Loan Voltage Drop & Wire Size Calculator
One page, both questions: how much voltage will this run lose, and what gauge should it actually be? Enter load, voltage and one-way distance, and get the drop for any AWG plus the recommended gauge that satisfies both the NEC ampacity table and a 3% drop limit.
The math is the standard NEC Chapter 9 method: VD = 2 × K × I × L ÷ circular mils for single-phase (K = 12.9 copper, 21.2 aluminum, Ω·cmil/ft). Ampacity checks use NEC Table 310.16 at 75 °C (copper: 12 AWG 25 A, 10 AWG 35 A, 8 AWG 50 A, 6 AWG 65 A, 4 AWG 85 A; aluminum runs roughly two sizes larger for the same amps). The 3% / 5% figures are the NEC’s informational-note guidance for reasonable efficiency — widely treated as design limits, not letter-of-law mandates. Breaker and termination rules (including the 14/12/10 AWG overcurrent caps of 240.4(D)) still apply on top.
How it works
Voltage drop grows with current and distance and shrinks with conductor cross-section. The recommendation works the shelf in order: starting from the smallest gauge whose 75 °C ampacity covers the load, it tests each size against your drop limit and returns the first that passes both. That is why long runs upsized for drop are normal — a 100 ft run at 16 A needs 8 AWG to hold 3% at 120 V even though 12 AWG carries the amps easily. Distance hurts at 120 V four times as much as at 240 V for the same wattage (half the amps, half the percentage) — the quiet argument for 240 V circuits to outbuildings.
Formula & worked example
Formula: Single-phase voltage drop VD = 2 × K × I × L ÷ CM, with K = 12.9 (copper) or 21.2 (aluminum) Ω·cmil/ft, I in amps, L one-way feet, CM the conductor’s circular mils (12 AWG 6,530; 10 AWG 10,380; 8 AWG 16,510; 6 AWG 26,240). Percent = VD ÷ circuit volts × 100. Recommended gauge = smallest size passing both NEC Table 310.16 (75 °C) ampacity and your drop limit.
Worked example: 16 A, 120 V, 100 ft on 12 AWG copper: VD = 6.32 V = 5.27% — and the recommended gauge for a 3% limit is 8 AWG.
When to use this
- You are wiring a garage, barn, shed or dock beyond the house wall.
- A well pump, compressor or EV charger sits far from the panel.
- Lights dim when a motor starts and you want the cause quantified.
- You are choosing between copper and aluminum feeders on price.
How to use it, step by step
- Enter the load’s actual running amps (nameplate, not breaker size).
- Pick the circuit voltage and measure the one-way run honestly — conduit path, not straight-line.
- Check your intended gauge; read the drop volts and percent.
- Read the recommended gauge for your chosen limit — that is the buy decision.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Load amps | Running current of the load | 16 A space-heater circuit; 32 A EV charger at 240 V |
| One-way distance | Panel to load along the actual route, feet | 50–150 ft outbuilding runs |
| Gauge | AWG size to test; recommendation searches all sizes | 12 AWG branch standard; 6–8 AWG long runs |
| Material / limit | Copper vs aluminum constants; drop ceiling for the recommendation | Cu K 12.9; Al K 21.2; limit 3% typical |
Another worked example
A 32 A, 240 V load at 150 ft: on 8 AWG copper the drop is 7.50 V = 3.13% — close, but over a strict 3% line, so the recommendation is 6 AWG. Relax the limit to the 5% feeder-plus-branch guidance and 10 AWG stands instead (35 A ampacity, 4.97% drop). Same run, answers two sizes apart — the limit is the decision, the math just enforces it.
Frequently asked questions
How much voltage drop on 100 ft of 12 AWG at 16 amps, 120 V?
VD = 2 × 12.9 × 16 × 100 ÷ 6,530 cmil = 6.32 V, which is 5.27% — over the 3% guidance. The amps are fine on 12 AWG; the distance is the problem. Holding 3% on this run takes 8 AWG. This is the single most common surprise in garage and outbuilding wiring.
What gauge do I need for 32 amps at 240 V over 150 ft?
On 8 AWG the drop is 7.50 V = 3.13% — just over a 3% limit — so the recommendation steps up to 6 AWG. At a 5% limit, even 10 AWG passes: its ampacity (35 A) covers the load and its drop is 4.97%, so 10 AWG is the recommended size there. The limit you choose is a design decision: motors and heaters tolerate 5%; electronics and long-term efficiency argue for 3%.
Why does 240 V halve the problem twice?
Percentage drop depends on amps and volts together. The same 3,840 W load draws 32 A at 120 V but 16 A at 240 V, and the percentage is computed against twice the voltage: the drop percentage falls to a quarter. Whenever a load can be served at 240 V over distance — well pumps, shop tools, EV chargers — it should be.
Copper or aluminum for a long feeder?
Aluminum’s K is 21.2 against copper’s 12.9, so an aluminum conductor needs about 64% more circular mils — roughly two AWG sizes — for the same drop, and its ampacity per size is lower. It is also much cheaper per amp-foot at feeder sizes, which is why service feeders are aluminum and branch circuits are copper. Terminations must be AL-rated and torqued to spec; that is an installer discipline item, not a math one.
Is 3% a code requirement?
It is an informational note in the NEC (210.19 and 215.2) recommending design so that branch-circuit drop stays under 3% and feeder-plus-branch under 5% "for reasonable efficiency of operation." Many specs and inspectors treat it as a limit; the enforceable code governs ampacity and protection. Design to 3% on branch runs and nobody argues.
What happens if I ignore voltage drop?
Motors run hot and start hard (a pump at 105 V works measurably harder), heaters under-deliver (output falls with the square of voltage: 5% drop ≈ 10% less heat), lights dim on load steps, and electronics complain. The wire itself also wastes the dropped energy as heat along the run — you pay for volts your tools never see.
How do I handle a run longer than the table likes?
Upsize the conductor (this tool’s recommendation), raise the voltage (240 V or a transformer), shorten the route, split the load across two circuits, or move the source (a subpanel fed by a properly sized feeder, with drops budgeted in two legs that must sum under your limit). Real designs often combine the first two.