Wire Size Calculator
Professional electrical wire sizing calculator with full NEC compliance. Calculate proper wire gauge based on ampacity, voltage drop, temperature corrections, and bundling factors.
Wire Size Calculator
Calculate proper wire size based on NEC requirements
Circuit Parameters
🔧 What is Electrical Wire Sizing?
Definition & Purpose
Electrical wire sizing is the process of selecting the appropriate conductor size (gauge) for a specific electrical circuit. Proper wire sizing ensures safe operation, prevents overheating, minimizes voltage drop, and complies with National Electrical Code (NEC) requirements.
The wire size must be adequate to carry the expected current load without exceeding temperature limits while maintaining acceptable voltage levels at the load.
Key Factors
- Current Carrying Capacity (Ampacity): Maximum current the wire can safely carry
- Voltage Drop: Voltage loss over the length of the conductor
- Temperature Rating: Insulation and termination temperature limits
- Installation Conditions: Ambient temperature and bundling effects
🎯 How to Use This Wire Size Calculator
Step-by-Step Instructions
Enter Circuit Parameters
Input your system voltage, phase configuration, load current, and circuit distance. Select the maximum allowable voltage drop percentage.
Choose Conductor Material
Select between copper or aluminum conductors. Copper has better conductivity but aluminum is more cost-effective for larger sizes.
Select Load Type
Choose continuous, non-continuous, or motor load. Continuous and motor loads require 125% safety factor per NEC requirements.
Configure Advanced Settings
Adjust ambient temperature, number of conductors in raceway, insulation rating, and termination temperature for precise calculations.
Review Results
The calculator provides the recommended wire size, detailed analysis, correction factors, and NEC compliance status with warnings if applicable.
Input Parameters Guide
Basic Parameters
- Voltage: System voltage (120V, 208V, 240V, 277V, 480V, 600V)
- Phase: Single-phase or three-phase system
- Current: Load current in amperes
- Distance: One-way circuit length in feet
- Voltage Drop: Maximum allowable voltage drop (1%, 2%, 3%, 5%)
Advanced Settings
- Ambient Temperature: Installation environment temperature
- Conductors in Raceway: Number of current-carrying conductors
- Insulation Rating: 60°C, 75°C, or 90°C insulation type
- Termination Rating: Equipment termination temperature rating
- Installation Type: Raceway, cable, direct burial, or free air
💡 Pro Tips
- • Use 3% voltage drop for branch circuits, 2% for feeders
- • Consider future load growth when sizing conductors
- • Always verify local code requirements
- • Check equipment termination ratings
⚙️ How This Wire Size Calculator Works
📊 Data Sources
- • NEC Table 310.15(B)(16) - Complete ampacity data
- • Temperature correction factors from NEC Table 310.15(B)(2)(a)
- • Bundling adjustment factors from NEC Table 310.15(B)(3)(a)
- • Circular mil values for all standard wire sizes
- • Resistance constants for copper and aluminum
🔄 Calculation Process
- • Real-time calculations as you type
- • Parallel ampacity and voltage drop analysis
- • Automatic correction factor application
- • NEC compliance verification
- • Warning generation for potential issues
✅ Validation & Safety
- • Input range validation
- • Temperature limitation checks
- • Termination compatibility verification
- • Load type safety factor application
- • Comprehensive warning system
Calculation Algorithm
1. Ampacity Calculation
Step 1: Determine required ampacity
Required = Load Current × Safety Factor
Step 2: Apply correction factors
Adjusted = Base Ampacity × Temp Factor × Bundling Factor
Step 3: Consider termination limits
Final = min(Adjusted Ampacity, Termination Rating)
2. Voltage Drop Calculation
Step 1: Calculate required circular mils
CM = (K × I × L × M) / VD
Step 2: Find matching wire size
Select smallest wire with CM ≥ required
Step 3: Verify actual voltage drop
VD% = (K × I × L × M) / (CM × V) × 100
🔍 Result Analysis
Final Wire Size Selection
The calculator selects the larger of the two wire sizes determined by:
- • Ampacity requirements (with all correction factors)
- • Voltage drop limitations
- • NEC compliance verification
- • Safety margin considerations
Warning System
Automatic warnings are generated for:
- • High voltage drop conditions
- • Temperature rating mismatches
- • Extreme ambient conditions
- • Heavy bundling situations
- • Potential code violations
📐 Wire Sizing Calculation Methods
1. Ampacity Method
Based on the current-carrying capacity of the conductor according to NEC Table 310.15(B)(16).
