I don't understand WHY you can apply the 125% rule.

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Elecestim123

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Been googling like hell to try to understand. I like to understand how/why things work/are the way they are, and not just do it because you're told to.

So I understand when the 125% rule applies, and how to size a breaker for it and all that. I even get youre sizing the wire/CB larger to overcome the heat problem. My question isn't about trying to understand the rule to apply it, or how to apply it. My question is probably more related to the theory of it.

The general idea of a breaker is to stop the flow of current when there's too much of a current draw. If you have a piece of equipment sized to draw 200A, that's stating it can safely handle UP to 200A. Any more current then that it needs a circuit breaker to stop the flow of current so it doesn't become damaged. I also know for motors there's a large inrush current, and breakers are designed to handle this SHORT burst of inrush current. But with the 125% rule for continuous loads, with the 200A example your breaker is now loaded to 250A. This means if there's a problem and there's say 235A being drawn by the load now, the breaker won't trip, and in the mean time the equipment is being damaged.

Am I completely overthinking this?

Is it as simple as the load can only draw what it draws, so there's no reason it would ever draw over 200A anyways, so there would never be more than that being drawn, and if there's a ground fault/short that would be a huge amount of current anyways so the breaker would trip? I feel like this is the answer, and therefore this is a dumb question, but in case it's not, I'm curious to know the answer.
 
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Elecestim123 said:
Is that because the equipment typically has its own protections internal to it?
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That can be the case but not not necessarily. Generally the conductor can overheat when the full capacity of the conductor is reached and damage could be done to the conductor if that load is continuous for long periods of time.

Circuit breakers, at least most of them, are not rated for 100% load at continuous use. A 20 amp breaker will not trip with 20 amps for short periods but if that load is on continuously the breaker will fail in time. It is not designed for that constant load.

Because we use 125% that doesn't mean we can load the circuit to that 125%. As I mentioned above a breaker cannot handle 100% load continuously so the wire and breaker needs to be sized 125% higher to cool things dow.

A 20 amp continuous load would have to be wired with #10 wire and a 25 or 30 amp breaker so the heat doesn't do damage to the wire or breaker
 
Dennis Alwon said:
Circuit breakers, at least most of them, are not rated for 100% load at continuous use. A 20 amp breaker will not trip with 20 amps for short periods but if that load is on continuously the breaker will fail in time. It is not designed for that constant load.
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If I may "fine tune" this a little. I believe the issue is There is a discrepancy in the standard testing procedure for circuit breakers, and the way they are typically installed and used. They are designed and tested to hold 100% of their current rating forever, however they are tested at a certain temperature and with a certain amount of free air flow, which is often not the case when we pack a panel board full of breakers right next to each other. Because of this, we need that 80% derating. A 100% breaker is actually the same, it is just the way it is used, typically in its own enclosure with an approved amount of free space and air flow that lets it run at 100% forever without issues. I think I have that right, but no doubt someone will jump in if I messed up 😉
 
Elecestim123 said:
Am I completely overthinking this?

Is it as simple as the load can only draw what it draws, so there's no reason it would ever draw over 200A anyways, so there would never be more than that being drawn, and if there's a ground fault/short that would be a huge amount of current anyways so the breaker would trip? I feel like this is the answer, and therefore this is a dumb question, but in case it's not, I'm curious to know the answer.
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100% is simply a reference number that our industry chose many decades ago. It is not an absolute. And there is very little science behind it.

After enough decades of basically trial and error, it was decided that sizing conductors to 125% provided a safe installation for continuous loading. Circuit breakers and fuses in enclosures were design tho protect the conductors chosen based on the 125% value.

Remember conductors fail by getting too hot over a time period. They can handle 100% forever but they can also handle 1000% for several seconds.
 
jim dungar said:
100% is simply a reference number that our industry chose many decades ago. It is not an absolute. And there is very little science behind it.

After enough decades of basically trial and error, it was decided that sizing conductors to 125% provided a safe installation for continuous loading. Circuit breakers and fuses in enclosures were design tho protect the conductors chosen based on the 125% value.
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I don't follow your comments and it sounds backwards to me. The 125% has nothing to do with conductor capabilities; a conductor is rated to carry 100% of ampacity continuously (see the definition of ampacity). The 125% is only about the limitations of circuit breakers in a thermal environment of a crowded panelboard; if you try to draw 100% of a breaker's rating in that thermal environment, you may get nuisance tripping.

Cheers, Wayne
 
wwhitney said:
I don't follow your comments and it sounds backwards to me. The 125% has nothing to do with conductor capabilities; a conductor is rated to carry 100% of ampacity continuously (see the definition of ampacity). The 125% is only about the limitations of circuit breakers in a thermal environment of a crowded panelboard; if you try to draw 100% of a breaker's rating in that thermal environment, you may get nuisance tripping.

