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Neutral Conductor Sizing

Calculate minimum neutral ampacity per NEC 220.61, accounting for harmonic loads and unbalanced load reductions.
Amps

NEC 250.24(C) Rule: The grounded (neutral) conductor can never be smaller than the required grounding electrode conductor, regardless of calculated reduction.

The 70% Neutral Reduction Explained ⚡

In a balanced 3-phase system, current cancels out in the neutral. However, since few systems are perfectly balanced, we size for the "Maximum Unbalanced Load." Following NEC 220.61, we can reduce the portion of the load exceeding 200 Amps by 30%, provided the load isn't non-linear (which generates additive harmonics that actually heat up the neutral).

Required Neutral Ampacity

150A
Full load ampacity

Min. Conductor Size

1 AWG
75°C Copper (THHN/THWN)
For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →
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What is NEC 220.61 Neutral Sizing?

The neutral conductor is sized to carry the maximum unbalanced load. In specific conditions, the National Electrical Code allows for a reduction in sizing for loads exceeding 200 Amps.

Mathematical Foundation

Neutral Reduction Formula
In=200+(Iunbal−200)×0.70I_{n} = 200 + (I_{unbal} - 200) \times 0.70
InI_{n}= Required Neutral Ampacity
IunbalI_{unbal}= Calculated Unbalanced Load > 200A

Laws & Principles

  • Reduction to 70% is allowed for the portion of the load over 200 Amps (Linear loads only).
  • NO reduction is allowed for non-linear loads (harmonics from LEDs, ballasts, VFDs, PCs).
  • Harmonic currents in the neutral are additive and can exceed phase currents.
  • The neutral can never be smaller than the required grounding electrode conductor (NEC 250.24).
  • Always verify voltage drop on the neutral if the run exceeds 100 feet.

Step-by-Step Example Walkthrough

" 400 Amp unbalanced load with no harmonic presence. "

  1. 1. First 200A: sized at 100% (200A).
  2. 2. Remaining 200A: (400 - 200) * 0.70 = 140A.
  3. 3. Total: 200 + 140 = 340A required.
Final Result: 340 Amp required neutral ampacity.

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