"HorneTD" wrote in message
nk.net...
On Mon, 3 Jan 2005 10:32:32 -0500, John Grabowski
wrote:

You might want to consider #4 wire with a 70 amp main breaker. I
don't know what you will be using the 50 amp receptacle for, but
if you draw 40 amps the 60 amp main will be at its maximum
continuous load. Circuit breakers are only rated for 80%
continuous load.


50 amp receptacle will be used for testing military surplus stuff,
or, alternatively, later, for a phase converter or some such.
Supposedly, #6 wires are rated for 75 amps.


i
You need to be aware that the 75 ampere value given in table 310.16
Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts,
60°C Through 90°C are not achievable in actual practice. This is because
everything that the conductor connects to has to be listed to be used at
the ninety degree centigrade temperature. In general such terminals are
not readily available in the market place and that column is used as a
starting point for derating the wire for high ambient temperatures or
for more than three current carrying conductors in a raceway or cable.
The values in the seventy five degree column of the table are the actual
maximums that can be practically achieved. Since the 65 ampere value is
not a standard size of Over Current Protective Device (OCPD) you are
allowed to use a seventy ampere breaker but the idea is that the circuit
will only draw sixty five amperes under the designed operating
conditions. If you were actually going to draw the full seventy amperes
you would go up to the next wire size.
--
Tom H


Absolutely great answer Tom...