rated at 80% or higher
- Thread starter Ponchik
- Start date
- Status
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
Are you saying that it the strip is rated for 15 amps that it would trip at either 12 amps or 15 amps?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
The power strip is rated for what the power strip says it is rated for, there is no mystery here. The "80% rated" thing with breakers is not really what people think it is.
If you have a 12A load, the code says that the conductors for that load must be sized for 125% of that value. 12 x 1.25 = 15A conductors. So the OCPD for that circuit, the breaker, must be rated to protect that conductor, therefore 15A. But you cannot load that breaker to more than 12A without overloading the conductors it is connected to, so INDIRECTLY, the breaker appears to be "rated" for 80%. In reality, the breaker testing and listing is done at 100% of the stated rating. But that testing is done in "free air", meaning that the breaker is all by itself with nothing next to it when it is tested. When you use it, you put it in a panel next to other breakers, which are also hot, so the breaker is then de-rated a little based on that. However since you STILL cannot load the conductors coming FROM that breaker to more than 80% of the breaker rating, that more than covers any heat de-rating anyway, so we still call it a "15A" breaker.
None of this applies to the little internal trip device in a power strip. That power strip is (should be) UL listed all by itself under it's own test criteria, UL1383, "Relocatable Power Taps" (RPTs). In that standard, an RPT is not even required to HAVE an OCPD included in it, but if it does, the standard SPECIFICALLY states that it CANNOT be a substitute for the Branch Over Current Protective Device. In other words it is there as "supplementary protection" only, so the rules on when and how it trips are irrelevant. The UL standard ALSO says that the RPT is only listed to be used when plugged directly into a hard wired receptacle. Most people don't know it (unless they have been popped by a Fire Marshal) but you are NOT allowed to put a plug strip on the end of an extension cord, nor are you allowed to plug an extension cord into a plug strip. So because the plug strip is plugged into a hard wired 15A receptacle, and IT is then ostensibly connected to a #14 wire and a 15A circuit breaker, you effectively cannot exceed 12A on that plug strip anyway.
If you have a 12A load, the code says that the conductors for that load must be sized for 125% of that value. 12 x 1.25 = 15A conductors. So the OCPD for that circuit, the breaker, must be rated to protect that conductor, therefore 15A. But you cannot load that breaker to more than 12A without overloading the conductors it is connected to, so INDIRECTLY, the breaker appears to be "rated" for 80%. In reality, the breaker testing and listing is done at 100% of the stated rating. But that testing is done in "free air", meaning that the breaker is all by itself with nothing next to it when it is tested. When you use it, you put it in a panel next to other breakers, which are also hot, so the breaker is then de-rated a little based on that. However since you STILL cannot load the conductors coming FROM that breaker to more than 80% of the breaker rating, that more than covers any heat de-rating anyway, so we still call it a "15A" breaker.
None of this applies to the little internal trip device in a power strip. That power strip is (should be) UL listed all by itself under it's own test criteria, UL1383, "Relocatable Power Taps" (RPTs). In that standard, an RPT is not even required to HAVE an OCPD included in it, but if it does, the standard SPECIFICALLY states that it CANNOT be a substitute for the Branch Over Current Protective Device. In other words it is there as "supplementary protection" only, so the rules on when and how it trips are irrelevant. The UL standard ALSO says that the RPT is only listed to be used when plugged directly into a hard wired receptacle. Most people don't know it (unless they have been popped by a Fire Marshal) but you are NOT allowed to put a plug strip on the end of an extension cord, nor are you allowed to plug an extension cord into a plug strip. So because the plug strip is plugged into a hard wired 15A receptacle, and IT is then ostensibly connected to a #14 wire and a 15A circuit breaker, you effectively cannot exceed 12A on that plug strip anyway.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
They should be able to carry their full rated load. If you're talking about de-rating to account for continuous use, that's another issue, and should be dealt with by properly managing the connected loads.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Jraef, it is rare that I disagree with you, but I believe that the conductor ampacities are their continuous capacities, and the '80%' thing is an aspect of the _breakers_, not an aspect of the _conductors_.
