What is voltage drop?

Voltage drop is the reduction in voltage that occurs as electrical current travels through the resistance of a conductor. Every wire has some resistance, so a portion of the source voltage is "lost" as heat over the length of the run, leaving less voltage available at the load. Longer runs, smaller wire gauges, and higher currents all increase voltage drop.

How is voltage drop calculated?

The standard approach multiplies the conductor's resistance per unit length by the current and by the round-trip (or three-phase equivalent) length of the circuit: VD = Multiplier × R(Ω per 1000ft) × Length(ft) ÷ 1000 × Current(A). The multiplier is 2 for DC and single-phase AC (current travels out and back) and √3 (≈1.732) for balanced three-phase AC.

What is an acceptable voltage drop percentage?

The National Electrical Code (NEC) does not mandate a maximum voltage drop for most installations, but its informational notes recommend keeping voltage drop to 3% or less on a branch circuit or feeder alone, and no more than 5% total for the combination of feeder and branch circuit conductors. These are widely followed design targets rather than strict code requirements, though some jurisdictions or specifications may enforce them.

How can I reduce voltage drop in a circuit?

The most effective options are: (1) Use a larger wire gauge (lower AWG number means lower resistance). (2) Shorten the conductor run where possible. (3) Reduce the current draw if the load allows it. (4) Increase the system voltage where feasible, since the same power at a higher voltage draws less current. (5) Use copper instead of aluminum, since copper has roughly 60% of the resistance of aluminum for a given size.

Does wire length affect voltage drop?

Yes, directly and linearly. Voltage drop is proportional to the one-way length of the conductor run — doubling the distance between the source and the load doubles the voltage drop for the same wire size and current. This is why long outdoor runs, detached garages, or well pumps often need larger wire than the ampacity rating alone would suggest.

Is voltage drop different for copper vs. aluminum wire?

Yes. Aluminum conductors have higher resistance than copper conductors of the same AWG size — roughly 1.6 times higher. This means an aluminum run needs a larger wire size than a comparable copper run to achieve the same voltage drop, in addition to any ampacity-based sizing requirements.

Does three-phase power have less voltage drop than single-phase?

For the same conductor size, current, and length, a balanced three-phase circuit has lower voltage drop than a single-phase circuit, because the three-phase voltage drop formula uses a multiplier of √3 (≈1.732) instead of 2. This is one reason three-phase distribution is often preferred for long runs and large loads.

What is the formula for voltage drop in a three-phase circuit?

For three-phase circuits: VD = √3 × R(Ω per 1000ft) × Length(ft) ÷ 1000 × Current(A). The √3 factor reflects the phase relationship between the three conductors and results in a lower drop than the equivalent single-phase calculation (which uses a factor of 2) for the same length, current, and conductor size.

Engineering

Voltage Drop Calculator

Calculate voltage drop and percentage drop across a wire run by AWG size, conductor material, length, current, and circuit type.

This calculator uses standard DC resistance values per AWG size and does not account for conduit fill derating, reactance, or skin effect on large conductors. For code-required sizing on real installations, verify with the applicable electrical code and a qualified electrician.

Circuit

Circuit type

System voltage (V)

Conductor

Material

Wire size (AWG / kcmil)

One-way circuit length

Length unit

Load current (A)

Parallel conductors per phase

Recommended max drop

Voltage drop result

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Voltage drop—

Percent drop—

Voltage at load—

Conductor resistance used—

Check vs. recommended limit—

How voltage drop is calculated

Every real conductor has resistance, so as current flows through it, some voltage is dissipated as heat before reaching the load. This calculator looks up the standard DC resistance per 1000 feet for the selected AWG size and material, scales it by the actual one-way run length and any parallel conductors, and multiplies by the load current to find the total voltage lost in the circuit.

The result is compared against your system voltage to get a percentage drop, which is then checked against the recommended maximum you select. Both DC and AC circuit types use conductor resistance directly; the multiplier differs because single-phase and DC circuits complete a round trip (out and back, factor of 2), while balanced three-phase circuits use a factor of √3 (≈1.732) due to the phase relationship between conductors.

