Nameplate Rated Current

Consider a 100 amp, 100% rated breaker. You could use a 3 AWG conductor with this breaker and run it continuously.
If you are not using a 100% rated breaker, you would have to use a 1 AWG and 125 Amp breaker.

Julia, I'm curious about the following statement:
This is the point I was overlooking, because in my line of work all loads are continuous and I forget that non-continuous loads even exist. Thanks!

What kinds of loads do you deal with?
 
eric stromberg said:
Consider a 100 amp, 100% rated breaker. You could use a 3 AWG conductor with this breaker and run it continuously.
If you are not using a 100% rated breaker, you would have to use a 1 AWG and 125 Amp breaker.
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I thought the conductor would still have to be sized 125% even if the breaker is rated 100% continuous
 
Dennis Alwon said:
I thought the conductor would still have to be sized 125% even if the breaker is rated 100% continuous
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Nope, see the exceptions to 210.19(A)(1)(a), 215.2(A)(1)(a), etc.

The exception is one of the reasons behind my post #14. The other is the definition of ampacity.

Cheers, Wayne
 
Not sure why everyone seems so baffled. As far as I know the 125% thing is mostly unique to the NEC and just covers OCPD's (and conductors). Julia does not appear to do building and premises electrical wiring.

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The only reason that the conductor is sized at 125% is for the amount of metal mass on the termination of the overcurrent device, for heat dissipation. Although conductors are rated for 100% continuously, running them at 80% results in 64% heating. (I*I*R)
 
eric stromberg said:
The only reason that the conductor is sized at 125% is for the amount of metal mass on the termination of the overcurrent device, for heat dissipation. Although conductors are rated for 100% continuously, running them at 80% results in 64% heating. (I*I*R)
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I don't follow that logic Eric. The conductor is rated at 125% because the OCPD has been increased by 125%. If we don't upsize the conductor it would no longer be properly protected.
 
don't follow that logic Eric. The conductor is rated at 125% because the OCPD has been increased by 125%. If we don't upsize the conductor it would no longer be properly protected.

You are correct. But sizing the conductor at 125% came from the OCPD manufacturers. The reason they did this was to have the conductor run at a lower temperature. If you look at the conductor sizing rules for continuous, there are a couple of options. If the conductor is adjusted for ambient and bundling, and results in a size that is larger than what 125% would have been, you don't need the 125%. It's all about metal mass on the termination. Think about a neutral conductor. Even if the neutral is carrying continuous current, it does not need to be sized at 125%. It is not connected to overcurrent protection, so it doesn't need the metal mass on the termination.
 
electrofelon said:
I don't follow that logic Eric. The conductor is rated at 125% because the OCPD has been increased by 125%. If we don't upsize the conductor it would no longer be properly protected.
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Where the issues Eric raises comes in is explaining why we can't use 240.4(B) with continuous loads. E.g. a 100A continuous load on a 125A panelboard; we have to protect the panelboard with a 125A breaker if the breaker is not 100% rated. #2 Cu 75C with an ampacity of 115A is normally allowed to be protected at 125A under 240.4(B) when the load is below 115A, and the 100A load is less than 115A.

But with a 100A continuous load, #1 Cu 75C is required as an additional step to reduce the heating at the breaker. Seems like overkill to me, since we already upsized the breaker by 125%, but that's the rule, upsize the conductor on the breaker by the full 125%, rather than just upsize the conductor enough to be safe to use with the upsized breaker.

Cheers, Wayne
 
If you look at the temperatures, running conductors at their maximum, in a 30 degC ambient, will result in 75 DegC at the termination of the OCPD. The OCPD manufacturers are not comfortable with that much heat on the terminations. After all, their devices are typically thermal devices and too much heat on the terminal could result in the OCPD tripping at a lower level. Last time i calculated it, when 125% conductor size was used, resulting in 80% current and 64% heating, the resultant temperature at the terminations was about 60 DegC. A 100% rated breaker is built so that it will trip at the correct level with the terminations at 75 DegC.
 
eric stromberg said:
But sizing the conductor at 125% came from the OCPD manufacturers. The reason they did this was to have the conductor run at a lower temperature
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Do you have any proof of this?

eric stromberg said:
A 100% rated breaker is built so that it will trip at the correct level with the terminations at 75 DegC.
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And do you have any proof of this? I have never gotten a conclusive answer. Some say there is NO difference in a 100% breaker other than the enclosure, and then some say they are built to a different thermal profile....
 
