I have been trying to crack the NEC rules for reading Table 310.16 for about
20 years. I think I have finally got it pinned down. I am now almost 65
years old so to save some time for you young whipple snappers I am revealing
what I have learned. I have read many articles and handbooks on this topic
and not one has addressed the issue to my satisfaction and most are so
confusing that an average electrician can't make heads or tails out of what
they are trying to tell you. The rules are scattered all over the NEC from
one end to the other making this one of the more difficult tasks for
engineers and electricians. You would think the NFPA would at least put the
rules in one location and provide more examples, too.

So below is some of the information from my notes to the Table 310.16 Calculator program that is in Code Calculators found at http://www.electriciancalculators.com I just completed another 5 weeks and many late night sessions writing my third program to read this table, this time in Visual Basic. My online one in JavaScript that I wrote in about 1999 is an abortion that works, but I don't know why. It is a force fed program. The VB program is a Click Once VB NET 3.5sp1 application.

Program Description:

For feeders and branch circuit that do not supply receptacles or multiple outlets this program solves a general program if statement by sorting from the smallest conductor size using ampacity to the largest conductor size using ampacity to find the smallest conductor size that satisfies each of the the following:

1. The conductor ampacity without derating for the insulation temperature shall be greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load 2. The derating factor times conductor ampacity for insulation temperature shall be greater than the overcurrent Device Rating that is the next size smaller than the overcurrent device that is the next greater size or equal to the continuous load multiplier times the continuous load plus noncontinuous load 3. The derating factor times the the conductor ampacity for insulation temperature is greater than or equal to the continuous load plus noncontinuous load 4. The corresponding ampacity for the terminal temperature is greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load.

For devices, terminations and overcurrent devices that are suitable for continuous loads the continuous load multiplier is equal to 1.0 otherwise it is equal to 1.25.

Derating factor (derfactor) is the product found by multiplying the ambient temperature derating factor found at the bottom of Table 310.16 for the respective conductor material and insulation temperature times the derating factor for over 3 current carrying conductors in a raceway, cable or directly buried as given in Table 310.15(B)(2)(a). For nipples that are 24 inches or less in length the derating factor is equal to the ambient derating factor, only. For installations above rooftops the ambient adder is added to the high limit of the ambient selection temperature. For example if the ambient range selected is 36 to 40 C and the rooftop adder is 22 then the ambient would be 40 plus 22 or 62 degrees C. Next 62 degrees C is used to determine the ambient derating factor found at the bottom of Table 310.16.

This general solution can be written for a conductor with 90 C insulation and a 75 degree terminal rating as:

If (cu90(x) >= terminalloadvb2 And (derfactor

Where the cu90(x) represents copper ampacity for 90 degree C for conductor No. x counting from 0 to 27 to cover sizes No. 14 through 2000 kcmil, and cu75(x) represents copper ampacity for the 75 degree C terminal.

Terminallaodvb2 is equal to continuous load multiplier times continuous load plus noncontinuous load. Totalloadk is equal to continuous load plus noncontinuous load. Stocpd is equal to the next higher standard overcurrent device that is greater than or equal to the continuous load multiplier times the continuous load plus noncontinuous load. stocpd(ocpd(x-1) is the next smaller OCPD that is less than stocpd from Section 240.6.

For feeders and branch circuit that do supply receptacles or multiple outlets this program solves a general program if statement by sorting from the smallest conductor size using ampacity to the largest conductor size using ampacity to find the smallest conductor size that satisfies each of the the following:

1. The conductor ampacity without derating for the insulation temperature shall be greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load 2. The derating factor times conductor ampacity for insulation temperature shall be greater than or equal to the Overcurrent Device Rating that is the next greater size or equal to the continuous load multiplier times the continuous load plus noncontinuous load 3. The derating factor times the the conductor ampacity for insulation temperature is greater than or equal to the continuous load plus noncontinuous load 4. The corresponding ampacity for the terminal temperature is greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load.

For devices, terminations and overcurrent devices that are suitable for continuous loads the continuous load multiplier is equal to 1.0 otherwise it is equal to 1.25.

Derating factor (derfactor) is the product found by multiplying the ambient temperature derating factor found at the bottom of Table 310.16 for the respective conductor material and insulation temperature times the derating factor for over 3 current carrying conductors in a raceway, cable or directly buried as given in Table 310.15(B)(2)(a). For nipples that are 24 inches or less in length the derating factor is equal to the ambient derating factor, only. For installations above rooftops the ambient adder is added to the high limit of the ambient selection temperature. For example if the ambient range selected is 36 to 40 C and the rooftop adder is 22 then the ambient would be 40 plus 22 or 62 degrees C. Next 62 degrees C is used to determine the ambient derating factor found at the bottom of Table 310.16.

This general solution can be written for a conductor with 90 C insulation and a 75 degree terminal rating as:

If (cu90(x) >= terminalloadvb2 And (derfactor

This program also follows the rules for small conductors No. 14, 12, and 10 AWG copper and No, 12, and 10 AWG aluminum as given in NEC Section 240.4(D). Table 210.24 is also applied for copper only conductors in sizes No. 8 and No. 6 AWG for circuits supplying two or more (multiple) receptacle and outlets.

New rules in the 2008 NEC do not require that 125 percent of the continuous load be applied when sizing the neutral or grounded conductor. Therefore since this conductor does not terminate on an overcurrent device the following general solution is used where the conductor insulation is 90 degrees C and the terminal is 75 degrees C.:

If (derfactor * cu90(j)) >= totalloadk) And (cu75(j) >= totalloadk)) Then select cu(j)

These rules are applied to 9 combinations of conductor insulation temperatures and terminal temperatures for copper and aluminum for a total of 18 combinations using 60, 75, and 90 degrees C..

So below is some of the information from my notes to the Table 310.16 Calculator program that is in Code Calculators found at http://www.electriciancalculators.com I just completed another 5 weeks and many late night sessions writing my third program to read this table, this time in Visual Basic. My online one in JavaScript that I wrote in about 1999 is an abortion that works, but I don't know why. It is a force fed program. The VB program is a Click Once VB NET 3.5sp1 application.

Program Description:

For feeders and branch circuit that do not supply receptacles or multiple outlets this program solves a general program if statement by sorting from the smallest conductor size using ampacity to the largest conductor size using ampacity to find the smallest conductor size that satisfies each of the the following:

1. The conductor ampacity without derating for the insulation temperature shall be greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load 2. The derating factor times conductor ampacity for insulation temperature shall be greater than the overcurrent Device Rating that is the next size smaller than the overcurrent device that is the next greater size or equal to the continuous load multiplier times the continuous load plus noncontinuous load 3. The derating factor times the the conductor ampacity for insulation temperature is greater than or equal to the continuous load plus noncontinuous load 4. The corresponding ampacity for the terminal temperature is greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load.

For devices, terminations and overcurrent devices that are suitable for continuous loads the continuous load multiplier is equal to 1.0 otherwise it is equal to 1.25.

Derating factor (derfactor) is the product found by multiplying the ambient temperature derating factor found at the bottom of Table 310.16 for the respective conductor material and insulation temperature times the derating factor for over 3 current carrying conductors in a raceway, cable or directly buried as given in Table 310.15(B)(2)(a). For nipples that are 24 inches or less in length the derating factor is equal to the ambient derating factor, only. For installations above rooftops the ambient adder is added to the high limit of the ambient selection temperature. For example if the ambient range selected is 36 to 40 C and the rooftop adder is 22 then the ambient would be 40 plus 22 or 62 degrees C. Next 62 degrees C is used to determine the ambient derating factor found at the bottom of Table 310.16.

This general solution can be written for a conductor with 90 C insulation and a 75 degree terminal rating as:

If (cu90(x) >= terminalloadvb2 And (derfactor

*** cu90(x) > stocpd(ocpd(x - 1) And (derfactor ***cu90(x)) >= totalloadk) And (cu75(x) >= terminalloadvb2)) Then select cu(x).Where the cu90(x) represents copper ampacity for 90 degree C for conductor No. x counting from 0 to 27 to cover sizes No. 14 through 2000 kcmil, and cu75(x) represents copper ampacity for the 75 degree C terminal.

Terminallaodvb2 is equal to continuous load multiplier times continuous load plus noncontinuous load. Totalloadk is equal to continuous load plus noncontinuous load. Stocpd is equal to the next higher standard overcurrent device that is greater than or equal to the continuous load multiplier times the continuous load plus noncontinuous load. stocpd(ocpd(x-1) is the next smaller OCPD that is less than stocpd from Section 240.6.

For feeders and branch circuit that do supply receptacles or multiple outlets this program solves a general program if statement by sorting from the smallest conductor size using ampacity to the largest conductor size using ampacity to find the smallest conductor size that satisfies each of the the following:

1. The conductor ampacity without derating for the insulation temperature shall be greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load 2. The derating factor times conductor ampacity for insulation temperature shall be greater than or equal to the Overcurrent Device Rating that is the next greater size or equal to the continuous load multiplier times the continuous load plus noncontinuous load 3. The derating factor times the the conductor ampacity for insulation temperature is greater than or equal to the continuous load plus noncontinuous load 4. The corresponding ampacity for the terminal temperature is greater than or equal to continuous load multiplier times the continuous load plus noncontinuous load.

For devices, terminations and overcurrent devices that are suitable for continuous loads the continuous load multiplier is equal to 1.0 otherwise it is equal to 1.25.

Derating factor (derfactor) is the product found by multiplying the ambient temperature derating factor found at the bottom of Table 310.16 for the respective conductor material and insulation temperature times the derating factor for over 3 current carrying conductors in a raceway, cable or directly buried as given in Table 310.15(B)(2)(a). For nipples that are 24 inches or less in length the derating factor is equal to the ambient derating factor, only. For installations above rooftops the ambient adder is added to the high limit of the ambient selection temperature. For example if the ambient range selected is 36 to 40 C and the rooftop adder is 22 then the ambient would be 40 plus 22 or 62 degrees C. Next 62 degrees C is used to determine the ambient derating factor found at the bottom of Table 310.16.

This general solution can be written for a conductor with 90 C insulation and a 75 degree terminal rating as:

If (cu90(x) >= terminalloadvb2 And (derfactor

*** cu90(x) >= ocpd1 And (derfactor ***cu90(x)) >= totalloadk) And (cu75(x) >= terminalloadvb2)) Then select cu(x)This program also follows the rules for small conductors No. 14, 12, and 10 AWG copper and No, 12, and 10 AWG aluminum as given in NEC Section 240.4(D). Table 210.24 is also applied for copper only conductors in sizes No. 8 and No. 6 AWG for circuits supplying two or more (multiple) receptacle and outlets.

New rules in the 2008 NEC do not require that 125 percent of the continuous load be applied when sizing the neutral or grounded conductor. Therefore since this conductor does not terminate on an overcurrent device the following general solution is used where the conductor insulation is 90 degrees C and the terminal is 75 degrees C.:

If (derfactor * cu90(j)) >= totalloadk) And (cu75(j) >= totalloadk)) Then select cu(j)

These rules are applied to 9 combinations of conductor insulation temperatures and terminal temperatures for copper and aluminum for a total of 18 combinations using 60, 75, and 90 degrees C..