Table of Contents
What Is Wire Sizing?
Wire sizing is the process of selecting the correct conductor gauge to safely carry electrical current over a given distance while keeping voltage drop within acceptable limits. For a 100-amp service, the National Electrical Code (NEC) requires specific minimum wire gauges depending on the conductor material and installation conditions.
Undersized wires overheat, waste energy, and create fire hazards. Oversized wires are unnecessarily expensive. The goal is to find the smallest wire that safely handles the load while maintaining voltage within 3% of the source voltage for branch circuits or 5% total for feeder plus branch circuits combined.
Wire Size Formula
Where VD is voltage drop in volts, K is the resistivity constant (10.37 for copper, 17.02 for aluminum), I is the current in amps, D is the one-way distance in feet, and CM is the circular mil area of the wire. After computing the required CM, the next larger standard AWG wire is selected.
AWG Wire Size Table
| AWG | Circular Mils | Copper Amps (75C) | Aluminum Amps (75C) |
|---|---|---|---|
| 14 | 4,110 | 15 | -- |
| 12 | 6,530 | 20 | 15 |
| 10 | 10,380 | 30 | 25 |
| 8 | 16,510 | 40 | 35 |
| 6 | 26,240 | 55 | 45 |
| 4 | 41,740 | 70 | 60 |
| 3 | 52,620 | 85 | 70 |
| 2 | 66,360 | 95 | 80 |
| 1 | 83,690 | 110 | 95 |
| 1/0 | 105,600 | 125 | 110 |
| 2/0 | 133,100 | 145 | 125 |
| 3/0 | 167,800 | 165 | 145 |
| 4/0 | 211,600 | 195 | 170 |
Factors Affecting Wire Size
- Distance: Longer runs require larger wire to compensate for voltage drop.
- Material: Aluminum has higher resistivity than copper, so it needs a larger gauge for the same current.
- Temperature: Higher ambient temperatures reduce the current-carrying capacity of a conductor.
- Conduit fill: Multiple conductors in a single conduit generate more heat, requiring derating.
Frequently Asked Questions
What wire size do I need for 100 amps?
For copper wire at 75C rating, you typically need 1 AWG for a short run (up to about 50 feet). For longer runs, such as 100 feet, you may need 1/0 or 2/0 AWG to keep voltage drop within 3%. Aluminum wire requires 1/0 AWG minimum for 100 amps at short distances.
Can I use aluminum wire for 100-amp service?
Yes, aluminum wire is commonly used for 100-amp service feeders. It is lighter and less expensive than copper. However, you must use a larger gauge (typically 1/0 AWG aluminum vs 1 AWG copper) and ensure all connections use anti-oxidant compound and are rated for aluminum.
What happens if the wire is too small?
An undersized wire will overheat under load, potentially melting insulation and causing a fire. It also causes excessive voltage drop, which makes appliances perform poorly and can damage sensitive electronics. The circuit breaker may not trip in time to prevent damage since it responds to current, not heat in the wire.