Chapter 11, Problem 10PP

Determine the ohms-per-mil-foot of an aluminum conductor located in an area with a temperature of 104°F (40°C).

Use the NEC to determine the ampacity of the following conductors.

To determine

To find:

The ohms-per-mil-foot of an aluminum conductor located in an area with a temperature of 104°F (40°C). Also, use the NEC to determine the ampacity of the following conductors.

Answer to Problem 10PP

The resistance of a 16 AWG copper conductor is 1.27 ohms.

Explanation of Solution

The resistivity, K of the conductor at temperature T is

$K=R_{ref}[1+\alpha (T-T_{ref})]$

   where, R_ref is the resistance at 20°C

     From Figure 10-16, the value of R_ref can be obtained

     $\alpha$is the temperature coefficient, $\alpha =0.004$

     T_ref is the reference temperature (20°C)

Substituting the values, we get

$\begin{array}{l}K=R_{ref}[1+\alpha (T-T_{ref})]\\ =17[1+\left(0.004\right)(\text{40}-20)]\\ =18.36ohms-CMperfoot\end{array}$

(a) From Table 310.15(B)(16), we know that the maximum ampacity of a 10 AWG copper conductor of Type RHW insulation is 35A and the rated temperature is ${\text{75}}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${\text{44}}^{0}C$ copper conductor of Type RHW insulation is 0.82

Thus, after correction, the ampacity will be

$\begin{array}{l}=35A\times 0.82\hfill \\ =28.7A\hfill \end{array}$

(b) From Table 310.15(B)(16), we know that the maximum ampacity of a 350 kcmil copper conductor of Type XHH insulation is 350A and the rated temperature is ${\text{90}}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${128}^{0}F$ copper conductor of Type XHH insulation is 0.76

Thus, after correction, the ampacity will be

$\begin{array}{l}=350A\times 0.76\hfill \\ =266A\hfill \end{array}$

If a raceway is to contain more than three conductors, the ampacity of the conductors must be de-rated. The correction factors for six copper conductors is 80%. Thus, after correction, the ampacity will be

$\begin{array}{l}=\left(266\text{ A}\right)\times 80\text{\%}\\ =212.8A\end{array}$

(c) From Table 310.15(B)(16), we know that the maximum ampacity of a 2 AWG aluminum conductor of Type TW insulation is 75A and the rated temperature is ${60}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${\text{86}}^{0}F$ aluminum conductor of Type TW insulation is 1.00

Thus, after correction, the ampacity will be

$\begin{array}{l}=75A\times 1.00\hfill \\ =75A\hfill \end{array}$

(d) From Table 310.15(B)(16), we know that the maximum ampacity of a 3/0 AWG aluminum conductor of Type XHHW-2 insulation is 175A and the rated temperature is ${90}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${\text{38}}^{0}C$ aluminum conductor of Type XHHW-2 insulation is 0.91

Thus, after correction, the ampacity will be

$\begin{array}{l}=175A\times 0.91\hfill \\ =\text{159}\text{.25A}\hfill \end{array}$

If a raceway is to contain more than three conductors, the ampacity of the conductors must be de-rated. The correction factors for nine aluminum conductors is 70%. Thus, after correction, the ampacity will be

$\begin{array}{l}=\text{159}\text{.25A}\times 70\text{\%}\\ =111.475A\end{array}$

(e) From Table 310.15(B)(16), we know that the maximum ampacity of a 500 kcmil copper conductor of Type THWN insulation is 380A and the rated temperature is ${\text{75}}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${48}^{0}C$ copper conductor of Type THWN insulation is 0.75.

Thus, after correction, the ampacity will be

$\begin{array}{l}=380A\times 0.75\hfill \\ =285A\hfill \end{array}$

If a raceway is to contain more than three conductors, the ampacity of the conductors must be de-rated. The correction factors for six copper conductors is 80%. Thus, after correction, the ampacity will be

$\begin{array}{l}=285\text{ A}\times 80\text{\%}\\ =228A\end{array}$

(f) From Table 310.15(B)(16), we know that the maximum ampacity of a 6 AWG copper conductor of Type THW-2 insulation is 75A and the rated temperature is ${90}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${150}^{0}F$ copper conductor of Type THW-2 insulation is 0.58.

Thus, after correction, the ampacity will be

$\begin{array}{l}=75A\times 0.58\hfill \\ =43.5A\hfill \end{array}$

(g) From Table 310.15(B)(16), we know that the maximum ampacity of a 2/0 AWG aluminum conductor of Type UF insulation is 115A and the rated temperature is ${60}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${\text{86}}^{0}F$ aluminum conductor of Type UF insulation is 1.00.

Thus, after correction, the ampacity will be

$\begin{array}{l}=115A\times 1.00\hfill \\ =\text{115A}\hfill \end{array}$

If a raceway is to contain more than three conductors, the ampacity of the conductors must be de-rated. The correction factors for 12 aluminum conductors is 50%. Thus, after correction, the ampacity will be

$\begin{array}{l}=\text{115A}\times 50\text{\%}\\ =57.5A\end{array}$

(h) From Table 310.15(B)(16), we know that the maximum ampacity of a 750 kcmil aluminum conductor of Type RHW-2 insulation is 435A and the rated temperature is ${90}^{0}C.$

Thus, from Table 310.15(B)(2)(a), the correction factor of the ambient air temperature at ${34}^{0}F$ aluminum conductor of Type RHW-2 insulation is 1.00.

Thus, after correction, the ampacity will be

$\begin{array}{l}=435A\times 1.15\hfill \\ =\text{500}\text{.25 A}\hfill \end{array}$

If a raceway is to contain more than three conductors, the ampacity of the conductors must be de-rated. The correction factors for 6 aluminum conductors is 80%. Thus, after correction, the ampacity will be

$\begin{array}{l}=\text{500}\text{.25A}\times 80\text{\%}\\ =400.2A\end{array}$

Thus, the table will be: