Would you mind giving me an example using a 20 amp branch circuit serving continuous and non continuous loads with NM-B. It will sink in my head when I see the calculations worked out on paper.
Well when you say a 20A branch circuit, you should know that 210.3 says branch circuit rating is in accordance with the ocpd rating... not the conductor ampacity. In effect, that adds another dimension to the determination.
210.20(A), disregarding the exception thereto, requires the OCPD rating to be not less than the non-continuous load plus 125% of the continuous load. So you have to consider 210.20(B) requires the circuit conductors to be protected in accordance with 240.4.
For a 20A branch circuit using NM-B, 240.4 typically doesn't affect the determination because of the 20A max ocpd requirement for 12AWG. When you get into higher rated circuits, there are instances where the conductor size has to be increased in order for it to be protected by the ocpd at its ampacity. For a 20A branch circuit, suffice it to say that conductor ampacity determined under 210.19(A)(1) has to greater than the next smaller standard ocpd rating: 15A.
Now, getting to the determination calculations:
First step is to multiply the continuous load by 125% and add the non-continuous. Using that result, look at the 60?C column of Table 310.15(B)(16). If it equals a value therein, its corresponding size is the minimum. If the result is in between column values, we use the next larger corresponding size as the minimum. Note this determines minimum conductor size, not sizing for ampacity.
Second step is sizing for ampacity. Here we will make allowances for the conditions of use, namely ambient temperature correction under 310.15(B)(2) and adjustment for current-carrying conductors in close proximity under 310.15(B)(3). For NM-B we can use the 90?C column values of Table 310.15(B)(16) as the basis for this determination...
Take the calculated load (no 125% factoring for continuous load) and divide it by adjustment and correction factors as applicable. Using the resulting value, if it equals a value in the 90?C column, its corresponding size is the minimum. If the result is in between column values, we use the next larger corresponding size as the minimum. Note this determines minimum conductor size for ampacity as it correlates to the calculated load.
Step three compare sizes of steps one and two. The larger of the two sizes is the required minimum size conductor.
Step four is to determine circuit ampacity. Take the 90?C column value of the minimum required size determined in step three and multiply by the adjustment and correction factors. Compare to the 60?C value. The smaller of the two is the circuit ampacity.
Step five is to make certain this conductor is protected by the ocpd. The minimum ocpd rating is the same value determine in step one, first sentence. For 800 ampere and lesser circuit ampacities, we can use the next larger standard ocpd rating. Whether using an adjustable or standard rating ocpd, if the circuit ampacity determined in step four is equal or greater than the next lower standard rating, the conductor is protected with this ocpd rating. Where the ocpd rating is greater than 800 amperes, the circuit ampacity must equal or exceed that rating. When an rated greater than the minimum ocpd is used, the conductor size and circuit ampacity will have to be adjusted using in part the method prescribed in the preceding steps.
I believe the preceeding to be a fairly comprehensive outline of the steps involved. In practice, the steps will seem much less involved once the principles are understood... :angel:
and thank for helping me learn.