How to use this calculator
- Choose circuit type. Select single-phase, three-phase or DC/two-wire so the correct multiplier is used.
- Choose conductor material and size. Pick copper or aluminum and the AWG/kcmil conductor area.
- Enter one-way length, current and voltage. Use one-way source-to-load distance, full load current and the system voltage at the source.
- 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.
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 and the transformer kVA calculator when the same load needs transformer capacity.
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.
Reference data
Common AWG circular-mil areas used by the voltage-drop shortcut. The calculator also includes larger kcmil sizes through 1000 kcmil.
| 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.
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.
- 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.