"Rick" wrote in message
ink.net...

"The Real Tom" Tom @ www.Love-Calculators.com wrote in message
...
On Wed, 09 Nov 2005 17:37:05 GMT, HorneTD
wrote:

Doug Miller wrote:
In article
.com,
wrote:

#1 is more than NEC calls for. If you have a 200amp panel in
the
house
and want to run a 100amp sub panel, you will need to run #2AL
or
#4
Copper.


Ummmm.... this depends a LOT on the specifics of the
installation. 100 amps on
#4 copper single insulated conductors in free air is OK, but
NOT
in raceway or
cable or earth. Ditto #2 aluminum, with the additional caveat
that it *also*
depends on the temperature rating of the conductor insulation,
e.g. #2
aluminum UF cable is limited to 75A, but #2 aluminum THHN is OK
at 100A.



Doug
I'm not trying to quibble when I say that the 90 degree column is
useful
only when derating the wire do to the number of conductors in a
cable or
raceway or the ambient temperature. Very few terminals are
actually
rated at 90 degrees centigrade so your final ampacity is limited
to
the
ampacity of the conductor at the temperature rating of the
terminals to
which it is connected. Since the ampacity of the #2 Al is ninety
amperes at 75 degree centigrade rating of most terminals and that
is a
standard breaker and fuse size a larger wire is required.



WOW!

I brought this up to a fellow electrician. Why are we allowed to
use
#2 AL for 100amp subpanel, and he said he's used #2 for servers
and
feeders for dwellings for years, per Table 310.15(B)(6). I
agreed,
and said I wasn't feeding a dewlling, just a sub panel in the same
dwelling, so per 110.14(c)(1) and table 310.16 and limited by 75C
terminals on the sub panel and feeder breaker aren't I limited to
90
amps?

The concensious is #2 AL is good for 100 amps because of Table
310.15(B)(6) reguardless if it's feeding a dewelling or a portion
of
a
dewelling, it's still a dewelling (loads).

Can anyone confirm this?

later,

tom


310-15(b)(6)

120/240 volt, 3 wire, single phase dwelling service and feeders

For dwelling units, conductors as listed in Table 310-15(b)(6) shall
be permitted as 120/240 volt 3 wire single phase service entrance
conductors, service lateral conductors, and feeder conductors that
serve as the main power feeder to a dwelling unit and are installed
in raceway or cable with or without an equipment grounding
conductor.
For application of this section, the main power feeder shall be the
feeder(s) between the main disconnect and the lighting and appliance
branch circuit panelboards...


The last line is kind of vague, does a subpanel qualify? I've always
considered Table 310-15(b)(6) as just for service drops...


OK, I looked at an older (1990) version of an NEC handbook. A footnote
in the handbook states in part "If there are panelboards on the load
side of the main service-entrance equipment supplied by feeders, Note
3 also permits a reduction in the conductor size of these feeders as a
result of a revision for the 1990 NEC".

Can't say if that's still in effect for 2005...