Voltage Drop Calculator
Calculate voltage drop in electrical circuits and find the right wire size to keep voltage within NEC and CEC guidelines. Works with both single-phase and three-phase systems.
Results
Voltage drop occurs when electrical current flows through a conductor with resistance. This calculator helps you determine the voltage drop in a circuit and select appropriate wire sizes to maintain voltage within acceptable limits per NEC and CEC requirements.
Voltage Drop Formulas
VD = 2 × K × I × L / CM
Factor of 2 accounts for both conductors
VD = 1.732 × K × I × L / CM
√3 factor for three-phase systems
Where:
VD = Voltage drop (volts)
K = 12.9 (copper resistance constant)
I = Current (amperes)
L = One-way length (feet)
CM = Circular mils of conductor
Voltage Drop Percentage
VD% = (VD / V_source) × 100
V_load = V_source - VD
- • Branch circuits: 3% maximum
- • Feeders: 5% maximum
- • Total system: 5% maximum
Wire Selection Considerations
Current Capacity (Ampacity)
- Wire must handle the full load current
- Consider temperature derating factors
- Account for conduit fill derating
- Include safety margin for future loads
Voltage Drop Limitations
- Larger wire sizes reduce voltage drop
- Shorter runs require smaller wire
- Higher currents increase voltage drop
- Three-phase systems have lower voltage drop
Example Calculation
A 20A load on a 120V single-phase circuit, 100 feet from panel using 12 AWG wire:
• VD = 2 × 12.9 × 20 × 100 / 6530 = 7.9V
• VD% = 7.9V / 120V × 100 = 6.6% (exceeds 3% limit)
• Recommendation: Use 10 AWG wire for 3.9V drop (3.3%)
How to Calculate Voltage Drop: Step-by-Step
Voltage drop is a critical calculation for any wire run. Follow these steps to determine whether your wiring meets NEC and CEC requirements.
Step 1: Gather Circuit Parameters
Collect the system voltage, load current in amps, one-way wire distance in feet, and the wire material (copper or aluminum). These values come from the circuit design or equipment nameplates.
Step 2: Find Wire Circular Mil Area (CMA)
Look up the circular mil area for your wire gauge in NEC Chapter 9, Table 8. For example, #12 AWG copper has 6,530 circular mils. Larger wire means more circular mils and less voltage drop.
Step 3: Apply the Voltage Drop Formula
For single-phase circuits, use VD = 2 x K x I x L / CM. For three-phase, replace the 2 with 1.732. The K constant is 12.9 for copper and 21.2 for aluminum at 75 degrees C.
Step 4: Convert to a Percentage
Divide the voltage drop by the source voltage and multiply by 100. This gives you the percentage of voltage lost in the wire run. For example, a 3.6V drop on a 120V circuit is 3.0%.
Step 5: Compare to NEC Limits
The NEC recommends no more than 3% drop for branch circuits and 5% total for feeder plus branch. If your percentage exceeds these limits, increase the wire size and recalculate.
Formula
VD = 2 x K x I x L / CM
Where: VD = Voltage drop (volts), K = Resistivity constant (12.9 copper, 21.2 aluminum), I = Current (amps), L = One-way length (feet), CM = Wire area (circular mils)
Worked Example
Scenario: A 120V, 20A circuit using #12 AWG copper wire runs 100 feet to a load.
- Step 1: V = 120V, I = 20A, L = 100 ft, Copper wire
- Step 2: #12 AWG CMA = 6,530
- Step 3: VD = 2 x 12.9 x 20 x 100 / 6,530 = 7.9V
- Step 4: Percentage = 7.9 / 120 x 100 = 6.6%
- Step 5: 6.6% exceeds the 3% limit. Upgrade to #8 AWG (16,510 CM) for VD = 3.1V (2.6%)
Result: #12 AWG fails at 6.6% drop. Use #8 AWG copper for a 2.6% drop that meets NEC limits.
Understanding Voltage Drop
Voltage drop happens because wire isn't perfect - it has resistance that "uses up" some voltage as electricity flows through it. Think of it like water pressure dropping as it flows through a long garden hose. The longer the wire or the more current it carries, the more voltage gets lost.
Here's what actually happens: When current flows through wire resistance, it creates heat and "eats up" voltage. A motor that needs 240 volts might only get 225 volts at the end of a long wire run. That 15-volt difference (6.25% drop) will make the motor run hot, draw more current, and probably fail early.
The NEC and CEC set limits to ensure equipment gets the voltage it needs to operate properly. These limits aren't just suggestions - inspectors check voltage drop calculations during plan review, and your equipment's warranty might be void if you exceed recommended limits.
How This Calculator Determines the Right Wire Size
This calculator uses the actual resistance values for different wire sizes and applies the voltage drop formulas that electricians have used for decades. Enter your load current, wire length, and system voltage, and it instantly shows you which wire sizes keep you within NEC and CEC limits.
