What is the NEC 220.61 70% rule?

NEC 220.61(B)(1) lets you take the portion of the maximum unbalanced neutral load that exceeds 200 A at 70% when sizing the neutral. The first 200 A is always counted at 100%, and only the excess is reduced.

When must the neutral be full size?

On a 4-wire, 3-phase wye feeder serving significant non-linear load, NEC 220.61(C)(2) blocks the 70% reduction because triplen (3rd) harmonics add in the neutral. The neutral must then be sized for the full maximum unbalanced load and counted as a current-carrying conductor.

What counts as a non-linear load?

Loads that draw current in pulses rather than a smooth sine wave — electronic power supplies, LED and fluorescent ballasts, computers, and variable-frequency drives. These generate 3rd-harmonic currents that combine in the shared neutral instead of cancelling.

Does the reduction apply to a 3-wire feeder?

No. A 3-wire feeder tapped from a wye (two phases plus neutral) carries full line current in the neutral, so the 70% reduction does not apply. The calculator handles this by sizing the neutral for the full unbalanced load.

Is this a substitute for a full feeder design?

No. This tool estimates neutral demand to catch undersized neutrals early. Final conductor selection must account for ampacity tables, ambient and bundling derates, voltage drop, and the authority having jurisdiction.

Neutral Conductor Sizing Calculator

The neutral of a feeder rarely carries the same current as the phase conductors, and the National Electrical Code lets you take advantage of that — but only in specific cases. This calculator applies NEC 220.61 to find the neutral demand load and warns you when a full-size neutral is mandatory because of harmonic (non-linear) loads.

How it works

The starting point is the maximum unbalanced load — the largest net computed load between the neutral and any single ungrounded (phase) conductor.

NEC 220.61(B)(1) permits a reduction on the part of that load over 200 A:

neutral demand = first 200 A (at 100%) + (load over 200 A) × 0.70

So a 260 A maximum unbalanced load becomes 200 + 60 × 0.70 = 242 A.

That reduction is not always allowed. NEC 220.61(C) removes it for:

The portion of load that is non-linear on a 4-wire, 3-phase wye system — triplen harmonics add arithmetically in the shared neutral rather than cancelling, so the neutral can carry as much current as a phase.
3-wire feeders from a wye, where the neutral carries full line current.

When those conditions hit, the tool flags a full-size neutral and counts it as a current-carrying conductor for ampacity-adjustment purposes.

Worked example

A 277/480 V wye panel feeds a mix of office lighting and receptacles. The most-loaded phase shows a 260 A unbalanced load and 60% of it is electronic (non-linear).

Because non-linear content is at or above 50% on a 4-wire wye, the 70% reduction is blocked. The neutral must be sized for the full 260 A, the same as the phase conductors — not the 242 A you would get from the reduction alone.

Notes and tips

The 200 A threshold is a fixed Code value, not a percentage — always count the first 200 A at 100%.
Harmonic-rich panels (lots of switch-mode supplies) often warrant an oversized neutral, sometimes 200% of the phase ampacity, even where the Code minimum would be smaller.
This is a planning aid. Confirm the final size against the applicable ampacity table, ambient and conductor-fill derates, and your local inspector.

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