Enter wire gauge, one-way distance, and load — see voltage drop percentage and whether you're inside NEC recommended limits (3% branch, 5% total).
Free PDF: voltage drop lookup tables for 14-AWG through 500-kcmil, copper and aluminum, at 120V/240V/480V — plus common EV charger and generator run examples.
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The NEC (Article 210.19(A) FPN #4) doesn't mandate a voltage drop maximum — it recommends. But the recommendation exists because dropping more than 3% on a branch or 5% total between service and load will show up in the field: motors run hot, LED drivers flicker, EV chargers throttle back, sensitive electronics reset.
Every homeowner call that says "the light dims when the fridge kicks on" is a voltage drop problem, dressed up as something else.
| Drop | What it means |
|---|---|
| Under 3% | Passes NEC recommended for branch circuits. |
| 3–5% | Passes 5% total (service to load) but exceeds 3% branch recommendation. |
| 5–7% | Exceeds NEC recommendation. Motors and sensitive loads will notice. |
| Over 7% | Guaranteed callbacks. Upsize the wire or shorten the run. |
EV chargers: Level 2 chargers pulling 40A at 240V over a 120-ft run in #8 copper drop 3.5% — right at the edge. Charger throttles down "for protection" and takes 12 hours instead of 8. Customer blames the charger, calls a different electrician.
Detached garage / shop: Long run from house main panel to subpanel. 60A subpanel feed at 100 ft in #6 copper = ~4% drop before any load is applied. Add a compressor and the shop lights dim every time it starts.
Well pumps: 240V motor on a long run to the pumphouse. Undersize the wire and the pump runs hot, dies in 3 years instead of 15.
Upsize the wire. Each AWG size up roughly halves the voltage drop. Going from #12 to #10 drops the loss ~37%. Going from #10 to #8 drops it another ~37%.
Shorten the run. Sometimes moving the subpanel closer to the load, or running from a different existing panel, gets you inside limits without upsizing. Not always possible, but always worth checking first.
If the calc says #12 fails at the required distance and #10 passes, quote #10 as the correct wire and explain the code-recommendation basis. Customers who understand "we're avoiding a callback" pay for it. Customers who just want the cheapest wire become someone else's problem later.
Bid panel upgrades and EV chargers with confidence, sell whole-house surge protection, and quote from a price book that keeps you off the truck at 8 PM writing estimates.