since the conductors sized for the circuit are over sized, that means the circuit cannot be allowed carry 100% current continuously anyway.
4) Since the breaker is there to protect those conductors, and they can only be loaded to 80% continuously, no need for the breaker mfr to design for heat dissipation at 100% load when mounted in a panelboard with other breakers, which is the difficult (translate; expensive) part for them.

This is why you will see that 100% ratings on breakers are usually restricted to being used as stand-alone breakers or main breakers, where there will be no other breakers directly adjacent to them. It's really the exact same breaker*, often just with different lugs that are rated for 90C, but not allowed to be feeders in a packed panelboard. All breakers are bench tested per UL standards at 100% current, it is the APPLICATION of them as feeders in panels that indirectly creates the "80%" rating.

* You will be hard pressed to find an employee of a breaker mfr who will state this, especially in writing. I used to work for Siemens, we were SPECIFICALLY trained to never say this, because it can lead to over interpretation and misapplication, then lawsuits where the sharks look for the biggest bleeders.
 
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iwire

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It's more than the NEC, it's also listing.

From the UL White book CIRCUIT BREAKERS, MOLDED-CASE AND CIRCUIT BREAKER ENCLOSURES (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 three hours or more.
 

mbrooke

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Is it really that hard to design a breaker to run over 3 hours? :blink:



Thanks for the response J-raef!


If the limiting factor for the breakers is the wire, than what makes the wire become less safe after 3 hours?
 
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mbrooke

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Thanks for the link!:)

My understanding is that under test conditions and design its 104 degrees in free air, but come panel board its derated because of heat build up.


IMO, it should be taken into account during testing so a breaker can be 100% even in a panel board. The 80% rule is silly imo. But gotta do what the NEC says.


Not to dive off topic but I believe this may also have dictated the 42 circuit rule.
 

jjs

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Location
Puryear, TN, USA
There is no such thing as an 80% rated breaker. They are all 100% rated. Now let me clarify. Some are 100% rated for both non-continous and continous loads, called 100% breakers. The rest are rated to hold 100% of the demand load for the combination of the non-continous loads and the continous loads. You take 100% of the non-continous loads and add them to continous loads at a 125% factor and the combination is the demand load. The regular breaker can handle that demand at 100% of its rating.

It all has to do with heat build up. The NEC states that a continuous load is expected to be at max wattage for 3 hours. So a server may be on 24 hours a day, but very seldom hits the max of its power supply so is not considered continuous.

I think this section of the code is one of the most misunderstood in the whole industry. People think you can only load a regular break to 80% of its rating no matter what. So they would say a 100A regular cb can never be loaded over 80A. As long as the demand load of all the different loads on the cb is 100A or less, then it is fine. So you can put 18000VA of lighting and 10000VA of receptacles plus another 6900VA of receptacles on a 100A/3P 208V cb.

Assume 208V 3P

Demand load for the receptacles
10000VA x 100% + 6900VA x 50% = 13450VA of receptacles, which is non-continuous. 35950 VA / (208*1.732) = 37.3A

Lighting
18000 VA / (208*1.732) = 49.96A

Load calculation for regular breaker.
13450VA x 100% + 18000VA x 125% =35950 VA / (208*1.732) = 99.8A
That meets the NEC requirements for the 100A CB.
 
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