Voltage Drop Calculator

Voltage drop is the voltage lost between the source and the load because the conductors carrying the current are not perfect. Every conductor resists the current passing through it, and pushing current through resistance costs voltage, so what arrives at the equipment is always less than what left the supply. The difference does no useful work; it leaves the circuit as heat in the cable. It is normally quoted as a percentage of the supply, because 3 V lost from 120 V matters far more than 3 V lost from 480 V.

What is voltage drop

Multiply the load current by the impedance of the run and by the multiplier for the circuit type. In practice that is five steps: look up the conductor resistance per unit length for the size and material, reduce the run to a one-way length, pick the multiplier (2 for DC and single-phase, √3 for balanced three-phase), multiply current × impedance × length × multiplier, then divide by the supply voltage for the percentage. The accuracy of the answer depends entirely on how well you know the current, the length and the conductor temperature.

How to calculate voltage drop·Worked examples

For DC and single-phase AC it is V_drop = 2 × I × Z × L. For balanced three-phase it is V_drop = √3 × I × Z × L. I is the load current in amperes, Z is the conductor impedance per unit length, and L is the one-way distance from source to load in the units Z is published in. The 2 accounts for current travelling out on one conductor and back on the other; √3 is smaller because a balanced three-phase circuit has no return conductor carrying full load current. Z is plain resistance for DC and for AC at unity power factor, and R × cos θ + X_L × sin θ below it.

Voltage drop formula

The same way as any other run, using the resistance of the cable conductors. Working in metric units, V_drop = 2 × I × (R_km ÷ 1000) × L, with R_km the conductor resistance in ohms per kilometre and L the one-way cable run in metres. Substitute √3 for the 2 on a balanced three-phase cable. One point catches people out: for a two-core cable the cable length is the one-way distance, because the second core is the return path the multiplier already accounts for.

Cable voltage drop calculator·Voltage drop in cable

Use Ohm’s law on its own: V = I × R. Multiply the current through the resistor in amperes by its resistance in ohms. A 470 Ω resistor carrying 20 mA drops 0.02 × 470 = 9.4 V. There is no length term, no conductor material and no multiplier, because you are looking at one component rather than a run of cable. Rearranged, I = V ÷ R gives the current from a known drop and R = V ÷ I gives the resistance needed to produce one. Power dissipated is P = V × I, which is what sets the wattage rating the part needs.

Resistor voltage drop calculator

Conductor resistance, and on AC a second contribution from inductive reactance. A wire is a lattice of metal atoms with electrons moving through it; the electrons collide with the lattice as they go and each collision gives up a little energy as heat. The bulk effect is resistance, and Ohm’s law turns it into a voltage difference between the two ends. On AC the alternating current also sets up a changing magnetic field around the conductor which opposes the change, adding to the drop without dissipating energy. Terminations, splices and switch contacts add resistance of their own that no calculation predicts.

What is voltage drop

Strongly, and inversely through cross-sectional area. A larger conductor gives the current more room, so resistance falls roughly in proportion to the increase in area and the drop falls with it. Each AWG step down in gauge number is about 1.26 times the area, and three steps roughly doubles it and halves the drop. A 120 V circuit carrying 20 A over 100 ft of 12 AWG copper drops 6.60%; the same run in 8 AWG copper drops 2.59%, because the conductor resistance falls from 1.98 to 0.778 Ω per 1000 ft.

Wire size calculator·Voltage drop chart

Yes, in direct proportion. Double the run and you double the drop; halve it and you halve it exactly. Resistance accumulates with every metre of conductor, so length is the one factor that scales the answer with no diminishing returns in either direction. This is why shortening a run, or moving a distribution point closer to the load, is often cheaper than upsizing the whole cable. Enter the one-way distance from source to load, not the total length of conductor there and back, or the result comes out doubled.

