Voltage Drop Calculator
How much voltage drop on a 30 A, 240 V circuit 100 ft away on 10 AWG copper?
About 5.99 V, or 2.50%, leaving 234.0 V at the load. That is inside the 3% guideline: 2 × 30 A × 0.999 ohms per 1,000 ft × 100 ft ÷ 1,000 = 5.99 V. The same wire on a 120 V circuit at 30 A would drop 5.0%.
Voltage drop is the voltage lost in the wire itself between the supply and the load. On a single-phase circuit it is 2 × current × resistance per 1,000 ft × run length ÷ 1,000. A 15 A load 100 ft away on 12 AWG copper drops 4.76 V, about 4% of a 120 V supply, which is past the 3% most people design to.
Choose a system voltage (120, 208, 240, 277 or 480 V), enter the current and one-way run length, pick a wire gauge from 14 AWG to 4/0, copper or aluminum, and single or three phase. The calculator returns the volts lost, the drop as a percentage, the voltage arriving at the load, and whether that lands inside the usual 3% and 5% limits. It is a planning estimate; your local electrical code and inspector have the final say.
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How to Use the Voltage Drop Calculator
- 1
Choose voltage and enter current
Pick the system voltage (120, 208, 240, 277 or 480 V) and type the load current in amps. Use the actual expected load, not the breaker size: a 20 A breaker feeding a 12 A load drops voltage for 12 A.
- 2
Enter the one-way run length
The distance from source to load in feet. Do not double it for the return conductor; the formula already accounts for the round trip.
- 3
Pick the wire gauge and metal
Choose the gauge you are planning, from 14 AWG to 4/0, and copper or aluminum. Aluminum is treated as 1.61 times the resistance of copper at the same gauge.
- 4
Set the phase and read the verdict
Single phase or three phase. The result shows volts dropped, percent, voltage at the load, and a green (3% or less), yellow (over 3% up to 5%) or red (over 5%) verdict. If it fails, step up one gauge and look again.
How voltage drop is calculated
Single-phase circuits use a multiplier of 2, because current travels out on one conductor and back on the other. Three-phase uses 1.732 (the square root of 3), because the three currents are 120 degrees apart and partly cancel.
Resistance comes from a table of copper ohms per 1,000 feet, from 2.525 for 14 AWG down to 0.0490 for 4/0. These are room-temperature (about 20°C) values. A conductor carrying full load runs warmer, and at 75°C copper resistance is roughly 24% higher, so treat results near the 3% line as optimistic. The wire gauge calculator uses the hotter NEC Chapter 9 values and also checks ampacity, so it is the better tool for choosing a size.
Single phase: VD = 2 × amps × (ohms per 1,000 ft) × length ÷ 1,000
Three phase: VD = 1.732 × amps × (ohms per 1,000 ft) × length ÷ 1,000
Percent drop = VD ÷ supply voltage × 100
Aluminum: copper resistance × 1.61Worked example: a 15 A circuit 100 ft away
- 120 V single phase, 15 A, 100 ft run, 12 AWG copper (1.588 ohms per 1,000 ft).
- VD = 2 × 15 × 1.588 × 100 ÷ 1,000 = 4.76 V.
- Percent: 4.76 ÷ 120 × 100 = 3.97%, over the 3% guideline.
- Voltage at the load: 120 − 4.76 = 115.2 V.
- Step up to 10 AWG (0.999 ohms): VD = 3.00 V, or 2.50%. Now inside the limit.
Voltage drop table: 120 V at 15 A and 20 A
Percentage drop on a 120 V single-phase circuit with copper conductors, by one-way run length, using the calculator's resistance values. At or under 3% is comfortable; over 5% needs fixing.
| Gauge | Load | 25 ft | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|---|---|
| 14 AWG | 15 A | 1.58% | 3.16% | 6.31% | 9.47% | 12.62% |
| 12 AWG | 15 A | 0.99% | 1.98% | 3.97% | 5.96% | 7.94% |
| 12 AWG | 20 A | 1.32% | 2.65% | 5.29% | 7.94% | 10.59% |
| 10 AWG | 20 A | 0.83% | 1.67% | 3.33% | 5.00% | 6.66% |
| 8 AWG | 20 A | 0.52% | 1.05% | 2.09% | 3.14% | 4.19% |
| 6 AWG | 20 A | 0.33% | 0.66% | 1.32% | 1.98% | 2.63% |
Voltage drop table: 240 V at 30 A
A typical 240 V, 30 A load such as a water heater, dryer or shop subpanel, single phase, copper. At 240 V the same current costs half the percentage it would at 120 V.
| Gauge | 25 ft | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|---|
| 10 AWG | 0.62% | 1.25% | 2.50% | 3.75% | 5.00% |
| 8 AWG | 0.39% | 0.78% | 1.57% | 2.35% | 3.14% |
| 6 AWG | 0.25% | 0.49% | 0.99% | 1.48% | 1.98% |
| 4 AWG | 0.16% | 0.31% | 0.62% | 0.93% | 1.24% |
12 V and 24 V DC: the same formula, much less margin
This calculator's voltage menu starts at 120 V, so it does not take 12, 24 or 48 V DC directly. A two-wire DC circuit uses the same single-phase formula, so the table below is worked by hand with the same copper resistances. At 12 V, 3% is only 0.36 V, which is why RV, boat and solar wiring looks so heavy. For low-voltage runs, the wire gauge calculator accepts 12, 24 and 48 V DC and picks the size for you; to see how long a battery lasts on that load, try the battery runtime calculator.
