As mentioned, the Common Grounded Conductor on a 3Ø 4W Wye system will carry at least the same level of Current of that found on the Ungrounded Conductor with the highest Load Current - for L-C connected Loads.
Under some circumstances, the Common Noodle will carry little to no Load Current (for L-C loads). These are very specific in nature, and mostly apply to Pure Resistance Loads.
For most applications, figure the Common Noodle to carry at least what the largest Load Current of any ØA, ØB or ØC line is.
Example:
Multiwire (4 Wire) circuit, 20 amp rating, reactive loads (no load is pure resistance only), THD is <1% overall:
ØA Load = 8 Amps,
ØB Load = 10 Amps,
ØC Load = 12 Amps.
Common Grounded Conductor will carry ± 12.3 Amps.
Another example:
If Loads with THD of 33% overall are connected to the same systems + Multiwire Circuitry, and:
ØA Load = 10 Amps,
ØB Load = 10 Amps,
ØC Load = 10 Amps.
Common Grounded Conductor will carry ± 20 Amps.
Harmonics are produced by many types of loads besides Fluorescent Lamp Ballasts. SMPS for Computers, AC Induction Motors, Linear Power Supplies, UPS Equipment, AC Coils for Relays and Contactors, Dimmers, and even TVSS devices are generators of Harmonics.
Magnetic (Reactor-Core type) Ballasts generate Harmonics, as well as Hybrids and straight Electronic Ballasts - for both Fluorescent and HID Lighting.
Multiwire Circuitry is Code Compliant, and if done properly, can be beneficial.
Installation requires knowledge of load types, and for the best installations / designs, those factors should be applied to determine maximum loads + conductor sizing.
Scott35
