Saturday, January 5, 2008

More on ShorePower

First, a quick note to anyone who has been patiently waiting for the photos of the steering pump replacement: I have gone back and inserted them in the original post, here.

After my post Thursday night about Jubitz, loyal reader Leland Bradley (who, incidentally, was another person who provided us with some early warning about the impending steering pump disaster) wrote in to inquire what sort of power arrangement involved 208 volts, as that is normally a three-phase rating. That's a great question, and so I thought I would answer it here, and provide some photos of our "hookups."

In most commercial applications, power is derived from a three-phase source, and while not all three phases are present in the connectors provided on these pedestals, that is still the source of their power. So the four-wire plug arrangement on these pedestals has two hot legs, with a voltage of 208VAC between them, while each hot leg has a voltage of 120VAC with respect to the neutral wire in the outlet (the fourth wire being the ground). The very same type of outlet is used in household applications, where the voltages are more commonly 240 (or sometimes 220) leg-to-leg and 120 (or 110) leg-to-neutral.

Here is a photo of the receptacle area of the ShorePower pedestal:

As you can see, there are some breakers (two 20-amp single pole, and one 30-amp double pole), two separate duplex 20-amp GFCI receptacles, and a four-wire, 30-amp "250-volt" receptacle.

Most "240-volt" household appliances will work on 240, 220, and 208 without problems, and almost nothing that takes 240/120 will notice that the 208-volt power involves two different "phases" 120 degrees apart. (As opposed to residential current, which involves two "halves" of the same (single) phase, or, as some people prefer to think of it, 180 degrees between them.)

The receptacle pictured is the very same one used for household electric dryers, and plugs or pigtails with this arrangement are available at most hardware stores and home centers. Note also the cable-TV "F-connector" adjacent. The translucent panel in the upper half allows for some illumination from a lamp in the pedestal. The pedestals here at Jubitz are doubles -- there is one of these arrangements on each side of the pedestal, and one pedestal serves two truck stalls.

A bit lower down the pedestal are the instructions, which is how I knew the pedestal was 208-volt (note the "Free for now" sticker -- pay-per-use is coming):

This is what the ShorePower area of the truckstop looked like from our stall this morning:

Somewhat reminiscent of a campground, actually. Come to think of it, these pedestals are way nicer than the ones we have seen at some commercial parks. If you look closely, though, you will notice only two cords in the photo (and one of them is ours), even though there are eight equipped stalls visible. The other trucks in the photo are just idling (despite numerous "No Idling Zone" signs). I imagine that once they start charging for the service, they will ask trucks who don't intend to use it to park elsewhere (there are only a couple dozen hookups, out of hundreds of stalls at Jubitz), or at the very least crack down on the idling.

If this idea sticks and the arrangement becomes common, I will fix myself a more permanent adapter to connect my 50-amp, 240-volt (nominal) shore cord to the 30-amp, 208-volt receptacle. In addition to giving us a tad more oomph, it will eliminate the annoying nuisance trips of the GFCI receptacles, which we have experienced several times here as the Portland "liquid sunshine" hits the pedestals. I have not heard what the price-point will be for this service, but truckers are notoriously cheap, and so I am guessing it will be less than what we would spend to run our genny all night, and certainly less than campground rates.


  1. 208/120 volt three phase power distribution is considered more a commercial rather than an industrial distribution system. Typically found in commercial buildings requiring lots of 120 volt distribution.
    The nice thing about it is that there is only one neutral with three hot legs available (A, B or C). The 240/120 distribution takes three separate neutrals.

    There is some loss of performance on three phase motors operating on 208 volts as opposed to 240 volts. About 10% as I recall. Also they can not operate in as severe a brownout condition with only 208 nominal available.

    BTW, the L-N voltage on a 208 volt system is 120 volts, not 110.


  2. Richard,

    You are right, of course, about the phase-to-neutral voltage on 120/208 (duh) and I have corrected the mistake in my original post.



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