150A Service

Sometimes when I see this kind of stuff the answer should be so elementary,
but I’m scratchin’ my head on this one when I see it…

House built in 1982.
No exterior main disconnect, it’s in the Square D distribution panel in the garage.
It is a 150 amp breaker, the cable size is 1/0 AL, NOT CU.
I believe 1/0 AL is rated for 125 not 150 amp.
What say you!
THANKS! :slight_smile:

You are correct. I will post the table once it opens…

THANKS Michael. :slight_smile:
I was using this table (attached).
That’s why I was rubbing my eyes when I took the cover off & saw 1/0 AL.

That is the table, but looking at the 2017 code book it appears to have been removed. From a quick google search as to why it was removed I am finding that it was being blindly followed without noting the exceptions in the text. Here is the relevant snippet from my book:

(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 sup-plied
by a single-phase, 120/240-volt system shall be permitted
to be sized in accordance with 310.15(B)(7)(1) through (4).
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 neu-tral
conductor from a 208Y/120 volt system shall be permitted
to be sized in accordance with 310.15(B)(7)(1) through (3).
**(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 require-ments
of 215.2 and 220.61 for feeder conductors are met.

Not much has changed in 50 years, this is from the 1965 NEC:

THANKS for chiming in Robert & Michael for taking the time :smiley:

Thanks for the replies Michael and Robert.

You’re welcome. To expand a little on what Michael has stated the 2014 NEC abandoned the table and went to the 83% rule. If you compare conductors sizes from the old tables to the 83% rule the sizes generated by the calculation are the same. When switching to 83% rule this made it easier to size the conductors when applying things like derating or when using parallel conductors.

Now it is more clear that when you have adjusted the conductor ampacity or used more than one set of parallel conductors the resulting conductor size must be a minimum of 83% of the service rating.