Cable Sizing Calculator

Size copper or aluminum cables by allowable voltage drop or check voltage drop for an existing cable in single- or three-phase systems.

Cable Inputs

Engineering Guide

Cable Sizing Calculator

Use this guide with the Cable Sizing Calculator to pick conductor sizes that satisfy current capacity, voltage-drop limits, and basic code assumptions—without over-sizing so much that you waste copper or aluminum.

10–15 min read Updated 2025

Quick Start

This section mirrors how the Cable Sizing Calculator is laid out. Work through these steps top-to-bottom and you will usually get a safe, efficient conductor size on the first try.

  1. 1 Select the system type that matches your circuit: DC, single-phase AC, or three-phase AC. The calculator updates the voltage-drop equation accordingly.
  2. 2 Set the nominal system voltage (for example 120 V, 230 V, 277 V, 400 V, 480 V). Use the line-to-line voltage for three-phase systems.
  3. 3 Enter the load current \(I_{\text{load}}\) in amperes. For motors or variable loads, use the design current rather than momentary inrush.
  4. 4 Enter the one-way cable length \(L\). The calculator handles the “there and back” path for single-phase and DC where appropriate.
  5. 5 Choose the conductor material (copper or aluminum) and installation type (system/layout). These affect the resistance, reactance, and allowable current.
  6. 6 Set the allowable voltage drop \(\%V_D\). Typical design limits are 3 % for branch circuits and 5 % end-to-end, but always check your local code and project specs.
  7. 7 Use the “variable to solve for” selector to choose whether you are sizing the cable, checking the voltage drop, or evaluating the drop in percent. Run the calculation and review the result, quick stats, and step-by-step breakdown.

Tip: Start with voltage-drop-based sizing, then cross-check that the chosen cable’s ampacity is still greater than \(I_{\text{load}}\) after derating for grouping, ambient temperature, and installation method.

Warning: This calculator is an engineering aid, not a substitute for mandatory national or local electrical codes. Always verify final selections against the applicable standard (for example NEC, IEC, or utility design rules).

Choosing Your Method

Cable sizing usually comes down to two independent checks: current-carrying capacity and voltage drop. The calculator focuses on the voltage-drop side but is structured so that you can loop back and confirm ampacity from tables.

Method A — Voltage-Drop Driven Sizing

Use this approach when loads are sensitive to undervoltage (motors, controls, lighting) or runs are long.

  • Makes long feeder runs practical without guesswork.
  • Directly reflects owner/specifier limits on \(\%V_D\).
  • Works for DC, single-phase, and three-phase with the same structure.
  • Requires resistance (and sometimes reactance) data for the chosen cable size.
  • May produce a cable larger than the minimum ampacity-based size.
\[ \%V_D = \frac{V_D}{V_{\text{nom}}}\times 100 \]

Method B — Ampacity-First, Then Check Drop

Here you pick a cable size from ampacity tables, then use the calculator to see if voltage drop is still acceptable.

  • Lines up directly with code ampacity tables.
  • Simple for short runs or small loads where drop is rarely critical.
  • Can under-estimate cable size on long runs if you forget the drop check.
  • Requires more trial-and-error for very long feeders.
\[ I_{\text{design}} = \frac{I_{\text{load}}}{C_g\,C_t\,C_o} \]

Method C — Verification Mode

Use the “solve for voltage drop” or “solve for \(\%V_D\)” modes when you already know the cable size and just want to check performance.

  • Great for sanity-checking existing installations.
  • Helps compare copper vs aluminum or alternate routes.
  • Does not by itself confirm short-circuit withstand or protection coordination.
\[ V_D = \begin{cases} 2 I L R_{\text{loop}}, & \text{DC or single-phase two-wire} \\ \sqrt{3}\, I L Z_{\text{line}}, & \text{three-phase} \end{cases} \]

What Moves the Number

The Cable Sizing Calculator uses standard voltage-drop formulae internally. Understanding the main variables helps you interpret why a small change in inputs can shift the recommended cable size by one or two steps.

Circuit length \(L\)

Voltage drop is proportional to length. Doubling \(L\) roughly doubles \(V_D\), all else equal. Very long feeders often drive cable size more than current does.

Load current \(I\)

The drop is directly proportional to current. High-demand circuits (large motors, EV chargers, distribution feeders) quickly push you into larger conductors.

