125 Amp Wire Sizes: What You Need to Know

Proper wire sizing is vital for safety

Electrician holding wires
Photo: Grispb / Adobe Stock
Electrician holding wires
Photo: Grispb / Adobe Stock
Highlights
  • Handling electrical installations requires a qualified electrician.

  • 125 amp wires are available in copper and aluminum.

  • 125 amp copper wire is thinner but more costly than aluminum.

  • Proper wire size is essential for electrical safety.

Get quotes from up to 3 pros!
Enter a zip below and get matched to top-rated pros near you.
Geo Icon

Installing a whole-house generator or extending electrical service in your home through a subpanel requires using the correct wire sizes, including 125 amp wire size. 

Hiring a qualified electrical contractor is necessary for this type of electrical work. However, having wire-sizing information, such as the proper 125 amp wire size, will help you better understand a price breakdown for subpanel work or the cost of installing a whole-house generator.

How Are Wire Sizes Measured?

Electrician stripping insulation from wire
Photo: Karyna Chekaryova / Adobe Stock

Electrical wires come in various sizes to accommodate different electrical loads or how much electricity they can safely handle. We measure wire sizes by their thickness and express the measurement in gauges. The loads they handle, we measure in amps (A), volts (V), and watts (W).

AWG means American Wire Gauge and is one of the standards that denotes wire gauge or thickness. Another measurement notation when dealing with large-diameter wires is kcmil or thousandths of circular mils where one kcmil equals 0.5067 mm squared.  

AWG Significance

The larger a wire's diameter, the lower its gauge number representation and the more amps it can safely handle. In general, household circuit wire sizes range from 14 AWG  for 15A circuits to 6 AWG for up to 60A ones. 

When discussing larger amp loads for electrical service, sub-panels, or whole-house generator feeder lines, we're talking about wire sizes between 4 AWG to larger than 1 AWG. Wires larger than 1 AWG use 1/0 AWG or 0 AWG up to 4/0 or 0000 AWG. The numbers get bigger as the diameter increases. Beyond 4/0 AWG, the measurement changes to kcmil, a direct measurement of the wire's diameter. 

125 Amp Copper vs. Aluminum Wire 

The current standard material for household circuitry wiring is copper. However, copper and aluminum wires are common and legal for use in large-gauge electrical applications, such as feeder wires to your home. 

Copper wire is more costly than aluminum but less resistant to electrical current. You can use smaller diameter copper wire than aluminum for the same amp load. Aluminum wiring requires specific hardware for installation to handle the metal's tendency to expand and contract. 

Small-gauge aluminum wires for household circuits were popular once but no longer meet building code requirements. 

125A Copper Wire125A Aluminum Wire
2 AWG1/0 AWG
0.2576-inch diameter0.3249-inch diameter
Max amps 130AMax amps 135A

Common Feeder Amp Wire Sizes

Installing the correct 125A wire size depends primarily on the amount of current passing through the wire. The chart below contains typical household copper and aluminum wire sizes based on amps.

100A4 AWG2 AWG
125A2 AWG1/0 AWG
150A1/0 AWG2/0 AWG
200A2/0 AWG4/0 AWG

Why is Wire Size Important?

Properly sizing wires to meet electrical current demands is necessary to avoid overheating and is more than a simple electrical safety tip. The movement of electricity through wires creates heat due to the friction of a wire's resistance. A wire's AWG or diameter affects its resistance measurement or ohms. Larger gauge wires have less resistance than smaller ones. 

Friction heat can be excessive in wires too small for the current passing through them. Electrical fires can result from this condition. So, it's crucial to install the correct wire size for the electricity that will travel through them. Luckily, your local electrician will know the requirements and code restrictions for your location and is indispensable when installing or servicing large electrical installations. 

Need professional help with your project?
Get quotes from top-rated pros.
The homeowners guide to electrical services
From average costs to expert advice, get all the answers you need to get your job done.
A family watching TV in living room
Aug 21, 2024
Aug 21, 2024
Find out how much it costs to install a cable outlet for TV or internet. Our expert guide breaks down the costs involved in installing a new coaxial outlet and explores how you can save and if you can do this project yourself.
Wood shed
Aug 21, 2024
Aug 21, 2024
Running electricity to a detached garage or shed brings life to new areas of your home. If you’re building a workspace, an office, or a loft, you could even see a return on your investment by making that space liveable or useful for projects.
Man in laundry room loading dryer
Aug 21, 2024
Aug 21, 2024
The cost to install a 240-volt outlet varies depending on the amperage and the home’s current wiring conditions. Use this guide to budget for your next project.
man with tool belt installing hanging lights
Jul 18, 2024
Jul 18, 2024
Did you know there are different types of electricians? If you need electrical work done but aren’t sure who to hire, click here to explore your options.
Smoke detector on ceiling
Jul 19, 2024
Jul 19, 2024
How long do smoke detectors last? Keep reading to find out when you should replace your smoke detectors and how to maintain them.
A family reading a book in front of a teepee tent
Sep 24, 2024
Sep 24, 2024
Knowing what an electrical emergency is in your home helps you take speedy and appropriate action to keep your family safe and get your power back on.
Share project details to get started.