Electrical Engineering Guides
Professional-grade electrical engineering education with interactive calculators, detailed explanations, and real-world examples.
Guide Topics
100 Amp Wire Size Guide
Complete specifications for 100A service entrance and subpanel installations
Quick Answer: Wire Size for 100 Amp Service
For 100 amp service, use #3 AWG copper wire or #1 AWG aluminum wire. For runs over 100 feet, increase to #2 AWG copper or #1/0 AWG aluminum to compensate for voltage drop. Always use #8 AWG copper for the equipment grounding conductor per NEC Table 250.122.
Wire Size by Distance - 100 Amp Service
| Distance | Copper Wire | Aluminum Wire | Voltage Drop |
|---|---|---|---|
| 0-50 ft | 3 AWG | 1 AWG | <1% |
| 50-100 ft | 2 AWG | 1/0 AWG | 1-2% |
| 100-150 ft | 1 AWG | 2/0 AWG | 2-3% |
| 150-200 ft | 1/0 AWG | 3/0 AWG | 2.5-3.5% |
| 200-250 ft | 2/0 AWG | 4/0 AWG | 3-4% |
| 250-300 ft | 3/0 AWG | 250 kcmil | 3.5-4.5% |
Note: NEC recommends maximum 3% voltage drop for branch circuits, 5% total for feeders and branch circuits combined.
Common 100 Amp Applications
Main Service
Small home main panel
Garage Subpanel
Detached garage workshop
Mobile Home
Manufactured home feeder
Pool House
Pool equipment subpanel
Cost Comparison - Copper vs Aluminum
| Wire Type | Per Foot | 100 ft Run | 200 ft Run |
|---|---|---|---|
| Copper 3 AWG | $8.50 | $850 | $1,700 |
| Copper 2 AWG | $11.20 | $1,120 | $2,240 |
| Aluminum 1 AWG | $3.85 | $385 | $770 |
| Aluminum 1/0 AWG | $4.60 | $460 | $920 |
Savings: Aluminum wire typically costs 50-65% less than copper for the same ampacity. Consider total project cost including terminations and labor.
Installation Requirements & Code References
| Component | Specification | NEC Reference |
|---|---|---|
| Main Breaker | 100A 2-pole breaker | NEC 230.79 |
| Wire Size (Copper) | #3 AWG minimum | NEC Table 310.16 |
| Wire Size (Aluminum) | #1 AWG minimum | NEC Table 310.16 |
| Ground Wire | #8 AWG copper | NEC Table 250.122 |
| Conduit Size | 1¼" minimum for 4 wires | NEC Chapter 9 |
| Grounding Electrode | #6 AWG copper GEC | NEC 250.66 |
Step-by-Step Installation Guide
- 1
Calculate Load Requirements
Verify that 100A is sufficient for your needs using NEC Article 220 load calculations.
- 2
Measure Distance
Measure the actual wire run distance including vertical runs and bends.
- 3
Select Wire Size
Choose wire based on distance: #3 AWG copper for short runs, upsize for longer distances.
- 4
Install Conduit
Run 1¼" conduit for 4 conductors (2 hots, 1 neutral, 1 ground).
- 5
Pull Conductors
Pull THHN/THWN-2 conductors through conduit, maintain proper color coding.
- 6
Make Terminations
Use proper torque specs and anti-oxidant compound for aluminum connections.
- 7
Install Grounding System
Connect #8 AWG equipment ground and #6 AWG grounding electrode conductor.
- 8
Test & Inspect
Test insulation resistance, verify connections, and schedule inspection.
Temperature & Environmental Factors
High Temperature Areas
- Attics over 40°C (104°F): Derate to 88% capacity
- Use THHN-2 rated for 90°C in dry locations
- Consider upsizing wire for extreme heat conditions
Wet Locations
- Use THWN-2 or XHHW for wet locations
- Underground: USE-2 or direct burial cable
- Schedule 80 PVC for physical protection
Related Calculators
Common Mistakes to Avoid
Sizing Errors
- ❌ Using #4 AWG copper (only rated for 85A at 75°C)
- ❌ Forgetting voltage drop for long runs
- ❌ Not accounting for continuous loads (125% rule)
- ❌ Using 60°C column instead of 75°C
Installation Errors
- ❌ Undersized conduit causing difficult pulls
- ❌ Missing anti-oxidant on aluminum connections
- ❌ Incorrect torque on terminals
- ❌ Wrong ground wire size (#10 instead of #8)