System voltage & phase

For the same kW, higher voltage circuits draw less current. Three-phase systems also use the \(\sqrt{3}\) form of the equation, which changes the effective drop compared with single-phase.

Conductor material

Copper has lower resistivity than aluminum. For the same cross-section, aluminum experiences more voltage drop, so the calculator usually suggests a larger area for aluminum on long runs.

Impedance \(R\) and \(X\)

AC circuits, especially three-phase, may need both resistance and reactance. The calculator combines them into an effective line impedance \(Z_{\text{line}}\) when needed.

Allowable voltage drop \(\%V_D\)

Tightening the limit from 5 % to 3 % forces a larger cable or higher operating voltage. Looser limits save material but may reduce equipment performance.

Worked Examples

Example 1 — Single-Phase Feeder to a Motor

  • System: 240 V single-phase AC
  • Load: 40 A motor circuit (continuous)
  • Length: 50 m one-way (panel to motor)
  • Material: Copper cable
  • Target voltage drop: \(\%V_D = 3\%\)
1

Compute the maximum allowed voltage drop:

\[ V_{D,\max} = \frac{\%V_D}{100} \, V_{\text{nom}} = 0.03 \times 240 = 7.2\ \text{V} \]

2

Use the single-phase voltage-drop equation with loop resistance \(R_{\text{loop}}\):

\[ V_D = 2 I L R_{\text{loop}} \]

Rearranged for the allowable loop resistance:

\[ R_{\text{loop,max}} = \frac{V_{D,\max}}{2 I L} \]

3

Substitute numbers (with \(L = 50\ \text{m}\), \(I = 40\ \text{A}\)):

\[ R_{\text{loop,max}} = \frac{7.2}{2 \times 40 \times 50} = \frac{7.2}{4000} = 0.0018\ \Omega/\text{m} \]

Any cable with loop resistance lower than this value will satisfy the drop limit.

4

In the calculator, you select candidate copper sizes and let it use tabulated \(R\) to compute the actual drop. When you reach a size where \(V_D \leq 7.2\ \text{V}\) and the ampacity still exceeds the derated load current, that is your minimum acceptable size.

Example 2 — Three-Phase Aluminum Feeder

  • System: 400 V three-phase, 50 Hz
  • Load: 120 A distribution feeder, power factor \(\cos\varphi = 0.9\)
  • Length: 80 m one-way
  • Material: Aluminum cable
  • Target voltage drop: \(\%V_D = 4\%\)
1

Compute the maximum allowable voltage drop:

\[ V_{D,\max} = 0.04 \times 400 = 16\ \text{V} \]

2

For three-phase AC, a common form of the equation is:

\[ V_D = \sqrt{3}\, I L \bigl( R_{\text{line}} \cos\varphi + X_{\text{line}} \sin\varphi \bigr) \]

Here \(R_{\text{line}}\) and \(X_{\text{line}}\) are per-metre resistance and reactance for the selected aluminum cable.

3

In the calculator, choose the aluminum material and system type “three-phase,” enter \(I\), \(L\), \(\cos\varphi\), and your voltage-drop limit. The script pulls the appropriate \(R\) and \(X\) values and computes \(V_D\) and \(\%V_D\).

If \(\%V_D\) is larger than 4 %, increase the cable size and recompute.

4

Once the chosen size satisfies both the drop limit and the derated ampacity, document the result and attach the calculator output to your design notes.

Common Layouts & Variations

Different installations (buried, duct banks, tray, single cores) affect both ampacity and impedance. The table below gives qualitative guidance for how typical layouts influence cable sizing decisions.

