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Voltage Drop Calculator: The NEC Formula and How to Use It
The Problem Voltage Drop Solves
Every foot of wire has resistance. Current flowing through that resistance loses voltage. By the time power reaches the end of a 200-foot run, the voltage at the outlet can be meaningfully lower than the voltage at the panel. Lights dim. Motors overheat. VFDs throw faults.
The NEC recommends keeping voltage drop under 3% on any branch circuit and under 5% total for feeder plus branch combined (NEC 210.19(A) Informational Note No. 4 and 215.2(A)(4) Informational Note No. 2). These aren’t hard mandates, but inspectors in many jurisdictions treat them as if they are. And for good reason: a circuit running at 6% voltage drop wastes energy as heat in the wire and shortens the life of everything connected to it.
The Formula
For single-phase circuits:
VD = (2 x K x I x D) / CM
For three-phase circuits:
VD = (1.732 x K x I x D) / CM
Where:
- VD = voltage drop in volts
- K = resistivity constant (12.9 for copper, 21.2 for aluminum)
- I = load current in amps
- D = one-way distance from panel to load, in feet
- CM = circular mils of the conductor (from NEC Chapter 9, Table 8)
The factor of 2 in the single-phase formula accounts for the round trip. Current travels out on the hot conductor and returns on the neutral. Both legs have resistance. The three-phase version uses 1.732 (the square root of 3) because of how the phase voltages relate in a balanced three-phase system.
To get percentage: (VD / source voltage) x 100.
Wire Gauge Reference Table
These circular mil values come from NEC Chapter 9, Table 8. You’ll need them for the formula.
| AWG | Circular Mils | Typical Use |
|---|---|---|
| 14 | 4,110 | 15A lighting circuits |
| 12 | 6,530 | 20A receptacles |
| 10 | 10,380 | 30A small appliances |
| 8 | 16,510 | 40-50A ranges, dryers |
| 6 | 26,240 | 50-65A sub-panels |
| 4 | 41,740 | 70-85A feeders |
| 3 | 52,620 | 100A sub-panels |
| 2 | 66,360 | 100-115A feeders |
| 1 | 83,690 | 130A feeders |
| 1/0 | 105,600 | 150A services |
| 2/0 | 133,100 | 175A services |
| 3/0 | 167,800 | 200A services |
| 4/0 | 211,600 | 230A services |
If you’re working with wire sizing for ampacity, remember that voltage drop is a separate check. A wire can pass the NEC 310.16 ampacity table and still fail the voltage drop recommendation.
Worked Example: Residential Detached Shop
You’re running a 240V, single-phase, 50-amp circuit to a detached workshop 175 feet from the main panel. The wire is copper.
Start with 6 AWG, since NEC 310.16 rates it at 65A at 75C, which covers the 50-amp load.
VD = (2 x 12.9 x 50 x 175) / 26,240
VD = 225,750 / 26,240
VD = 8.6 volts
Percentage: 8.6 / 240 = 3.6%
Over the 3% recommendation. A table saw on that circuit will notice. Try 4 AWG:
VD = (2 x 12.9 x 50 x 175) / 41,740
VD = 225,750 / 41,740
VD = 5.41 volts
Percentage: 5.41 / 240 = 2.25%
That’s under 3%. Use 4 AWG copper.
Notice what happened: the wire that passed ampacity (6 AWG at 65A for a 50A load) failed voltage drop. This is the most common wiring mistake on long runs. I’ve seen it on job sites more times than I can count, usually discovered when someone complains their welder won’t arc properly.
Worked Example: Three-Phase Commercial Feeder
A 208V three-phase feeder carries 80 amps over 250 feet using copper conductors.
Start with 4 AWG (85A capacity at 75C).
VD = (1.732 x 12.9 x 80 x 250) / 41,740
VD = 446,976 / 41,740
VD = 10.71 volts
Percentage: 10.71 / 208 = 5.15%
Way too high for a feeder. If the branch circuit adds any drop at all, you’ll blow past the 5% combined limit. Try 2 AWG:
VD = (1.732 x 12.9 x 80 x 250) / 66,360
VD = 446,976 / 66,360
VD = 6.74 volts
Percentage: 6.74 / 208 = 3.24%
Still leaves only 1.76% for the branch circuits. On a large facility with long branch runs, you’d want 1 AWG or 1/0 to give yourself breathing room.
