Temperature adjustment for Effective Z?

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kenjsil

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Note 2 to Table 8 of Chapter 9 gives the formula for calculating DC Resistance for conductor temperatures other than 75C.

Can this formula also be used for temperature adjustment of the Effective Z values of Table 9?

Can anyone point me to more information on AC reactance and resistance data and calculations?

Thanks all,
Kenneth
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Temperature adjustment for Effective Z?

There are three components to the ?Effective Z? values in Table 9. Only one of the three, DC resistance, varies with temperature. The other two components, the inductive reactance and the power factor, are independent of temperature. Therefore, the formula shown in Note 2 of Table 8 cannot be used for the Effective Z values in Table 9.

The mathematics needed to correct an Effective Z value for temperature are more complicated than I can explain here. Indeed, I would have to work them out myself, before I could try to explain them, and I haven't the free time for that effort. Perhaps someone else already has a method. In general, you would have to start with the Effective Z value, get rid of the power factor component, separate the AC and DC resistance, correct the DC resistance for temperature (using the formula shown in Note 2 of Table 8), add the original value for AC resistance, and reinsert the power factor component.

The bottom line is, why bother? Do you really have an installation with an ambient temperature over 167F? If so, you might need to ask for assistance from an engineer. On the other hand, if the ambient temperature is lower than 167F, then you don?t need to correct for temperature, and it is best that you not try to take advantage of the lower temperature to gain ampacity.
 

bob

Senior Member
Location
Alabama
Re: Temperature adjustment for Effective Z?

The Effective Z as shown in table 9 is caculated by Ze = R x Cos(PF) + X x Sin(PF) where PF is the power factor of the circuit. In table 9 the notes specifies that the PF = 0.85. The R is the AC resistance of the conductor. This resistance is
obtained from the DC resistance x AC factor.
This AC factor takes into account the skin effect,
the proximity effect of conductors and the effects of metal conduit. The formula given as Note 2 in table 8 allows for calculating the DC Resistance for conductor temperatures other than 75C. The AC factor can be applied to this new DC value and used in the Ze formula give above to obtain a new Ze.
Unless you have a significant difference in temperature you won't see a change in Ze.
 
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