MachineCalcs

Voltage Drop Calculator

Estimate voltage drop and percent drop from phase, material, wire size, current or transformer kVA, voltage and one-way circuit length. Copper and aluminum AWG/kcmil sizes, with formulas shown.

Electrical 9 inputs 8 results

Calculator

Inputs

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Single-phase/DC uses a two-conductor path. Three-phase uses the balanced line-to-line shortcut.
Uses 75 C resistance constants: copper K=12.9, aluminum K=21.2 ohm-cmil/ft.
Conductor circular-mil area used for voltage drop. This is not an ampacity selection.
One-way distance from source to load, not round-trip length.
ft
Use direct amps when known, or derive balanced load current from kVA and voltage.
Load current in amperes.
A
Line-to-line voltage for three-phase, or source voltage for single-phase/DC.
V
Used for the max-length and suggested-area outputs. Common design screens use 3% for branch or feeder and 5% total.
%

Results

Default result
Edit inputs
Voltage drop(VD)
2.63%
Pass

At or below the 3% target.

Also computed

Voltage lost(V_drop)3.161V

Load-end voltage(V_load)116.84V

Load current used(I)16A

direct current input

Direct input current, or derived from kVA and phase.

Max one-way length(L_max)56.95ft

For the selected wire and target percent drop.

Suggested conductor area(CM_min)Pass6.53kcmil

nearest listed size: #12 AWG

Selected conductor area(CM)6.53kcmil

#12 AWG

Method notes 3 notes
  • single-phase shortcut using copper conductor constant and one-way length.
  • Load current is used directly as entered.
  • This screens voltage drop only. It does not check ampacity, insulation temperature, conduit fill, derating, equipment grounding conductors, termination ratings or local code requirements.

Voltage drop can be screened with the circular-mil shortcut VD = M×K×I×L/CM, where M is 2 for single-phase/DC or √3 for balanced three-phase, K is the copper or aluminum constant, I is current, L is one-way length in feet and CM is conductor circular-mil area. This calculator can use direct amps or derive load current from transformer/load kVA and voltage, then returns volts lost, percent drop, load-end voltage, max length and the first listed conductor area that meets your target drop.

Continue workflow

All Electrical
Next 1 Check ampacity derating After selecting a conductor for voltage drop, apply correction, adjustment and terminal-cap factors to check usable ampacity. Next 2 Check parallel ampacity When the same feeder uses parallel paths, multiply terminal-limited per-conductor ampacity by path count and sharing allowance. Next 3 Size access control power Turn low-voltage door-hardware current draws into supply size, wire drop and standby battery capacity. Chart Use reference data Transformer kVA chart

How to use this calculator

  1. Choose circuit type. Select single-phase, three-phase or DC/two-wire so the correct multiplier is used.
  2. Choose conductor material and size. Pick copper or aluminum and the AWG/kcmil conductor area.
  3. Enter one-way length, load and voltage. Use one-way source-to-load distance, then enter direct amps or derive current from load/transformer kVA and system voltage.
  4. Check drop and target. Compare percent drop with your target and review the suggested conductor area if the selected size is too small.

How it works

Voltage drop is the voltage lost in the conductors before power reaches the load. This calculator uses the circular-mil shortcut: VD = M x K x I x L / CM where M is 2 for single-phase/DC or sqrt(3) for balanced three-phase, K is the copper or aluminum constant, I is current, L is one-way length in feet and CM is conductor circular-mil area.

If you are starting from transformer kVA instead of measured amps, switch Current basis to Load / transformer kVA. The calculator derives single-phase current as I = kVA x 1000 / V and balanced three-phase current as I = kVA x 1000 / (sqrt(3) x VLL), then uses that current in the same voltage-drop screen. Use the load or design kVA for the feeder, not automatically the largest transformer nameplate, unless that is the load you are actually checking. The exact kVA-current workflow is also available as a voltage drop calculator from transformer kVA.

