Knowing Which Wire Gauge Works at 100 Amps

When wiring a new construction project or upgrading electrical service in an existing home, one critical decision is selecting the proper wire gauge that can safely handle the expected amp load. For many residential and light commercial applications, 100 amps is a common service size. But determining the right gauge wire for 100 amp circuits requires understanding key electrical codes and ratings.

We'll also look at recommendations from electricians for wire installation best practices when working with 100 amp capacities.

Electrical Basics - Voltage, Current and Wire Gauge

First, let's quickly review some basic electricity terms that relate wire thickness to amps and volts...

• Voltage - The "pressure" that pushes electrical current through a wire.
• Amps - The amount of current flowing through a wire.
• Gauge - The diameter size of the conductor (wire).

For a given voltage, more amps (current) require a larger diameter wire size to prevent overheating hazards. Wire gauge thickness increases as the AWG number decreases. So a 3 AWG wire is thicker than a 10 AWG wire.

Ohm's Law and 100 Amp Service

Understanding basic Ohm's law also helps explain the relationship between voltage , current and wire thickness. As the formula shows:

• Voltage (Volts) = Current (Amps) X Resistance (Ohms)

So for a given voltage ( usually 120V or 240V ), higher current loads demand lower resistance wires to prevent excess heat buildup. That's why the 100 amp load in our example needs a larger minimum wire gauge versus smaller 20 amp branch circuits common in homes.

Wire Gauge Rating Systems

Determining what size wire can safely handle 100 amps relies first on understanding wire gauge rating standards...

American Wire Gauge (AWG)

In the US, the American Wire Gauge (AWG) scale is most often used to specify wire diameter. With AWG, the larger the gauge number, the smaller the physical wire diameter:

• 1 AWG wire has a larger thickness than 4 AWG wire.
• 10 AWG wire is thinner than 4 AWG wire.

Why does this inverse scale exist? As wire manufacturing improved to make smaller diameter wires, more gauge numbers were added. So a 4 AWG wire once was the "largest" gauge until technology enabled even thicker wires like 1, 2 or 3 AWG sizes for high capacity electrical loads.

Code Requirements for 100 Amp Service

For any new constructions or service upgrades, following approved wire specifications is critical for safety. In the United States, the National Electrical Code (NEC) sets legal standards for the correct wire thickness needed based on expected electrical load.

NEC Minimum Size Rules

Under most conditions, the NEC requires a minimum 3 AWG copper or 1 AWG aluminum wire for 100 amp services less than 120/240V. However, some electricians may increase this wire gauge depending on:

• Length of wire run from main panel to subpanel
• If wires are bundled inside shared conduit
• Capacity needs exceeding 100 amps
• Area temperature extremes effecting wire ratings

Choosing Correct Wire Size

Use the following as a guide when selecting suitable 100 amp feeder wires for your specific project...

Voltage Drop Based on Run Length

The longer the distance between a 100 amp subpanel and the main electrical meter/panel, the greater the potential voltage drop. Electricians may recommend thicker wires than legally required to account for this:

• Up to 50 ft run - Minimum code wire size usually fine
• 50 to 100 ft - Increase wire thickness 1 size
• Over 100 ft - Increase wire thickness 2 sizes

Number of Conductors Bundled

When running multiple feeder wires through shared conduit, the combined heat may reduce safe amp capacity. De-rating amp tables adjust for this:

• 3-5 wires usually need 1 wire gauge increase
• 6-9 wires usually need 2 wire gauge sizes increase

Aluminum vs Copper Wire

Although less conductive per size than copper, aluminum wire is lighter in weight and lower cost. Follow specialty termination procedures when using aluminum feeder wires:

• Minimum 1 AWG for 100A aluminum wire
• Approved connectors to prevent oxidation issues
• Upsized breaker panel lugs to match wire diameter

Installation Considerations

Properly installing feeder wires for 100 amp subpanels or upgrades involves a few key steps covered in our wire gauge guide:

Pulling Wires Through Conduit

Use approved lubricants when pulling thicker 3 AWG or larger wires through conduit runs:

• Ensures less friction and tension on wire insulation
• Avoids scrape damage and conductor breaks

Securing and Connecting Wires

When securing feeder wire ends, avoid over-torqueing terminal screws. Use oxide inhibitors and anti-seize compounds when terminating aluminum wires.

Verifying Proper Installation

Double check electrical connections meet torque specs. Verify suitable wire gauge with an amp probe or infrared scan after energizing.

Safety Factors

Why does the NEC specify such large wire diameters for 100 amp services? Let's look at a few key reasons to select feeder wires able to handle capacities above rated loads...

Prevents Overheating Risks

Thicker wires reduce resistance to pass the maximum expected amp load without reaching unsafe temperatures.

Accounts for Overcurrent Conditions

Short circuits or even 30 year-old kitchen appliances may temporarily draw much more than 100 amps. Adequate wire gauge prevents momentary spikes from starting electrical fires.

Considers Ambient Temperature

The hotter your climate, the lower the amp rating permitted for a given wire gauge per NEC standards. Using the next larger size as recommended provides a safety buffer.

Typical Wire Size Examples

To give you a better idea of what wire gauge gets commonly used for 100 amp residential or commercial electrical service, check out these quick examples:

Typical Home 100A Feeder Wire

3 AWG copper wire meets code for a 10 foot feeder run from a main panel to a 100A subpanel for lighting, receptacles and equipment like an AC condenser or welder.

Commercial Kitchen 100A Wiring

2 AWG aluminum wire may feed several large electric ovens, hot water heaters and other equipment requiring 100A 220V service.

Garage or Workshop 100A Subpanel

1 AWG aluminum wire twisted as a "mobile home" feeder easily handles 100A for multiple power tools plus a vehicle charger or heater.

We've covered the essential information needed to safely size copper and aluminum wire gauges for 100 amp electrical service. Be sure to follow local codes and consider voltage drop, conduit fill percentage and ambient temperatures when selecting your project's feeder wires.

Install feeders carefully to avoid damaging insulation or causing hazardous loose connections. For any complex new wiring or upgrades, don't hesitate to consult a professional electrician or inspector to ensure you pick the ideal gauge wire for 100 amps in your home or business.