Voltage Drop Calculator
Check how much voltage a long run loses before it reaches the load. Enter the conductor size, material, load current, and one-way run length to see the volts dropped, the percentage, the voltage actually arriving, and how far you could run that same wire before hitting the 3% design target.
Your project
Estimate
7.9V
Voltage drop · Volts lost in the conductors
- Drop percentage
- 6.58%
- Voltage at the load
- 112.1V
- Max one-way run at 3%
- 45ft
Estimates are guidance only and exclude tax, labor, and site conditions unless stated. Confirm with a local pro.
How it's calculated
No black boxes — here is the exact math behind your estimate, so you can check it or adapt it for your own quote.
- 1
Phase factor = 2 for single phase, 1.732 (√3) for three phase - 2
Voltage drop = phase factor × K × amps × one-way length ÷ circular mils - 3
K = 12.9 for copper, 21.2 for aluminum (ohm-cmil per foot at 75 °C) - 4
Drop % = voltage drop ÷ system voltage × 100, and voltage at load = system voltage − voltage drop - 5
Max run at 3% = 0.03 × system voltage × circular mils ÷ (phase factor × K × amps)
Worked example
A 20 A, 120 V single-phase circuit run 100 feet on 12 AWG copper to a detached shed.
Inputs
- Conductor size
- 6530
- Conductor material
- 12.9
- Load current
- 20 A
- One-way run length
- 100 ft
- System voltage
- 120
- Phase
- 2
Result
- Voltage drop
- 7.9 V
- Drop percentage
- 6.58 %
- Voltage at the load
- 112.1 V
- Max one-way run at 3%
- 45 ft
Tools & materials for this project
As an Amazon Associate we may earn from qualifying purchases, at no extra cost to you.
- Digital multimeter Measure the actual voltage at the load and compare it to the estimate.
- THHN copper building wire Upsize the conductor when a long run pushes past 3% drop.
Get real quotes
Compare local pros for voltage drop
An estimate gets you a budget. A few competing quotes get you the real price. Tell us about the job and we'll help you find vetted contractors nearby.
Frequently asked questions
What is an acceptable voltage drop?
Three percent on a branch circuit and five percent total from the service to the far end of the load is the number the trade designs to. Be clear about where it comes from: it appears in Informational Notes at NEC 210.19(A) and 215.2(A), and informational notes are explanatory — they are not enforceable requirements. Designers still hold to 3% because motors lose torque, LED drivers dim and flicker, and heating elements lose output roughly with the square of the voltage. Some jurisdictions, utilities, and equipment listings do impose a hard limit, so confirm what your local authority has adopted.
Does voltage drop decide my wire size?
No — ampacity does. NEC Table 310.16, together with the small-conductor limits in 240.4(D), sets the minimum conductor for a given overcurrent device: 14 AWG copper for 15 A, 12 AWG for 20 A, 10 AWG for 30 A. A voltage drop calculation can only push you to go bigger than that minimum on a long run. It can never justify going smaller. Size for ampacity first, then run the drop check and upsize one or two gauges if the run is long.
Why is the detached garage or well pump always the problem?
Drop is directly proportional to one-way distance, so tripling the run triples the loss. A 20 A, 120 V circuit on 12 AWG copper reaches 3% at roughly 45 feet one way; at 150 feet that same wire is near 10%, and a motor trying to start on 108 volts draws more current and gets hotter. Detached garages, barns, well pumps, and yard lighting are the classic complaints. The two standard fixes are running 240 V instead of 120 V, which halves the current for the same watts, and upsizing the conductor.
How much bigger does aluminum wire need to be?
Aluminum has a K of 21.2 against copper at 12.9, about 64% higher resistance for the same size, so it drops roughly 1.64 times as much voltage. In practice installers step up about two AWG sizes: 4/0 aluminum commonly replaces 2/0 copper, and 2 AWG aluminum replaces 4 AWG copper. Aluminum also carries a lower table ampacity and requires terminations listed AL/CU or CO/ALR plus an antioxidant compound on the connections. Confirm the breaker and lug ratings before you buy aluminum feeder cable.
Do I enter one-way or round-trip length?
One-way — panel to load. The factor of 2 in the single-phase formula already accounts for current flowing out on the ungrounded conductor and back on the grounded conductor. Three phase uses 1.732 instead of 2 because the return currents in a balanced three-phase set partly cancel. Measure the path the cable actually takes, up walls and along joists and around obstructions, rather than the straight-line distance across the yard. That routing difference is usually 15 to 30 percent and is the most common reason a real measurement misses an estimate.
Can I design a circuit from this calculator?
Treat it as a planning estimate only. It uses the standard circular-mil approximation at 75 °C and ignores conductor heating under load, power factor, the added reactance of steel raceway, harmonics, and any ampacity correction for high ambient temperature or conduit fill. It does not perform a load calculation. The locally adopted code and a licensed electrician govern the final design, and work on a service, feeder, or panel needs a permit and inspection in most jurisdictions.