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Topic: Some Trivia on 50 Amp Services (Read 490 times) previous topic - next topic
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Some Trivia on 50 Amp Services

Yahoo Message Number: 21255
I have been reading the recent discussions of the theory behind a 50- amp service. I was a Power Engineer working for a Public Utility for 35 years. Just a couple of points.
Whether or not the two legs of a 120/240 volt system are 180 degrees out of phase is on the edge of where technical approaches philosophical. Normally only two or more voltages are considered as having a phase relationship where the same points on their sine waves occur at different points in time. Theoretically this is true for a 120/240 volt service but it is points on the same sine wave.
In this case it is more common to think of it as a difference in instantaneous polarity rather than a phase difference. One leg is changing in a positive direction while at the same point in time the other leg is changing negatively. Because the differential math involved with AC voltages they are usually expressed as their RMS (Root Mean Square) value and polarity is not considered. It is, none the less, there.

Another point is that it is actually possible for someone to encounter a 50a service where the two legs are 120 degrees out of phase. It is unlikely but that would be on the fringe areas of a type of service known as a secondary network. This type of service is commonly found in the downtown areas of large cities. It is always underground. Here the transformers step down the 3 phase primary voltage to a secondary voltage of 120/208.3 phase 4 wire wye. Note that the phase to neutral voltage is not ½ of the phase to phase voltage. Within the Secondary Network Area normally 120/240 single phase services are supplied by bringing in 2 phases and the neutral. Here the customer should be made aware but often isn't that the voltage is 208 and not 220. Stove elements will not cook as fast and 220v equipment that is sensitive to low voltage may have a problem. The equivalent 208 volt equipment is available but you have to be aware of the problem. In this type of service if 50 amps were flowing in each phase then 50 amps would be flowing in the neutral due to the phase angle difference. For that reason the neutral must be the same size wire as the phase wire.

Again it would be highly unlikely to find an RV Park or campground serviced within a Secondary Network Area but it is possible. Another interesting thing about a Secondary Network Grid is that many large transformers are all connected in parallel on their secondary side.
For that reason the available short circuit current (the maximum current that will flow through a solid or direct short is vary high commonly in excess of 200,000 amps. Someone that causes a short in the service panel on the street side of the main fuses is in for a big time surprise. A screwdriver dropped across the internal bus bars will literally sublime (a condition when something goes form it's solid state straight to it's gaseous state without passing through it's liquid state). Just poof and it disappears.

Sorry to get technical but some may find it interesting.

Don Seager

2004 Allure 31046