Typically, the Common Grounded Conductor (AKA "Neutral") of a 3Ø 4 Wire Wye system will carry between 70% and 100% of the highest L-N load value.
Most commonly, the Grounded Conductor will be carrying 100% of the highest L-N load Ampere rating.
If there are extreme Harmonics across all 3 L-N circuits, expect upto 173% of the L-N load Amperes flowing on the Grounded Conductor (this would be very rare, and would exist if all 3 L-N circuits were carrying the same load Amperes, and had the same Harmonic Distortion values on the connected loads).
If the loads are connected as an "Open Wye" (1Ø 3 Wire), the Grounded Conductor will carry 100% of the highest L-N load Amperes.
If the connected loads happen to be _ALL_ Pure Resistance types (1.0 PF / < 0.5% Reactive / 99.99999% True Power / etc.), then the load Amperes on the common Grounded Conductor may be found using Vectors.
In these cases, the load Amperes will be lower than the highest L-N load Ampere value.
With unbalanced loads, the results will be different per L-N load values; and with connected loads made across 1Ø 3 Wire circuitry - vs - 3Ø 4 Wire circuitry.
If a 3Ø 4 Wire multiwire branch circuit was driving 3 L-N connected _PURE RESISTANCE_ loads of the exact same characteristics
(total circuit Impedance ((Z)) is exactly the same for all 3 connected loads),
there will be nearly zero load on the common Grounded Conductor - provided the Voltage across each load is exactly the same as the other two L-N loads.
FYI: "Pure Resistance" loads = things like:
- Incandescent Lamps,
- Quartz-Halogen Lamps (in normal operation),
- Electric Resistance Heating Elements for Stoves and Ovens,
- Electric Water Heater Elements.
To me, Incandescent Lamps and especially Quartz-Halogen Lamps have enough Inductive Reactance to qualify them as 95% Pure Resistance (0.98 PF).
This is due to the coiled filaments.
Along with this, many tightly wound Resistance Elements would have similar characteristics.
As for 1Ø 3 Wire, from a Single Phase source (derived from a single center tapped coil), the "Center Tapped Grounded Conductor (AKA "Neutral") will carry the "Unbalanced" load Amperes from two L-N load connections - as connected between the end of the coil leads to the center tap.
This would describe the typical 120/240V 1Ø 3 Wire Center Tapped Transformer, found in most Residential settings;
The center Tapped winding of a 240/120V 3Ø 4 Wire Delta Transformer arrangement.
208/120V 3Ø 4 Wire Open Delta "Tee" Transformer setups, may have odd resulting L-N Unbalanced load values found on the Grounded Conductor -
may be whatever the unbalanced load is,
may be 100% of the highest L-N load,
may be 141% of the balanced L-L-L-N load.