⚡ Voltage Drop Calculator

NEC-Compliant Wire Sizing for Branch Circuits & Feeders

Overview

Calculator

Wire Sizing

Reference Tables

Examples

Field Tips

📐 What is Voltage Drop?

Voltage drop is the reduction in voltage as electrical current flows through a conductor. All wires have resistance, and this resistance causes some voltage to be "lost" as heat along the length of the wire. Too much voltage drop means equipment won't operate properly - motors run hot, lights dim, and energy is wasted.

⚠️ Effects of Excessive Drop

Motors overheat and fail early
Lights flicker or dim
Electronics malfunction
Wasted energy (heat in wires)
Nuisance tripping

✓ NEC Recommendations

≤3% for branch circuits
≤5% total (feeder + branch)
These are recommendations, not code requirements
Many specs require ≤2% or ≤3% total

📋 Factors Affecting Drop

Wire size: Larger = less drop
Length: Longer = more drop
Current: Higher = more drop
Material: Copper vs aluminum
Temperature: Hotter = more resistance

🔌 How Voltage Drop Works

💡 Key Point

Voltage drop occurs in both the hot and neutral conductors. For single-phase circuits, the total circuit length for calculation is 2× the one-way distance.

📊 The Formula

VD = (2 × K × I × L) / CM

VD = Voltage Drop (volts)

K = Resistivity constant (12.9 Cu, 21.2 Al)

I = Current (amps)

L = One-way length (feet)

CM = Circular mils (wire size)

VD% = (VD / V_source) × 100

Percentage drop relative to source voltage

📏 Understanding the 3% and 5% Rules

✓ Feeder: ≤2%

From service entrance to panel/MCC. Allows 3% remaining for branch circuits.

✓ Branch: ≤3%

From panel to final outlet. This is the most common calculation.

⚠️ Total: ≤5%

Combined feeder + branch. NEC 210.19(A) & 215.2(A) Informational Notes.

🔢 Voltage Drop Calculator

System Voltage

Phase Configuration

Load Current Amperes (A)

One-Way Distance Feet (one-way, not round trip)

Conductor Material

Wire Size (AWG/kcmil)

Conduit Type (for AC resistance)

📊 Wire Size Comparison

Compare voltage drop across different wire sizes for your inputs:

Enter values and calculate to see comparison

💡 Recommendation

Select your parameters and calculate to get a wire size recommendation.

📈 Voltage Profile

⚡ Quick Reference

Drop %	Status	Action
≤ 2%	Excellent	Ideal for sensitive loads
2-3%	Good	Acceptable for branch circuits
3-5%	Marginal	OK if feeder drop is minimal
> 5%	Excessive	Increase wire size or reduce length

⚠️ Motor Circuits

Motors are especially sensitive to voltage drop. A 10% drop can reduce motor torque by 19% and increase current draw, causing overheating.

🔍 Find Minimum Wire Size

Calculate the minimum wire size needed for a target voltage drop:

System Voltage

Phase

Load Current (Amps)

Distance (Feet)

Material

Target Max Drop (%)

📏 Maximum Distance Calculator

Find the maximum distance for a given wire size and load:

System Voltage

Phase

Load Current (Amps)

Wire Size

Material

Max Allowable Drop (%)

📋 Common Circuit Quick Reference

Maximum one-way distances for 3% drop with copper conductors:

Wire Size	120V Single Phase (feet)				240V Single Phase (feet)				480V Three Phase (feet)
Wire Size	15A	20A	30A	40A	20A	30A	40A	50A	30A	50A	100A
14 AWG	50	37	-	-	75	-	-	-	-	-	-
12 AWG	79	59	40	-	118	79	-	-	137	-	-
10 AWG	125	94	63	47	188	125	94	75	217	130	-
8 AWG	199	149	100	75	299	199	149	120	345	207	104
6 AWG	316	237	158	119	474	316	237	190	548	329	164
4 AWG	503	377	251	188	754	503	377	302	871	523	261
2 AWG	798	599	399	299	1198	798	599	479	1384	830	415
1/0	1267	950	634	475	1901	1267	950	760	2197	1318	659
4/0	2016	1512	1008	756	3024	2016	1512	1210	3494	2097	1048

💡 Using This Table

Find your voltage and current, then read the maximum distance. If your actual distance exceeds the table value, you need a larger wire size. Dash (-) indicates wire is undersized for that current per NEC ampacity tables.

