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Re: Electronics shielding
Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>
>What I have on the drawing board is electronic control of AC current in a
>main control station that will be at least 20 to 25 feet from the coil
>itself After following the advice in previous posts that leaves the main
>output cable, 30 feet 4/4, as my last concern that will provide up to 40
>amps to a 10Kva pig further down the line. Electronics is one thing but
>electrical is quite another and that's what I'm weak in. How can I protect
>that cable? I'd hate to have to buy something with shielding like romex to
>handle that much current.
Do you need real RF shielding, or just protection against mechanical abuse
and the stray ground strike streamer?
If the latter, then why not use flex metallic conduit and run your 4 wires
inside it. Good mechanical protection if someone steps on it (or drives
over it), and it't tough enough to take a ground strike.
Why 4 wires to the pole transformer, by the way? You need 2 wires for the
240VAC, presumably, and another for safety ground, but that's only 3. Or,
do you have some sort of 110V load at the load end that you want to
supply? (or, is it that you happen to have some 4/4 cable and want to use
Also, AWG 4 is pretty big... 10kVA at 240V is only 50Amps or so. The NEC
shows AWG10 is good for 40A, and you could probably go one bump up to AWG 8
for 55A. If all 4 conductors were carrying power in your cable, then you'd
need to derate a bit, also because it's jacketed, but even so, with AWG4
you're paying for a lot more copper than you need. AWG4 is twice the
diameter of AWG10, and for big wire, copper cost dominates, so it would
cost 4 times as much (per foot).
You might already have your wire, in which case it's moot, but for others
contemplating this sort of thing, a pair of AWG12s is equivalent to a AWG9
(3 gauges is twice the area, 6 is twice the diameter), and from an NEC
standpoint, two AWG12s would be 30Amps*2 = 60A (although, again, if they're
run in the same raceway/conduit/etc, you'd need to derate (30%, I
think)). AWG12 wire is very inexpensive due it's huge popularity.
And of course, our TCs, particularly high power ones, are hardly stuff that
is designed with NEC compliance in mind (if for no other reason than it's
not a permanent installation inside a wall, where you need more design margin).
The real issue is going to be heating and voltage drop along the wire.
AWG10 is 1 ohm/1000ft, a 50 foot run puts 0.1 ohm in series (50ft out and
50ft back). 50Amps would be a 5V drop, pretty close to the 2% drop allowed
by NEC for a permanent installation, and quite acceptable for a tesla coil.
Looking at heating.. 5V*50A is 250W, distributed along that 50 feet of
wire, for 5W/ft. This is comparable to a string of christmas lights,
dissipation wise, for comparison.
At 50A, I might feel more comfortable running a bit more than AWG10, but I
don't know that AWG4 would be required...