Does the NEC mandate a maximum voltage drop?

No. The NEC provides informational notes recommending 3% maximum drop on branch circuits and 5% for feeder plus branch combined, but these are not enforceable code requirements unless adopted by a spec.

What method does this calculator use?

It uses the NEC K-constant method based on Chapter 9, Table 8 conductor properties. K=12.9 for copper and K=21.2 for aluminum at 75°C. This provides a practical and commonly accepted approximation.

Is the input length one-way or round-trip?

Enter one-way length (source to load). The calculator applies the appropriate factor (2 for DC and single-phase, √3 for three-phase) to compute loop impedance for voltage drop.

Can I consider power factor and reactance?

This version focuses on the K-constant method. For projects requiring reactance and precise AC impedance, consult NEC Chapter 9 Table 9 or manufacturer data and use an impedance-based method.

Does Auto-size consider ampacity limits?

No. Auto-size selects the smallest conductor meeting your target voltage drop only. Always verify ampacity, temperature, termination ratings, and installation conditions per NEC Article 310 and related sections.

Why do copper and aluminum give different results?

Aluminum has higher electrical resistance than copper. Using the NEC K constants, aluminum (K=21.2) exhibits greater voltage drop for the same size, length, and current.

How accurate is the K-constant method?

It aligns with NEC guidance and is widely used for design approximations. For long runs, large conductors, or critical loads, consider detailed impedance calculations and coordination with equipment voltage tolerances.

NEC Voltage Drop Calculator | Engineering Tool (US NEC)

Results

Awaiting input…

Voltage Drop —

Percent Drop —

Load Voltage —

Estimated Loop Resistance —

Estimated Power Loss —

Selected/Recommended Size —

Data Source and Methodology

Authoritative Source: NFPA 70 — National Electrical Code (NEC), 2023 Edition.

Chapter 9, Table 8: Conductor Properties (resistivity and circular mil areas)
Chapter 9, Table 9: Alternating-Current Impedance and Reactance (reference for advanced methods)
Informational Notes to 210.19(A), 215.2(A), and 310 Articles regarding recommended voltage drop (3% branch; 5% feeder+branch)

Access NEC (NFPA 70) - Official site

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

For the NEC K-constant method (75°C):

DC and Single-Phase AC:
$$ V_{drop} = \frac{2 \cdot K \cdot I \cdot L}{\mathrm{CMA}} $$ Three-Phase AC:
$$ V_{drop} = \frac{\sqrt{3} \cdot K \cdot I \cdot L}{\mathrm{CMA}} $$ Percent drop and load voltage:
$$ \%V_{drop} = \frac{V_{drop}}{V_s} \times 100 \qquad V_{load} = V_s - V_{drop} $$ Minimum required circular mil area for a target drop:
$$ \mathrm{CMA}_{min} = \begin{cases} \dfrac{2 \cdot K \cdot I \cdot L}{V_s \cdot \%/100}, & \text{DC or 1-Phase} \\ \dfrac{\sqrt{3} \cdot K \cdot I \cdot L}{V_s \cdot \%/100}, & \text{3-Phase} \end{cases} $$ where K = 12.9 (copper) or 21.2 (aluminum) in ohm·circular-mil/ft, I is current (A), L is one-way length (ft), CMA is circular mil area.

Glossary of Variables

Vs (Source Voltage): The nominal supply voltage at the panel or source (V).
I (Load Current): The circuit load current in amperes (A).
L (Length): One-way conductor length from source to load (ft). The loop factor is applied by the formula.
K (Material Constant): NEC K-constant at 75°C: Copper 12.9; Aluminum 21.2 (ohm·circular-mil/ft).
CMA: Conductor area in circular mils (from AWG/kcmil size).
Vdrop: Calculated voltage drop (V).
%Vdrop: Voltage drop as a percentage of source voltage (%).
Vload: Voltage available at the load after the drop (V).
Loop Resistance: Effective round-trip resistance used by the K method (Ω).
Power Loss: I²R loss along the effective loop (W).

How It Works: A Step-by-Step Example

Scenario: 1-Phase AC, Vs = 120 V, I = 20 A, Copper, one-way length L = 150 ft, target = 3%, size = 8 AWG (CMA = 16510).

Factor for 1-Phase = 2. Copper K = 12.9.
Voltage drop: $$ V_{drop} = \frac{2 \cdot 12.9 \cdot 20 \cdot 150}{16510} \approx 4.69 \text{ V} $$
Percent drop: $$ \%V_{drop} = \frac{4.69}{120}\times 100 \approx 3.91\% $$
Load voltage: $$ V_{load} = 120 - 4.69 \approx 115.31 \text{ V} $$
Since 3.91% exceeds the 3% target, auto-size would recommend the next larger conductor until: $$ \mathrm{CMA} \ge \frac{2 \cdot 12.9 \cdot 20 \cdot 150}{120 \cdot 0.03} \approx 22{,}973 $$ The smallest common size with CMA ≥ 22,973 is 6 AWG (CMA = 26240).

Frequently Asked Questions (FAQ)

Is voltage drop a code violation?

No. The NEC provides informational notes (not enforceable requirements) recommending 3% maximum drop on branch circuits and 5% for feeders plus branches combined. Specifications may mandate these limits.

Do I enter one-way or round-trip length?

Enter one-way length. The formulas include the round-trip factor internally.

Why doesn’t the calculator require power factor?

The NEC K-constant method is based on conductor resistivity and length and does not directly use power factor. For AC impedance methods including reactance, PF influences the result.

Can I mix copper and aluminum in the same run?

Not typically in a single continuous conductor. If feeders and branches differ by material, calculate each segment separately and consider total voltage drop.

Does auto-sizing check ampacity or derating?

No. This tool focuses on voltage drop. Verify ampacity per NEC Article 310, temperature ratings, ambient conditions, bundling, and terminations separately.

How precise is the result vs. field measurements?

Results are design approximations using NEC values at 75°C. Actual conditions (temperature, conduit type, frequency effects) can vary. For critical systems, perform detailed analysis.

What if my load voltage tolerance is narrow?

Use a lower target percent drop, shorten runs, increase conductor size, or raise distribution voltage with step-down transformers to stay within equipment tolerances.

Strumento sviluppato da Ugo Candido,. Contenuti verificati da, CalcDomain Engineering Editorial Board.
Ultima revisione per l'accuratezza in data: 2025-09-14.