Wire Temperature

[Dale Hayes]

From NEC Table 310.16, 10GA. 75C wire is rated for 35Amps at a 30C Ambient.

Does this mean that at 30C ambient, the 12Ga. wire itself will reach a temperature of 75C if a 35Amp continuious load is applied to the wire?

Is this also true for Type SA (SRML) 200C wire?

From NEC Table 310.18, 10GA. SA (SRML) 200C wire is rated for 60Amps at 40C Ambient. At 60Amps will the SA (SRML) wire temperature reach 200C?

 

[barclayd]

No. That is the Temperature Rating of the wire based on the type of insulation. The actual temperature of the wire would be lower. If you have a #10 carrying 35 amps, it will be 'hot-to-the-touch', but nowhere near 75 deg C (167 deg F)
I have contacted several wire manufacturere, and none of them have any meaningful data on "Temperature vs Current Flow" for various types of wire.
db

 

[petersonra]

From NEC Table 310.16, 10GA. 75C wire is rated for 35Amps at a 30C Ambient.

Does this mean that at 30C ambient, the 12Ga. wire itself will reach a temperature of 75C if a 35Amp continuious load is applied to the wire?

Is this also true for Type SA (SRML) 200C wire?

From NEC Table 310.18, 10GA. SA (SRML) 200C wire is rated for 60Amps at 40C Ambient. At 60Amps will the SA (SRML) wire temperature reach 200C?

Not necessarily. It means that it won't exceed that temperature if you have installed it in accordance with all the other rules.

For instance, a wire in free air will be cooler with the same amount of current flowing through it as a wire in conduit packed inside 6 inches of foam insulation.

 

[pbeasley]

Not necessarily. It means that it won't exceed that temperature if you have installed it in accordance with all the other rules.

For instance, a wire in free air will be cooler with the same amount of current flowing through it as a wire in conduit packed inside 6 inches of foam insulation.

And here I thought it meant the temperature at which the insulation and it's insulating properties would begin to degrade electrically and/or mechanically.

 

[winnie]

A _slight_ disagreement with the above two answers. They are correct in practise but not in theory.

The ampacity numbers in tables 310.16 -310.18 are the current which, under the conditions assumed when the tables were generated will heat the conductors up to the temperature rating of the conductors.

If you have a #10 installed as assumed under 310.16, in a 30C ambient, and you run 35A through it, then it will reach 75C.

The reason that db and Bob are correct in _practise_ is that the assumptions used to create the tables are extremely conservative, and assume far more thermal resistance than you would have in a normal installation. Because real world installations almost always have better thermal conductivity and lower ambient temperature than assumed in 310.16, and additionally are rarely permitted to be used at rated ampacity, conductors in real world installations are almost always much cooler than the insulation temperature rating.

-Jon

 

[barclayd]

If you have a #10 installed as assumed under 310.16, in a 30C ambient, and you run 35A through it, then it will reach 75C.

I strongly disagree.
If I have three pieces of #10 - one is TW (65 deg C), one is THWN (75 deg C), and one is THHN (90 deg C).
I put 35 amps through each wire.
Are you seriously asking me to believe that the TW will only get to 65, while the THHN will reach 90?
The insulation has, basically, nothing to do with the Temperature vs Current.
What about a bare wire, eh? (I gotcha there, Winnie)
db

 

[Dale Hayes]

Increase the amps and the temperature goes up

Increase the amps and the temperature goes up

If I have three pieces of #10 - one is TW (60 deg C rated for 30A), one is THWN (75 deg C rated for 35A), and one is THHN (90 deg C Rated for 40A).

Calculations based upon the NEC Table 310.16 30C ambient temperature.

If I put 30 amps through each wire should'nt I get 60 deg C in either of the wires?

If I put 35 amps through each wire should'nt I get 75 deg C in either of the wires?

If I put 40 amps through each wire should'nt I get 90 deg C in either of the wires?

 

[winnie]

I strongly disagree.
I put 35 amps through each wire.
Are you seriously asking me to believe that the TW will only get to 65, while the THHN will reach 90?

No, I am not saying that.

I am saying that if you install a piece of #10 TW in the conditions assumed by table 310.16, and you run 30A through it continuously, then it will heat up to 60C.

If you install a piece of #10 THHN in the conditions assumed by table 310.16, and you run 40A through it continuously, then it will heat up to 90C.

If you were to take that #10 TW conductor, in the conditions assumed by 310.16, and run 40A through it, it would heat up to 90C, exceeding the allowed temperature for the insulation.

The allowed ampacity depends upon the amount of heat generated in the wire, how well that heat is carried away from the wire, and how much temperature the insulation can tolerate. Type TW insulation can only tolerate 60C, and so its ampacity is that current which would heat it up to 60C. Type THHN can tolerate 90C, so its ampacity is that current which would heat it up to 90C.

The 'conditions' assumed by 310.16 include both the ambient temperature, surrounding heat sources (other conductors in the same raceway), and _thermal_ insulation between the conductors and the surrounding air. These assumed conditions are very conservative. Thus my statement that in the real world, with real levels of _thermal_ insulation, the conductors will not get as hot as the limit temperatures.

I note that 310.16 is rounded to multiples of 5A; I am sure that the 'real' values which hit the limiting temperatures above are not _exactly_ the values given in 310.16.

-Jon

 

[Pierre C Belarge]

No, I am not saying that.

I am saying that if you install a piece of #10 TW in the conditions assumed by table 310.16, and you run 30A through it continuously, then it will heat up to 60C.

If you install a piece of #10 THHN in the conditions assumed by table 310.16, and you run 40A through it continuously, then it will heat up to 90C.

If you were to take that #10 TW conductor, in the conditions assumed by 310.16, and run 40A through it, it would heat up to 90C, exceeding the allowed temperature for the insulation.

The allowed ampacity depends upon the amount of heat generated in the wire, how well that heat is carried away from the wire, and how much temperature the insulation can tolerate. Type TW insulation can only tolerate 60C, and so its ampacity is that current which would heat it up to 60C. Type THHN can tolerate 90C, so its ampacity is that current which would heat it up to 90C.

The 'conditions' assumed by 310.16 include both the ambient temperature, surrounding heat sources (other conductors in the same raceway), and _thermal_ insulation between the conductors and the surrounding air. These assumed conditions are very conservative. Thus my statement that in the real world, with real levels of _thermal_ insulation, the conductors will not get as hot as the limit temperatures.

I note that 310.16 is rounded to multiples of 5A; I am sure that the 'real' values which hit the limiting temperatures above are not _exactly_ the values given in 310.16.

-Jon

From what I have read and been taught, I agree with Jon, that the values he stated will generally occur within the specified values.

 

[Dale Hayes]

current/temperature

current/temperature

My original inquiry also questioned the current/temperature relationship of type SA (SRML) 200C wire.

From NEC Table 310.18, 10GA. SA (SRML) 200C wire is rated for 60Amps at 40C Ambient.

At 60Amps will the 10GA. SA (SRML) wire temperature reach 200C?