HOW TO SIZE A RESIDENTIAL SERVICE PER SECTION 310.15(B)(7), USING THE 83% RULE
Sizing of Service and/or Feeder Conductors for a dwelling unit PER SECTION 310.15(B)(7) see Example D7 in ANNEX D in the back of the Code book.
Service conductors and feeders for certain dwellings are permitted to be sized in accordance with 310.15(B)(7).
If a 200-ampere service is required, to select the service conductor size is as follows:
200 amperes (the size of required main) × 0.83 = 166 amperes per 310.15(B)(7). (THIS IS THE 83% RULE)
If no other adjustments or corrections are required for the installation, then, in accordance with Table 310.15(B)(16), a 2/0 AWG Cu which is good for 175 amperes or a 4/0 AWG Al which is good for 180 amperes will meet this rating at 75°C (167°F).
The following is the section from the 2017 Code book.
310.15(B)(7) Single-Phase Dwelling Services and Feeders.
For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (4).
As you can see this section it is for “one and two family dwellings, and multifamily dwellings”that use 120/240 voltages for services and/or feeders. In this section a reduced neutral is allowed.
For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of 2 ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.15(B)(7)(1) through (3).
In this section we can use it for the same types of units, but the voltage is 208Y/120 and a reduced neutral “can not be used”. So you will be required to pull the same size neutral under this section. Here in South Florida this voltage is used in Multifamily buildings.
(1) For a service rated 100 through 400 amperes, 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 amperes, 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) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).
(4) Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, if the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.
The table below comes from ANNEX D in the back of the 2017 CODE book.
This table was made using the 83% rule for sizing the conductors to a residential dwelling unit.
Coming in the 2020 Code Book this Table will have a home at 310.12 which will be nice to see it back in the Code book where it belongs. The inclusion of the 83% rule confused many people and was mis-interpreted all the time. I was often asked if it was 83% of the load calculation, 83% of the wire size and a number of other mis-understood interpretations.
One of the most important parts of this 83% rule is that the load calculation can exceed the ampacity of the conductors chosen.
An example is a 200a load calculation comes out to 190 amperes. We are allowed to use the 83% rule because it is a residential service or feeder. The 200 ampere service rating is multiplied by the 83% ( 200 X 83% = 166. amperes) which gives us 166 amperes. We then go to Table 310.15(B)(16) and go down the 75 degree column and find the a 2/0 copper conductor has the ampacity of 175 amperes which exceeds the 166 ampere used in the 83% rule, so a 2/0 Cu. can be used for a 200 ampere service or feeder.
Hope this helps.
Till next time be safe work safe.
Jake
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