I have a subpanel that is about 80 feet away from the main panel. It
uses 6 gauge copper THHN wires conducting 220V current, in a 3/4"
raceway. What will happen if I upgrade the breakers to 100 A and try
to use close to 100A of 220V. Thanks, Dave J.
That at first reading seems surprising?
Using I^2 * R = 780 watts.
Then 100 * 100 * R = 780
And R = 780/10,000 = 0.078 ohms
If 0.078 ohms represents 80 times 2, feet (two conductors totalling 160
feet) then 0.078/160 = 0.49 ohms (per 1000 feet). This is close to 0.395
ohms per 1000 feet given in one available wire table.
So the cable dissipating (780/160 = 4.9) nearly 5 watts per conductor foot
and about 10 watts per 'cable' running foot will get hot!
NB: Referring to wire tables in an older version of Northern Electric's
'Electrical Conductors Handbook',
also presuming we are referring to Number 6 AWG (American Wire Gauge) and
The AMPACITY of #6 AWG (Copper) is listed in various configurations ranging
from bare and weather resistant conductors in free air, to "Not more than 3
conductors in a raceway or cable" as ranging from 55 to 100 amps.
So IMHO doubt that anything will actually 'burn off' but will not be in
accordance with your current electrical codes for new construction?
The other consideration however will be voltage drop; 80 feet is quite a
distance! Also using the 780 watt figure, of a total unity power factor VA
of 100 * 220 = 22,000 watts; 780/22,000 = 3.5% will be lost as heat.
Not likely. Resistance of 6 gauge wire is 0.47 ohms per 1,000
feet. I have 160 feet (going both ways), which makes resistance equal
to 160*0.47/1000 = 0.075 ohm. At 100 amps, a 0.047 ohm resistor
would produce 7.5 watts of power total, a negligible amount.
Is my calculation wrong?
750 watts over 80 feet of wire isn't so bad.
NEC lists the ampacity of 6 gauge wire as 55 amps or 70 amps depending
environment and raceway. 100 amps is certainly not up to code but
won't cause an instant fire!
Upgrading to a larger wire size may well require a larger raceway, too.
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