Voltage Drop Calculator

How voltage drop is calculated

Current flowing through a conductor's resistance uses up some of the source voltage before it reaches the load. The more current a load draws and the longer or thinner the cable, the more voltage is lost in the wire itself, so the load receives less voltage than the supply puts out. This calculator's DC mode runs that relationship directly; AC single-phase and three-phase modes use the same underlying idea but also factor in the load's power factor and the conductor's reactance (see the AC section below).

DC voltage drop

Enter the supply voltage, the load's maximum current (or its power in watts, if you don't have the current figure), the one-way cable length, and the conductor. The calculator looks up single-conductor resistance from the AWG or mm² table (or uses a custom resistance you supply), doubles it to account for the outgoing and return legs of the circuit, and multiplies by the one-way length to get total loop resistance. Loop resistance multiplied by current gives the voltage drop; subtracting that from the supply voltage gives the voltage actually delivered at the load.

This is the same relationship that applies to any two-conductor DC circuit — a 12V or 24V camera, a fire alarm initiating device circuit, a battery-charging run, or a solar DC distribution run. Low-voltage systems are more sensitive to voltage drop than high-voltage ones because the same 1V of drop is a much larger fraction of a 12V supply than of a 120V one.

Cable size and conductor selection

AWG (American Wire Gauge) and mm² (cross-sectional area in square millimeters) are two independent sizing systems, and this calculator supports both. In AWG, a lower number means a thicker conductor with lower resistance — 12 AWG is thicker than 18 AWG. In mm², it's the reverse: a larger number is always a thicker conductor. Resistance is looked up independently from whichever system you select; the calculator does not convert an AWG size into an approximate mm² equivalent or vice versa, because manufactured cable in each system is specified and tested against its own standard, and treating the two as interchangeable can understate or overstate real resistance.

Both copper and aluminum are supported, in both AWG and mm², where the underlying resistance data has been independently verified at this calculator's 20°C reference basis. Copper has lower resistance than aluminum at the same size, so a copper conductor shows less drop than an aluminum one of the same size, length, and current. AWG aluminum uses NIST Handbook 109's EC-0 resistivity constant derived across the standard AWG size progression (see the assumptions section below); it is a derived dataset, not a transcription of a published per-size table, and is documented as such.

If your cable's datasheet gives a resistance different from the standard tables — for a specific manufacturer's construction, for example — switch the conductor system to Custom resistance and enter it directly. Custom resistance is always resistance per unit length for one conductor, never the total resistance of the run, and you choose whether your figure is in ohms per 1000 feet or ohms per kilometer; the calculator converts both to a common internal unit before calculating, so either entry method gives the same result for the same physical cable.

AC voltage drop

AC Single Phase and AC Three Phase modes account for both conductor resistance (R) and inductive reactance (X), combined through the circuit's power factor (PF), since an AC load's current and voltage are not necessarily in phase. The reference data is NEC (NFPA 70) Chapter 9, Table 9 — “Alternating-Current Resistance and Reactance for 600-Volt Cables, 3-Phase, 60 Hz, 75°C — Three Single Conductors in Conduit.” That basis is fixed and disclosed on screen whenever an AC mode is selected: 60 Hz only, 75°C, stranded conductors in conduit — it is not the same temperature or construction as the 20°C solid-wire DC tables above, so AC and DC results for the same nominal size are not directly comparable.

Conduit matters for AC. Steel conduit is magnetic and raises reactance measurably compared to PVC or aluminum conduit, so this calculator requires you to pick a conduit type rather than assuming one — there is no single universal AC reactance value for a given wire size. Conductor coverage is narrower than DC: NEC Table 9 has no data below 14 AWG, and aluminum has no 14 AWG row at all (14 AWG aluminum conductor is not manufactured per NEC Table 10), so AC mode's size list is 14 AWG through 4/0, with 12 AWG through 4/0 for aluminum. AC mode requires the AWG conductor system; mm² and custom resistance don't have a verified AC dataset, so they're not offered while an AC mode is selected. 50 Hz is not supported. No verified reactance dataset was found for 50 Hz at the same conductor construction and installation basis as Table 9, and scaling the 60 Hz reactance by 50/60 is not something this calculator's sources support — rather than show an unverified approximation, AC stays 60 Hz only, and the page says so on screen.

