Reason behind 125% overrating of conductors and overcurrent devices

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Pilot Super Grip

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What could be the reason(s) behind the 125% overrating factor required to be applied on conductors and overcurrent devices?

Is it only because UL listed overcurrent devices cannot carry more than 80% of continuous load?

If it is also because a 100% continuous load might not trip the device because of its tripping curve, then how do we explain the exception found in NEC that if the device is listed for operation at 100% of its rating, the ampere rating of the overcurrent device may be equal or greater than the sum of the continuous load plus the noncontinuous load?
 
by sizing conductors for 125% of continuous loads, the conductor is larger and acts like a heat sink so the breaker doesn't trip.
 
Pilot Super Grip said:
What could be the reason(s) behind the 125% overrating factor required to be applied on conductors and overcurrent devices?

Is it only because UL listed overcurrent devices cannot carry more than 80% of continuous load?

If it is also because a 100% continuous load might not trip the device because of its tripping curve, then how do we explain the exception found in NEC that if the device is listed for operation at 100% of its rating, the ampere rating of the overcurrent device may be equal or greater than the sum of the continuous load plus the noncontinuous load?
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"Overrating?" Could you clarify your reference to Overrating.
When sized conductor based upon 125% of the continuous current the rating of the OCPD is based upon the rated as ampacity of the conductor. The results will be an OCPD will be applied at a maximum of 80% of the continuous current.

To me OCP is a gray area when you consider that a breaker is calibrated based upon a 40degC ambient temperasture and its ampacity will increase above its rating as the temperature decreases meaning that a breaker in a panel in a basement for example may be at 60-65degF or about 18degC. Then you run wire through hot attic that may be in excess of 110decF or 43degC minimum.you will not it weouldn',t take much to exceed the old 60degC TW tasting.

What it amounts to is you have wonder how wires is protected at all. As I see it a lot of leway is allowed such as the that 125% continuos load rule etc. A 15at breaker at 25degC (75degF) moves up to 17a and even higher at 18degC or about 65degF. Going from 60de,C to 75degC rated wire is a big improvement and the 90degC rasted THHN even better for insulation lifer.
I have not foud it to be an exact science.
 
But an OCPD listed for operation at 100% of its rating has also been tested for some ambient temperature and its rating also changes according to the ambient temperature it operates in, so the 125% rule should also apply to it. Yet we find that OCPD that can operate at 100% of its rating is exempted from the 125% rule by NEC itself. This means the rule is only for devices able to operate at less than 100% of their ratings.

But why 125%? Is it because the standard construction of circuit breaker allows operation up to 80% of its rating?

It is mentioned in Marking and Application Guide Moulded Case Circuit Breakers:

100 Percent Continuous Rated - Unless otherwise marked for continuous use at 100 percent of its current rating, a circuit breaker is intended for use at no more than 80 percent of its rated current where in normal operation the load will continue for three hours or more.
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Pilot Super Grip said:
But an OCPD listed for operation at 100% of its rating has also been tested for some ambient temperature and its rating also changes according to the ambient temperature it operates in, so the 125% rule should also apply to it. Yet we find that OCPD that can operate at 100% of its rating is exempted from the 125% rule by NEC itself. This means the rule is only for devices able to operate at less than 100% of their ratings.

But why 125%? Is it because the standard construction of circuit breaker allows operation up to 80% of its rating?

It is mentioned in Marking and Application Guide Moulded Case Circuit Breakers:
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Keep in mind any application of a 100% rated breaker will be in its own enclsure. There is no application of 100% rated breakers in a panel ganged together with other breakers.

This means they are not effected by the heat of other breakers.

Why 125%?

Why any number? Like why is it four or more conductors requires derating?

Why is it not bundling at 23" but it is bundling at 25"?
 
iwire said:
Why 125%?

Why any number? Like why is it four or more conductors requires derating?

Why is it not bundling at 23" but it is bundling at 25"?
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I was asking if 125% is by any chance related to the CB that can be loaded only up to 80%. (80% being reciprocal of 125%)

http://ecmweb.com/basics/sizing-circuit-breaker?page=1

In summary, a CB either carries a standard rating (80%) or a 100% rating. The standard rating is subject to the NEC sizing rules we've just discussed. CBs that are 100%-rated are permitted to be loaded continuously at their full rating as long as the assembly is listed and conductors are properly connected.
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http://ecmweb.com/basics/sizing-circuit-breaker?page=2

Example 3: 200A continuous load.

OCPD = 100% noncontinuous load + 125% continuous load = (1.00 x 0A) + (1.25 x 200A) = 250A

Therefore, a 250A device is needed. If a 100%-rated CB is selected, a 200A rating is permitted.
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So from the above a CB shall be selected with a rating that is equal or more than 125% of continuous load amperes ONLY if it has the standard 80% rating. Am I correct?
 
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Pilot Super Grip said:
...So from the above a CB shall be selected with a rating that is equal or more than 125% of continuous load amperes ONLY if it has the standard 80% rating. Am I correct?
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Ditto on the yes.

FWIW, a 100%-rated assembly [enclosure, panelboard] is required to use the 100%-rated breaker option. The main difference is the enclosure will be designed with ventilation and have clearance specifications above what is required by the NEC. This amounts to reduction in operating temperature rise.

If you research availability of 100%-rated assemblies, you will find your choices are further limited... and for the most part the reason why 100%-rated installations are uncommon.
 
100% rated breakers are misunderstood by many.

100% rated breakers are misunderstood by many.

iwire said:
Keep in mind any application of a 100% rated breaker will be in its own enclsure. There is no application of 100% rated breakers in a panel ganged together with other breakers.

This means they are not effected by the heat of other breakers.

Why 125%?

Why any number? Like why is it four or more conductors requires derating?

Why is it not bundling at 23" but it is bundling at 25"?
Click to expand...

Intersting enough the 100% rated breaker is no different internally than a std breaker only that it has been tested to meet the requirements to be listed as a 100% beaker by UL. It is the name plate that is. The beaker has gone through addition tests to allow it to be applied ayt 100% provided additional conditions be met.
What make the difference is that a 100% rated breaker must be installed in a suitable enclosure per the installation instruction as you had pointed out. In addition the wire must be rated 90degC, not 75degC.
But the real reason for a 100% rated breaker is the possibility to reduce the cost of wire. Normally wire is sized based upon the 125%+100% rule, the wie size selected must carry that current, the breaker is sized to protect the rated wire.
With 100% rated breakers 100%+100% can be used which can relate to reducing the wire size what may equate to a significant savings depending upon the length. But, the wire must be rated 90degC.
Then a 100% breaker is used to protect the wire. 100% breaker though are more expensive.
But depending upon the frame rating groups there may be an opportunity to even downsize a frame size. This may save space as well as using a smaller cable.
So when considering the use of a 100% rated breaker, 100% rated breakers are not superior but it is the entire installation cost that must be evaluated. Those who specify the use of 100% rated breakers based upon there thinking that they are better are mistaken. The instasllation instructions for a 100% breaker will provide details on this.
 
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