Formula: Required Ampacity = Load Current × Safety Factor
Safety Factors:
- • Continuous loads: 125%
- • Motor loads: 125%
- • Non-continuous: 100%
2. Voltage Drop Method
Ensures adequate voltage at the load by limiting voltage drop to acceptable levels.
Formula: CM = (K × I × L × M) / VD
- • CM = Circular Mils
- • K = Resistance constant
- • I = Current (A)
- • L = Length (ft)
- • M = Phase multiplier
- • VD = Voltage drop (V)
3. Combined Method
Uses the larger of the two wire sizes determined by ampacity and voltage drop methods.
Process:
- • Calculate ampacity size
- • Calculate voltage drop size
- • Select larger wire size
- • Apply correction factors
- • Verify NEC compliance
📋 NEC Compliance & Standards
Key NEC Requirements
Section 210.19(A) - Branch Circuits
Branch circuit conductors must have an ampacity not less than the maximum load to be served, with continuous loads calculated at 125%.
Section 215.2(A) - Feeders
Feeder conductors must have an ampacity not less than required to supply the load as calculated per Article 220.
Section 110.14(C) - Temperature Limitations
Conductor ampacity must be based on the lowest temperature rating of any connected termination, conductor, or device.
NEC Tables Used
Table 310.15(B)(16)
Allowable ampacities of insulated conductors rated up to 2000 volts.
- • Not more than three current-carrying conductors
- • In raceway, cable, or earth (directly buried)
- • Based on ambient temperature of 30°C (86°F)
Table 310.15(B)(2)(a)
Ambient temperature correction factors for conductors.
- • Multiply base ampacity by correction factor
- • Based on actual ambient temperature
- • Separate factors for different insulation ratings
Table 310.15(B)(3)(a)
Adjustment factors for more than three current-carrying conductors.
- • Applies when more than 3 conductors in raceway
- • Reduces ampacity due to heat buildup
- • Factors range from 80% to 35%
🧮 Practical Calculation Examples
Example 1: Residential Branch Circuit
Given:
- • Load: 20A continuous
- • Voltage: 120V single-phase
- • Distance: 75 feet
- • Max voltage drop: 3%
- • Copper conductors, 75°C terminations
Solution:
- • Required ampacity: 20A × 1.25 = 25A
- • Ampacity method: 12 AWG (25A at 75°C)
- • Voltage drop method: 10 AWG
- • Result: 10 AWG copper
Example 2: Commercial Feeder
Given:
- • Load: 100A
- • Voltage: 480V three-phase
- • Distance: 200 feet
- • Max voltage drop: 2%
- • Aluminum conductors, 6 in conduit
- • Ambient temperature: 40°C
Solution:
- • Bundling factor: 0.80
- • Temperature factor: 0.88
- • Required base ampacity: 100A ÷ (0.80 × 0.88) = 142A
- • Result: 1/0 AWG aluminum
🔌 Wire Types & Applications
Building Wire
THHN/THWN-2
90°C dry, 75°C wet. Most common building wire for branch circuits and feeders.
XHHW-2
90°C dry and wet. Excellent for underground and wet locations.
THW
75°C wet and dry. Traditional choice for many applications.
Cable Types
NM-B (Romex)
90°C rating. Residential wiring in dry locations only.
UF-B
90°C rating. Underground feeder cable for wet locations.
MC Cable
90°C rating. Metal-clad cable for commercial applications.
Service Entrance
USE-2
90°C rating. Underground service entrance cable.
SER/SEU
75°C rating. Service entrance cable for overhead connections.
RHH/RHW-2
90°C dry, 75°C wet. Heavy-duty applications.
❓ Frequently Asked Questions
What's the difference between ampacity and voltage drop sizing?
Ampacity sizing ensures the wire can safely carry the current without overheating. Voltage drop sizing ensures adequate voltage reaches the load. Both must be satisfied, so use the larger wire size.
Why do I need temperature correction factors?
Wire ampacity decreases as ambient temperature increases. NEC requires derating the ampacity when ambient temperature exceeds 30°C (86°F) to prevent overheating.
When do bundling adjustment factors apply?
When more than three current-carrying conductors are in the same raceway, cable, or conduit. This accounts for additional heat buildup that reduces ampacity.
What's the maximum allowable voltage drop?
NEC recommends maximum 3% for branch circuits and 2% for feeders, with a combined maximum of 5%. Some applications may require tighter limits.
Should I use copper or aluminum conductors?
Copper has better conductivity and is easier to work with but costs more. Aluminum is lighter and less expensive but requires larger sizes and special terminations.
How do I handle motor loads?
Motor circuits require 125% sizing factor for continuous operation. Also consider starting current, which can be 6-8 times running current, for voltage drop calculations.
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