A 125-amp panel or subpanel requires either a 1 AWG copper wire or a 2/0 AWG aluminum wire. Conductor sizing is based on the 75 °C temperature rating. These sizes apply regardless of whether the feeders supply a main service panel or a subpanel.

Note that if a 125-amp-rated panel is fed by a 100-amp breaker, the conductors are sized for 100 amps, not 125 amps. Panel rating does not determine wire size; the breaker rating does. If this is the case, you should use the sizes recommended for 100 amps.

Clarification & Applications: Why 125-Amp Sizing Is Different From 100-Amp Sizing

A 125-amp panel and a subpanel provide flexibility and a higher capacity margin compared to a 100-amp panel.

This is also why a 125-amp panel may operate just like a 100-amp panel when fed at 100 amps, but still requires larger conductors if the breaker is ever upgraded to 125 amps.

A 125-amp main service panel is typically found in homes where a 100-amp service is no longer sufficient, but a complete 200-amp upgrade is neither necessary nor practical.

Common applications include:

• Small to mid-size homes with electric ranges or induction cooktops

• Homes with an electric dryer combined with other household loads

• Houses planning for one EV charger (typically 32–40 amps)

• Homes with heat pump systems and limited electric auxiliary heat

• Properties that are solar-ready but not fully electrified

This, however, is not a popular solution, and a full 200-amp upgrade to your panel is significantly more efficient. 200 amps is a solid, popular choice that provides more options for future upgrades.

Now, a 125-amp subpanel is a way more popular option that is fed from a main panel with a higher service capacity, such as 200 amps.

125-amp subpanels are most commonly used for:

• Garages

• Basement remodels

• Home additions or ADUs

• EV charging locations

• Solar, battery, or inverter equipment

• Mechanical rooms with heat pumps or air handlers

Although rated for 125 amps, these subpanels are most often fed at 100 amps, allowing for future expansion without the need to replace the panel later.

Types of Cable Used for 125-Amp Feeders

Copper Cable Options (1 AWG)

Common copper conductors used for 125-amp feeders include:

• THHN / THWN-2 copper (1 AWG)

• In EMT, PVC, or rigid conduit, for both indoor and outdoor runs. In PVC conduit, it is the most common option for underground runs.

• Copper SER cable (1-1-1-3 AWG)

• Used for indoor residential feeders where conduit is not required

Aluminum Cable Options (2/0 AWG)

Aluminum cables are widely accepted for residential feeders when properly installed. Common aluminum conductor options include:

• XHHW-2 aluminum or THHN aluminum in conduit

• Aluminum SER cable for residential feeders (2/0-2/0-2/0-1)

• Aluminum URD cable (USE-2 rated) (2/0-2/0-2/0-1) used for underground distribution (direct burial or installed in conduit) between a transformer, meter base, and service equipment

Installation requirements for aluminum conductors specifically are:

• AL-rated lugs and breakers

• Antioxidant compound on terminations

• Proper torque per manufacturer specifications

All of the cables for 125 amp service are available at Nassau National Cable at excellent prices and in top quality.