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From: (no name) (no email)
Date: Tue Apr 15 2008 - 07:25:11 EDT
Hi Ken -
I beg to say that there are not 2 phases in a 120/240 transformer-derived
neutral circuit. It cannot be 2-phase because the primary winding is only
single-phase. The 3-wire secondary in this instance is called "split-phase."
Regards,
John
"Seahorse"
Using the example of the center tapped 240 volt transformer is a very good
way to supply the 50 amp service, and in fact perhaps the only correct way
in my opinion. Here is why:
1.) The two 120 volt phases will be 180 degrees out of phase with each
other, as they should be, and this is critically important.
2.) The reasons this is critically important is that the standard Marinco
shorepower cable provides the two 120 volt antiphase supplies on three
wires. What this means is that the two phases (black and red) share the
common white return wire.
3.) When properly 180 degrees antiphase, consider if the two 120 VAC
circuits are each loaded to their max at exactly 50 amps on each, -- then
the white wire will be conducting zero amps since the antiphase cancel. By
the same token, if one 120 VAC phase is at 50 amps, and the second is at
zero amps, then the white wire will be conducting 50 amps. In either case,
the white wire is nver required to conduct more than its rated ampacity.
This allows it to be (usually) an AWG #6 stranded wire.
4.) If you have something other than 180 degrees out of phase, then there
can be portions of the two waveforms that will be additive and the white
wire can be required to carry more than its rated amperage. You know where
that leads.
To make a long story short, it is important to remember that the phase angle
matters as well as the RMS voltage difference between two phases.
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