Technical article from Blue Sea Systems
AC Current flow in 120/240 V AC single phase three or four wire systems is frequently misunderstood. This technical brief goes through a step by step process of adding one load at a time to a 120/240 V AC system and looking at the resulting current flows.
There is an assumed understanding of basic DC theory, Ohms Law (V=IR), the Power Law (P=VI) and Kirchhoff's Voltage and Current Laws.
 The sum of the voltage rises equals the sum of the voltage drops around any closed loop.
 The sum of the currents entering a junction equals the sum of the currents leaving the junction.
In a DC circuit we assign a polarity to Voltage and a direction to current flow. By convention current flows out of the positive of the source through the load, where there is a voltage drop, and returns to the negative of the source. It is easy to visualize the current flowing out of a battery, through a light bulb, and back to the battery. There is a voltage rise across the battery and a drop across the light bulb. If we connect another light bulb, it too requires current. The total current will be sum of the current flowing in each bulb.
When we start thinking about AC current flow our mind boggles trying to comprehend the fact that the current flow changes direction 60 times a second (or 50 for our European friends). That is why we call it alternating current. In fact it makes virtually no difference. Thanks to the mathematical magic of our electrical forefathers we use exactly the same laws and rules for AC and DC, with one exception in the Power law called power factor (A topic for another discussion). In fact for resistive loads everything is exactly the same. We simply assign a current flow direction, and voltage rises and drops, as long as we stay consistent everything works just like DC.
Current Flow in a 240 V Load
First consider the current flow in a 240 V load, in this example Load 3. Assume it is purely resistive. The red arrows indicate the direction we are assigning to the current flow from the AC source, which might be shore power or a generator. We could have reversed the current flow assignment but we once a direction is assigned we cannot change it. We have intentionally not shown the neutral.
Adding a 120 V Load
Now let's add the Neutral and a 120 V load that is connected between L2 and Neutral. The current flow for this load is shown with green arrows. The current flows through the load and returns back to the source (shore power or a generator) via the neutral. We see that the current in L2 is now equal to the current flowing in the 240 V Load 3 and the 120 V Load 1. We also see that the current flow in Line 1 is unchanged.
Current Flows in a 120/240 Single Phase Three Wire System
Finally, in this diagram we add another 120 V load, Load 2 connected to L1 and Neutral. Since we have already assigned a direction to the current flow in L1 we must use the same current flow direction to supply Load 2. We see that the current flow in L1 is equal to the sum of the current flow for Load 3 and Load 2. Just like the current in L2 was the sum of the currents supplying Load 1 and Load 3.
The current flow in the Neutral deserves a special look. Since the currents "flow" in opposite directions we subtract to get the net current flowing in the Neutral. If Load 1 and Load 2 are exactly the same size the Neutral current will be Zero. In fact when the loads are assigned electricians try to balance the load on Line 1 and Line 2 so that the neither leg is overloaded, the result of this is that the Neutral current should be about zero in a well designed system.
Based on the previous diagram we see that if we want to measure all of the relevant currents in a 120/240 V system we need to place our current sensing Current Transformers (CT) in the appropriate positions.
To see the current associated with 240 V loads alone, you must bus the distribution panel with this bus arrangement. The current measured by CT1 and CT2 measures the current flow total in L1 and L2. This is the sum of the currents of the 120 V loads on that line and the 240 V current. Where as the current measured by CT3 is only that associated with 240V AC loads.
Original article from Blue Sea Systems