So much absolute misinformation.

Go to stanards like UL 489 or NEMA AB-1 and look at the standard require testing for circuit breakers. You will find they are tested at 100% loading in open air with 4ft of conductor, sized per NEC, attached to their terminals. The conductor terminals never ever reach the temperature rating of the conductor insulation. In fact the lugs are often the coolest part of the breaker.

The NEC requires you to chose the conductor based on the load, and environmental factors, then you chose it's protection. I have never seen where the NEC says pick the OCPD and then pick the conductor.
 
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jim dungar said:
You will find they are tested at 100% loading
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And calibrated trip curves show 100% loading may occur for several hours.

Calibration also depends on <=40C ambient, so sun baked fuse boxes may alter trip curves or damage electronics.
jim dungar said:
The conductor terminals never ever reach the temperature rating of the conductor insulation. In fact the lugs are often the coolest part of the breaker.
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IMHO, less thermal imaging reports would show overheated wire, if electricians paid attention to 220.18 for inductive ballast & motor loads, where current rises with voltage drop.
 
jim dungar said:
So much absolute misinformatio.

Go to stabdards like UL 489 or NEMA AB-1 and look at the standard require testing for circuit breakers. You will find they are tested at 100% loading in open air with 4ft of conductor, sized per NEC, attached to their terminals. The conductor terminals never ever reach the temperature rating of the conductor insulation. In fact the lugs are often the coolest part of the breaker.

The NEC requires you to chose the conductor based on the load, and environmental factors, then you chose it's protection. I have never seen where the NEC says pick the OCPD and then pick the conductor.
Click to expand...

Ok so still some questions remain:

1. I would like to know the exact chain of events around continuous loads and the NEC. Did the 80/125% thing come about because there were problems arising from many breakers being jammed in an enclosure and the assembly getting too hot? Is the conductor increased just because now the ocpd is increased, or was there a specific argument or testing done to show that a larger conductor acted as a heat sink?

2. Is there any actual internal difference between a regular and 100% rated breaker, or is it just the enclosure?
 
electrofelon said:
Ok so still some questions remain:

1. I would like to know the exact chain of events around continuous loads and the NEC. Did the 80/125% thing come about because there were problems arising from many breakers being jammed in an enclosure and the assembly getting too hot? Is the conductor increased just because now the ocpd is increased, or was there a specific argument or testing done to show that a larger conductor acted as a heat sink?
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I don't think you will ever find it. It is almost for surely not based on science but rather it is a feel good number.

electrofelon said:
2. Is there any actual internal difference between a regular and 100% rated breaker, or is it just the enclosure?
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For the past 50 years or so it has just been the enclosure and the breaker labeling. If you search on-line you will find the trip curves are identical between 80 and 100%.

In the past one way to make 100% breakers was to put small trip units into larger breakers, but electronic trips have pretty much done away with this need except for small trip units, like <200A. However some day some manufacturer may find a way to cost reduce their products and this might change.
 
electrofelon said:
Ok so still some questions remain:

1. I would like to know the exact chain of events around continuous loads and the NEC.
Click to expand...
When I researched the history of it the first mention of continuous loads was in the 1951 NEC and just branch circuits, that year had two sub committees one on electric space heating the other on motion picture studios.
Later in the 1959 NEC it was expanded to feeders, The term 'Continuous load' was added to the definitions in 1965. Prior to 1965 I think they were just loads that ran for 'long periods of time’.
 
tortuga said:
When I researched the history of it the first mention of continuous loads was in the 1951 NEC and just branch circuits, that year had two sub committees one on electric space heating the other on motion picture studios.
Later in the 1959 NEC it was expanded to feeders, The term 'Continuous load' was added to the definitions in 1965. Prior to 1965 I think they were just loads that ran for 'long periods of time’.
Click to expand...
Great info, thanks. So so OCPD's came later? That is interesting.
 
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