Circuit Breaker Tripping

[flashlight]

While we were doing a service call at a hair salon that had had some
problems with overloads, we found that they consistently used two 13.5 amp
hair dryers on the same 20A branch circuit. This was not the circuit that was tripping (that turned out to be something else) !
We put an ammeter on this circuit and it was pulling 27 amps without the breaker tripping. We observed this for at least 10 minutes.
We double checked with another meter, same reading.
I remember years ago as an apprentice reading that CBs are designed
to carry 110 % of their rated load indefinitely. (Even though the designed load cannot exceed 80 %) That seemed odd to me at the time. But 27A is 135 %. I switched it with the breaker next to it, and same result. These are fairly new Cutler Hammer HACR. What gives ? Is this a bad batch ? We didn't have time for a full survey and had to schedule to come back when they weren't open. Before we left, I put a 15 in there so they wouldn't run 2 dryers.
I'd appreciate your insights.

 

[JohnJ0906]

I think breakers are allowed to carry 135% of their rated ampacity indefinitely. Someone has posted that here, and I'm sure someone more knowledgeable than me will say for sure one way or the other.

 

[ptonsparky]

The time current curve for a CH120 shows approx 135% overload extending past 2 hrs. Depending on the ambient temp of your enclosure it may never trip.

 

[mdshunk]

I plotted the 27 amp overload on a QO breaker's trip curve. You can see that the breaker will take between 35 and 200 minutes of this overload before it will trip:

 

[jim dungar]

FYI, fuses also have this type of time response to a 135% loading.

 

[flashlight]

Thank you for this info.

Since #12 THHN has an ampacity of 25A at 75 degreesC, and since the customers will continue to use their appliances in this fashion, loading the circuit to 27A, and since for practical purposes the breaker will not trip at 27A, for the short term I should change out their 20A breakers to 15A to prevent them loading the circuit like this ?

For the long term I will strongly recommend a dedicated circuit for each
hair dryer.

 

[mdshunk]

I think you should make no changes to the existing circuit of any sort, since everything functioning within design parameters. I'd advise them to get a dedicated circuit to each hair dryer location. I've done 10 or 12 beauty shops in the past, and I've wired each one with two circuits to each styling station. That seems like the least I can get away with, considering the stylist will have a hair dryer and a curling iron running at the same time, along with whatever other miscellany.

(Is this a shop that was never really originally wired as a beauty shop?)

 

[iwire]

I think you should make no changes to the existing circuit of any sort, since everything functioning within design parameters.

I agree, I will also say if you were to change it to a 15 amp breaker they would blame you for its tripping.

I'd advise them to get a dedicated circuit to each hair dryer location.

I agree

 

[augie47]

I plotted the 27 amp overload on a QO breaker's trip curve. You can see that the breaker will take between 35 and 200 minutes of this overload before it will trip:

Is that not seconds ?? 200 seconds ?

 

[flashlight]

I think you should make no changes to the existing circuit of any sort, since everything functioning within design parameters....

(Is this a shop that was never really originally wired as a beauty shop?)

I don't believe it was originally wired as a beauty shop. We were just
called in recently to trouble-shoot.

But is it really functioning within design parameters ? Ok, the breaker is functioning within the parameters posted, but the circuit is not supposed to be carrying 135% of the CB rating, which exceeds the ampacity of the conductors.
It worries me.

 

[iwire]

But is it really functioning within design parameters ? Ok, the breaker is functioning within the parameters posted, but the circuit is not supposed to be carrying 135% of the CB rating, which exceeds the ampacity of the conductors.
It worries me.

The NFPA and UL work together, they are well aware that breakers do not trip right at the rating. The rating we see in 310.16 takes all these things into account.

 

[ptonsparky]

Yes, you will be blamed for "it worked before *!@^! was here". We cannot limit what is plugged into a receptacle by a customer. Suggestions are all we can do.

 

[templdl]

My reference is NEMA AB-4, "Guidelines for the inspection and preventive maintenance of molded case circuit breakers used in commercial and industrial applications."
Section 5, Test procedures.
5.5.4 Test, table 5.3. The maximum trip time for a >250v 20at rated breaker at 300% of the rated current is 50 sec if one were to to field test the breaker.
However, the trip curve as included in a previous post provides a very good example as to what would be expected with the current as measured.
I believe it should provide a rude awakening to most that these devises do not spit hairs as illustrated by the 35-200 sec trip time bases upon the calibration as allowed.
And now to throw in a little extra twist, that's at 40deg C 0r 104degF The trip curve will slide to the right as the ambient temperature is decreased, 25degC=77degF, 20degC=68degF.
In other words it will be even longer. A breaker with a 20at trip rating calibrated at 40degC will have a 22at rating a 25degC.
So if there are those asking for an exact answer there isn?t one except to be aware of the variables and to anticipate where a breaker is expected to trip based upon the calibration range as included with the trip curve of the breaker along with the ambient temperature.

 

[KentAT]

Here is the time/current curve for the Cutler-Hammer Type CH breakers. It appears somewhat longer than the QO.

Funny thing, I was just troubleshooting a tripping 20A QO breaker at a friend's house about 3 weeks ago. They were pulling about 12A, then began turning on the loads that were causing the breaker to trip. Went to 17A, then 28A as they turned on some plug-in space heaters. Breaker tripped in about 20-30 secs at the 28A load the first time I checked it. It was earlier each successive time we tested it, down to about 15-20 secs on 3rd try. No problem there, breaker was doing it's job.

 

[mdshunk]

Looks like the CH is on the "forever" portion of the curve for 135%. The QO was almost there, and if it was cooler, it probably would be.