ConfigurationTypical UseVoltage-Drop ConsiderationsPros / Cons
Cu, three-core cable in trayShort industrial feeders, MCCs, indoor runsLow resistance, modest reactance. Often drop is not critical for < 30 m runs.Excellent performance but higher material cost.
Al, single-core cables in trefoilMedium-current three-phase feedersHigher resistance than copper; watch drop on longer runs > 50–70 m.Cost-effective, lighter; needs correct terminations and corrosion management.
Buried direct in soilOutdoor feeders, utility lateralsSoil thermal resistivity affects ampacity; impedance remains similar for sizing.Good aesthetics and protection, but repairs require excavation.
Duct bank with multiple circuitsCampus or plant distributionMutual heating and coupling increase derating. Voltage drop can grow as you group more circuits.Highly organized, but design is sensitive to installation details.
Long feeder to remote buildingOutbuildings, EV chargers, pumpsLength dominates; voltage-drop sizing almost always controls conductor size.Use higher system voltages where allowed to keep cable sizes manageable.
  • Confirm whether the design standard specifies a maximum \(\%V_D\) per feeder or for the whole path.
  • Check if neutral and protective conductors need separate sizing checks.
  • Account for future load growth—especially for feeders to panels or EV infrastructure.
  • Coordinate cable sizing with protection settings and breaker selection.
  • Verify short-circuit withstand and thermal limits for the chosen cable.
  • Document assumptions for soil, ambient temperature, and grouping factors.

Specs, Logistics & Sanity Checks

A correct cross-section is only part of a good cable design. Use the calculator outputs as one piece of a broader technical specification.

Specification Checklist

  • Rated voltage and insulation class (for example 0.6/1 kV).
  • Conductor material and class (Cu/Al, stranded, flexibility requirements).
  • Number of cores, screen/armour type, and jacket material.
  • Installation method and route (tray, buried, duct bank, ladder).
  • Design ambient temperature and grouping factors used for derating.

Using the Calculator in Practice

  • Run at least two scenarios: one for current-carrying capacity, one for voltage drop.
  • Capture the calculator’s steps output as a design record for peer review.
  • Test “what-if” cases—changing material, length, or \(\%V_D\) to see sensitivity.
  • Round up to the next standard cable size offered by your supplier.

Field & Commissioning Checks

  • Measure end-to-end resistance and compare against design values.
  • Verify terminations, lugs, and glands match cable material and size.
  • Confirm that breakers, fuses, and relays are set according to the final cable choice.
  • During commissioning, log actual operating voltage at the load during full-load tests.

As a last step, sanity-check the calculator’s recommended cable size against similar projects. If the result looks unusually small or large, revisit assumptions about length, current, and design limits before locking in the design.

Frequently Asked Questions

What inputs do I need for the Cable Sizing Calculator?
You need at least the system type (DC, single-phase, or three-phase), nominal voltage, load current, one-way circuit length, conductor material, and either a target cable size or an allowable voltage drop. For more advanced checks you may also include power factor, installation method, and design ambient temperature.
What is a good voltage drop percentage to use?
Common design guides suggest keeping individual branch circuits within about 3 % voltage drop and the total path (service plus feeders plus branch) within about 5 %. However, some sensitive loads or long rural feeders may require tighter limits. Always follow the limits in your governing code or project specification.
Does the Cable Sizing Calculator replace electrical code calculations?
No. The calculator is a design and teaching tool. It uses standard voltage-drop equations and typical impedance values, but it does not replace the requirements of NEC, IEC, or local regulations. You still need to verify ampacity, short-circuit ratings, and protective device coordination directly against the applicable standard.
How do temperature and grouping affect cable size?
Higher ambient temperatures and grouped circuits reduce the allowable current in each cable. In practice you apply correction factors for temperature, grouping, and installation method, then size the cable so the derated ampacity is still higher than the design load current. If the derated ampacity becomes marginal, the calculator will typically recommend moving up one conductor size.
What is the difference between sizing by ampacity and sizing by voltage drop?
Ampacity-based sizing ensures the cable can carry the current without overheating, while voltage-drop-based sizing keeps the delivered voltage within acceptable limits at the load. Short circuits are often limited by ampacity alone, but long feeders are usually controlled by voltage drop. In a complete design you check both and choose the larger cable size that satisfies every requirement.
Can I use aluminum instead of copper for long runs?
Yes, aluminum can be a cost-effective choice, especially for large cross-sections. However, aluminum has higher resistivity than copper, so for the same current and length you typically need a larger cross-section to meet the same voltage-drop target. The Cable Sizing Calculator lets you switch materials and immediately see the impact on recommended size and voltage drop.
How accurate are the results from the Cable Sizing Calculator?
For typical low-voltage engineering work, the results are usually within a small margin of detailed hand calculations, assuming your inputs match the actual installation. Real-world differences in cable construction, temperature, soil conditions, and terminations can shift performance slightly, so treat the calculator as a design tool rather than a guarantee. When in doubt, verify against manufacturer data and applicable standards.
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