Formula steps

Step 1 — Look up conductor resistance R (Ω per 1000 ft) for the chosen AWG size and material. Step 2 — Divide by the number of parallel conductors per phase, since paralleling reduces effective resistance: R_eff = R ÷ (parallel conductors) Step 3 — Apply the circuit multiplier: DC or single-phase AC: Multiplier = 2 Three-phase AC: Multiplier = √3 (≈1.732) Step 4 — Calculate voltage drop: VD (V) = Multiplier × R_eff × Length(ft) ÷ 1000 × I(A) Step 5 — Calculate percentage drop: %VD = (VD ÷ System voltage) × 100 Step 6 — Voltage at load = System voltage − VD

Safety derating & sizing margin

Voltage drop is only one part of safe conductor sizing — the wire must also be rated to carry the load current continuously without overheating. Two derating principles commonly apply alongside a voltage drop check:

Continuous load factor: for loads expected to run 3 hours or more, conductors and overcurrent devices are commonly sized so the continuous load does not exceed 80% of the rating (equivalently, the conductor/breaker is rated for at least 125% of the continuous load).
Ambient temperature and bundling: conductor ampacity tables assume a reference ambient temperature and a limited number of current-carrying conductors in a raceway. Higher ambient temperatures or multiple bundled conductors require additional derating factors, which can further reduce the safe current for a given wire size beyond what a simple voltage drop check would suggest.

If a calculated voltage drop is within the recommended limit but the conductor is already near its derated ampacity, upsizing the wire addresses both concerns at once and is generally the more robust choice.

Electrical code cautions

Voltage drop limits of 3% and 5% referenced by this calculator come from NEC informational notes and are widely used design targets, not universally enforced code minimums — always check the requirements that apply in your jurisdiction, utility, or project specification. Conductor ampacity, overcurrent protection sizing, and continuous-load rules should be verified against the applicable code (for example, NEC Article 210, 215, and Table 310.16 in the United States) rather than relying on a voltage drop percentage alone. This tool is intended for planning and comparison purposes; final conductor selection for any fixed wiring, feeder, or branch circuit should be confirmed by a licensed electrician or engineer.

Design limitations of this calculator

Resistance only, no reactance: for larger conductors and higher frequencies, inductive reactance becomes significant and DC resistance alone underestimates impedance. This calculator ignores reactance.
No conduit fill or bundling derating: ampacity adjustment factors for multiple conductors in a raceway or high ambient temperature are not applied here.
Standard temperature resistance values: conductor resistance used is based on a standard reference temperature and does not adjust for actual operating temperature under load.
Balanced loads assumed for three-phase: unbalanced three-phase loading is not modeled.
No connector or termination resistance: voltage drop at splices, lugs, and breaker terminals is not included.
Not an ampacity or code-compliance tool: this calculator estimates voltage drop only — it does not verify that a wire size is legally permitted to carry the specified current.

Frequently asked questions

Voltage drop is the reduction in voltage that occurs as electrical current travels through the resistance of a conductor. Every wire has some resistance, so a portion of the source voltage is "lost" as heat over the length of the run, leaving less voltage available at the load. Longer runs, smaller wire gauges, and higher currents all increase voltage drop.
The standard approach multiplies the conductor's resistance per unit length by the current and by the round-trip (or three-phase equivalent) length of the circuit: VD = Multiplier × R(Ω per 1000ft) × Length(ft) ÷ 1000 × Current(A). The multiplier is 2 for DC and single-phase AC (current travels out and back) and √3 (≈1.732) for balanced three-phase AC.
The National Electrical Code (NEC) does not mandate a maximum voltage drop for most installations, but its informational notes recommend keeping voltage drop to 3% or less on a branch circuit or feeder alone, and no more than 5% total for the combination of feeder and branch circuit conductors. These are widely followed design targets rather than strict code requirements, though some jurisdictions or specifications may enforce them.
The most effective options are: (1) Use a larger wire gauge (lower AWG number means lower resistance). (2) Shorten the conductor run where possible. (3) Reduce the current draw if the load allows it. (4) Increase the system voltage where feasible, since the same power at a higher voltage draws less current. (5) Use copper instead of aluminum, since copper has roughly 60% of the resistance of aluminum for a given size.
Yes, directly and linearly. Voltage drop is proportional to the one-way length of the conductor run — doubling the distance between the source and the load doubles the voltage drop for the same wire size and current. This is why long outdoor runs, detached garages, or well pumps often need larger wire than the ampacity rating alone would suggest.
Yes. Aluminum conductors have higher resistance than copper conductors of the same AWG size — roughly 1.6 times higher. This means an aluminum run needs a larger wire size than a comparable copper run to achieve the same voltage drop, in addition to any ampacity-based sizing requirements.
For the same conductor size, current, and length, a balanced three-phase circuit has lower voltage drop than a single-phase circuit, because the three-phase voltage drop formula uses a multiplier of √3 (≈1.732) instead of 2. This is one reason three-phase distribution is often preferred for long runs and large loads.
For three-phase circuits: VD = √3 × R(Ω per 1000ft) × Length(ft) ÷ 1000 × Current(A). The √3 factor reflects the phase relationship between the three conductors and results in a lower drop than the equivalent single-phase calculation (which uses a factor of 2) for the same length, current, and conductor size.