For example, 20 A over 25 ft on 10 AWG at 12 V loses about 1.0 V, or 8.3%: far too much. A 24 V system at 20 A drops the same percentage as 12 V at 10 A.
| Gauge | 12 V, 10 A, 10 ft | 12 V, 10 A, 25 ft | 12 V, 20 A, 10 ft | 12 V, 20 A, 25 ft |
|---|---|---|---|---|
| 12 AWG | 2.65% | 6.62% | 5.29% | 13.23% |
| 10 AWG | 1.67% | 4.16% | 3.33% | 8.33% |
| 8 AWG | 1.05% | 2.62% | 2.09% | 5.23% |
| 6 AWG | 0.66% | 1.65% | 1.32% | 3.29% |
| 4 AWG | 0.41% | 1.04% | 0.83% | 2.08% |
| 2 AWG | 0.26% | 0.65% | 0.52% | 1.30% |
Why 3% and 5%
- The figures come from informational notes in the NEC rather than enforceable rules: 3% on a branch circuit, and no more than 5% for feeder and branch circuit combined. Local codes and the authority having jurisdiction have the final say.
- Aluminum needs roughly two gauge sizes larger than copper for the same drop, and it needs terminations rated for it (marked AL or CU-AL).
- This calculator sizes for voltage drop, not ampacity. A conductor must also be rated to carry the current under NEC Table 310.16. Check both and use whichever gauge is larger.
When to call a licensed electrician
Use this page to understand a run and price materials, not to sign off on a design. Call a licensed electrician, and check whether a permit is needed, before you add or extend a circuit, run a feeder to a garage, shop or subpanel, wire an EV charger, well pump or hot tub, or work in a panel. Also get help if lights dim when a motor starts, breakers trip, or outlets, cords or switch plates feel warm: those can be signs of an undersized or loose connection, not just voltage drop. Always switch off and verify power is off before touching wiring.
Features
- Calculate voltage drop percentage
- Flags drops above the 3% guideline
- Copper and aluminum support
- Single and three phase circuits
Frequently Asked Questions
What is an acceptable voltage drop?
Three percent or less on a branch circuit, and no more than 5% across feeder and branch combined. These come from NEC informational notes, which are recommendations rather than enforceable requirements, so your local code or inspector may set something stricter.
How do I calculate voltage drop for a 12V circuit?
Use the single-phase formula: 2 × amps × ohms per 1,000 ft × length ÷ 1,000, then divide by 12 for the percentage. This calculator's voltage menu starts at 120 V, so do it by hand or use the wire gauge calculator, which accepts 12, 24 and 48 V DC. At 12 V, 3% is only 0.36 V: a 10 A load 25 ft away on 12 AWG drops about 6.6%.
Does the length mean one way or the total wire?
One way: the distance from the source to the load. The formula multiplies by 2 for single phase to account for the return conductor, so doubling the length yourself would double the answer wrongly.
How much worse is aluminum than copper?
The calculator treats aluminum as 1.61 times the resistance of copper at the same gauge, so roughly 61% more voltage drop. Compensating usually means going up about two gauge sizes.
Why is three-phase 1.732 instead of 2?
Because the three conductor currents are 120 degrees out of phase and partly cancel, so there is no separate return path carrying the full current. The multiplier is the square root of 3, about 1.732, which is why a three-phase run drops about 13% less voltage than single phase at the same current and distance.
Why does the wire gauge calculator show a bigger drop for the same wire?
This voltage drop calculator uses room-temperature resistance (for example 0.999 ohms per 1,000 ft for 10 AWG copper). The wire gauge calculator uses NEC Chapter 9 values at 75°C (1.24 ohms for 10 AWG), which reflect a conductor warmed by load. The hotter figure is about 24% higher and is the more conservative one for sizing.
What do I do if the drop is too high?
In order of cost: increase the wire gauge, shorten the run, or raise the supply voltage. One gauge size up cuts resistance by about 37% through the common 14 to 6 AWG range. On long runs, moving a load from 120 V to 240 V halves the current and cuts the percentage drop to a quarter.
Sources and review
Prepared by the Codezyra editorial team. The figures and rules on this page were checked against these official sources:
- NFPA 70, National Electrical Code (NEC): Table 310.16 ampacity, Chapter 9 Table 8 conductor resistance, and the 3%/5% informational notes
- OSHA: Electrical safety
This is general information, not personal tax, legal or medical advice.
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