Maximum cable length calculator·Voltage drop in cable

Yes, and also in direct proportion. Drop and current rise together, so a circuit carrying twice the current on the same conductor drops twice the voltage. Power lost as heat behaves differently: it follows I² × R, so doubling the current quadruples the heating. This is why the drop should be calculated at the actual load current rather than at the rating of the breaker protecting the circuit, which is usually higher and will overstate the result.

Calculate voltage drop

It is the voltage lost along the conductors of a direct-current circuit, where resistance is the only thing opposing the current. There is no reactance and no power factor, which makes DC the simplest case and the one where a resistance-only calculation is exactly right rather than an approximation. It matters more than the arithmetic suggests because DC systems usually run at low voltage: losing half a volt on a 120 V circuit is 0.42% and nothing notices, while losing the same half volt on a 12 V battery circuit is 4.2%.

DC voltage drop calculator

Use V_drop = 2 × I × R × L, with I the load current, R the conductor resistance per unit length and L the one-way run. The 2 is there because current leaves along one conductor and returns along the other, meeting the resistance of the run twice. No power factor term appears, so the calculation is exact for the conditions you give it. Low-voltage DC systems also draw much higher current for the same power, and drop rises with current, so the two effects compound on battery, solar and vehicle circuits.

DC voltage drop calculator

Start from the same formula but replace resistance with effective impedance: V_drop = 2 × I × Z_e × L for single-phase, or √3 × I × Z_e × L for balanced three-phase. Below unity power factor, Z_e = R × cos θ + X_L × sin θ, where X_L is the inductive reactance of the conductor and θ is the power factor angle. That is the component of the impedance lying along the direction of the load current, not the magnitude √(R² + X²). At unity power factor sin θ is zero, reactance drops out, and the result collapses back to a resistance-only calculation.

AC voltage drop calculator

Divide the drop in volts by the supply voltage and multiply by 100: %V_drop = (V_drop ÷ V_source) × 100. A 7.92 V drop on a 120 V supply is (7.92 ÷ 120) × 100 = 6.60%, leaving 112.08 V at the load. The percentage is the figure worth working in, because it is what design targets are written against and what tells you whether a run is usable. The same volts lost mean very different things on a 12 V circuit and a 480 V one.

Calculate voltage drop percentage

In everyday use the two are interchangeable and both mean the volts that fail to reach the load. If you want to be precise, voltage drop is the potential difference measured across the conductors, in volts, while the associated power loss is what that difference costs you, in watts. Drop is measured with a voltmeter; loss is calculated as I² × R over the current-carrying conductors. Some equipment manufacturers also use "voltage loss" for the drop inside a device rather than in the wiring feeding it.

Voltage drop vs voltage loss

By reading the voltage at two points of an energised circuit while it is carrying its normal load, and taking the difference. A circuit at rest carries no current and therefore drops nothing, which is why a reading taken at an idle outlet tells you nothing useful. Measurement and calculation answer different questions: the calculation models an ideal run, while a reading includes terminations, splices, contact resistance and the real conductor temperature. Live testing carries genuine risk and belongs to people trained and equipped for it, working to the safety procedures that apply where they are.

How to measure voltage drop

A diagnostic check that looks for excessive resistance in a circuit that is under load, rather than measuring resistance directly. Because the reading is taken with current flowing, it reveals problems a resistance measurement on a dead circuit can miss: a corroded joint or a loose terminal may read near zero ohms yet drop a significant voltage once real current passes through it. Comparing the measured figure against a calculated one is what makes the result meaningful, since a reading far above the calculation points at a connection rather than an undersized conductor.

Voltage drop testing guide·Calculate the expected figure

Four things work. Increase the conductor size, since drop falls roughly in proportion to the increase in cross-sectional area. Shorten the run, which reduces it exactly in proportion. Split the load across conductors in parallel, which divides the impedance of the path by the number of sets. Or supply the load at a higher voltage, which cuts the current needed for the same power and reduces the drop twice over. Switching from aluminium to copper at the same size is a fifth option and cuts the drop by about 39%.

