What voltage drop is acceptable?

The NEC recommends 3% on branch circuits and 5% total.

How do I reduce voltage drop?

Upsize the wire or shorten the run.

How do I convert AWG to mm²?

12 AWG = 3.31 mm², 10 AWG = 5.26 mm², 8 AWG = 8.37 mm².

Does voltage drop waste energy?

Yes — it is lost as heat in the wire (I × VD watts).

What happens with excessive drop?

Equipment underperforms, motors run hot and lights flicker.

Does the neutral count?

Yes — that's why single-phase uses ×2.

Voltage drop calculator

System, load, voltage, run length, conductor size and material: you get voltage drop in volts and percent and the voltage at the load.

Voltage drop

2.9 %

Voltage drop6.67 V
Voltage drop (%)2.9 %
Current16 A
Voltage at the load223.33 V

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How it's calculated (visible formula)

single-phase: VD = 2 × L × I × PF ÷ (γ × A); three-phase: VD = √3 × L × I × PF ÷ (γ × A)

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Choose single-phase or three-phase.
Enter the load and voltage.
Measure one-way cable length along the real route.
Enter conductor size and material.
Work out the drop and compare with the allowed maximum.

Worked example: 1,920 W on 120 V, 82 ft of 12 AWG (3.31 mm²) copper

I = 16 A
VD = 2 × 25 × 16 ÷ (48 × 3.31) ≈ 5 V

→ 4.2% — over the NEC's 3% recommendation; go to 10 AWG.

Common mistakes

Forgetting the ×2 (out and back) on single-phase.
Using 20 °C conductivity for loaded cable.
Measuring run length on plan without drops and rises.

Pro tips

Measure along the actual route, including rises and drops.
If it's too high, upsize the wire.
On long outdoor lighting runs, voltage drop usually governs over ampacity.
Check the cumulative drop from the service too.

From calculation to quote

Add labor, overhead and your margin, and generate the professional PDF.

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FAQs

What voltage drop is acceptable?: The NEC recommends 3% on branch circuits and 5% total.
How do I reduce voltage drop?: Upsize the wire or shorten the run.
How do I convert AWG to mm²?: 12 AWG = 3.31 mm², 10 AWG = 5.26 mm², 8 AWG = 8.37 mm².
Does voltage drop waste energy?: Yes — it is lost as heat in the wire (I × VD watts).
What happens with excessive drop?: Equipment underperforms, motors run hot and lights flicker.
Does the neutral count?: Yes — that's why single-phase uses ×2.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.