Conductors and temperature adjustment

[Plano12345]

Using 90 C XHHW-2 wires.
Ambient temperature 25 C.

When determining wire resistance for voltage drop calculations, do I apply 90 C or 25 C to the temperature forumla? Does the temperature (in this case) only have to do with the copper wire type you use or does it involve the ambient temperature?


Example:

R2=R1(1+alpha(t2-t1))

Size #2 copper uncoated:
T2=25
T1=75
R1=.194
R2=.163
alpha = .00323

or

T2=95
T1=75
R1=.194
R2=.204
alpha = .00323

Thanks

 

[Gregg Harris]

Using 90 C XHHW-2 wires.
Ambient temperature 25 C.

When determining wire resistance for voltage drop calculations, do I apply 90 C or 25 C to the temperature forumla? Does the temperature (in this case) only have to do with the copper wire type you use or does it involve the ambient temperature?


Example:

R2=R1(1+alpha(t2-t1))

Size #2 copper uncoated:
T2=25
T1=75
R1=.194
R2=.163
alpha = .00323

or

T2=95
T1=75
R1=.194
R2=.204
alpha = .00323

Thanks

FPN: The temperature rating of a conductor [see Table 310.13(A) and Table 310.13(C)] is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The allowable ampacity tables, the ampacity tables of Article 310 and the ampacity tables of Annex B, the correction factors at the bottom of these tables, and the notes to the tables provide guidance for coordinating conductor sizes, types, allowable ampacities, ampacities, ambient temperatures, and number of associated conductors. The principal determinants of operating temperature are as follows:

(1)

Ambient temperature ? ambient temperature may vary along the conductor length as well as from time to time.

(2)

Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents.

(3)

The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation.

(4)

Adjacent load-carrying conductors ? adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation.

 

[rcwilson]

If you are trying to get that precise, use the actual temperature of the wire during operation.

If it has had no load on it for a long time, the copper will be at ambient temperature, use 25C resistance to calculate the voltage drop when the circuit first turns on.

If it is carrying 130 Amps in a 30C ambient, with not more than 3 conductors in an isolated conduit in air, the wire may be at 90C; its rated temperature for 130 A under those standard conditions. Use resistance at 90C.

If the wire is applied at its 75C rating, so we don't overheat the circuit breaker, the load is 115A, the conductor is at 75C under full 100% load of 115A. Use impedance for 75C.

Or you can do like many of us, and either use 75C for most calculations or the worst case 90C figure. Most of the data we put into the equations is only accurate to +/-10% and the difference in temperature doesn't make a lot of difference in the results.

 

[Julius Right]

I agree with Bob, of course.
NEC does not recommend any way to calculate the voltage drop. Art.210.19 and 215.2 recommend 3% for branch and total 5% for branch and feeder.
IEEE Std 141/1993 in ch. 3.11 "Calculation of voltage drops" it is mentioned:
"the temperature of the conductor (normally 75 dgr.C for average loading and 90 dgr.C for maximum loading)"
The voltage drop in a cable depends upon conductor resistance and phase reactance. The resistance has to be calculated for rated temperature-maximum temperature at which conductor insulation can withstand without any damage for indefinite duration. For a.c. system, skin effect and proximity effect factors have to be employed, also.