DBoone
Senior Member
- Location
- Mississippi
- Occupation
- General Contractor
Why does 310.15(B)(7) allow 2/0 copper for a 200A service or feeder but then flip over one page to 310.15(B)(16) and 2/0 copper isn't rated 200A even at 90C.
310.15(B)(7)
- Thread starter DBoone
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That is only for a residential service, also the 2014 NEC scrubbed that table so now you must do the math.
jaylectricity
Senior Member
- Location
- Massachusetts
- Occupation
- licensed journeyman electrician
It scrubbed the table, but the math results in almost exactly the same numbers as the table. Cut the table out of your old book and glue it to your new book.
Just remember that you can only do this when it is feeding the entire load of the service.
Just remember that you can only do this when it is feeding the entire load of the service.
- Occupation
- Licensed Electrician
Ever look at the size of wire the POCO attaches to those 2/0 conductors?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
This is true but that table will work without doing the math. The math will come out to be what the table would be. For a 200 amp service you multiply by 83% and get 166 amps. Table 310.15(B)(16) allows 2/0 copper since it is rated 175 amps at 75C. The catch here is if you use se cable and it is rated 60C you will have issues where in the Table that really never addressed all the other factors.
The reason the NEC allows this reduction for residential work is because of the diversity of the loads. Commercial loads such as lighting etc are going all the time. In residential loads are on and off so frequently that there generally is never a high demand on the service conductors.
- Location
- Illinois
- Occupation
- retired electrician
Actually it is because the calculations in Article 220 are very very conservative and result in the requirement for a service that is much larger than needed in the real world. The residential service wire sizes came from actual loads from the utility companies.
The calculations are also very conservative for commercial occupancies, but the code does not recognize that fact. Look at the utility wire size for a commercial service as compared to what we are required to install.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Line trucks only have two sizes for service drops, 2 and 6. POCO has been making these connections for a long time so they know what works
But they are not insulated, but rather covered, and are considered single conductors in free air.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
I agree with Don on being conservative for commercial, the NEC requirements for lighting loads are perhaps 2 w/sq ft, and in the energy codes we are use are way below those numbers.
True that, But the table is gone for the 2014 edition, I'm not sure why the would scrub the table, It seems it made it more simple with the table.
- Occupation
- Licensed Electrician
You are right and that was my point. Don explained it better and more completely.
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