How much voltage drop is acceptable

There is no single universal answer. 3% and 5% are commonly used as planning figures in various contexts — 5% is often cited as a general guideline for a full circuit, 3% for a stricter split or for more voltage-sensitive equipment — but neither is a blanket electrical code requirement for every circuit. The correct limit for a specific job depends on the applicable standard, the equipment's own tolerance, and the system design. This calculator lets you set 3%, 5%, or any custom percentage, and separately lets you enter a minimum or target end voltage when you know it from the equipment's datasheet — that direct figure is a stricter and more accurate check than any percentage guideline.

Maximum cable length

Maximum Cable Length mode runs the same relationship backward: instead of computing drop from a known length, it solves for the one-way length at which the drop would first reach your allowable percentage, your entered minimum end voltage, or both. Because voltage drop is directly proportional to length in this formula, solving for length is a direct calculation, not an estimate. If you supply both a percentage and a minimum voltage, the calculator shows both limits separately and reports which one is actually more restrictive for your inputs, rather than silently combining them into one number.

Thinnest passing cable size

Thinnest Passing Cable Size mode checks each available conductor size, from thinnest to thickest, and reports the first one whose calculated drop passes your selected limit, along with the next thicker size for comparison. It's called "thinnest," not "smallest AWG," because a smaller AWG number is actually a thicker conductor — the thinnest passing size is the one with the highest AWG number (or the smallest mm² figure) that still meets your limit. If none of the available sizes pass, the calculator says so directly rather than guessing at an unsupported larger size.

CCTV and ELV applications

CCTV cameras, fire alarm circuits, access control equipment, and other extra-low-voltage (ELV) systems are all ordinary DC circuits from a voltage-drop standpoint, and this calculator applies to all of them the same way.

12V and 24V CCTV cameras are especially sensitive to voltage drop because the supply voltage is small to begin with. Many cameras draw noticeably more current at night, when IR illuminators (and sometimes heaters) switch on. A run checked only against a camera's idle daytime current can look fine and still fail after dark, once the added IR current pushes the actual drop past what was verified. Always enter the camera's maximum operating current, including IR and any other auxiliary load, not a typical or idle figure.

Fire alarm and access control circuits commonly apply their own voltage and current requirements from the specific panel or device manufacturer, which can be stricter than a general planning percentage. Check the applicable documentation for the actual system before relying on a default threshold. Other low-voltage DC systems — battery charging runs, solar DC distribution, general signaling circuits — use the same loop-resistance model shown above. Always verify actual equipment requirements against the manufacturer's own documentation rather than relying on this calculator's defaults alone.

Worked examples

Illustrative only. Your actual result depends on your own supply voltage, current, length, conductor, and limits.

Example 1 — 12V CCTV camera

12V DC supply, 0.5A maximum current, 150 ft one-way run, 18 AWG copper (6.385 Ω/1000 ft), 10% allowable drop, no minimum voltage entered.

Example 2 — 24V ELV run

24V DC supply, 1A maximum current, 250 ft one-way run, 16 AWG copper (4.016 Ω/1000 ft), 10% allowable drop, no minimum voltage entered.