Here's the step-by-step process:
- Takes your current and distance to calculate the voltage drop for each wire size
- Compares that drop to your system voltage to get a percentage
- Checks which wire sizes keep you under 3% (or your custom limit)
- Shows you the cost difference between wire sizes so you can make smart decisions
- Accounts for single-phase vs three-phase systems (three-phase is more efficient)
The calculator also handles temperature correction factors automatically. Wire resistance increases with heat, so if you're running wire in a hot attic or near a furnace, you might need to go up a wire size to compensate.
The Math Behind Voltage Drop
Basic Formula Breakdown
Single-Phase: VD = 2 × K × I × L ÷ CM
Three-Phase: VD = 1.732 × K × I × L ÷ CM
VD = Voltage drop in volts
K = 12.9 for copper, 21.2 for aluminum
I = Current in amps
L = One-way distance in feet
CM = Wire area in circular mils
1.732 = √3 for three-phase efficiency
Why These Numbers Matter
The "2" factor: Accounts for current flowing through both hot and neutral wires
Wire constant (K): Based on the actual resistivity of copper or aluminum
Circular mils: How wire area is measured - bigger number means less resistance
Three-phase advantage: More efficient power delivery means less voltage drop
Electrical Code Limits You Must Follow
- Branch Circuits: 3% maximum
- Feeders: 3% maximum
- Total Combined: 5% maximum for branch + feeder
- Critical Circuits: Many engineers aim for 2% or less
Real-World Application Guide
Common Scenarios and Solutions
Residential Panel to Subpanel (100 feet):
For a 100-amp subpanel 100 feet away, you'd need 1 AWG copper or 1/0 aluminum to stay under 3% voltage drop. Going smaller means voltage sag and potential equipment problems.
Workshop with 240V Equipment:
240V circuits handle voltage drop better than 120V. A 3-volt drop is only 1.25% on 240V but 2.5% on 120V. That's why big tools and appliances use 240V.
Outdoor Lighting Circuits:
Long runs to outdoor lights often exceed voltage drop limits with standard 12 AWG. Use 10 AWG for runs over 80 feet to prevent dim lighting and early ballast failure.
Troubleshooting Voltage Drop Issues
Symptoms of Excessive Voltage Drop:
- Motors running hot or drawing high current
- Lights dimming when equipment starts
- Reduced heating element capacity
- Electronic devices malfunctioning
- Frequent breaker trips under load
Quick Field Test:
- Measure voltage at panel with load running
- Measure voltage at equipment with same load
- Difference is your voltage drop
- Calculate percentage: (drop ÷ source voltage) × 100
- Over 3%? You need bigger wire
Frequently Asked Questions
Does wire length mean total length or one-way distance?
Always use one-way distance in calculations. The formula already accounts for current traveling through both the hot and neutral wires. If you enter total wire length, you'll get double the actual voltage drop.
Can I use aluminum wire instead of copper to save money?
Yes, but aluminum has higher resistance, so you need larger wire sizes. The calculator can show you aluminum requirements, but factor in the cost of larger conduit and the special terminations aluminum requires. Sometimes copper ends up cheaper overall.
How does temperature affect voltage drop calculations?
Wire resistance increases with temperature, but the standard formulas use resistance values at 75°C, which covers most installations. In very hot environments like attics or near furnaces, consider going up one wire size to compensate for higher resistance.
Should I calculate voltage drop based on full load or actual load?
Always use the full rated load of the circuit or equipment, not what it's currently drawing. A 20-amp circuit should be calculated at 20 amps even if you're only using 10 amps now. This ensures proper performance when the circuit is fully loaded.
What if my actual load varies throughout the day?
Use the maximum expected load, not the average. For circuits with motors, include starting current which can be 3-6 times the running current. For lighting circuits with electronic ballasts, use 125% of the nameplate rating per NEC requirements.
Can voltage drop cause nuisance breaker trips?
Absolutely. When voltage drops, motors and other equipment draw more current to maintain power output. This increased current can trip breakers that were properly sized for normal voltage operation. Fixing voltage drop often eliminates mysterious tripping issues.
Is it worth spending extra for larger wire to reduce voltage drop?
Usually yes, especially for motor loads and long runs. The extra wire cost is typically recovered through reduced energy costs and longer equipment life. Equipment running at proper voltage uses less power and lasts longer than equipment starved for voltage.
Related Tools
Wire Sizing Calculator
Calculate the correct wire gauge for your electrical installation. Includes ampacity, derating factors, and NEC/CEC compliance.
Calculate Wire Size →Load Calculation
Calculate electrical loads for residential and commercial installations. Determine feeder and service sizes based on NEC requirements.
Calculate Load →Branch Circuit Calculator
Design branch circuits with proper breaker sizing, wire selection, and outlet calculations for residential and commercial projects.
Calculate Branch Circuit →