Neutral Load Calculator

Electrical Purpose

On a three-phase wye system or a split-phase three-wire system, the neutral conductor carries return current only when the phase currents are unequal or contain harmonic content. A balanced three-phase linear load produces zero neutral current; a balanced split-phase load with equal 120V branch currents produces the same result. In practice, phase loading is rarely equal, and the Neutral Load Calculator quantifies how much current the neutral actually carries under a given imbalance, using the phase currents entered for Phase A current, Phase B current, and Phase C current.

This is a screening calculation, not a substitute for measured neutral current with a clamp meter or a full harmonic analysis. It applies to linear loads with a balanced 120-degree phase displacement assumption on the wye model. Nonlinear loads (variable frequency drives, switching power supplies, LED drivers) generate triplen harmonics that add arithmetically in the neutral rather than canceling vectorially, and this tool does not model that condition.

System Model Selection

The System model field selects the neutral-current formula:

• Three-phase wye linear load — uses all three phase currents (A, B, C) with vector addition based on balanced 120-degree displacement.
• Split-phase three-wire model — uses the difference between the two current-carrying legs; Phase C current is ignored in this mode, matching a single-phase 120/240V panel with two ungrounded conductors and a shared neutral.

Selecting the correct model is required before the result has any field meaning. A split-phase panel evaluated with the wye formula, or vice versa, produces a number with no relationship to the actual neutral loading.

Inputs

Field	Description
Phase A current	Measured or estimated current on Phase A, in amperes
Phase B current	Measured or estimated current on Phase B, in amperes
Phase C current	Phase C current for three-phase models; ignored for split-phase neutral difference

Currents should be RMS values taken at the panel, feeder termination, or branch circuit under review — typically from a clamp-on ammeter reading or a load schedule derived from connected nameplate loads and demand factors.

Formula and Outputs

For the three-phase wye linear load model, the calculator derives neutral current from the vector sum of three phase currents at 120-degree displacement:

Estimated neutral current: \(I_N = \sqrt{I_A^2 + I_B^2 + I_C^2 – I_AI_B – I_BI_C – I_AI_C}\)

Average phase current: \(I_{avg} = \dfrac{I_A + I_B + I_C}{3}\)

Maximum phase deviation: \(\max(|I_A – I_{avg}|,\ |I_B – I_{avg}|,\ |I_C – I_{avg}|)\)

Phase-current unbalance: \(\dfrac{\text{Maximum phase deviation}}{I_{avg}} \times 100\)

The unbalance percentage follows the same logic used in motor and feeder unbalance checks: the largest deviation from the average phase current, expressed as a percentage of that average.

Calculation Example

With Phase A current = 100 A, Phase B current = 90 A, and Phase C current = 80 A on the three-phase wye model:

\(I_N = \sqrt{100^2 + 90^2 + 80^2 - (100)(90) - (90)(80) - (100)(80)}\) \(I_N = \sqrt{10{,}000 + 8{,}100 + 6{,}400 - 9{,}000 - 7{,}200 - 8{,}000} = \sqrt{300} = 17.3205\ \text{A}\)

Average phase current = (100 + 90 + 80) / 3 = 90 A Maximum phase deviation = 10 A (both Phase A and Phase C deviate 10 A from the average) Phase-current unbalance = 10 / 90 × 100 = 11.1111%

The estimated neutral current of 17.32 A is what the neutral conductor is carrying under this load split, even though no single phase is overloaded relative to its own rating. This is the number that matters for neutral conductor sizing on a feeder or branch circuit with a shared neutral, and for confirming that a neutral is not undersized relative to actual return current on multiwire branch circuits.

Practical Use in Load and Conductor Review

The estimated neutral current feeds directly into several field workflows:

• Neutral conductor sizing — on a feeder or service where the neutral is not a full-size conductor, the calculated neutral current confirms whether the installed neutral ampacity, after applicable adjustment and correction factors, covers the actual unbalanced return current.
• Current-carrying conductor count — where the neutral carries current under an unbalanced linear load, it counts as a current-carrying conductor for raceway fill and conduit ampacity adjustment factor purposes; a neutral assumed to be non-current-carrying on a balanced three-phase circuit does not qualify for that exemption once the load is unbalanced.
• Load review and panel rebalancing — an 11% phase-current unbalance flags a load distribution problem worth correcting at the panel schedule level before it shows up as excess neutral heating or voltage-drop asymmetry between phases.
• Voltage-drop review — unbalanced phase currents produce unequal voltage drop across phases downstream of the panel; the average phase current and per-phase deviation help isolate which branch is driving that asymmetry.

Field and Code Limitations

The calculation assumes linear loads and a balanced 120-degree phase displacement, which holds for resistive and induction-motor loads but not for loads with significant harmonic distortion. Nonlinear loads can produce neutral current well above what this vector-sum formula predicts, since triplen harmonic currents (3rd, 9th, 15th) add directly rather than canceling across phases.

The tool does not evaluate NEC neutral-sizing rules, grounded conductor derating exceptions, or service-neutral load calculations under Article 220 — those require confirmation against the applicable code edition and AHJ interpretation, using the calculated current as one input rather than a final answer. Field verification with a true-RMS clamp meter on the actual neutral conductor remains the reference measurement whenever the connected load includes electronic ballasts, drives, or switching supplies.