What is the fourth wire in a three-phase four-wire system?
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What is the fourth wire in a three-phase four-wire system? We have 3 phase wires + 1 PE wire. Is this a cable designed for 380V? Such a cable cannot supply 220V. If 220V is needed, should another wire be connected from the distribution cabinet, namely 1 phase wire + 1 N wire? 3 phase conductors busbars, 1 PE busbar, and 1 neutral conductor busbar – all 5 types of busbars are available. Then why not make each cable use a three-phase five-wire system? That way, there would be ground protection as well as the possibility to obtain 380V and 220V voltages, which would be very convenient. Why use 3 phase conductors plus 1 PE conductor instead?
Because in some places, 220 volts aren’t necessary; large cables are very expensive.
A 4-core cable with 3 phase wires + 1 PE wire is generally used for motors, while power supply cables consist of 3 phase wires + 1 N wire + 1 PE wire; typically, the cross-sectional area of the N wire and PE wire is half that of the phase wires.
Can the same cable power both 220V and 380V devices? In other words, can 220V equipment and 380V equipment be powered by the same cable? Will this cause an imbalance in the three-phase load?
It can only be said that it enables the three-phase load to be balanced for this cable； If the capacity of this circuit is low while the transformer capacity is high, it will not have much impact on the low-voltage bus.
Currently, power supply mostly uses a three-phase five-wire system, namely 3 phase wires + 1 N + 1 PE. Regarding these issues, you can take a look at TN—S, TN—C, TN—C—S. A cable is delivered to the site; if there are both 220V and 380V electrical devices at the site, a small distribution box is required there. During power distribution, it is necessary to allocate power to the devices properly to ensure three-phase balance.