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On 9/4/15 5:47 AM, David Rieben wrote:
John, Depending on the size and manufacturer of the variac, it is may
very well be the inrush of initial power up of the variac itself that
is tripping your breaker. I'm had much issues with this with variacs
of 15 amps or more rating. The real "poor boy's: way to address this
issue is to run your system via an appropriate guage 100 ft.
extension cord. The added relatively small impedance of the extension
cord itself is often enough to squelch the instantaneous current rise
at power up enough to correct this problem. David
a 14 ga 100 foot cord will add about 0.5 ohm to the circuit..
That will limit the current peak to 120/0.5 ohm or 240A, which is
probably good enough to keep the transient from tripping the breaker
(esp since your load has some series R, too)
A big variac can look like a dead short when you first energize it.
Handy numbers:
10AWG -> about 0.1" in diameter and 0.001 ohm/ft
3 AWG gauges is half the area
6 awg gauges is half the diameter, 4x the resistance
so 10 AWG 100 ft is 0.1 ohm.
16 AWG 100 ft is 0.4 ohm
an extension cord has two runs of wire, so
100 ft 16 AWG is 0.8 ohm
if you're converting back and forth between US and European units, some
handy approximates:
AWG 14 is 2 sq mm
AWG 17 is 1 sq mm
AWG 24 is 0.5 sq mm
AWG 30 is 0.05 sq mm (it's 0.01", 0.255mm diameter)
Most people consider AWG 16 (which is a standard "even size") is
basically the same as a 1 mm2 .. things like screw terminals and such
are the same size.