allowable temperature rise for conductors

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rhamblin

rhamblin

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Kaukauna, Wisconsin
A few weeks ago we lost power at one of our plants because of a seemingly loose connection between a buss bar and a breaker. 200 Amp 240 Volt single phase breaker. Because the switch gear is quite old, we had an electrical contractor find something that will get us through temporarily, they found a breaker that doesn't match or fit properly but got the lights on so we could run production.

We have been also been tinkering with an Infrared Thermography camera lately. So I curiously looked at the breaker and wires with the camera. Both had elevated temperatures above ambient. Lets say ambient was probably 80?F and the hottest spot was 117?F. An amp clamp showed the breaker was running 137 amps on one phase and 129 amps on the other phase. All of the other 5 breakers and connecting feeders were basically ambient temperature. I am thinking that any temperature rise above ambient should be seen as a problem.

Also these same feeders that were terminated in a panel say 100 feet away also head these elevated temperatures, i.e. the temperature is constant along the length of the conductors as opposed to one hot spot with the heat lessening as it gets further away.

I know that as current runs through a conductor it heats up, but these feeders off the load side of the breaker are 3/0 THHN. In my mind there shouldn't be that much increased heating on these wires or the breaker.

Can anyone shed some light on this issue?

Recently we changed a lot of the lighting in this area of the plant from T12 8' HO fixtures to a T8 4' fixture, utilizing electronic ballasts. I know that these new ballasts can add some heating to the system because of Harmonics.

Other questions rolling around in my head:
  • Knowing that breakers and fuses operate on basically heat generated by current, how hot should a conductor get say at 50%, 75%, 90%, 100% ampacity? I know there are a lot of variables included in answering this question properly, but generally speaking, what could one expect. I guess in my mind the temperature of a conductor at 99% ampacity would be pretty close to ambient temperature.
  • I know that some manufacturers have allowable temperature rises in the 50?C range and above, but the conductor should be able to safely handle this current for a very long time.
  • I have read a little about Harmonics online and it seems to really affect the neutral wire, the lighting in this area is feed 240 volts 1 phase. I don't know if that means Harmonics won't affect it as much.
 
rhamblin said:
A few weeks ago we lost power at one of our plants because of a seemingly loose connection between a buss bar and a breaker. 200 Amp 240 Volt single phase breaker. Because the switch gear is quite old, we had an electrical contractor find something that will get us through temporarily, they found a breaker that doesn't match or fit properly but got the lights on so we could run production.
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Just curious, how long will this temporary breaker be left in place?

rhamblin said:
We have been also been tinkering with an Infrared Thermography camera lately. So I curiously looked at the breaker and wires with the camera. Both had elevated temperatures above ambient. Lets say ambient was probably 80?F and the hottest spot was 117?F. An amp clamp showed the breaker was running 137 amps on one phase and 129 amps on the other phase. All of the other 5 breakers and connecting feeders were basically ambient temperature. I am thinking that any temperature rise above ambient should be seen as a problem.
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Simple answers first.

Get comfortable with measuring using degrees C instead of F, as it it easier to compare to equipment ratings.

Parts of fully loaded circuit breaker can easily exceed 40?C (104?F) with some parts, like the handle, exceeding 50?C. So it sounds like your breaker may be operating correctly.


rhamblin said:
Also these same feeders that were terminated in a panel say 100 feet away also head these elevated temperatures, i.e. the temperature is constant along the length of the conductors as opposed to one hot spot with the heat lessening as it gets further away.

I know that as current runs through a conductor it heats up, but these feeders off the load side of the breaker are 3/0 THHN. In my mind there shouldn't be that much increased heating on these wires or the breaker.
Click to expand...

If current is flowing through it it will get hot. Warm to the touch electrical equipment is extremely common, too hot to touch is not unusual.


rhamblin said:
Recently we changed a lot of the lighting in this area of the plant from T12 8' HO fixtures to a T8 4' fixture, utilizing electronic ballasts. I know that these new ballasts can add some heating to the system because of Harmonics.
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Harmonics are nothing special, they are simply currents, and currents cause heat. A common problem with harmonics is what happens to neutral bars where several circuits join together 9the currents may added together instead of cancelling)

Keep asking questions, and the discussion can then move into more detail.
 
There is mention of degrees F and degrees C in the post; so make sure you are talking apples to apples.

Cable with a manufacturer rating of 90deg C means that it can continuously carry that amount of current without damage. There are cable thermal damage curves which will show the additional current they can carry for discrete amounts of time also without damage.

The terminations are another question, and since most are not rated for higher than 75 deg c, then the ampacity of the cable is limited to keep the temperature no greater than 75 deg C.

What this means is that the temperature can reach 167 deg F (75 deg C) or 194 deg F (90 deg C) continuously without damage.

The rated temperatures are far above those listed in the OP from your scan.
 
Rhamblin,

Did any of the conductors show signs of exposure to extreme temperature?