My reasoning is the exceptions for '100% rated' breakers. If you have a conductor with a rating of 300A, and you have a 100% rated breaker with a trip of 300A, then you can put 300A of calculated load on that system.
If, on the other hand, you only have normal 80% rated breakers, then the same 300A of load could only be placed on a 375A (or larger) breaker, and you would therefore need 375A conductors.
-Jon
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
You are absolutely right, but you will not find a 15A 1P load center breaker rated 100%. Most mfrs start at 400AF, a few go down to 250AF. That concept has very specific uses and restrictions, not applicable to the question asked.
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
Do you mean 300 amps of calculated continuous load? For non-continuous loads can't the CB be loaded to it's full ampacity?
- Location
- Connecticut
- Occupation
- Engineer
:thumbsup:
GerryB
Senior Member
- Location
- woodbridge, ct. USA
Little bit lost here. Sounds like you are saying a 15 amp breaker trips after 12 amps, (and 20a breaker at 16a). That is not so. You could plug enough things into the power strip to get to 15 and the breaker wouldn't trip.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
No, never said that. I specifically said a 15A breaker is tested based on 15A.
I hate this "80% breaker" issue, it causes way more confusion than it is worth in my opinion. Hardly anybody understands it.
Lets try this:
A standard breaker is referred to as being an "80%" breaker only because that is how it can be used without a number of other considerations and issues. The breaker is designed based on its full rated load, the trip settings are based on its full rated load. But when applied to protect a conductor, which is breaker's primary purpose, the required over sizing of the conductor leads to the breaker only being able to be loaded to 80% of its rating. Will it trip at 100%? No. But that still does not mean you can USE it at that current continuously (without the other considerations) because you would be over loading the conductors.
GerryB
Senior Member
- Location
- woodbridge, ct. USA
Yeah I kinda new you weren't saying that
. I get the continuous load concept easier, like a 13amp load if it was continuous would wind up being on a 20amp breaker with #12'swinnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
We may be debating chickens versus eggs here.
In normal situations with normal hardware, a '100A' conductor, protected by a normal '100A' breaker, may only be loaded up to 80 amps on a continuous basis. I believe that we both agree on this.
I believe that we also agree that the '100A' rating of the conductor is an approximation pulled out of NEC tables, and only approximates the self heating limit of the conductor. Further, the 100A rating on a breaker is also a 'nominal' value, which roughly describes how a breaker operates but glosses over things like trip curve tolerance and inverse time characteristics.
Where we disagree is on which comes first:
I claim that the 100A _conductor_ would be capable of carrying 100A on a continuous basis without overload, and that the limitation to 80A is necessitated by limitations in the design of normal breakers.
I believe that you saying: "A nominal '100A' conductor must be limited to operation at 80 Amps on a continuous basis, and that a nominal 100A conductor only has 80A continuous ampacity. While a nominal 100A conductor may be protected by a nominal 100A breaker, the conductors may only be used at 80% of their nominal capacity, then leaving the breaker used at only 80% of its capacity."
Do I understand your point correctly?
Thanks
Jon
In normal situations with normal hardware, a '100A' conductor, protected by a normal '100A' breaker, may only be loaded up to 80 amps on a continuous basis. I believe that we both agree on this.
I believe that we also agree that the '100A' rating of the conductor is an approximation pulled out of NEC tables, and only approximates the self heating limit of the conductor. Further, the 100A rating on a breaker is also a 'nominal' value, which roughly describes how a breaker operates but glosses over things like trip curve tolerance and inverse time characteristics.
Where we disagree is on which comes first:
I claim that the 100A _conductor_ would be capable of carrying 100A on a continuous basis without overload, and that the limitation to 80A is necessitated by limitations in the design of normal breakers.
I believe that you saying: "A nominal '100A' conductor must be limited to operation at 80 Amps on a continuous basis, and that a nominal 100A conductor only has 80A continuous ampacity. While a nominal 100A conductor may be protected by a nominal 100A breaker, the conductors may only be used at 80% of their nominal capacity, then leaving the breaker used at only 80% of its capacity."
Do I understand your point correctly?
Thanks
Jon
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