Voltage Drop Calculations Reference

Why this matters

Wire has resistance. Current through resistance produces voltage drop. On long runs, voltage drop becomes significant - motors run hot, electronics misbehave, lights dim, energy is wasted as heat in the conductors. NEC recommends ≤3% drop on branch circuits and ≤5% total (feeder + branch). These limits are in informational notes (not code-enforceable), but ignoring them produces real customer complaints and reduces equipment life.

The basic formula

Single-phase (120 V or 240 V):

VD = (2 × K × I × L) / CM

Where:

• VD = voltage drop in volts
• K = resistance constant: 12.9 for copper at 75 °C, 21.2 for aluminum at 75 °C
• I = current in amps
• L = one-way length of run in feet (the 2× accounts for current going AND returning)
• CM = conductor cross-section in circular mils

Three-phase (208 V, 480 V, etc.):

VD = (1.732 × K × I × L) / CM

Three-phase drops less than single-phase for the same conductor because the return current is shared across multiple phases.

Circular mils by AWG (copper / aluminum same)

Worked example - branch circuit

20 A load, 75 ft one-way, 12 AWG copper, 120 V:

VD = (2 × 12.9 × 20 × 75) / 6,530 = 5.93 V

Percentage drop = 5.93 / 120 = 4.94%

Exceeds the 3% recommendation. Upsize to 10 AWG:

VD = (2 × 12.9 × 20 × 75) / 10,380 = 3.73 V → 3.11% drop

Still slightly over 3%. Go to 8 AWG:

VD = (2 × 12.9 × 20 × 75) / 16,510 = 2.34 V → 1.96% drop

8 AWG is comfortable.

Worked example - sub-panel feeder

100 A continuous load, 75 ft, 2 AWG copper, 240 V single-phase:

VD = (2 × 12.9 × 100 × 75) / 66,360 = 2.92 V → 1.22% drop

Within 3% comfortably.

100 A continuous load, 150 ft, 2 AWG copper, 240 V:

VD = (2 × 12.9 × 100 × 150) / 66,360 = 5.83 V → 2.43%

Still acceptable at this length, but approaching the limit.

100 A continuous load, 250 ft, 2 AWG copper, 240 V:

VD = (2 × 12.9 × 100 × 250) / 66,360 = 9.72 V → 4.05%

Now we're past 3%. Upsize to 1/0:

VD = (2 × 12.9 × 100 × 250) / 105,600 = 6.11 V → 2.55%

Common scenarios that need voltage drop calculation

EV charger circuits: 32-48 A continuous loads, often long runs from panel to garage or driveway. A 40 A charger 75 ft from panel on 8 AWG drops 6.4% - needs 6 AWG minimum.

Detached structures: garage, workshop, pool equipment. Underground runs of 100-300 ft are common; voltage drop drives wire size more than ampacity does.

Well pumps: deep submersible pumps + long underground runs. Manufacturer install manuals specify minimum wire size to maintain ≥95% voltage at the motor for proper starting torque.

Outdoor lighting: LED loads are small but long runs in landscape lighting concentrate the drop. Low-voltage transformers have their own derating tables.

Air conditioning: start-up inrush current is 5-7× running current. Excessive drop during start makes the contactor chatter and the compressor struggle.

When upsizing is required by code (not just recommendation)

NEC 210.19(A)(1)(b) Informational Note 4 and NEC 215.2(A)(1)(b) IN 2 give voltage-drop recommendations but they are not enforceable.

However, some applications have hard limits:

• EV charging equipment per NEC 625.41: sized for continuous load (125% of EVSE nameplate)
• Motor branch circuits per NEC 430: sized for 125% of motor FLA
• Fire pump circuits per NEC 695: strict voltage limits during start

For these, voltage drop calculation isn't optional - it's part of sizing the conductor properly.

Continuous load consideration

NEC defines a continuous load as one that operates 3+ hours at full nameplate. For continuous loads:

• Conductors sized at 125% of the load current (NEC 210.19(A)(1))
• THEN apply voltage drop calculation
• Use the larger of the two requirements

A 32 A EVSE running 6+ hours nightly:

• Ampacity per NEC: 32 × 1.25 = 40 A → 8 AWG copper minimum (NEC 310.16 at 75 °C)
• Voltage drop at 75 ft on 8 AWG: VD = 2 × 12.9 × 32 × 75 / 16510 = 3.75 V → 1.56%. Comfortable.
• Voltage drop at 150 ft on 8 AWG: 7.5 V → 3.1%. Upsize to 6 AWG.

Adjustments

Temperature: K = 12.9 at 75 °C. At higher conductor temperatures (e.g., 90 °C insulation in a hot attic), resistance climbs. Use 13.6 for 90 °C copper if precision matters.

Power factor: for motors with poor PF, the impedance (not just resistance) matters. For most residential and standard commercial loads, the resistance approximation is close enough.

Conduit fill: more than 3 current-carrying conductors in a raceway requires ampacity derating per NEC 310.15(C). Doesn't change voltage drop, but the ampacity requirement may force upsize anyway.

Direct burial: wire in conduit underground runs cooler than aerial, slightly less resistance.

Common voltage drop mistakes

• Forgetting the 2× for one-way length on single-phase. Single-phase has current going AND returning; the formula already includes this with the 2 factor, BUT you only enter ONE-way length. Easy mistake to enter round-trip length and double-count.
• Using K = 12.9 for aluminum. Aluminum is K = 21.2 (~65% more resistive). Calculation is off by 65%.
• Sizing only for ampacity, ignoring drop. Conductor meets NEC table; circuit performs poorly; customer complains.
• Calculating drop at expected average load instead of maximum continuous load. Drop should be evaluated at worst case.
• Skipping the 125% continuous-load factor. Conductor undersized for sustained operation.

Quick reference - voltage drop calculator approaches

Online calculators (Mike Holt, Southwire, Cerrowire) - paste numbers, get result. Easier than the formula in the field.

NEC handbook table - pre-calculated drop per 1000 ft for various AWG / current. Multiply by your length / 1000.

Manual formula - handy when you don't have the calculator and need to estimate.

For commercial / large projects, drop is calculated in design software during engineering, not by the installer.

Aluminum vs copper sizing comparison

For the same voltage drop limit, aluminum requires roughly one AWG size larger than copper. Common service entrance conductors:

These are NEC 310.12 reduced-conductor ampacities for residential 120/240 V services; verify per local AHJ.

When the customer asks "why is the new wire bigger than the old?"

References

• NEC Article 210.19 (branch circuit conductors)
• NEC Article 215.2 (feeder conductors)
• NEC Article 310.16 / 310.12 (conductor ampacity)
• NEC Article 625 (EV charging equipment)
• NEC Article 430 (motors)
• Mike Holt Enterprises voltage drop training
• Southwire / Cerrowire / Encore Wire technical bulletins