Wire size calculator·Which lever to pull

There is no single answer, because the size depends on the current, the run length and the supply voltage together. Work backwards instead: set the drop limit you want to hit and solve for the smallest conductor that clears it. Bear in mind that the answer has to satisfy two independent checks. Voltage drop is an engineering calculation; ampacity, the current a conductor may carry, is a separate question answered by published tables and then modified for ambient temperature, conductor bundling and terminal ratings. A size that passes one says nothing about the other.

Wire size calculator

3% on a branch circuit and 5% for feeder and branch circuit combined are the figures most often quoted. In the National Electrical Code they appear in Informational Notes to 210.19(A) and 215.2(A), which are explanatory rather than enforceable, so within the code itself they are recommendations. Some local amendments, energy codes and utility requirements make equivalent limits mandatory, and individual equipment may specify something tighter. Treat 3% and 5% as engineering targets, and check what actually applies to your installation and jurisdiction rather than assuming these figures are binding.

Where 3% and 5% come from

Yes, though the effects are usually gradual rather than dramatic. Motors draw more current at reduced voltage, start harder and run hotter, which shortens their life. Incandescent and some LED lighting dims. Resistive heating elements take longer to reach temperature because power falls with the square of the voltage. Electronics with a specified minimum input voltage may reset, behave erratically or refuse to start. Separately, the energy lost in the conductors is paid for at the meter and never delivered: a 20 A load on 100 ft of 12 AWG copper turns about 158 W into heat in the wiring.

What voltage drop does to equipment

Aluminium drops noticeably more at the same conductor size. It has roughly 61% the conductivity of copper, so about 1.6 times the resistance and about 1.6 times the drop. For example, 2 AWG copper is 0.194 Ω per 1000 ft against 0.319 Ω per 1000 ft for 2 AWG aluminium. To match a copper conductor on drop you generally need to go up about two AWG sizes in aluminium, which is why aluminium runs are physically larger for the same electrical result even though the conductor itself is lighter and cheaper.

Copper vs aluminium conductors

Conductor resistance rises with temperature, so a hot conductor drops more voltage than a cold one carrying the same current. The correction published in NEC Chapter 9, Table 8 is R₂ = R₁ × [1 + α × (T₂ − 75)], with α of 0.00323 for copper and 0.00330 for aluminium against a 75 °C reference. Going from 75 °C to 90 °C raises copper resistance by about 4.8%. The reference data used here is quoted at 75 °C, which represents a loaded conductor; calculating at room temperature because that is what a data sheet quotes will understate a cable that is actually working.

Data and assumptions

Yes, and the arithmetic does not change with distance, but long runs are where the result usually decides the design. Because drop scales directly with length, a run several times longer needs a conductor several times larger in area to hold the same percentage, and it is common for voltage drop rather than current-carrying capacity to set the size. Two other things become worth checking on long runs: the conductor temperature you assume, and on AC the reactance term, which is negligible on small conductors but not on large ones at low power factor.

Maximum cable length calculator·Cable voltage drop calculator

The multiplier. Single-phase uses 2, because the current goes out along one conductor and returns along the other, meeting the resistance of the run twice. Balanced three-phase uses √3, about 1.732, because no single conductor carries the full return current. For the same current, conductor and distance, the three-phase drop is √3 ÷ 2, or about 86.6%, of the single-phase drop, and the figure is the line-to-line voltage drop. The comparison is a little misleading in practice, since a three-phase circuit delivers considerably more power at that same current.

AC voltage drop calculator·Where 2 and √3 come from

It can tell you the smallest conductor that keeps the drop inside a limit you set, which is one of the two checks a conductor has to pass. The other is ampacity, and the two are answered from different data and can give different answers; on a long run the drop limit usually decides, while on a short heavily loaded run ampacity does. The tools here report each separately rather than merging them into a single recommendation, because doing so would obscure which constraint is binding. Neither output is a determination of code compliance.

Wire size calculator·How the calculations are made