heat on electric wire
Since the electric code requires wires and devices to be rated for 600
volt, how could a 600 volt wire melt when household voltage is 240 volt?
The answer is Heat caused by resistance of the wire.
- The
voltage did not increase during the short circuit.
The
voltage didn't spike over 600 volts. It's still 120-240 volt, because
the grid transformer didn't
suddenly re-wire itself and deliver more voltage.
- The amp rating of the copper wire did not not change. For example 10 gauge wire is rated 30 amps. NEC Code
says: safe maximum ... 80%
x 30 amps = 24 amps for 10 ga wire.
- Wires are like lanes
on
a freeway, and amps are like the cars. Wires only allow so many cars
before the lanes are full, so 10 gauge wire has a maximum 30 amps, while larger 6 gauge wire has a maximum 60 amps.
- So what happens when a short circuit occurs? Answer: The flow of amps
on wire
increases because the short is allowing electrons to flow into
earth, and voltage will push electrons into earth in runaway numbers
until the wire melts
or the breaker's magnetic coil causes breaker to trip.
- But why does the wire get hot enough to melt?
- The answer is, if a circuit is shorted to ground, then voltage keeps
pushing more and more electrons against
the atomic structure of
a
conductor (copper wire). Except the copper wire is a
fixed size,
and the copper only allows so many electrons to flow before the
resistance begins to limit the flow of electrons. Electrons
begin to pile up, like an impatient traffic jam. The
more electrons trying to flow, the higher the resistance.
Resistance
is like friction that causes heat. So the runaway flow of electrons
causes more heat on the wire.
- Mathematics of heat
First formula: Volts x Amps = P (Power or watts). Volts do not change,
but more electrons (amps)
are flowing down the wire, so amps are rising. This means P (power or
watts) is rising. Power goes up during a short circuit.
Second
formula: IČ (amps) x R (resistance) = P
(Power or watts). The first formula tells us that Power is going up. If
P is going up, then the second formula says [amps x resistance] must be
going up. But the wire
limits how many amps (electrons) can flow on the wire. Once the number
of electrons approaches the maximum, then electrons are getting
restricted. This means the resistance must go up../. which slows the
electrons, like too many cars on the freeway. Too many
electrons on the wire and the wire gets hot, causing the breaker to
trip. If there is an arc caused by loose connection, then the excess
power can melt the wire almost instantly before the breaker trips.
One of the functions of the ground wire is to absorb the runaway flow
of
electrons, letting them into earth. This is how the ground wire
protects the breaker and helps preserve the electrical installation.
If
the circuit does not have a circuit breaker or ground wire, or if wires are
too small for the amp rating of breaker (each a strict violation of
electrical code), then the wire will overheat, melt, arc, or cause fire
... depending on unpredictable nature of runaway electricity.
The math
tells us that amps (the flow of electrons on the wire), held back by
resistance, is the cause of heat. If there was no amperage flowing,
there would be no heat.