The 80% rule and how it is applied to circuits.

There seems to be a lot of confusion, urban legend and simply false information floating around. OK let's start in article 210.19(A)(1) "General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load."
Ampacity is defined in article 100 as "The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating".
Notice this says "conductors". Overcurrent protection (breakers or fuses) are not mentioned.
Now lets go look at the common conductors used on circuits used for general lighting, 12ga and 14ga. If I look an article 315, I see a 14ga conductor is 20a and a 12 ga conductor is 25a (in the 60c column, the strictest category). OK so where does the 20a and 15a standard come from? That is in 240.4(D) where they tell you what breakers size you should use on "small conductors". With a few exceptions mostly where motors or welders are involved, you have that limit we know and love. Since the installer has no control over what a user will plug in, they build that 80% safety factor into the maximum breaker allowed.
I have never seen that mythical "80% breaker" but I would be willing to look at a catalog that lists one. I am not sure why that would even exist since the NEC.
There are a lot of other rules about specific circumstances, motor loads, ambient temperatures etc but generally they will apply in industrial applications. Where the homeowner will usually see that is the conductors going to their A/C compressor. Uninformed home inspectors will write up a perfectly legal installation because the breaker does not seem to match the wire they used and they do not understand the label on the unit. They will be wildly wrong if they also try to apply "the 80% rule" to the breaker when the real rule is as much 250%.
Light your torches and flame away ;-)
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On Tuesday, January 10, 2017 at 11:23:00 PM UTC-5, snipped-for-privacy@aol.com wrote:

It appears that the typical breakers we buy every day are the 80% rated ones. I think I figured this out as I posted yesterday in the other thread. I think I just confirmed it with this:
http://static.schneider-electric.us/docs/Circuit%20Protection/0600DB0101.pd f
They explain that the rule you cited applies for our everyday breakers which are rated for use subject to the 80%/125% rule for continuous loads. However if you have a 100% rated breaker and panel, then you don't have to apply the rule. In that case, with a 100% rated breaker and panel you could have a 20 amp continuous load on a 20 amp breaker.
In the above link, they quote the following exception to the 125/80% rule:
“Exception: Where the assembly, including the overcurrent devices p rotecting the branch circuit(s), is listed for operation at 100 percent of its rating, th e ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the non-continuous load.”
So, apparently there are these 100% rated breakers, but they are not what we typically use everyday and they probably are not common or we'd see them, see them marked that way, etc. But whether the breaker is 80% or 100%, it will still only open at over 100%, ie a 20A breaker of either type will carry a full 20A. It's a rating, listing, rule application thing . That is my conclusion, anyway.
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On Wed, 11 Jan 2017 06:54:35 -0800 (PST), trader_4

As I stated in the other note when they say 100% they mean it will handle rated current at 60c instead of the 40c that regular breakers are rated at. You notice all the references to enclosure size and ambient temperatures. The 80% rule does not apply to breakers, It applies to the conductors and the breaker does not change that with the exception that in large conductors, the terminal rating may be the limiting factor. THHN and most modern conductors are actually rated 90c but there are few devices with 90c terminals. You can still use the 90c column to derate the conductor. When you read the examples you also notice they all talk about the "round up" rule which seldom applies to residential circuits and never applies to small conductors in general lighting circuits.
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On Wednesday, January 11, 2017 at 11:38:25 AM UTC-5, snipped-for-privacy@aol.com wrote:

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Then why does Schneider say this:
“Exception No. 1: If the assembly, including the overcurrent device s protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowa ble ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the non-continuous load.”
In short, these article says that a standard-rated circuit (circuit breaker and wire) can carry 80% of the circuit breaker and conductor current rating. In contr ast, 100% rated circuits can carry 100% of the circuit breaker and conductor current rating."
Seems clear to me. The exception in NEC and Schneider are saying that with a 100% rated breaker, the circuit can use that breaker and you don't have to derate the ampacity for continuous loads. Eg if you have a 20A, 80% breaker, then that circuit is limited to a 16A continuous load. If you have a 20A, 100% breaker, then it's OK to have up to 20A continuous load. Isn't that what NEC and Schneider are saying?
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On Thu, 12 Jan 2017 07:06:16 -0800 (PST), trader_4

You also notice they say "feeder"? I already showed you in that article that they say a "standard" breaker can carry 100% of it's rating. This still has to do with the terminals on the breaker which is usually the limiting factor when you are using 90c conductors. Find me a trip curve that shows a standard breaker tripping at less than it's rating on that site.
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On Thursday, January 12, 2017 at 11:30:14 AM UTC-5, snipped-for-privacy@aol.com wrote:

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No, I don't see where they say that in what I've quoted.

You keep going back to something that I'm not arguing. I've said quite clearly that I agree that both an 80% breaker and a 100% breaker will carry the full, 100%, current. According to Schnieder and what I see elsewhere, the difference is that the 80% breaker has to have the 125/80% de-rating rule applied for continuous loads, while the 100% breaker does not.
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On Thu, 12 Jan 2017 10:56:04 -0800 (PST), trader_4

Again, they are talking about feeders, not branch circuits and we are talking about industrial sized breakers so it is really just an OT curiosity here in Home Repair.
You win OK?
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On Thursday, January 12, 2017 at 2:19:24 PM UTC-5, snipped-for-privacy@aol.com wrote:

