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elevate:
If you have a two terminal device, such as an ordinary incandescent lamp, then you have a series circuit of one wire to one side of the lamp, the lamp, and the other wire from the other side of the lamp. The same current flows thru each of these elements in this series circuit. You can measure the current anywhere in this series path and the reading will be the same. So as stated by SG-1 your load or lamp current is 1 A.
Now suppose you have a 3 phase Y source with 4 wires, one is a neutral. If you have a load, like one lamp, from one hot wire to the neutral, and no other loads on any of the other hot wires, then this the same as I described in the first paragraph.
Next consider the load of paragraph two, call it load A, and add a different load between one of the other hot wires and the neutral. Now the neutral current will be different than the current thru load A. However, the wire on the neutral side of load A, but not yet part of the neutral will have the same current as the load A.
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