Voltage drop & wire size calculator
Every conductor has resistance, so some voltage is lost as current travels from the panel to the load. On a short run that loss is negligible; on a long run, a high current, or a low-voltage system it can be large enough to dim lights, overheat cable, and stop motors and electronics from working properly. Sizing the conductor to keep that loss within an accepted limit is the point of a voltage-drop check.
This calculator works two ways. In Check mode it takes a wire size and tells you the drop on the run as volts and as a percentage, with a pass/fail against your allowable limit. In Size mode it searches the conductor catalog and returns the smallest size that keeps the drop within your target. It handles copper or aluminium, AWG or mm², and single-phase, three-phase or DC — imperial or metric, entirely in your browser.
What is voltage drop?
Voltage drop is the reduction in voltage between the supply and the load caused by the resistance of the conductors carrying the current. By Ohm's law the drop is proportional to the current, to the conductor's resistance per unit length, and to the length of the run — so it grows with a longer cable, a heavier load, or a thinner wire. Because current flows out to the load and back, the calculation uses the round-trip path, not just the one-way length.
It matters because the load sees the supply voltage minus that drop. Lights run dimmer and warmer filaments age faster; motors draw more current and run hotter to make the same power; sensitive electronics can reset or misbehave. Excess drop also wastes energy as heat in the cable itself. Codes therefore recommend keeping the drop to a few percent — most commonly 3% on a branch circuit and 5% overall — which is the target this tool checks against.
The voltage-drop formula
The drop is a direct application of Ohm's law over the run. Multiply the load current by the conductor's resistance for the round-trip path and you get the volts lost; divide by the supply voltage to express it as a percentage. The only nuances are the phase factor — 2 for single-phase and DC, √3 for three-phase — and that the resistance is tabulated per 1000 length units, so it is scaled by ÷1000.
Both modes are built from the same two transparent formulas — no black box.
Voltage drop
Vdrop = k × I × (R ÷ 1000) × L- I is the load current in amps; L is the one-way run length (ft for AWG, m for mm²).
- R is the conductor's tabulated resistance — Ω per 1000 ft for AWG, Ω per km for mm² — so it is scaled by ÷1000.
- k is the phase factor: 2 for single-phase and DC, √3 ≈ 1.732 for three-phase. It accounts for the round-trip path, so you enter the one-way length only.
Percentage drop and voltage at the load
%VD = Vdrop ÷ V × 100 · Vload = V − Vdrop- V is the system (nominal) voltage; %VD is what the pass/fail check compares to your allowable limit.
- Voltage at the load is simply the supply minus the drop — the voltage the equipment actually receives.
Conductor resistance and acceptable limits
The calculator ships a resistance value for every conductor size in each system: AWG sizes in ohms per 1000 ft (NEC Chapter 9, Table 8, at 75 °C) and mm² sizes in ohms per km (IEC 60228, at 20 °C). A smaller cross-section means a higher resistance and a larger drop, which is why sizing up a gauge is the usual fix when a run fails. Aluminium has a higher resistance than copper for the same size, so an aluminium conductor drops more voltage than the copper equivalent.
The acceptable-drop limits below are what pass/fail is measured against. They are recommendations, not always mandatory rules, and your local code or the jurisdiction that governs your installation has the final say.
Direct-current resistance for common AWG conductors, copper and aluminium, in ohms per 1000 ft (NEC Chapter 9, Table 8, 75 °C). A thinner wire has a higher resistance and drops more voltage; aluminium always sits above copper for the same size.
| Size | Copper (Ω/kft) | Aluminium (Ω/kft) | ≈ Metric |
|---|---|---|---|
| 14 AWG | 3.07 | 5.06 | ≈ 2.5 mm² |
| 12 AWG | 1.93 | 3.18 | ≈ 4 mm² |
| 10 AWG | 1.21 | 2.00 | ≈ 6 mm² |
| 8 AWG | 0.764 | 1.26 | ≈ 10 mm² |
| 6 AWG | 0.491 | 0.808 | ≈ 16 mm² |
| 4 AWG | 0.308 | 0.508 | ≈ 25 mm² |
| 2 AWG | 0.194 | 0.319 | ≈ 35 mm² |
| 1/0 AWG | 0.122 | 0.201 | ≈ 50 mm² |
| 2/0 AWG | 0.0967 | 0.159 | ≈ 70 mm² |
| 4/0 AWG | 0.0608 | 0.100 | ≈ 95–120 mm² |
| 250 kcmil | 0.0515 | 0.0847 | — |
The recommended limits the calculator checks against. 3% is both the NEC branch-circuit figure and the IEC/BS 7671 lighting figure — the single best default for either regime.
