T&T: Electricity at Dock and Phase Angle

[Ken Bloomfield]

GB makes very good points in his well written article.  There is one 
additional point that I would like to bolster his article with, as it is a 
point that can have profound effect.

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.

Cheers,
Ken.

Ken Bloomfield
Cell# 865-293-2174
MTOA# 2062
AGLCA# 3529
M/V Tellico Lady, 50' Marine Trader-Walkaround
Maryville, TN