As a result of extensive review by Code Making Panel #6; the familiar table was deleted and replaced with code language describing a calculation required by the code user. The efforts behind this change is to ensure that code users apply the correct corrections and adjustments factors to service and feeders that use 310.15(B)(7) and to simplify the intent of the code. Previous references to "main power feeder" have been removed.
Let's establish that 310.15(B)(7) only applies to 120/240 Volt, Single-Phase Dwelling Services and Feeders that are associated with one-family dwellings, and the individual dwelling units of two-family and multifamily dwelling units. This code allowance only applies to services and feeders rated 100 through 400 Amps that serve 100 % of the dwelling load.
First we have to understand which conductors this rule applies too, examine the code text below.
"the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling" and " the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling, unit in a two-family or multifamily dwelling"
Once established that 310.15(B)(7) can apply; we bring in the calculation values to be considered. shall be permitted to have an ampacity not less than 83 percent of the service rating or feeder rating ; depending on which we are working with. Lets look at an example.
Example:
If a 175-ampere service rating is selected, a service conductor is then sized as follows:
175 amperes × 0.83 = 145.25 amperes per 310.15(B)(7).
If no other adjustments or corrections are required for the installation, then, in accordance with Table 310.15(B)(16), a 1/0 AWG Cu or a 3/0 AWG Al meets this rating at 75°C (167°F).
Lastly it is important to emphasize that any adjustments or corrections do have to be applied in determining the final ampacity value of a conductor, and you do have to use the standard ampacity table at 310.15(B)(16). The move from a table to a calculation ensures that the end user understands that other conditions may apply that can change a conductors ampacity versus simply pulling a figure from a table and assuming it is the final acceptable value.
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