R-o-T: If you can't keep your finger on a warm spot, then for most persons, temperature is above 105o F.

Regards, Phil Corso
 
Jim Dungar, temporary being how ever long it takes to find a refurbished breaker to fit into a Kinney brand Switchgear.

Thanks for the pointers about C and F. Everything in my question was accurate as far as C or F, albeit maybe not so easy to convert from one temp scale to the other.

Regarding the Harmonics, does the fact that we don't technically use a neutral for all the new electronic ballasts, likely increase or decrease or neither the amount of harmonics on the system?
 
Phil Corso said:
Rhamblin,

Did any of the conductors show signs of exposure to extreme temperature?

R-o-T: If you can't keep your finger on a warm spot, then for most persons, temperature is above 105o F.

Regards, Phil Corso
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None of the conductors showed signs excessive heat that I saw. I used a Thermal Imagery Camera, so I am fairly confident of the temperatures I was scanning.
 
Let me start with this question specifically? If I take a 10' length of a wire, say #6, and load it to 30 amps what would be the temperature of the wire in a room that's 70?F, with no air movement. Then the same question at 45 amps and the same at 60 amps and 65 amps and 70 amps.

Again in my mind, you can take any wire, any size, that isn't too long (so we don't have to worry about excessive resistance); and apply 99% of its rated ampacity, the temperature of the wire would be near ambient. The reason I am thinking this is because if the temperature gets too high, we could have insulation breakdown and problems stemming from this. I guess I am looking for an agree or disagree on this. Just trying to educate myself here, sometimes looking at it from different angles makes it easier for me to understand things.

Thanks again everyone.
 
If you take a wire an place in in a room it will be at ambient. When you start to flow current through the wire based on the resistance of the wire heat will be generated (P=I squared R) altough R is very small per foot of wire, there is some resistance. So double the current, four times the energy(not temperature). How hot the wire will get will depend on the insulation thickness, and thermcal conductivity of the wire. Thank fully we have tables to look up and find an accetable factor as compared to all the math that would go into figuring this out. However, it you want to determine the wire temperature, hook up a welding machine and see what happens. Althought this won't be exact it might get you a faily good number. The temperature rise is typically limitted by the type of insulation and wet or dry conditions.

the thing I look for on an infared survey if a three phase breaker has a balanced load, the conductors shall be about the same temperature. If there is a difference that is the indication of a problem. the wire itself is rairly the problem, but the connections. Every surface on a connection has some amount of resistance. A little loose and it has a lot more. The wire typically has no connections internally and is the same diameter so it should be consistant throughout the conductor if the conductors are installed in the same space. (not those up high in a building may have a higher temperature because of the heat rise in the building). You will commonly see the conductors warmer near the connector ends as some of the heat in the connector is conducted down the conductor. I find a lot of improper termination this way. If the lug is to big or to small for the wire size it seems to always show up as a hot spot.

Also note some breakers have circuit breakers or small soleniods in them that may generate heat.
 
Any conductor carrying any current will be above ambient.

P = I^2*R right?

Any conductor has resistance, the more current the power, which is dissipated as heat. Since conductors aren't perfect dissipators of heat the temperature of the conductor will rise. Think of wire as heating elements.


#6 wire is 0.49 ohms per 1000', so 10 feet is 0.0049 ohms. So 30A of current would be:

P = I^2*R = 30^2 * 0.0049 = 4.41 W

So, your 10 feet of wire is a 4.41 watt heating element.

At 45A, 60A, 65A, and 70A it would be 9.9W, 17.6W, 20.7W, and 24W, respectively.

Unfortunately it isn't easy to figure out the temperature rise of the conductor based on current because it depends how efficiently the conductor can rid itself of this heat. Even fundamental values such as the wire's resistance changes versus temperature as well. Thermodynamics is well outside the scope of this post.

Anyway back to the numbers. 4.5 watts on some copper isn't going to rise the temperature much but it will be above ambient. When you start to get large amounts of power those conductors are gonna get warm and even hot.
 
rhamblin said:
Again in my mind, you can take any wire, any size, that isn't too long (so we don't have to worry about excessive resistance); and apply 99% of its rated ampacity, the temperature of the wire would be near ambient.
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In my mind, if you take a wire of any size and apply current that is 99% of its rated ampacity, and the ambient was in the 76-86F range, then the wire temperature would be near the temperature rating of the conductor.
 
Tarbaby and Timbert, thank-you for the response. Now things make sense again. Sometimes you gotta hear it explained a certain way and then it makes sense. Funny thing is I Knew all that stuff but didn't put it all together in my head. Thanks again.
 
david luchini said:
In my mind, if you take a wire of any size and apply current that is 99% of its rated ampacity, and the ambient was in the 76-86F range, then the wire temperature would be near the temperature rating of the conductor.
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I believe that's the concept in a nutshell for ampacity tables... :slaphead::D
 
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