🔌 Wire Size - Circular Mils

Circular mils (CM) is the cross-sectional area used in voltage drop calculations:

AWG/kcmil	Circular Mils	Diameter (in)
14	4,110	0.064
12	6,530	0.081
10	10,380	0.102
8	16,510	0.128
6	26,240	0.162
4	41,740	0.204
3	52,620	0.229
2	66,360	0.258
1	83,690	0.289
1/0	105,600	0.325
2/0	133,100	0.365
3/0	167,800	0.410
4/0	211,600	0.460
250	250,000	0.500
300	300,000	0.548
350	350,000	0.592
400	400,000	0.632
500	500,000	0.707
600	600,000	0.775
750	750,000	0.866

⚡ NEC Ampacity (75°C)

Maximum allowable current per NEC Table 310.16:

AWG/kcmil	Copper	Aluminum
14	20A	-
12	25A	20A
10	35A	30A
8	50A	45A
6	65A	50A
4	85A	65A
3	100A	75A
2	115A	90A
1	130A	100A
1/0	150A	120A
2/0	175A	135A
3/0	200A	155A
4/0	230A	180A
250	255A	205A
300	285A	230A
350	310A	250A
400	335A	270A
500	380A	310A

⚠️ Important

Wire size must satisfy BOTH ampacity AND voltage drop requirements. Size for ampacity first, then verify voltage drop.

🔧 Resistivity Constants (K)

Material	K Value	Notes
Copper (Cu)	12.9	Standard at 75°C
Aluminum (Al)	21.2	Standard at 75°C

💡 Temperature Effect

K values increase with temperature. At higher operating temperatures, resistance increases and voltage drop worsens. Values shown are for 75°C conductor temperature.

Temp	Copper K	Aluminum K
60°C	12.2	20.1
75°C	12.9	21.2
90°C	13.6	22.4

📐 Phase Multipliers

🔄 Copper vs Aluminum Comparison

Same voltage drop requires larger aluminum wire:

Copper Size	Equivalent Aluminum	Cu Ampacity	Al Ampacity	Notes
10 AWG	8 AWG	35A	45A	Residential branch circuits
8 AWG	6 AWG	50A	50A	Subpanels, ranges
6 AWG	4 AWG	65A	65A	HVAC, welders
4 AWG	2 AWG	85A	90A	Service feeders
2 AWG	1/0 AWG	115A	120A	100A services
1/0 AWG	3/0 AWG	150A	155A	150A services
4/0 AWG	250 kcmil	230A	205A	200A services
250 kcmil	350 kcmil	255A	250A	Large feeders
350 kcmil	500 kcmil	310A	310A	Industrial feeders

💰 Cost Consideration

Aluminum is typically 60-70% cheaper than copper per foot, but requires 1-2 sizes larger for equivalent voltage drop. For long runs, aluminum often provides the best value despite the larger conduit required.

🏠 Example 1: Residential Subpanel

Scenario

100A subpanel in detached garage, 150 feet from main panel, 240V single-phase. What size copper wire is needed for 3% drop?

Given:

• V = 240V, 1φ | • I = 100A | • L = 150 ft

• Max VD = 3% = 7.2V | • K = 12.9 (copper)

CM = (2 × K × I × L) / VD

CM = (2 × 12.9 × 100 × 150) / 7.2 = 53,750 CM

Required Wire Size

2 AWG

Copper (66,360 CM) - Actual drop: 2.4%

🏭 Example 2: Industrial Motor

Scenario

50 HP motor at 480V 3-phase, FLA = 65A, 400 feet from MCC. Target ≤3% voltage drop with copper.

Given:

• V = 480V, 3φ | • I = 65A | • L = 400 ft

• Max VD = 3% = 14.4V | • K = 12.9

CM = (√3 × K × I × L) / VD

CM = (1.732 × 12.9 × 65 × 400) / 14.4 = 40,361 CM

Required Wire Size

4 AWG

Copper (41,740 CM) - VD: 2.9%, Ampacity: 85A ✓

💡 Example 3: Lighting Circuit

Scenario

277V lighting circuit, 20A load, 250 feet to furthest fixture. What's the voltage drop with #10 copper?

