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125 Amp Sub Panel Wire Size (All You Need to Know)

Electricity is one of the essential power sources that we use on a daily basis, therefore it is crucial for us to know which electronic components and wiring should be used around our houses.

The amps in your electrical panel signify how much electrical service size is available to you – and this article discusses the wire requirements for a 125-amp sub panel.


The ampacity of a wire is referred to the carrying capacity of a wire and defined as a threshold as to how much electricity can a wire carry safely.

It is important to not exceed the ampacity of a wire or cable because that could compromise on the electrical safety and may result in damages to the devices the wires are connected to.

Besides damages to the device, it could also expose you to risks such as electrocution and even fire risks.


Another important property of a wire is its thickness which is known as its gauge, represented by a number assigned to it on the insulation.

The higher the number is, the thinner the wire is – therefore, if you want a thicker wire, you need to pick a wire with a lower number.

The number assigned to the wire is known as the AWG or American Wire Gauge which is used to standardize the thickness of a wire.

The thickness of a conductive wire is a crucial property because changes in thickness of a wire also affects its other properties such as resistance and carrying capacity.

The AWG is a useful metric to determine which wire is suitable for use in which application and maintain electrical safety.

125-amp sub panel wire size:

125-amps sub panel means 125-amps of electricity can be supplied safely into your home and how many devices you can run. You must have an appropriately sized wire to capacitate the electricity from the sub panel – a 1/0 AWG aluminum wire or 1 AWG copper wire is suitable for 125-amps.

The 125-amp rating in your panel is an indicator that your panel has the electrical service capacity of 125-amps – which is the total volume of electricity the wires can carry to your home.

This ampacity also determines how many devices or electrical equipment can you safely use in your house.

When the power from the main grid reaches your house, they go through the service panel where they get distributed to the branch circuits which can be determined by the pair of bus bars the panel has.

One bus bar provides 120 volts whereas two bus bars can provide 240 volts worth of electricity. Therefore, the panel crucially determines the service size of electricity into your house.

In the 1960s, the amp size of these electrical sub panels was much smaller within the range of 30 – 60 amps since the demand for high power was much less due to the lack of electronic devices in homes.

But now, that demand has increased by a huge margin and people use more and more electronic devices at their homes – which has put 125-amps sub panels to be a standard. 

Along with that, the size of the wire also plays a vital role in the service size since the wire must be capable enough to supply all the electricity from the sub panel.

Aluminum wires of 1/0 AWG or copper wire of 1 AWG are sufficient to carry 125-amps of electricity from the sub panel.

NEC code for wire size:

In order to regulate a standard power supply to the houses, the NEC has set standard codes for houses of sizes starting from 1,200 feet and more.

The NEC code specifies that the minimum amperage for houses of this size should have 100-amp sub panels to accommodate for the essential electronic components.

100-amp is still too low for modern houses, especially those which have heavy-duty kitchens, laundry appliances and air conditioning units.

100-amp sub panels will barely meet the minimum energy requirements and you’d need to upgrade your sub panels to 125-amps or higher.

For a typical household, a 125-amp sub panel is sufficient to meet all the energy requirements of the electrical appliances without overloading the panel. However, if your house has a higher electrical demand, you can increase the capacity to 200-400 amps.

Although 400-amp sub panels are unnecessary, a house with high power consuming appliances such as hot tubs and such but your house has a sub panel of lower amperage – you need to contact the local electrical power supplier to upgrade the panel to a higher amperage panel that suits your house’s power demands to ensure safety of your house and the appliances.

Using a sub panel with amperage not sufficient to provide enough power to the appliances would overload the sub panel and damage it. Besides that, appliances getting supplied with lower power than required may also be vulnerable to damage.

Wire size for 125-amp sub panelDistance/Length of wire
1/0 AWG100 feet away
6/2 AWG150 feet away
4/3 AWG200 feet away
3/3 AWG400 feet away

How to determine a 125-amp sub panel wire size?

There are several ways to determine the wire size of a 125-amp sub panel, those methods have been described in detail below:

Depending on load and temperature:

The load size of the electrical service line heavily affects which size wire must be used to supply it since the wires need to be thick enough to handle the load size.

In case of 125-amp sub panels, you need to have a minimum 2/0 AWG aluminum wire to capacitate the electrical supply size but it is safer to use a wire with higher thickness – which would make it less prone to heating up over time.

Higher electrical flow causes the wire to heat up, therefore you need to use a thicker wire made of heat-tolerant material in order to keep the connection safe.

Depending on the distance:

This is an important factor to keep in mind because as the distance the wire has to cover increases, the overall resistance of the wire increases as well because the length of the wire and resistance are directly related.

However, the thickness of the wire and the resistance is inversely related to each other – meaning, if you make the wire thicker, the resistance will decrease.

Therefore, if you extend the wire far away from the sub panel – you need to make sure that your wire is thick enough.

Spacing in the sub panel:

Fitting the wire in your sub panel is an issue most people tend to overlook and they end up buying wires that are too large to fit into the panel.

The space in the panel may not be sufficient enough to fill up the panel and if that happens, you may need to get another panel to make up space for the extra wiring.

Make sure that the new sub panel has the same ampacity rating as the panel the wiring is coming in from.

Why Is wire size important?

There are several factors to why you need to consider the wire size and those factors have been discussed down below:

To prevent damage to appliances:

A wire that is inappropriately sized to carry the electricity to the appliance can be both dangerous to the wire and the appliance as well.

If large amounts of electricity flows through the wire, it will not only cause the wire to melt down due to the excessive heat generated but may also cause the power ports of the appliances to get damaged as a result.

In other cases, if the electricity is too low compared to the wire size, the appliance may not get sufficient power to operate optimally and thus end up with damaged internal circuitry.

To prevent fire:

Wrong wiring can cause excessive heat generation due to resistance which can easily catch fire. Therefore, you need to pick a thick wire if you are connecting heavy-duty appliances.

To meet appropriate power demands:

You must choose the wire size appropriately according to the power demands, such as use 18 – 12 AWG wires for light power consuming appliances whereas, use 10 – 4 AWG wires for heavy-duty appliances.

Sub panel wire size & Amp rating chart:

Amp ratingAluminum wire gauge size (AWG)Copper wire gauge size (AWG)
100 amp13
60 amp46
50 amp68
40 amp88
30 amp810
20 amp1012

Final thoughts

A 125-amp sub panel indicates how many items you may run and how much electricity can be securely provided into your home at 125-amps. A 1/0 AWG aluminum wire or 1 AWG copper wire is acceptable for 125-amps, thus you must have the right size wire to handle the current from the sub panel.