Dimitrios Tzortzakakis said:
The ampacities are for normal conditions, thus residence.It's a thumb rule
which cable to use for what load.A cooking range needs 6 mm^2, a water
heater 4 mm^2, a washing machine and dishwasher and most space heaters 2.5
mm^2 and incadescent lighting 1.5 mm^2.I suppose these calculations are
for
the climate of Greece, and for three wires (live, neutral, earth) in a
conduit.
"Normal conditions" is a broad term. In the USA the NEC (National
Electrical Code) has many specifics for ampacity tables. For instance, the
main Table 310.16 that we use for building wire ampacity defines the ambient
as 30 degrees C. and has derating factors where the temperature exceeds this
for long runs. If the temperature is excessive for no more than 10 per cent
of the circuit length to a maximum of ten feet whichever is less, then no
excessive temperature derating is required. The ampacities are also defined
for no more than three current carrying conductors in a raceway or cable.
If conductors are bundled or cables with more than three current carrying
conductors are bundled for longer than 24 inches or if there are more than
three current carrying conductors in a raceway or cable there are additional
derating factors. In some cases where there is excessive temperatures and
more than three current carrying conductors double derating is required. We
also have a rule for continuous loads requiring a 80 per cent derating.
There are also rules defining a current carrying conductor. If a neutral
only carries unbalanced current it is not counted as a current carrying
conductor. But where the majority of current in a neutral is from discharge
lighting, third harmonics, or computer loads it is counted. Also, the
grounding conductor is not counted as a current carrying conductor. Also,
ampacities are listed for three temperatures for copper and for aluminum.
One column is for 60 degrees insulation, one for 75 degrees C. and one
column for 90 degree C. The maximum operating temperature of a conductor
cannot exceed the maximum allowed rated operating temperature for the
terminals, equipment or insulation whichever is the lesser. We can use the
90 degree column for derating purposes. For example if there are 9 No. 12
AWG (American Wire Gauge) 90 degree C. rated current carrying conductors in
a raceway we can multiply the derating factor of .7 times the 90 degree
ampacity of 30 amperes to get 21 amperes and use this as the ampacity but
other rules still apply. Our rules are so complicated that it took me about
5 weeks to write an Excel program to determine ampacity. This program is
accessible at
http://www.electrician.com/calculators/T310-16.xls
But in general for dwelling units (where people live, eat, sanitize, and
sleep), we use No. 8 AWG copper on a two pole 40 ampere circuit breaker for
ranges, No. 10 AWG copper on a two pole 30 ampere circuit breaker for hot
water heaters, and No. 12 AWG copper on single pole 20 ampere circuit
breakers for general purpose lighting and appliance circuits.
The NFPA (National Fire Protection Association) that writes the NEC wants
the international community to adopt the NEC as an International Electrical
Code!
So you too may become acquainted with our famous NEC someday!