Maximum cable length calculator — voltage drop limit

Size the longest run a cable can cover before voltage drop breaks your limit: enter load current, system voltage, conductor area and allowed drop, and read the maximum one-way length.

Cable & circuit

Load current

Current drawn by the load

Conductor cross-section

mm² of the conductor

System voltage

Line voltage — pick a preset below or type one

Allowed voltage drop

NEC 210.19 suggests 3% for branch circuits

Required cable length

One-way run you need to cover

Conductor material

Copper ρ ≈ 0.0172 Ω·mm²/m · aluminium ≈ 0.0282

Circuit type

V_drop = 2 × I × R × L

Maximum one-way length: 3.13m
Maximum one-way length: 10.28ft
Voltage drop per 100 m: 22.99V
Voltage drop per 100 m: 95.78%

Your 50 m run exceeds the 3.13 m limit — step up a conductor size, raise the voltage, or lower the current.

Quick note: 12 V / 24 V runs are almost always voltage-drop-limited — a 15 A, 24 V, 3% limit over copper 4 mm² reaches only about 5.6 m, far less than ampacity tables suggest.

About voltage-drop limits

NEC 210.19 recommends keeping branch-circuit drop to 3% and total drop (feeders plus branch) to 5%. At low voltages the limit bites fast: 3% of 24 V is only about 0.7 V of slack, so the allowable run collapses even when ampacity is comfortably met. Long low-voltage runs usually need a larger gauge than ampacity alone would suggest.

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Frequently asked questions

How is maximum cable length calculated from voltage drop?

The tool computes conductor resistance per kilometre as R = ρ × 1000 ÷ A, with copper at about 0.0172 Ω·mm²/m and aluminium at 0.0282. Maximum one-way length is the allowed voltage (V × drop% ÷ 100) divided by the drop factor k × current × R, where k is 2 for single-phase or √3 for three-phase.

Why is 3% the default voltage drop limit?

NEC 210.19 recommends keeping branch-circuit drop to 3% and total drop (feeder plus branch) to 5%. Because the default system voltage is 24 V, 3% leaves only about 0.7 V of slack, which is why low-voltage runs reach their maximum length very quickly.

Why do 12 V and 24 V runs need such thick conductors?

Low-voltage circuits are almost always voltage-drop-limited before ampacity. As the tool notes, a 15 A, 24 V run at a 3% limit over copper 4 mm² reaches only about 5.6 m — far less than ampacity tables suggest. Raising the voltage or lowering the current lengthens the allowable run fastest.

How it works

IC Source Direct sizes long runs by conductor resistance rather than ampacity alone. Line resistance follows R = ρ × L ÷ A with copper at ≈ 0.0172 Ω·mm²/m and aluminium at ≈ 0.0282, and the drop is 2 × I × R × L for single-phase or √3 × I × R × L for three-phase. Working the allowed drop back against these terms gives the longest one-way run the cable can cover.

IC Source Direct provides this tool for reference only.

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