If I remember correctly, we never could use T310.15(B)(7) when the calculated load exceeded the ampacity of the conductors [in T310.15(B)(7)].
Per the table we can use a #4 for 100A service conductors but only IF the calculated load did not exceed the ampacity value of the #4s. [I don't think I'm nuts about remembering this topic in other forum discussions].
I don't recall seeing that discussion, but I disagree with that conclusion.
The illogical yet appropriate use of the table is straightforward; calculate the load, as you would anything else, and then use that number to select a service rating. Then, throw caution to the wind and connect a conductor to the service OCPD with an insufficient ampacity for the load in an approved fashion.
David Luchini said:
That doesn't sound right to me. If that was the case, then the reduction should also apply to 120/208V main feeders or services. The diversity on the main panel wouldn't be different if the service was 120/240 or if it was 120/208 single phase.
Here is what the panel has said about this section over the years.
CMP-6 said:
Panel Statement: The original data that was used to establish 310.15(B)(6), formally Note 3, was actual utility company data for 120/240 volt 3-wire single phase systems. It established that the conductors specified in the table could be used on a calculated dwelling unit load as shown. The submitter has not provided any technical data to show that this is true for 120/208 4-wire three phase systems.
CMP-6 said:
The submitter is incorrect in his assumption that the conductors in 310.15 (B)(6) are not permitted to be paralleled in accordance with 310.4. The conductor ampacities listed in 310.15(B)(6) are based on the diversity of the total load of an individual dwelling. This means that the conductors of a 120/240-volt, single-phase dwelling service or feeder with a calculated load of 200 amps will never carry 200 amps. Due to this fact, the language and table in 310.15(B)(6) will permit the use of a 2/0 conductor, which has an ampacity of 175 amps in the 75 degree C column...
CMP-6 said:
The language in this section, along with the table, has been in the code for many years and, as the submitter states, has been modified to provide more clarity over the past several cycles. When appropriately applied, there has been no evidence that the sizing of the conductors shown in the table creates a problem. Table 310.15(B)(6) deals with service loads, not with ampacities.
Now, they are inconsistent - in one response they say it does not list ampacities, and in the next breath they say it does. But the Table is based upon load diversity. There was a proposal in the last cycle to delete 310.15(B)(6) and just give a 10% reduction on the load calc to achieve the bonus for diversity, but it was rejected for being arbitrary.
I'm going to try again this cycle.