Running electricity through wires always comes with a little loss—and that loss is what we call voltage drop. It can affect performance of appliances, dim lights, or even cause motors to struggle. Fortunately, working out voltage drop is fairly straightforward, and this guide will walk you through each step clearly and simply.
Why Voltage Drop Matters
When current flows through a conductor—like copper or aluminum—it meets resistance. That resistance creates a small voltage loss along the line. If the drop gets too high, devices might not get the voltage they need, which can lead to underperformance or damage. That’s why knowing how to calculate it helps electricians and DIYers plan safer, more efficient wiring.
Step‑by‑Step: How to Calculate Voltage Drop
1. Gather the key values
You’ll need:
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I — the current in amperes (A)
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L — the one‑way length of the wire in feet (or meters)
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Rᵢ — resistance per unit length of the wire material (Ω/ft or Ω/m)
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VS — the supply voltage (e.g., 120 V, 240 V)
2. Find resistance per unit length
Wire tables list resistance by gauge. For example, 12 AWG copper might be around 0.001588 Ω/ft. Aluminum will be higher. Make sure you match the right material and gauge.
3. Use the voltage drop formula
For a single‑phase circuit:
The factor of 2 is because the current travels out and back along the conductor.
If you want the drop percentage:
4. Check against acceptable drop
Codes often recommend staying under a 3–5 percent voltage drop at full load. If your calculation is higher, you may need a larger conductor (lower resistance), shorten wire run, or increase supply voltage.
Example Calculation
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Current (I): 10 A
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Wire length (one‑way): 50 ft
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Wire: 12 AWG copper, Rᵢ = 0.001588 Ω/ft
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Supply voltage (VS): 120 V
Then:
That’s comfortably under 3 %, so it’s acceptable.
Handy Table: Voltage Drop at Different Lengths for 12 AWG Copper at 10 A
| One‑Way Distance | Voltage Drop (V) | Percentage Drop |
|---|---|---|
| 25 ft | 0.794 V | 0.66 % |
| 50 ft | 1.588 V | 1.32 % |
| 75 ft | 2.382 V | 1.99 % |
| 100 ft | 3.176 V | 2.65 % |
This shows how increasing the run length increases loss. Anything above 3 % may need reassessment.
Quick Tips
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Double your length when calculating, because current travels both ways.
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Use wire charts to quickly look up resistance values.
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Consider voltage drop for both AC and DC — the principle is the same, though sometimes codes treat AC slightly differently.
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If you can’t reduce the distance or boost voltage, upsizing wire gauge is often the easiest fix.
People Also Ask‑Style Q&A
What is the formula for voltage drop?
The basic formula is:
Where I is current, R is resistance per length, and L is the one‑way run length. The “2 × L” accounts for the round trip of current.
How much voltage drop is acceptable?
Common guidelines suggest keeping the drop under 3 % for branch circuits and 5 % for total drop (including feeder and branch). Your local regulations may vary, so double‑check code or standards for your area.
Why is voltage drop important in electrical circuits?
Voltage drop ensures the proper voltage reaches your appliances and equipment. Too much drop can cause dim lights, overheating motors, and reduced efficiency, potentially shortening the lifespan of devices.
How do wire size and length affect voltage drop?
Thinner wires have higher resistance, so they cause more voltage drop over the same distance. Longer runs mean current travels farther, multiplying the loss. Using thicker wiring or reducing distance helps lower the drop.
How do I calculate voltage drop for AC and DC circuits?
The formula is the same for both: multiply current by resistance times the round‑trip length. For AC, sometimes you may consider reactance too (especially in long runs at high frequencies), but for typical single‑phase power circuits, resistance alone is usually sufficient.
To Sum Up
Calculating voltage drop doesn’t have to be complicated. You just need the current, wire length, and the resistance per foot (or meter). Plug into the formula, check your percentage drop, and see whether you need to adjust sizing or layout. That simple check brings better safety, improved performance, and reduced energy losses.