VD with 1/0 = 446,976 / 105,600 = 4.23 volts = 2.03%
That leaves a full 3% for branch circuits. Better.
Aluminum vs. Copper
Aluminum wire has a K value of 21.2, compared to 12.9 for copper. That’s 64% more resistance per foot. In practice, you go up about two wire sizes when switching to aluminum.
A 4 AWG copper circuit that passes voltage drop at 2.25% becomes:
VD = (2 x 21.2 x 50 x 175) / 41,740 = 8.88 volts = 3.7% with the same gauge in aluminum.
To match the copper performance, you’d need 2 AWG aluminum (66,360 CM):
VD = (2 x 21.2 x 50 x 175) / 66,360 = 5.59 volts = 2.33%
Aluminum is cheaper per foot but the larger gauge eats some of that savings. For runs under 100 feet, copper usually wins on total installed cost because the conduit and labor stay the same. Past 200 feet, aluminum starts making financial sense on feeders, especially at higher amperages where the copper price gap widens.
Factors That Make Voltage Drop Worse
Distance is the obvious one. Every 10 feet adds proportional voltage drop, and there’s no trick to get around it besides upsizing the wire.
Current matters just as much. A 100-amp circuit drops twice as much voltage as a 50-amp circuit on the same wire. Double the amps, double the drop.
Smaller wire gauges have fewer circular mils and more resistance per foot. This is why the formula works in circular mils rather than AWG numbers, since CM scales linearly with cross-sectional area.
Temperature affects resistance too. The K values of 12.9 and 21.2 assume 75C. In extreme heat (like an attic in Phoenix or conduit on a roof in July), actual resistance climbs higher than the formula predicts.
Power factor on AC circuits is a subtler issue. For most construction work with resistive or near-resistive loads (heaters, lighting, receptacles), the simplified formula is accurate enough. Highly inductive loads like large motors running at poor power factor will see more voltage drop than calculated. If you’re sizing feeders for a motor control center, talk to an engineer and use the full AC impedance method from NEC Chapter 9, Table 9.
Rearranging the Formula to Find Minimum Wire Size
You can flip the formula around. Instead of checking a wire gauge, solve for the minimum circular mils needed to hit your target voltage drop.
CM = (2 x K x I x D) / VD
Say you need less than 3% drop on a 120V, 20A, single-phase circuit at 100 feet. Your target VD is 3.6 volts (120 x 0.03).
CM = (2 x 12.9 x 20 x 100) / 3.6
CM = 51,600 / 3.6
CM = 14,333
Look at the table: 10 AWG has 10,380 CM (too small). 8 AWG has 16,510 CM. Use 8 AWG.
That 20-amp kitchen circuit at 15 feet? Different answer entirely:
CM = (2 x 12.9 x 20 x 15) / 3.6 = 2,150
Even 14 AWG (4,110 CM) clears it. The NEC minimum of 12 AWG for a 20-amp circuit is already more than enough. Distance changes everything.
Common Mistakes
Forgetting the round trip. The “2” in the formula exists because current travels to the load and back. Leave it out and your calculation is half of reality. I’ve reviewed plans where someone calculated voltage drop at 1.5% and felt good about it, but the real number was 3%.
Using the wrong K value. Copper and aluminum have different resistivity. Mixing them up gives you a number that’s off by 64%.
Ignoring voltage drop on feeders. The 5% combined limit means the feeder gets a budget and the branch circuit gets the rest. If your 200-foot feeder eats 4%, your branch circuits only get 1% before you’re over.
Not rechecking after a design change. The HVAC guy moves the air handler from 50 feet to 150 feet from the panel. The original wire size doesn’t work anymore. Voltage drop calculations need to follow the final as-built distances, not the original plan.
Running These on Site
SiteCalc has a voltage drop calculator built in. Enter the amps, voltage, distance, and wire material, and it returns the drop percentage and recommends a wire gauge. It shows the formula breakdown so you can verify the math, and tags it with the relevant NEC code reference. If you’re up on a lift or have your hands full, voice input lets you call out the numbers instead of typing them.