Cheers, Wayne
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This sounds correct. At 80% with continuous loads the amount of heat generated in the panelbaord will be acceptable. At 100% it will not therefore you would need equipment designed to operate at 100% continuous load.
 
electrofelon said:
If I may "fine tune" this a little. I believe the issue is There is a discrepancy in the standard testing procedure for circuit breakers, and the way they are typically installed and used. They are designed and tested to hold 100% of their current rating forever, however they are tested at a certain temperature and with a certain amount of free air flow, which is often not the case when we pack a panel board full of breakers right next to each other. Because of this, we need that 80% derating. A 100% breaker is actually the same, it is just the way it is used, typically in its own enclosure with an approved amount of free space and air flow that lets it run at 100% forever without issues. I think I have that right, but no doubt someone will jump in if I messed up 😉
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I think the 80% breaker is just calibrated to a slightly different temperature curve because it may have current flow at 100% of its rating for extended periods of time. The thermal part of thermal magnetic breaker means there is an actually a little heater inside the breaker that is used to simulate the damage curve for the conductors the breaker is protecting. If that heater is running all the time, it will create some heat and that heat has to go somewhere.

As I understand it, one of the places the heats goes to get out of the breaker is through the wires connected to it, which is why there are some special wire rules for some 100% rated breakers.

I don't think there is any prohibition on mixing 80% and 100% rated breakers in the same panel though.

I can't recall offhand if I have ever used a 100% rated breaker.
 
wwhitney said:
I don't follow your comments and it sounds backwards to me. The 125% has nothing to do with conductor capabilities; a conductor is rated to carry 100% of ampacity continuously (see the definition of ampacity). The 125% is only about the limitations of circuit breakers in a thermal environment of a crowded panelboard; if you try to draw 100% of a breaker's rating in that thermal environment, you may get nuisance tripping.

Cheers, Wayne
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I said the 125% had to do with how the NEC tells you to size conductors and their protection based on if the load is or isn't continuous. If you have a 13A continuous load, can you feed it with a 15A rated conductor? If you have decided to use a conductor rated for 100A, what size protective device do you use?

All circuit breakers, and fuses, are tested and calibrated for 100%.
The 125% fudge factor comes into play when the protective device and it's terminations are put into an enclosure. 100% rated protective device enclosures have more ventilation than standard enclosures.
 
jim dungar said:
I said the 125% had to do with how the NEC tells you to size conductors and their protection based on if the load is or isn't continuous. If you have a 13A continuous load, can you feed it with a 15A rated conductor?
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You could, if you could find a 100% rated 15A OCPD.

jim dungar said:
If you have decided to use a conductor rated for 100A, what size protective device do you use?
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For normal (non-100% rated) OCPD, with the above 13A continuous load, any OCPD between 20A and 100A. [Not sure what you point is.]

My point was that the way the NEC's presentation leads with the conductor sizing rule (215.2 comes before 215.3, for example) can give rise to the misperception that a 20A ampacity conductor would be overloaded by a 20A continuous load. The conductor sizing rules exist only because of the limitations of the normal OCPD.

Cheers, Wayne
 
jim dungar said:
100% is simply a reference number that our industry chose many decades ago. It is not an absolute. And there is very little science behind it.

After enough decades of basically trial and error, it was decided that sizing conductors to 125% provided a safe installation for continuous loading. Circuit breakers and fuses in enclosures were design tho protect the conductors chosen based on the 125% value.

Remember conductors fail by getting too hot over a time period. They can handle 100% forever but they can also handle 1000% for several seconds.
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I like this explanation.

If you increase the conductor to 125% and the overcurrent to 125% of the actual load then it kind of takes care of any inaccuracies of any kind that could lead to unintended operation of the overcurrent device and yet prevents overloading the conductor. If the load is not continuous, which NEC has determined is 3 hours or more, the components involved have enough chance to cool that potential heating damage is lessened, but don't confuse that heating with the resistive heating that can happen at a bad connection, those can heat up to extremes in just minutes or even seconds.
 
So say you have a 50A continuous load, and you put a single 50A 2 pole breaker in a 2 position enclosure, and use it to supply #8 Cu in a 75C rated wiring method to the load. Even though there's no chance that breaker/enclosure has been tested for a 100% rating, I would expect if it is installed in conditioned space (say 25C ambient), there's a good chance that it won't ever trip at 50A continuous. Assuming that's the case, while the above installation violates 210.19(A)(1)(a), is there any safety problem with the installation?

Cheers, Wayne
 
Dennis Alwon said:
Try and find a standard breaker in that size is not possible, afaik. Someone told me dim rail cb's are rated 100% continuous but I have no proof of that.
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Again, all breakers are tested and calibrated at 100%. Any derating is to accommodate the lack of air flow when a protective device, and it's terminations, are put into an enclosure. 100% rating requires 'special' enclosure sizing and ventilation requirements. I have never seen UL 489 listed DIN rail breakers at 100%. UL 1077 listed supplementary breakers have different application considerations, they are not recognized for NEC branch circuit protection.

Historically, 100% rated breakers have been electronic style with frames sizes 400A and larger, in recent years you can find some 100% rated breakers in 125A frames.
 
wwhitney said:
So say you have a 50A continuous load, and you put a single 50A 2 pole breaker in a 2 position enclosure, and use it to supply #8 Cu in a 75C rated wiring method to the load. Even though there's no chance that breaker/enclosure has been tested for a 100% rating, I would expect if it is installed in conditioned space (say 25C ambient), there's a good chance that it won't ever trip at 50A continuous. Assuming that's the case, while the above installation violates 210.19(A)(1)(a), is there any safety problem with the installation?

Cheers, Wayne
Click to expand...
If the breaker is installed in a 25C ambient, the NEC allows you to consider how it affects the breaker.
 
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