Temperature of wire

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olly

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Master Electrician
Does anyone know of a chart, table, or literature out there that will give me the actual temperature of wire based on the current running through it? I know a lot comes into play like resistance and surrounding elements. Not sure if such a thing exists?

If you look at table 310.15(B)(16) take #12 CU for instance if 30A is running through it, it will be 90 degree C? Someone once told me that and I'm not sure i believe it.

Thank you.
 
That is not how the table works.

If you have a #12 wire with 90C insulation and an ambient temp of 30C and all terminations and enclosures rated for 90C than you could load the wire to 30A (non-continuous). The actual temp of the wire is not shown, or relevant IMO.
 
jumper said:
That is not how the table works.

If you have a #12 wire with 90C insulation and an ambient temp of 30C and all terminations and enclosures rated for 90C than you could load the wire to 30A (non-continuous). The actual temp of the wire is not shown, or relevant IMO.
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I think under those conditions of use the conductors could aproach 90c
 
You could Google "hot wire anemometer" and see if you can find design information. You might be able then to answer your question based on first principals. As others have said, the conditions of the installation are of paramount importance.
 
mbrooke said:
In open air rarely do you approach an actual temp listed in table 310.15 B 16.
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Of course not because that table is not based on free air use.

Table 310.15(B)(16) Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000
Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or
Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)*
Click to expand...

But under the conditions listed above you might right?
 
iwire said:
Of course not because that table is not based on free air use.



But under the conditions listed above you might right?
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Of course. And I doubt the conduit, raceway, ect is either. Table is built around worst case scenario, ie, conduit in contact with thermal insulation. In such a case you might actually come close to 90*C.
 
161209-1425 EST

oily:

You need a fundamental understanding of various aspects of physics.

There is no useful single equation for what you want.

You need to study some of the following:
1. Electrical resistivity of various materials.
2. Temperature coefficient of resistivity.
3. Laminar and turbulent air flow.
4. Thermal resistance of various materials.
5. Heat transfer under different flow and surface conditions.

For some of these factors you may find some useful numerical data.

You can create some of this data by your own experiments.

Need a definition for free air conditions.

Some broad general information:

Approximate average temperature of a wire can be obtained by measuring its electrical resistance.
Laminar airflow should produce a lower thermal resistance to a material the air is flowing over than turbulent flow.
Insulation over a wire in most cases would increase the temperature rise in the wire vs no insulation.

Measuring transformer DC coil resistance is a method to get an approximation of transformer temperature rise. This does not provide maximum hot spot temperature, but adding a constant like 10 C might provide an estimate.

.
 
olly said:
Does anyone know of a chart, table, or literature out there that will give me the actual temperature of wire based on the current running through it? I know a lot comes into play like resistance and surrounding elements. Not sure if such a thing exists? .... Thank you.
Click to expand...

To answer your question, no.

Calculating temperature rise would be much easier then calculating actual temperature.
 
90C is way too hot for a person to be able to grab a hold of without being burned.

Is it possible for 12 AWG to get that hot with a 30 amp load, IDK, I would think you need a situation where there is limited heat transfer away from said conductor. A metal raceway will sink away some of that heat from the contained conductors. A non metallic raceway will too but likely at a much different rate. Also consider that ampacity tables in 310 are considering heat contributed from other conductors in the same raceway or cable when determining minimum ampacity. Those tables in 310 are there primarily to protect the insulation on the conductors being selected, not the conductor itself.
 
161210-1059 EST

You guys can experiment and get some actual answers.

One easy test ---

Take a new 250 ft roll of #12 copper Romex, leave it coiled, at one end short black and white together, at the other end at room temperature with the wire stabilized at room temperature (possibly 12 hours) accurately measure the resistance of the 500 ft #12 wire loop with a DC source.

Measure room temperature, and keep room temperature constant within about 1 F. Most homes will have a short time average this good. Note that the thermal time constant of a roll of wire or transformer coil is moderately long.

Next pass a constant 20 A thru the 500 ft black-white loop for a time long enough for the copper wire temperature to stabilize (probably an hour or more). The current can be either AC or DC, we just want heat.

Measure the resistance of the stabilized heated wire. This will require multiple measurements to determine when the wire temperature stabilizes. Using the known resistance temperature equation (you do the research to find it) calculate the wire temperature rise. Add the temperature rise to ambient temperature and you have an average value for the wire temperature.

.
 
161210-1525 EST

K8MHZ:

The 100 footer if coiled in a small package. The 3 and 100 footers are about the same if stretched in a straight line. If there was lots of heatsinking on the 3 footer ends, then its average might be just slightly less than the 100 footer.

.
 
K8MHZ said:
Which wire would get hotter, a 3 foot long 12 AWG or a 100 foot long 12 AWG with a 30 amp current passing through it?
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The 100' has more voltage drop and more resistance.
The 100' is my answer.
 
Specific heat quantity C
heat q required a mass T degrees
q = m C delta T
where
q is heat in J
T in deg C
m in grams
C in J/g Deg C (for Cu 0.385)

q canbe calculated from v, i and R
may want to use a unit length so mass is fixed
 
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