125 Amp Wire Sizes: What You Need to Know

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?

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. 

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.

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.