What is acceptable voltage drop for a circuit?

The NEC recommends 3% or less for branch circuits and a combined total of 5% across feeders plus branch circuits. Below 3% has no perceptible effect; 3–5% may cause light dimming and motor heating; above 5% risks equipment malfunction and shortened component life.

What wire size should I use for a long run to avoid voltage drop?

For runs over 100 feet, upsize the wire by one AWG beyond what ampacity requires. A 20A circuit normally uses 12 AWG; for a 100+ foot run, use 10 AWG. Run the voltage drop calculation using the formula or calculator to confirm compliance with the NEC 3% guideline.

Does voltage drop affect 240V circuits the same way as 120V?

The voltage drop in volts is the same, but as a percentage it's half as significant. 7 volts of drop on a 120V circuit is 5.8%; on a 240V circuit it's only 2.9%. This is why 240V circuits for electric dryers, ranges, and EV chargers are less prone to drop problems over long runs.

How does wire material affect voltage drop?

Aluminum has higher resistivity than copper — about 1.64× higher. To achieve the same voltage drop, aluminum wiring must be one or two gauges larger than copper. The NEC already accounts for this in ampacity tables, but voltage drop calculations must use separate K values (12.9 for copper, 21.2 for aluminum).

Should I worry about voltage drop on short runs?

For runs under 25 feet, voltage drop is rarely significant even with standard wire gauges. Voltage drop becomes a real concern around 50 feet and beyond. Focus your calculations on subpanels, outbuildings, long garage or workshop runs, and outdoor circuits where 100+ foot runs are common.

Voltage Drop Explained: NEC Limits, Wire Sizing & Worked Examples

Learn how voltage drop is calculated, what the NEC 3% and 5% limits mean, and how wire gauge and run length affect your circuit. Includes worked examples.

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What Is Voltage Drop?

Voltage drop is the reduction in electrical voltage that occurs along a conductor due to its resistance. Every length of wire resists electron flow to some degree — the longer the run and the smaller the wire, the higher the resistance, and the greater the voltage difference between the source and the load.

A circuit that starts at 120V at the panel might deliver only 113V at the end of a long run — a 5.8% drop. This matters because electrical equipment is rated to operate within a specific voltage range. Motors run hotter, lighting dims, and sensitive electronics can malfunction when supply voltage falls too low.

NEC Voltage Drop Guidelines

The National Electrical Code (NEC) doesn't mandate a hard maximum for most circuits, but provides recommended limits through two sections:

NEC 210.19(A): Recommends keeping voltage drop on branch circuits to 3% or less
NEC 215.2(A): Recommends that the combined voltage drop across feeders and branch circuits not exceed 5%

These are recommendations, not code violations in most installations, but exceeding them is a sign of undersized wiring.

Drop Level	Effect
< 3%	No noticeable effect on equipment
3–5%	Possible dimming, motor heating
5–8%	Equipment malfunctions, shortened motor life
> 8%	Risk of equipment damage, code compliance issues

The Voltage Drop Formula

For single-phase AC circuits:

Vd = 2 × K × I × L ÷ CM

Where: - Vd = voltage drop (volts) - K = resistivity constant (12.9 for copper, 21.2 for aluminum) - I = current in amps - L = one-way conductor length in feet - CM = conductor cross-sectional area in circular mils

Alternatively, using resistance from NEC Chapter 9 Table 8:

Vd = 2 × I × R × L ÷ 1,000

Where R is the resistance in ohms per 1,000 feet for the chosen wire gauge.

Example: A 20A circuit running 100 ft of 12 AWG copper:

R for 12 AWG copper = 1.98 ohms/1,000 ft
Vd = 2 × 20 × 1.98 × 100 ÷ 1,000 = 7.92 volts (6.6% on a 120V circuit)

This exceeds 5% — you'd need to upsize to 10 AWG (R = 1.24 ohms/1,000 ft), which gives Vd = 2 × 20 × 1.24 × 100 ÷ 1,000 = 4.96 volts (4.1% — acceptable).

Wire Size vs. Voltage Drop Reference Table

Voltage drop (%) for 120V circuits at various AWG sizes and one-way run lengths, at 20A load:

AWG	25 ft	50 ft	75 ft	100 ft
14	1.6%	3.3%	4.9%	6.6%
12	1.0%	2.1%	3.1%	4.1%
10	0.6%	1.3%	1.9%	2.6%
8	0.4%	0.8%	1.2%	1.6%
6	0.3%	0.5%	0.8%	1.0%

Copper vs. Aluminum Conductors

Aluminum wiring is permitted by the NEC but requires larger gauge wire for the same current capacity:

Copper AWG	Equivalent Aluminum AWG	Ampacity
12	10	20A
10	8	30A
8	6	40A
6	4	55A

Aluminum requires anti-oxidation compound at connections and rated aluminum-compatible terminals. Residential aluminum wiring (particularly the small-gauge wiring from the 1960s–70s) is a known fire risk at connections — consult an electrician before working with it.

How the Calculator Works

The calculator takes: - Voltage (120V or 240V) - Phase (single-phase or three-phase) - Current (amps) — your load - One-way run length (feet) - Wire gauge (AWG) - Conductor material (copper or aluminum)

It outputs the voltage drop in volts, the percentage drop, and whether the result meets the NEC 3% and 5% guidelines.

Practical Rule: When to Upsize Wire

As a practical shortcut: for runs longer than 100 feet, upsize the wire by one gauge from the minimum required for ampacity. This covers most residential situations and keeps voltage drop within NEC recommendations without running the full calculation.

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Frequently Asked Questions

What is acceptable voltage drop for a circuit?: The NEC recommends 3% or less for branch circuits and a combined total of 5% across feeders plus branch circuits. Below 3% has no perceptible effect; 3–5% may cause light dimming and motor heating; above 5% risks equipment malfunction and shortened component life.
What wire size should I use for a long run to avoid voltage drop?: For runs over 100 feet, upsize the wire by one AWG beyond what ampacity requires. A 20A circuit normally uses 12 AWG; for a 100+ foot run, use 10 AWG. Run the voltage drop calculation using the formula or calculator to confirm compliance with the NEC 3% guideline.
Does voltage drop affect 240V circuits the same way as 120V?: The voltage drop in volts is the same, but as a percentage it's half as significant. 7 volts of drop on a 120V circuit is 5.8%; on a 240V circuit it's only 2.9%. This is why 240V circuits for electric dryers, ranges, and EV chargers are less prone to drop problems over long runs.
How does wire material affect voltage drop?: Aluminum has higher resistivity than copper — about 1.64× higher. To achieve the same voltage drop, aluminum wiring must be one or two gauges larger than copper. The NEC already accounts for this in ampacity tables, but voltage drop calculations must use separate K values (12.9 for copper, 21.2 for aluminum).
Should I worry about voltage drop on short runs?: For runs under 25 feet, voltage drop is rarely significant even with standard wire gauges. Voltage drop becomes a real concern around 50 feet and beyond. Focus your calculations on subpanels, outbuildings, long garage or workshop runs, and outdoor circuits where 100+ foot runs are common.

Last updated 7/28/2026