| Limit | Applies to | Code basis |
|---|---|---|
| 3% | Branch circuits; lighting circuits | NEC 210.19 (Informational Note); IEC 60364-5-52 Annex G; BS 7671 |
| 5% | Feeders; other loads; combined feeder + branch total | NEC 215.2 (Informational Note); IEC 60364-5-52; BS 7671 |
| 6% / 8% | Privately supplied installations (own transformer or generator) | IEC 60364-5-52 Annex G; BS 7671 (lighting / other uses) |
Worked example
A single-phase branch circuit at the calculator's default inputs. Your own numbers will differ once you set the material, length, current and voltage for your run — the drop gauge and the wire-size ladder update live as you type.
Copper, 12 AWG, one-way length 100 ft, load 20 A, 120 V single-phase, checked against a 3% allowable drop.
- Given
- Copper conductor, 12 AWG (R = 1.93 Ω/kft)
- One-way length 100 ft, load 20 A, 120 V, single-phase (k = 2)
- Allowable drop 3%
- Steps
- Vdrop = 2 × 20 × (1.93 ÷ 1000) × 100 = 7.72 V
- %VD = 7.72 ÷ 120 × 100 = 6.43% → fails the 3% limit
- Voltage at load = 120 − 7.72 = 112.28 V
- Sizing up: 8 AWG gives 3.06 V (2.55%) → the smallest size that passes at 3%
- Result
- 6.43% drop — fails 3%. The smallest passing copper size is 8 AWG (2.55%).
How to check or size a conductor with this calculator
- Choose units and modePick imperial (AWG, feet) or metric (mm², metres), then Check drop to test a known size or Size wire to find the minimum.
- Enter the circuitSet the conductor material, the one-way run length, the load current, the system voltage and the phase (1-phase, 3-phase or DC).
- Set the allowable dropKeep the 3% default, choose the 5% preset, or type a custom limit such as 6% for a privately supplied installation.
- Read the resultCheck shows the drop in volts and percent with a pass/fail verdict; Size returns the smallest passing conductor. The drop gauge and wire-size ladder visualise both.
Frequently asked questions
What is voltage drop?
Voltage drop is the voltage lost to conductor resistance between the supply and the load. It grows with a longer run, a higher current, or a thinner wire, and it means the equipment sees less than the nominal voltage. Keeping it within a few percent avoids dim lights, overheating cable and poor motor performance.
How much voltage drop is acceptable?
The common recommendations are 3% on a branch or lighting circuit and 5% for feeders or the combined total. These come from the NEC informational notes and IEC 60364-5-52 / BS 7671; privately supplied installations allow 6%/8%. They are recommendations rather than universal mandates, so confirm the limit your local code enforces.
Does it handle copper and aluminium?
Yes. Choose the conductor material and the calculator uses the matching resistance. Aluminium has a higher resistance than copper for the same size, so it drops more voltage — expect to size an aluminium conductor up one or more gauges compared with copper for the same run.
Does it check ampacity too?
No. This is a voltage-drop calculation only. It does not check the conductor's ampacity (current-carrying capacity), overcurrent protection, or full code compliance. A wire that passes the drop check can still be undersized for the circuit's current rating, so confirm ampacity separately against your local code.
What about single-phase, three-phase and DC?
The phase setting changes the multiplier applied to the one-way length: 2 for single-phase and DC (current out and back through two conductors), and √3 ≈ 1.732 for three-phase line-to-line. Pick the phase that matches your circuit and enter the one-way run length; the round trip is handled for you.