Voltage Drop Calculation Explained: Formula, Examples and Why 3% Matters

A voltage drop calculation tells you how much voltage a wire loses between the source and the load. Lose too much and inverters shut off at low voltage, motors run hot, and chargers never finish. The math is just Ohm’s law applied to the wire, and you can do it with a calculator and one table.

Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.

Voltage Drop Calculation Formula

Vdrop = 2 × L × I × R ÷ 1,000

• L = one-way length in feet (the 2 covers the round trip)
• I = current in amps
• R = conductor resistance in ohms per 1,000ft (NEC Chapter 9, Table 8)

Percent drop = Vdrop ÷ system voltage × 100

For 240V loads on two hots, the formula is the same: both conductors carry the current.

Example 1: 12V Fridge (Illustrative)

5A fridge, 15ft from the battery, 12 AWG (1.98Ω/1,000ft):

Vdrop = 2 × 15 × 5 × 1.98 ÷ 1,000 = 0.30V, which is 2.5% of 12V. That’s acceptable.

Example 2: 12V Inverter (Illustrative)

150A inverter, 6ft away, 2/0 (0.0967Ω/1,000ft):

Vdrop = 2 × 6 × 150 × 0.0967 ÷ 1,000 = 0.17V, about 1.5%. On 4 AWG (0.308) it would be 0.55V, about 4.6%, which is too much for a high-current inverter feed.

Example 3: 120V Shed Circuit (Illustrative)

15A over 100ft on 12 AWG: Vdrop = 2 × 100 × 15 × 1.98 ÷ 1,000 = 5.9V, about 4.95%. Going to 10 AWG (1.24) brings it to 3.7V, about 3.1%.

Where 3% Comes From

The NEC doesn’t require a specific voltage drop for most circuits. Informational notes in Articles 210.19 and 215.2 suggest sizing to keep drop to about 3% on a branch circuit and 5% total (feeder plus branch) for “reasonable efficiency of operation.” For low-voltage DC, many manufacturers and the ABYC marine standard use 3% for critical loads like electronics and inverters, and up to 10% for non-critical loads like some lighting. Follow your equipment manual.

How to Fix Excess Drop

Fix	Effect
Thicker wire	Lower resistance, so less drop
Shorter run	Drop scales with length
Higher system voltage	Less current for the same power
Move the battery or inverter	Shortens the high-current DC side

Measuring It

With the load running, measure voltage at the battery terminals and at the device terminals. The difference is your actual drop, connections included. Corroded lugs and loose terminals often add more drop than the wire itself.

Quick Formula Reference

Voltage drop (V) = current (A) × round-trip length (ft) × resistance (Ω per 1,000ft) ÷ 1,000.

Percent drop = voltage drop ÷ system voltage × 100.

Copper AWG (stranded)	Ω per 1,000ft (NEC Ch. 9 Table 8)
14	3.14
12	1.98
10	1.24
8	0.778
6	0.491
4	0.308
2	0.194
1/0	0.122
2/0	0.0967
4/0	0.0608

Worked Example: Solar Panels to a Charge Controller

Illustrative inputs: two panels in series at 37V and 10.5A each at maximum power (check your panel label), 40ft one-way to the controller, 10 AWG PV wire.

Step	Math	Result
String voltage	37 × 2	74V
Round trip	40 × 2	80ft
Voltage drop	10.5 × 80 × 1.24 ÷ 1,000	about 1.04V
Percent	1.04 ÷ 74	about 1.4%
Same panels in parallel (37V, 21A)	21 × 80 × 1.24 ÷ 1,000 ÷ 37	about 5.6%

Wiring in series kept the drop low on the same wire. That’s a big reason long panel runs use higher-voltage strings, within the controller’s voltage limit. See series vs parallel wiring and solar panel specs explained.

Percent Drop Changes With System Voltage

The same 1V lost is:

• 8.3% of 12V
• 4.2% of 24V
• 2.1% of 48V
• 0.8% of 120V

That’s why the wire gauge chart for 12V, 24V and 48V lists shorter maximum runs at lower voltages.

Where Drop Matters Most

• Inverter cables: high current; drop can trip low-voltage cutoff under surge.
• Charger output: drop makes the charger think the battery is fuller than it is.
• Long 120V runs to sheds and pumps: motors draw more current when voltage sags.
• LED lighting on 12V: brightness can visibly fall at the end of a long run.

Aluminum Wire

Aluminum has about 1.6 times the resistance of copper for the same gauge (compare NEC Table 8 columns), so it needs to be about two sizes larger. It’s common for long service and feeder runs, and it requires terminals rated for aluminum.

FAQ

What is an acceptable voltage drop?

Commonly 3% for branch circuits and critical DC loads, and 5% total for feeder plus branch, per NEC informational notes. Check your equipment’s requirements.

Does voltage drop waste power?

Yes. The lost power is Vdrop × I, turned into heat in the wire.

Does temperature change voltage drop?

Yes. Copper resistance rises as it heats, and the NEC table values are at 75°C.

What voltage drop is acceptable on 12V?

NEC informational notes suggest 3% on a branch circuit and 5% total; many DC designers aim for 3% or less on critical runs.

Does voltage drop change with load?

Yes. Drop is proportional to current, so it’s worst at full load.

Sources: NFPA 70 (NEC) 2023, 210.19 and 215.2 informational notes, Chapter 9 Table 8; ABYC E-11 (marine DC); NFPA 70 (NEC) Chapter 9 Table 8 and 210.19 informational note. Accessed Oct 2026.

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