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Voltage drop and wire size

Enter load, distance and installation to get the smallest conductor that meets ampacity and voltage drop, and see whether one size up pays back in lower losses.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Load and circuit
Cable and installation

For NEC, current-carrying conductors in the raceway = grouped circuits × 3 (three-phase) or × 2 (single-phase).

Cost of losses
Economic analysis
—Minimum conductor size
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Voltage drop and ampacity by size

Green bars meet both conditions: corrected ampacity and voltage drop within the limit.

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Total yearly cost by size

Cable annualized at the rate and life entered, plus Joule losses. The lowest bar is the economic size.

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Next step

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Project payback

Cash flow of the improvement: investment in year 0 and annual savings escalated at the rate entered, discounted at the MARR.

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The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Current: I = P ÷ (√3 · V · PF) three-phase; I = P ÷ (V · PF) single-phase
Ampacity: Iz = I_table × temperature factor × adjustment factor ≥ design current (× 1.25 for continuous loads)
Drop: ΔV = k · I · L · (R cos φ + X sin φ), k = √3 (three-phase) or 2 (single-phase); R = ρ20 [1 + α (Tc − 20)] ÷ S × 1.02; X = 0.08 Ω/km
Losses: P = n · (I · lf)² · R · L; total cost = cable × CRF(i, n) + losses × hours × rate

Assumptions and limits

  • Ampacities from NEC Table 310.16 (up to three current-carrying conductors, 30 °C) and the IEC 60364-5-52 tables for three loaded conductors; always confirm in the code book.
  • Short-circuit withstand, neutral and equipment grounding conductor sizing, harmonics and motor starting drop are not checked.
  • Preliminary sizing: the electrical design must be prepared by a qualified engineer and comply with the NEC as adopted locally.

Standards and technical documents

  • IEC 60364-5-52:2009: Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems
  • ABNT NBR 5410:2004: Instalações elétricas de baixa tensão
  • NFPA 70, National Electrical Code (NEC) 2023: Table 310.16, 310.15(B) and (C), Chapter 9 Table 8
  • IEC 60287-3-2: Electric cables — Calculation of the current rating — Economic optimization of power cable size
  • UNE-HD 60364-5-52:2014 e ITC-BT-19 (Reglamento Electrotécnico para Baja Tensión)

U.S. regulations

  • NFPA 70 (NEC) as adopted by state and local jurisdictions
  • OSHA 29 CFR 1910 Subpart S: electrical safety in general industry
  • NFPA 70E: electrical safety in the workplace

Frequently asked questions

What is the maximum allowed voltage drop?

The NEC informational notes in 210.19 and 215.2 recommend 3% for branch circuits or feeders and 5% overall. They are recommendations, but many specifications make them mandatory.

How do adjustment factors change the wire size?

Ambient temperatures above 30 °C (86 °F) and more than three current-carrying conductors in a raceway reduce allowable ampacity. With 4 to 6 conductors, ampacity drops to 80% of the table value.

Is it worth using a larger wire than required?

For loads that run many hours a year, yes: I²R losses fall with conductor size, and one size up often pays back in a few years. This is the economic sizing concept in IEC 60287-3-2.