There are many different kinds of fuses and breakers. One of the
major differences is how quickly they trip with current over their
nameplate rating.
Lots of electrical loads draw high "in-rush" currents when first
turned on, but for only a few hundred ms. For example, large audio
power amplifiers and the power supplies for our ham power amps can
easily draw more than 100A or more if they are turned on at the
peak of the AC cycle, but that "in-rush" surge won't last very
long.
You would be quite upset if your 15A breaker tripped each time you
tried to turn those power supplies on. A breaker or fuse SHOULD
withstand short surge currents without tripping. Lots of mechanical
equipment (motors) require lots of current to get them started, but
settle to much lower values once they are running. Most breakers
use a combination of magnetic and thermal mechanisms, and some are
designed to withstand higher surge currents than others. A breaker
rated for 15A will typically withstand 30A or so for at least 60
seconds. That is as it should be -- fires start due to overheating,
so just as the wire takes time to heat up, so does the fuse or
breaker.
Let's do some simple math. To keep the arithmetic simple, let's
assume our RF power amp is 50% efficient, and it's putting out
1,5kW. That's 3 kW input, which is 12.5A at 240 volts. Most amps
are more efficient than that, so the current draw will be less. And
remember, that's key-down, CW. From a SAFETY point of view, #14
copper is entirely adequate, assuming that's the only load on the
circuit. Assume a duty cycle of 50%, and the heat produced is one
half that. Heating will be increased a bit because the current is
non-sinusoidal, but that effect is still well within the rating of
the copper.
As noted in my earlier post, there's nothing WRONG with using
bigger copper -- you'll have less voltage sag with keydown, and
your power supply will be happier when the power company's line
voltage sags. It's all Ohm's law -- if you have long run from the
breaker panel, use bigger copper. #10 is 1 ohm/1,000 ft. At 100 ft,
that's 0.2 ohms and a 6 volt drop with a 30A load, which is 2.5% of
240 volts. No big deal. #14 copper would be about 2.5X the drop,
which is still only 6.2%. That's equivalent to a 120v circuit
dropping to 112.5 volts. And that's keydown with 3kW input!
As to fire safety -- poor connections, deteriorated insulation, and
damage to wiring by rodents are of far greater concern than wire
size. If you want to protect yourself, put your wiring in conduit,
preferably steel. Steel has the added advantage of keeping RF off
the power wiring.
73,
Jim Brown K9YC
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