What is an acceptable voltage drop for a residential circuit?

The NEC recommends no more than 3% voltage drop on any single branch circuit and 5% total from the service entrance to the farthest outlet. On a 120V circuit, 3% means the voltage at the outlet should not fall below 116.4V. Most electricians design for 2-3% to leave margin for future load increases. For sensitive equipment like medical devices or server rooms, aim for 2% or less to ensure stable operation.

Does aluminum wire have more voltage drop than copper?

Yes — aluminum has about 61% of copper's conductivity, which means aluminum wire has roughly 1.6 times the resistance of copper wire in the same gauge. A 100-foot run of 10 AWG copper with 2.5% drop would have about 4.1% drop if you swapped in 10 AWG aluminum. To get equivalent voltage drop performance, use aluminum wire that is two AWG sizes larger than the copper it replaces (for example, 8 AWG aluminum instead of 10 AWG copper). Aluminum is still cost-effective for long feeder runs because the wire itself costs less per foot despite the larger size.

How does voltage drop affect LED lights and electronics?

LED drivers and switching power supplies are generally tolerant of voltage drop because they regulate their own internal voltage. A laptop charger rated 100-240V works fine with 10% drop on a 120V circuit. Simple LED strips running directly on 12V DC, though, are very sensitive — a 1V drop from 12V to 11V causes visible dimming at the far end of a long run. The solution is to either use heavier wire, inject power at both ends of the strip, or switch to a 24V system that provides more voltage headroom.

Can I reduce voltage drop without replacing the wire?

There are several approaches that avoid rewiring. First, reduce the load on the circuit by moving high-draw devices to a closer outlet or a dedicated circuit. Second, raise the voltage if the system allows it — switching a 12V DC system to 24V cuts the current in half for the same power, quartering the voltage drop. Third, for long DC runs to LED lighting, you can add a second power injection point at the far end. Fourth, for motor circuits, installing a soft starter reduces the peak current draw during startup, which is when voltage drop is worst.

Voltage Drop Calculator — Free Online

7 min read

Check the voltage drop for any wire run before you pull cable. Select your wire gauge, enter the distance and load current, and see instantly whether your wiring stays within acceptable limits. The calculator handles both copper and aluminum conductors across all standard voltages.

Voltage drop benchmarks showing 3 percent limit, 5.2V drop, and 4.3 percent result.

How to Calculate Voltage Drop Step by Step

Identify the wire gauge. Check the cable markings or your plan specs. The gauge determines the wire's resistance per foot — thicker wire (lower AWG number) has less resistance. If you haven't chosen a gauge yet, start with the minimum required by NEC ampacity tables and check if the drop is acceptable.
Measure the one-way run distance. This is the physical distance from the power source (panel, battery, charge controller) to the load. The calculator doubles it internally to account for the return path — current flows through both conductors.
Determine the load current. Use the actual expected current draw, not the breaker rating. A 20A breaker circuit may only carry 12A in practice. For motor loads, use the running amps from the nameplate, not the starting surge. If you need to convert watts to amps first, our amps draw calculator handles that conversion. For motors rated in horsepower, convert to watts first with the HP to watts calculator.
Check the result against NEC recommendations. For branch circuits, keep voltage drop under 3%. For the combined feeder-plus-branch path, stay under 5% total. If the drop exceeds your target, either increase the wire gauge (lower AWG number = thicker wire) or reduce the distance.
Consider temperature derating. Copper resistance rises about 4% for every 10°C above 20°C. Wire running through a hot attic in summer operates well above standard temperature — add a safety margin by going one gauge larger than the calculator suggests.

Voltage drop percentage over 100ft for AWG 8, 10, 12, and 14 copper wire at 20A on 120V. — At 100ft and 20A, only AWG 8 keeps voltage drop under the 3% NEC recommendation for branch circuits.

Wire Resistance by Gauge (Ohms per 1,000 Feet)

AWG	Copper (Ω/1000 ft)	Aluminum (Ω/1000 ft)	Typical Max Amps (75°C)
14	3.14	5.17	15
12	1.98	3.25	20
10	1.24	2.04	30
8	0.778	1.28	40
6	0.491	0.808	55
4	0.308	0.508	70
3	0.245	0.403	85
2	0.194	0.319	95
1	0.154	0.253	110
1/0	0.122	0.201	125
2/0	0.0967	0.159	145
3/0	0.0766	0.126	165
4/0	0.0608	0.100	195

Values are DC resistance at 25°C from NEC Chapter 9 Table 8. AC resistance is slightly higher due to skin effect, but the difference is negligible for wire sizes 4/0 and smaller at 60 Hz. Aluminum resistance is approximately 1.6x that of copper for the same gauge, which is why aluminum installations typically use wire two sizes larger than copper for equivalent performance.

NEC Voltage Drop Recommendations and Why They Matter

The NEC Article 210.19(A) Informational Note No. 4 recommends a maximum 3% voltage drop on branch circuits and 5% total for the combined feeder and branch circuit path. These are recommendations, not hard code requirements — but they exist because excessive voltage drop causes real problems.

