Voltage Drop Calculator

How this voltage drop calculator works

Voltage drop happens because every real conductor has resistance, and that resistance eats a small amount of voltage over the length of the run. The longer the wire and the higher the current, the more voltage is lost by the time it reaches the load. This calculator uses the same formula referenced in NEC Chapter 9, Table 8, and used by most professional voltage drop calculators:

Single-phase or DC circuits: Voltage drop (V) = (2 × K × I × L) ÷ CM

Three-phase circuits: Voltage drop (V) = (1.732 × K × I × L) ÷ CM

Where K is the resistivity constant of the conductor material (12.9 ohm-circular-mils per foot for copper, 21.2 for aluminum, both at roughly 75°C), I is the load current in amps, L is the one-way length of the run in feet, and CM is the cross-sectional area of the conductor in circular mils, looked up from the wire gauge you select. The factor of 2 in the single-phase formula accounts for the round trip the current makes through both the hot and the neutral (or return) conductor. The factor of 1.732 (the square root of 3) accounts for the phase relationship between conductors in a balanced three-phase circuit.

Once the voltage drop in volts is known, the calculator divides it by your entered supply voltage to get the voltage drop percentage, and subtracts it from the supply voltage to get the voltage actually available at the load. Everything above runs locally in your browser as you type, no input is sent to or stored on a server, see the privacy policy for details. For the full algebra, including how to rearrange the formula to solve for wire size or maximum distance instead, see the voltage drop formula page.

Worked examples

A few real scenarios show how the numbers move:

• 120V single-phase, 15A load, 50 ft one-way run, 12 AWG copper: about 2.96V drop (2.47%), inside the 3% branch-circuit guideline.
• 120V single-phase, 20A load, 100 ft one-way run, 12 AWG copper: about 7.90V drop (6.58%), over both the 3% and 5% guidance, this is exactly the case where moving up to 10 AWG (4.97V, 4.14%) or 8 AWG (about 2.6%) brings it back under target.
• 480V three-phase, 60A load, 250 ft one-way run, 2 AWG copper: about 5.05V drop (1.05%), comfortably inside the recommended limits even at this distance, because the three-phase factor (1.732 instead of 2) and the high supply voltage both work in its favor.
• 12V DC landscape lighting, 5A load, 100 ft one-way run, 12 AWG copper: about 1.98V drop, which is 16.46% of a 12V supply. Low-voltage runs like landscape lighting are the case where voltage drop matters most in percentage terms, because the starting voltage is so small.

NEC voltage drop recommendations (3% and 5%)

The National Electrical Code addresses voltage drop as a recommendation, not a strict numeric requirement, through an informational note rather than a mandatory rule. Informational Note No. 4 to NEC 210.19(A) suggests sizing branch-circuit conductors so the voltage drop does not exceed 3% at the farthest outlet, and that the combined voltage drop of both feeder and branch-circuit conductors together does not exceed 5%. These figures provide what the NEC describes as "reasonable efficiency of operation," not a pass/fail electrical safety limit written into the mandatory text of the Code. This calculator flags your result against both thresholds so you can see where a given wire size lands, but always check your local jurisdiction's amendments and an inspector's expectations before finalizing a real installation.

Wire gauge (AWG) to circular mils reference table

Circular mils (CM) describe a conductor's cross-sectional area and come directly from NEC Chapter 9, Table 8. Larger circular-mil values mean a thicker conductor and less resistance per foot, which is why moving up a wire gauge reduces voltage drop for the same current and distance.

The calculator above already uses this exact table internally when you pick a wire size, so you do not need to look anything up separately, this reference is here for anyone who wants to see or double-check the underlying numbers, or size a run by hand. For a maximum-distance-by-current version of this table at common branch-circuit amperages, see the full voltage drop chart page.

Why voltage drop matters

Excess voltage drop is rarely dramatic, it shows up as small, cumulative problems: incandescent and LED lighting that looks slightly dim or flickers under load, motors that run hotter than they should because they draw more current to make up for lower voltage, electronics that behave erratically near their minimum operating voltage, and heating elements that take longer to reach temperature. None of these are always dangerous by themselves, but a motor running consistently under-voltage for years will wear out its windings faster, and a long undersized run to an outbuilding, well pump or detached garage is one of the most common real-world places voltage drop becomes a genuine problem rather than a rounding error.

Voltage drop by application

The same formula applies everywhere, but which threshold matters, and how much headroom to build in, changes with what the wire is feeding:

• Branch circuits (outlets, lighting, small appliances): target the 3% guideline at the farthest device on the circuit. A long extension cord run to a garage or shed is effectively an extra length added to the branch circuit, and is a common place a previously fine circuit crosses 3%.
• Feeders (subpanels, detached garages, well pumps, outbuildings): the NEC guidance is for the feeder and branch circuit together to stay at or below 5% combined, so a feeder run alone should usually leave headroom under that, not use the whole 5% by itself.
• Motor circuits: motors are more sensitive to under-voltage than resistive loads, low voltage from excess drop makes a motor draw more current to deliver the same torque, which raises heat and shortens the life of the windings, so it's worth sizing motor feeders conservatively rather than to the bare 3% line.
• Solar and other low-voltage DC wiring: DC voltage drop uses the same single-phase-style formula (factor of 2), but panel and battery circuits often run at 12V, 24V or 48V, so the same volt-for-volt drop is a much bigger percentage than on a 120V/240V AC circuit, this is the same effect as landscape lighting above, just for solar rather than lighting.
• Extension cords and temporary power: cord and reel manufacturers publish their own voltage-drop-based amp ratings by length, this calculator's cable-length input works the same way for a cord as it does for permanent wiring, just remember a cord's conductor size is usually printed as AWG on the jacket.

How to reduce voltage drop on a run

If your calculated percentage is higher than you would like, a few changes bring it down, roughly in order of how much they typically help:

1. Move up one or more wire gauges (a lower AWG number, or a larger kcmil size) to reduce resistance for the same length.
2. Shorten the run where practical, for example by relocating a subpanel or a transformer closer to the load.
3. Increase the supply voltage where the equipment allows it (for example running 240V instead of 120V, or a higher-voltage landscape lighting transformer), since the same voltage drop in volts becomes a smaller percentage at a higher supply voltage.
4. Split a long run into shorter parallel branch circuits instead of a single long daisy chain, which is common practice for landscape and low-voltage lighting.
5. Switch from aluminum to copper conductors where practical, since copper's lower resistivity constant (12.9 versus 21.2) directly reduces the calculated drop for the same gauge and length.