Assumptions and limitations

• DC uses 20°C; AC uses 75°C. DC resistance values assume nominal conductor resistance at 20°C (68°F). AC resistance and reactance come from NEC Table 9 at 75°C. Real installation temperature (hot attics, direct sun, bundled cable, conduit) can push actual resistance above either reference figure, and the two temperature bases are never mixed.
• Resistance is independently sourced per system, material, and electrical mode. AWG copper, AWG aluminum, mm² copper, mm² aluminum, and the AC NEC Table 9 dataset each come from their own reference data; none is derived from another by conversion.
• AWG aluminum (DC) is a derived dataset. It comes from NIST Handbook 109's EC-0 resistivity constant (16.782 Ω·cmil/ft @20°C) applied across the standard AWG size progression — not a transcription of a published per-size table, because NIST's own per-size aluminum table could not be reliably extracted. See the research notes for the full derivation.
• One-way length. Cable length is always the one-way distance from source to load — the formula's ×2 (single phase) or √3 (three phase) factor already accounts for the circuit's other conductor(s). Entering a there-and-back total would double-count it.
• AC is 60 Hz only. No verified reactance dataset was found for 50 Hz at the same conductor construction and installation basis as NEC Table 9, so 50 Hz is not offered rather than approximated by scaling the 60 Hz figures.
• AC conductor coverage is narrower than DC. NEC Table 9 only covers 14 AWG through 4/0 (12 AWG through 4/0 for aluminum), and only the AWG conductor system — mm² and custom resistance have no verified AC dataset and aren't offered in AC mode.
• Planning calculation, not a field measurement. Real connections, splices, and supply voltage variation are not modeled. This calculator does not certify compliance with any electrical code or standard — check the standard that actually applies to your installation.

Common mistakes

Recurring mistakes worth watching for: using the total there-and-back length instead of one-way length; assuming a percentage-based PASS means the load is guaranteed to work when its actual minimum voltage was never checked; treating an AWG size and its approximately similar mm² size as having identical real resistance; misreading AWG numbering (a smaller number is a thicker, not thinner, conductor); checking a CCTV run only against idle current and missing the night-time IR/heater increase; applying one voltage-drop percentage to every circuit without checking what the applicable standard or equipment actually requires; and treating this calculator's output as a field measurement rather than a planning estimate.

Frequently asked questions

How do you calculate DC voltage drop?

Take the resistance of one conductor at your wire size, double it to account for the outgoing and return legs of the run, and multiply by the one-way cable length to get total loop resistance. Multiply loop resistance by current to get voltage drop, then subtract that from the supply voltage to get the voltage delivered at the load. This calculator runs that exact formula for you.

What is the difference between AWG and mm²?

AWG and mm² are two independent conductor sizing systems. AWG is numbered so that a lower number means a thicker conductor — the opposite direction from mm², where a larger number is always a thicker conductor. This calculator looks up resistance from whichever system you select using its own independently sourced data, rather than converting one system's size into the other.

How much voltage drop is acceptable?

There is no single universal limit. 3% and 5% are commonly used planning figures in various contexts, but the applicable limit depends on the governing standard, the specific equipment, and the system design. When you know the load's actual minimum operating voltage, entering it directly gives a more accurate check than any percentage guideline.

How do I calculate 12V CCTV camera voltage drop?

Use DC mode with your camera's maximum current draw (including IR illuminators or heaters, not just idle current), your one-way cable length, and your wire gauge. The same loop-resistance formula used for any DC circuit applies directly to 12V and 24V camera runs.

Does this calculator support AC or three-phase circuits?

Yes. AC Single Phase and AC Three Phase modes use NEC (NFPA 70) Chapter 9 Table 9 resistance and reactance data at 60 Hz and 75°C, three single conductors in conduit, combined with your entered power factor. Coverage is 14 AWG through 4/0 (12 AWG through 4/0 for aluminum) and requires picking a conduit type, since steel conduit's magnetic reactance differs from PVC or aluminum conduit. 50 Hz is not supported — no verified reactance dataset was found for that basis, so only 60 Hz is offered.

How does cable length affect voltage drop?

Conductor resistance grows in direct proportion to length, so for the same current and wire size, doubling the one-way length doubles the loop resistance and the voltage drop. The calculator counts both the outgoing and return conductors.

Can I enter a custom cable resistance instead of a wire size?

Yes. Switch the conductor system to "Custom resistance" and enter your cable's known resistance per unit length, choosing whether your figure is in ohms per 1000 feet or ohms per kilometer. This is useful when a cable's datasheet gives a resistance that differs from the standard AWG or mm² table.

Related tools

Running CCTV cameras over PoE instead of DC? Check switch headroom with the PoE Budget Calculator. Sizing a battery for the same run? Use the Battery Backup Calculator or the Amp Hour Calculator. Planning a full CCTV install? See the CCTV Storage Calculator and the IP Camera Bandwidth Calculator.

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