For conductor-led searches, use the voltage drop calculator by wire size. For code-style query wording, use the NEC voltage drop calculator. For DC PV string and array runs, use the solar PV wire size calculator as a voltage-drop screen before checking PV ampacity and equipment limits.

The suggested conductor area is the first listed AWG/kcmil size that meets the entered voltage-drop target. It is not an ampacity result. After a voltage-drop screen, use the conduit fill calculator for raceway space, the electrical box fill calculator for enclosure volume, the conduit bend offset calculator for offset layout, the conduit pull tension calculator for long or bend-heavy pulls, and the transformer kVA calculator when the same load needs transformer capacity. For standalone sources, use the off-grid solar calculator before checking long DC or AC runs.

Worked example

Verified against the live calculator

A 120 V single-phase circuit carrying 16 A on #12 copper for 50 ft one-way has VD = 2 x 12.9 x 16 x 50 / 6530 = 3.16 V. The percent drop is 3.16 / 120 x 100 = 2.63%, so the load-end voltage is about 116.8 V. That passes a 3% target in this simplified screen.

For a transformer-kVA workflow, a 45 kVA, 480 V, three-phase load gives I = 45,000 / (sqrt(3) x 480) = 54.1 A. That derived current is the value used in the same VD = M x K x I x L / CM shortcut.

Reference data

Common AWG circular-mil areas used by the voltage-drop shortcut. The calculator also includes larger kcmil sizes through 1000 kcmil.

Common AWG conductor circular-mil areas.
Wire size Area (kcmil)
#14 AWG 4.11
#12 AWG 6.53
#10 AWG 10.38
#8 AWG 16.51
#6 AWG 26.24
#4 AWG 41.74
#3 AWG 52.62
#2 AWG 66.36
#1 AWG 83.69
1/0 AWG 105.6
2/0 AWG 133.1
3/0 AWG 167.8
4/0 AWG 211.6

Source: AWG circular-mil areas following NEC Chapter 9 Table 8 style values; verify conductor construction and code tables for final design.

Frequently asked questions

How do you calculate voltage drop?

For a circular-mil screening calculation, single-phase voltage drop is VD = 2*K*I*L/CM and balanced three-phase voltage drop is VD = sqrt(3)*K*I*L/CM. K is the conductor material constant, I is load current, L is one-way length in feet and CM is circular-mil conductor area.

Should I use one-way or round-trip length?

Enter one-way distance from the source to the load. The calculator applies the two-conductor multiplier for single-phase/DC circuits and sqrt(3) for balanced three-phase circuits.

What voltage drop target should I use?

A common design screen is 3% for a feeder or branch circuit and 5% total from service to load. Sensitive equipment, motors and local requirements may need tighter limits.

Can I calculate voltage drop from transformer kVA?

Yes. Choose the kVA current basis, enter the apparent load or transformer kVA, and the calculator derives load current from voltage and phase before running the voltage-drop formula. Use actual connected/design load kVA when it is lower than transformer nameplate capacity.

Does this choose a code-compliant wire size?

No. The suggested conductor area is only the minimum listed size for the entered voltage-drop target. Ampacity, insulation temperature, conduit fill, derating, equipment grounding conductors and protection must be checked separately.

Does this include reactance or power factor?

No. It uses the common DC-resistance circular-mil shortcut. Large conductors, long feeders, low power factor and parallel/conduit arrangements may need an impedance method using resistance and reactance tables.

Method & assumptions

  • Uses one-way circuit length, not round-trip length.
  • kVA mode derives balanced line current from apparent power and system voltage before running the voltage-drop shortcut.
  • Uses K = 12.9 for copper and K = 21.2 for aluminum in the circular-mil shortcut.
  • Does not include conductor reactance, harmonic heating, parallel conductors, temperature correction, ampacity or protection sizing.
  • Final electrical work needs the adopted code edition, local amendments, equipment instructions and qualified review.
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