310.15(B)(7) 120/240-Volt, Single-Phase Dwelling Services and Feeders.
I keep getting calls from contractors that tell me that this town or that county no longer allows them to use 2/0 Cu. for a 200a dwelling service or feeder. This section changed in the 2014 Code but the only thing that changed is they took out the Table and made it into a calculation using 83%. This 83% rule has seemed to throw off a lot of people in our trade (plan reviewers, inspectors, and contractors). Just so you know the old table from 2011 Code will still work nothing has changed. It is not just the 2/0s its all the conductor we use to use in the old Table (see the Table below) they all still work for serving the loads that are shown to serve in the old Table. They work for both feeders and services.
EXAMPLE:
A 2/0 conductor in the 75 degree column in 310.15(B)(16) is good for 175 amperes
A 200 ampere service rating multiplied by 83% equals a 166 amperes.
So as you can see a 2/0 will be large enough to serve 166 ampere service rating.
As long as your load calculation does not exceed 200 amperes the “CODE” allows you to exceed the rating of the 2/0 conductors ampacity, by applying the 83% rule in 310.15(B)(7)(1)(2).
See #1 and #2 below to see what the conductors ampacity is based on “SERVICE RATING” and FEEDER RATING”
310.15(B)(7)
(1) For a service rated 100 through 400 A, 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, shall be permitted to have an ampacity not less than 83 percent of the service rating.
(2) For a feeder rated 100 through 400 A, 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, shall be permitted to have an ampacity not less than 83 percent of the feeder rating.
3/0 is not needed for a dwelling service rated 200 amperes unless it is not feeding the entire load to that dwelling.
Below you will find a dwelling service table from the 2014 Florida Building Code 5th Edition that was taken out in the 6th Edition of the FBC, but it still works and it includes both copper and aluminum conductors plus the size of the required grounding electrode conductor. Use it in good faith that it will give you the right size service and/or feeder to your dwelling unit. .
Till next time be safe work safe
Jake