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No, they are not talking about feeders. Schneider reference NEC 210.20 which covers *branch circuits*. 210.20 (a) says that you have to apply the 125/80% rule to continuous loads.
210.20 (a) Where a branch circuit supplies continuous loads or any combination of continuous loads and non-continuous loads, the rating of the overcurrent device shall not be less than the non-continuous load, plus 125% of the continuous load.
Exception: Where the assembly, including the overcurrent device protecting the branch circuit, is listed for operation at 100% of it's rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the non-continuous load.
I read that to mean that if you have a normal, 80% breaker, then per code, the breaker is limited to a continuous load of 80% of what's stamped on the handle. . But if you have a 100% rated breaker and panel, then it's code compliant to the full value stamped on the handle. You raise a good point as to what size these 100% breakers come in, IDK and they may not even exist for residential size circuits.
We are in agreement that a 20A breaker you buy at HD, while it's not the special 100% rated type, will carry the full 20A circuit load. I said that at the beginning, when the other poster was saying that a 20A breaker will normally trip at 16A. It just has the 80% limitation applied with regard to being code for continuous loads.
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On Thu, 12 Jan 2017 12:00:18 -0800 (PST), trader_4

The part you are missing is 210.20 refers to the breaker and the enclosure assembly, not the conductors and the breaker is sized to the conductor it protects.
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On Thursday, January 12, 2017 at 4:49:54 PM UTC-5, snipped-for-privacy@aol.com wrote:

I know it applies to the breaker and the enclosure. I never said anything about sizing the conductors. Do you disagree that it says that if you have a regular breaker on a branch circuit, then the breaker has to be sized to 125% of the continuous load, while if you have a 100% rated breaker, then the breaker is sized to 100% of the continuous load, allowing a smaller breaker?
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On Thu, 12 Jan 2017 14:04:59 -0800 (PST), trader_4

I would more properly say the conductors are sized to the load and the breaker is sized to protect the conductor. That is the way the NEC is laid out.
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On Thu, 12 Jan 2017 14:04:59 -0800 (PST), trader_4

Here is another guy from ECN explaining this
This is perhaps one of the most misunderstood aspects of circuit breakers.
The so called "80% rule" is actually not stated anywhere in circuit breaker listing and labeling rules. UL489, the standard for Molded Case Circuit Breakers, requires that all MCCBs are tested at 100% load.
YOU must size CONDUCTORS at 125% of the continuous load you are feeding per the NEC. You then size the breakers for protecting those CONDUCTORS. So INDIRECTLY the breaker will never carry more than 80% of the load that you selected the conductors for. Ergo people refer to them as being rated for 80%, but that's just because of the way the NEC is worded.
You cannot use a 100% rated breaker unless you have also met all of the conditions for the CONDUCTORS that allow you to size the CONDUCTOR at 100% of the load. One of those conditions will end up being that the conductors are rated for 90C. In order for you to use conductors rated and sized for 90C, all components in the circuit must also be rated for 90C. Lugs on standard circuit breakers are NOT rated for 90C, they are rated at best, 75C. So one thing you get when you buy a 100% rated breaker is that the lugs will be 90C (usually copper, not Al). The other thing is that panelboards are NEVER rated for 90C conductor use. So the only way to use a 100% rated breaker is to buy it as a separately enclosed stand-alone breaker, or as an open breaker that YOU will build into an assembly, such as a switchboard or MCC, that you will have listed for use with 90C rated conductors at 100% load.
So for example where you see this done most often is that the MAIN breaker in a switchboard or MCC is selected to be rated 100%, because the load side of that breaker is going to bolt directly to the bus bar, not cables, so that connection can be rated for 90C. The incoming cables to that breaker can then be sized, selected and rated based on 100% load as well.
But if you have a breaker in a panelboard and you think you can simply upgrade it to 100% rated to get more current out of it, you are going to be disappointed because you will find that the 100% rated breaker cannot be used that way.
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On 1/14/17 8:19 PM, snipped-for-privacy@aol.com wrote:

Isn't there a provision requiring engineering supervision if one builds more than two components into a panel?
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On Sat, 14 Jan 2017 20:51:45 -0600, Dean Hoffman

-
If you are not using a listed assembly it is up to the AHJ what they will live with. There are a number of things in the NEC where they specify engineering supervision tho.
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On Saturday, January 14, 2017 at 9:20:08 PM UTC-5, snipped-for-privacy@aol.com wrote:

Didn't see this until today. I agree with what's there, and see your point, that in addition to the circuit breaker and the enclosure having to be 100% rated, the conductors would have to be too. So, in a practical sense, with residential circuits, you probably never see these 100% special breakers, because they don't exist for the typical residential size panel. The ones we use every day are the 80% ones. Which, again, doesn't mean that they will open at anything over 80% of the load on the handle, which is what the other poster claimed. It only means that the 125% rule has to be applied for continuous loads when using that breaker. And as you point out, the conductors have to be sized for the continuous load too.
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On Mon, 16 Jan 2017 10:44:49 -0800 (PST), trader_4

You actually size the conductor to the load and then size the breaker to protect the conductor. Since 240.4(D) ends up being the limitation for breakers on "small conductors" virtually all of them only have 60c rated lugs so the 80 vs 100% breaker thing is totally unrelated to anything you will ever see in a home. They never have 90c rated lugs (not even the main).
This is an example of the typical breakers you will see in residential panels.
http://gfretwell.com/electrical/breakers.jpg
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On 01/12/2017 12:56 PM, trader_4 wrote:

...

...

...
It's in the above...
"...the allowable ampacity of the FEEDER CONDUCTORS (emphasis added) shall be permitted ..."
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On Thursday, January 12, 2017 at 2:39:06 PM UTC-5, dpb wrote:

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That is a totally different section of NEC code they are talking about. Yes, they have a section where they talk about feeders, but they also have a section where they specifically talk about 210.20 A which is about *branch circuits*. THAT is the part I'm talking about and referencing.
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On 01/12/2017 2:03 PM, trader_4 wrote: ...

Whatever.
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