A 120V circuit with 5% drop delivers only 114V at the outlet. Most electronics handle that fine, but motors are less forgiving. An induction motor running at reduced voltage draws proportionally more current to maintain torque. That extra current generates heat in the windings, accelerating insulation breakdown and shortening the motor's life. A well pump, compressor, or HVAC blower running on chronically low voltage may fail years before its expected service life. Starting these motors already draws 5-8 times the running current, and low voltage makes the inrush even worse.

Low-voltage DC systems face the challenge at a different scale. A 12V solar circuit with 3% drop loses only 0.36V, but that fraction represents charging current the battery never receives. Over a full day of solar production, a 3% wiring loss on a 1,000W array wastes 30W continuously — roughly 150-180 Wh per day. Over a year, that is 55-65 kWh of energy turned to heat in the wires instead of stored in batteries.

Sizing wire correctly is not just a safety measure — it is an efficiency investment that pays for itself. For long solar wire runs, our wire distance calculator recommends the minimum gauge to meet your drop target.

Worked Examples

120V Kitchen Circuit Voltage Drop Check

Context

You are wiring a kitchen remodel with a dedicated 20A circuit for countertop outlets. The panel is 75 feet from the farthest outlet. You plan to use 12 AWG copper (standard for 20A circuits). Does the voltage drop stay under 3%?

Calculation

Wire resistance: 12 AWG copper = 1.98 Ω per 1,000 ft

Round-trip distance: 75 ft x 2 = 150 ft

Total resistance: (1.98 / 1,000) x 150 = 0.297 Ω

Voltage drop: 20 A x 0.297 Ω = 5.94 V

Drop percentage: 5.94 / 120 = 4.95%

Interpretation

At 4.95%, this circuit nearly hits the 5% total limit with the branch circuit alone — leaving zero margin for feeder drop. The outlets at the far end would receive only 114V under full load. Appliances like toasters and microwaves that draw close to 20A would underperform noticeably.

Takeaway

Upgrade to 10 AWG copper. At 1.24 Ω/1,000 ft, the drop falls to 3.72V (3.1%) — within the 3% branch circuit recommendation with minimal margin. If your loads typically draw less than 15A, 12 AWG is technically adequate, but the upgrade costs only $30-40 more in wire. To add up all loads on the circuit and see the total draw, use our electrical load calculator.

12V Solar Panel Run to Charge Controller

Context

You have a pair of 200W solar panels mounted 30 feet from your charge controller. The panels produce up to 30A at 12V nominal (Vmp ~18V, Imp ~11A per panel, wired in parallel). You are using 10 AWG copper wire.

Calculation

Wire resistance: 10 AWG copper = 1.24 Ω per 1,000 ft

Round-trip distance: 30 ft x 2 = 60 ft

Total resistance: (1.24 / 1,000) x 60 = 0.0744 Ω

Voltage drop at 30A: 30 x 0.0744 = 2.23 V

Drop percentage at 12V nominal: 2.23 / 12 = 18.6%

Drop percentage at Vmp 18V: 2.23 / 18 = 12.4%

Interpretation

Even at the panels' maximum power voltage (18V), a 12.4% drop is severe. At 12V nominal, 18.6% drop means the charge controller receives only 9.77V — too low to charge a 12V battery. This is a common mistake with low-voltage solar installations over even moderate distances.

Takeaway

Wire the panels in series for 24V (or use a higher voltage configuration) and use an MPPT charge controller. At 24V and 15A (same power, half the current), the drop on 10 AWG falls to a manageable 4.7%. To estimate your panels' actual daily output after accounting for losses, use our solar panel output calculator.

Frequently Asked Questions

Glossary

Circular Mils

A unit of wire cross-sectional area equal to the area of a circle with a diameter of one mil (0.001 inch). Larger circular mil values mean thicker wire with lower resistance. The AWG system is derived from circular mil measurements — 4/0 AWG has 211,600 circular mils while 14 AWG has only 4,110.

Resistivity

A material's inherent resistance to electrical current flow, measured in ohm-circular mils per foot. Copper has a resistivity of 10.75 Ω·cmil/ft at 20°C; aluminum is 17.7 Ω·cmil/ft — about 64% more resistive. Resistivity is a fixed property of the metal; resistance depends on resistivity plus the wire's length and cross-section.

Branch Circuit

The portion of a wiring system between the final overcurrent device (breaker or fuse) and the outlets or equipment it serves. NEC recommends no more than 3% voltage drop on a branch circuit. A typical 15A or 20A residential circuit running from the panel to a room's outlets is a branch circuit.

Designing a solar battery bank and need to size the wire runs between panels, charge controller, and batteries? Our solar battery bank size calculator helps you plan the full system. Try it now →

Voltage drop is one of the most overlooked factors in electrical installations. It does not trip breakers or blow fuses — it quietly wastes energy and degrades equipment performance over years. Checking the drop before pulling wire takes 30 seconds with this calculator and can save hundreds of dollars in wasted energy and premature equipment failure. For any permanent wiring at permit-required voltages, have a licensed electrician verify your wire gauge and connections. These results are theoretical estimates based on standard resistivity values at 25°C — actual performance varies with temperature, connection quality, and conductor condition. Our voltage drop guide explains the formula, walks through a shed-wiring example, and compares copper vs aluminum conductors.

Last updated: April 1, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.