5/22/2014 – Original Post
The “single phase, center-tapped, 3-wire circuit” is the residential and light commercial power distribution standard across all of North America. This circuit originates – as far as end-users are concerned – at a nearby utility power transformer. In the ordinary residential situations with which we are all familiar, it is the form of power that is delivered from the transformer to our homes and light commercial businesses. The feed is from the “secondary winding” of the power transformer.
In these 240VAC, “single phase, center-tapped, 3-wire circuits,” the two 120VAC legs (“L1,” “L2”) ARE NOT out-of-phase with respect to each other. It is very commonly thought – incorrectly – that the two “hot” legs are 180° out-of-phase. They are NOT. The rise and fall of the common magnetic field in the core of the power transformer simultaneously creates the voltages in both legs. As the magnetic field rises, the voltages in the legs rises; as the magnetic field then falls, the voltages in the legs fall. The wave forms appear out-of-phase because the measurement is inevitably taken from the center-tap connection (the “neutral,” often referred to – also incorrectly – as “the ground”). The transformer’s center tap connection is electrically 1/2 way along the entire secondary winding of the transformer secondary. Measuring from that mid-point connection makes one end of the winding appear out-of-phase with the other, but that appearance is only a phenomena of the manner in which the measurement is taken. Across the entire length of the secondary winding (“L1” and “L2,” the black and red “hot” wires), the measurement is 240V. From either of the winding ends to the center tap (black or red “hot” wire to the white wire, L1-to-N, L2-to-N), the measurement is 120V.
Why do I continue to emphasize and re-emphasize the point? Because 240V, 3-wire circuits are designed and intended to support complex parallel load attachments in which currents divide and follow different available paths. This is certainly the case found on boats with 240V, 50A shore power service cords. Understanding the electrical behavior of the neutral depends on understanding the electrical behavior of the power transformer’s entire secondary winding.
The electrical behavior of parallel circuits adheres to well defined laws of physics called Kirchoff’s Laws (Gustav Kirchoff, German Physicist, Scientist, 1824–1887). Kirchoff’s Law tells us that if there are loads on both 120V legs of a 240V, 3-wire circuit, the returning currents divide at the mid-point center-tap. That center-tap point is, by definition, the “neutral.” For a 240V, 3-wire circuit, the current flowing in the neutral can never exceed the largest current flowing in one of the “hot” legs. In fact, in most operating situations, the current flowing in the neutral will be well below that maximum.
Let me emphasize that this monograph applies ONLY to 120V/240V, single phase, 3-wire electric services, derived from the secondary of a single-phase transformer. For boats, this discussion applies ONLY to those powered by 120V/240V, 50A, single phase, 3-wire shore power cords. This discussion DOES NOT APPLY to boats with two correctly wired, individual 120V, 30A shore power connections. It also does not apply to boats wired for 120V/240V, 50A, single phase, 3-wire circuits when they are operating connected to 120V/208V, 50A power derived directly from two phases of a three-phase network.
Some discussion examples:
1. With a pure 240V device like the heating element of a hot water heater or the 240V motor of a deep well pump, there is no neutral. These devices are designed to operate on 240V and are wired with two “hot” wires only; no neutral. During one AC half-cycle, AC current arrives at the device on the black “hot” wire (L1) and returns to it’s source on the red “hot” wire (L2). During the other AC half-cycle, the red “hot” wire feeds and the black “hot” wire returns.
2. With a hybrid 240V/120V device like a electric range/oven or an electric clothes dryer, there are both 240V loads (heating elements) and 120V loads (appliance light, motors, timer control circuits) in the device. In such hybrid devices, the heating element current alternately arrives and returns in the black (L1) and red (L2) wires (240V). The 120V load currents alternately arrive and return in one of the “hot” legs and in the neutral. Lets assume that the 120V light bulb, fan and timer controller of a range/oven are connected inside the device to the black “hot” wire (L1). Also assume the heating element draws 20A and the light/fan/timer controller draws 2A. In that case, there will be 22A flowing in the black “hot” wire (L1), 20A of the 22A total will return in the red “hot” wire (L2), and 2A will return in the neutral (N). The direction of current flow reverses on the other AC half-cycle, but not the wires in which the currents flow.
3. Finally, consider only pure 120V loads on both “hot” legs of the 240V circuit; that is, no 240V loads connected. For example, assume that a 120V battery charger, 120V space lighting and 120V TV/DVR are operating on the house side (black “hot” wire, “L1,” in this example) and the A/C compressors and raw water circulator are operating on the other side (red “hot” wire, “L2,” in this example). Further, assume that house loads in this case are drawing a total of 17A, and the A/C system components are drawing a total of 26A. In this scenario, there are 26A flowing in the red “hot” wire (L2), 17A returning in, and powering the house loads, via the black “hot” wire (L1). In the neutral (N) in this example, there are (26-17)=9A flowing. EVEN THOUGH NONE OF THE DEVICES IN THIS SCENARIO ARE 240V devices, the base 17A will feed in the red “hot” wire and return in the black “hot” wire on one half cycle, and reverse direction in the other half-cycle, flowing through both 120V loads, BUT NOT FLOWING IN THE NEUTRAL. ONLY THE UNBALANCED CURRENT FLOWS IN THE NEUTRAL. The electrical presence of the center-tap (neutral) stabilizes the voltage on both halves of the 240V circuit at 120V.
All of the above examples assume all wiring is correct (code-compliant) and performing correctly . Let me re-emphasize, this discussion only applies to 240V, 50A, 3-wire (black, red and white) circuits. Because of differences in the way they are wired, two separate, independent 120V, 30A, shore power circuits DO NOT behave in the same way.
The only time the neutral in a 240V, 3-wire circuit carries the entire load current is when only one side (one 120V “hot” leg) of the circuit is powering loads. In example 3, above, if the house side were totally, completely powered “off,” but the A/C compressors were powered “on,” the A/C compressors/raw water pump would draw 26A through the red “hot” wire, but without a counterbalancing load path available in the black “hot” wire, the neutral would, in that case, carry the full 26A.