Given:

• V = 277V, 1φ | • I = 20A | • L = 250 ft

• Wire = #10 AWG (10,380 CM) | • K = 12.9

VD = (2 × K × I × L) / CM

VD = (2 × 12.9 × 20 × 250) / 10,380 = 12.4V (4.5%)

Voltage Drop

4.5%

⚠️ Exceeds 3% - Upgrade to #8 AWG (2.8%)

🔌 Example 4: Aluminum Feeder

Scenario

200A feeder to remote building, 480V 3-phase, 500 feet. Compare copper vs aluminum for 2% max drop.

Material	K	Required CM	Wire Size	Cost
Copper	12.9	116,597	2/0 AWG	$$$$
Aluminum	21.2	191,594	4/0 AWG	$$

💰 Cost Analysis

Aluminum 4/0 is approximately 50% cheaper than copper 2/0 for this run, even with larger conduit.

🎯 Quick Calculation Shortcuts

🔢 The "3-4-5 Rule" (Copper, 3%, 120V)

#14: ~50 ft max at 15A
#12: ~80 ft max at 15A
#10: ~125 ft max at 15A
Double distance for 240V
Halve distance for double current

⚡ Quick Mental Math

For copper at 3% drop:

Max feet ≈ (CM × VD%) / (25.8 × I)

Or use: Double wire size = quadruple distance

⚠️ Common Gotchas

Distance is ONE-WAY, not round trip
3-phase uses √3 ≈ 1.732, not 2
Aluminum needs 1-2 sizes larger
Motor starting current can be 6× FLA

📏 Measuring Voltage Drop

How to verify voltage drop in the field:

Measure voltage at the panel/source with load ON
Measure voltage at the load/equipment with load ON
Calculate: VD = V_source - V_load
Calculate %: VD% = (VD / V_source) × 100

⚠️ Important

Measurements must be taken under load. No-load measurements will show minimal voltage drop.

🔧 Troubleshooting High Drop

Common Causes

Loose connections: High resistance at terminations
Undersized wire: Original design or load increase
Long runs: Distance exceeds design
Damaged conductors: Nicks, corrosion, overheating
Poor splices: Wire nuts not properly tightened
Aluminum oxidation: Oxide layer increases resistance

✓ Solutions

Re-torque all connections to spec
Replace undersized conductors
Add parallel conductors
Move equipment closer to source
Use anti-oxidant on aluminum

⚡ Motor Voltage Sensitivity

⚠️ Critical

Torque varies as voltage squared. 10% drop = 19% torque loss. Motors may not start or may stall under load.

📋 NEC Code References

Section	Topic	Key Point
210.19(A) Note 4	Branch Circuit VD	≤3% recommended
215.2(A) Note 2	Feeder VD	≤3% recommended
Combined	Total System	≤5% max
310.16	Ampacity	Conductor sizing
Ch.9, Table 8	DC Resistance	Resistance values

💡 Note

The 3% and 5% limits are recommendations (Informational Notes), not requirements. Many specs make them mandatory.

🎯 Quick Field Reference Card

Voltage Drop Limits

≤2%	Excellent
≤3%	Good (branch)
3-5%	Marginal
>5%	Excessive

K Values (75°C)

Copper	12.9
Aluminum	21.2

Phase Multipliers

1φ	× 2
3φ	× √3 (1.732)

Formulas

VD = (M×K×I×L) / CM
CM = (M×K×I×L) / VD
L = (VD×CM) / (M×K×I)

⚡ Voltage Drop Calculator

⚠️ Effects of Excessive Drop

✓ NEC Recommendations

📋 Factors Affecting Drop

💡 Key Point

✓ Feeder: ≤2%

✓ Branch: ≤3%

⚠️ Total: ≤5%

Voltage Drop

💡 Recommendation

⚠️ Motor Circuits

Minimum Wire Size Required

Maximum One-Way Distance

💡 Using This Table

⚠️ Important

💡 Temperature Effect

💰 Cost Consideration

Scenario

Required Wire Size

Scenario

Required Wire Size

Scenario

Voltage Drop

Scenario

💰 Cost Analysis

🔢 The "3-4-5 Rule" (Copper, 3%, 120V)

⚡ Quick Mental Math

⚠️ Common Gotchas

⚠️ Important

Common Causes

✓ Solutions

⚠️ Critical

💡 Note

Voltage Drop Limits

K Values (75°C)

Phase Multipliers

Formulas