What Size Wire Do I Need for a 100-Amp Sub Panel?


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Home improvement is a part of any homeowner’s experience. Electrical wiring and maintenance is one such way of improving a home and making life just that little bit more comfortable. One issue you might encounter is deciding what size wire you need for a 100-amp subpanel.

Depending on the wire’s length and the temperature rating, you will need around a 3-gauge copper wire to power a 100-amp subpanel. As the length and temperature increases from subpanel to main, you will require a lower gauge, thicker wire to manage the load. Ultimately, you should consult an electrician to ensure which wires are capable of the task.

In this article, we will look into the process of subpanel installation and the required wiring involved. We will consider the specifics of the situation and the risks involved with these installations. Safety should be the primary concern when working on anything with electricity within the home. It is recommended to always get a professional to do any work with electricity.

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Reasons for Subpanel Installation

So, you are thinking of adding a subpanel to your house for one reason or another. These additional panels can undoubtedly be handy for some situations, but they can strain the main panel, so installers should approach with caution. Let us look at some of the main reasons you might want to add a subpanel.

The first of these reasons is simply that there is no more space on your main panel. A subpanel here adds extra space for additional circuits or even lighting. This could be because you are remodeling or expanding your home and require more circuits running to these areas.

The next advantage is providing greater proximity to the powered section of your home. Instead of having to run wires all the way from the main panel, which could cause a drop in voltage across the long cables, you can run these to a more central subpanel and then separate the necessary circuitry from there.

This also helps in the event of tripping a breaker, where you can head across to the closer subpanel and reset it there. This makes general use and control over the wiring a good deal easier across a larger area.

There is also a cost-saving factor to consider. Running the main panel connected to several subpanels is generally cheaper than running specific circuits across to each location required (source). This option can save you both in usage and set up costs.

Calculating the Load for the Subpanel

Now that you are fully committed to setting up a subpanel, it is important to note the load required for the subpanel and if the main panel will be able to support it. The way you can do this is to calculate and add the total wattage to use through the subpanel.

Add each of the different inputs that will connect to the circuit board and draw power from this subpanel — generally given in watts. 

The National Electric Code (NEC) recommends that, for overcurrent protection, you should multiply the continuous load, in watts, by 1.25. These are the systems that will run continuously, like a refrigerator or freezer.  

Once you have calculated this overall wattage, it is time to convert it into amps. The formula for this is to divide the total watts by the voltage on the service board. This will usually be listed on the board and generally sits at the 240-volt mark. 

If it is above a single-phase, take the square root of phase number, and multiply this by the total voltage.

Whatever the load, divide the total watts of the appliances and circuits that the subpanel will power by the voltage to get the expected amps required. It is important to ensure that the subpanel amperage is higher than this expected load. 

For example, if total watts of the expected load equals 15000 watts, then the minimum amp requirement for a panel connected to a 240-volt, single-phase service board is 15000/240, which comes out at 62.5 amps. 

This means that a 100-amp subpanel with this load and service board voltage would be able to handle the strain comfortably. 

The next step is to ensure your main panel has enough amperage supply to power the required subpanel. Electricians recommend a 100-amp subpanel with at least a 200-amp main panel, depending on both panels’ strain.

Wire Size

The wire size involved with the connection between the subpanel and the main panel is an important part of the setup. When in doubt, go for a lower gauge cable with a greater width that can support a greater amperage. The best material for these conductors is copper.

The NEC recommends that those installing panels use a 4-wire feeder cable. This means the cable consists of two live wires, commonly referred to as hots, a neutral, and a ground wire. 

The ground wire has been a more recently enforced addition to these cables and is primarily used for grounding the equipment itself, providing a low-resistance path for any faults that may occur.

The next factor is the temperature rating of the insulation system. These ratings are the maximum heat that the insulation can support and affect the amount of amperage that they can carry. Increased current can increase the temperatures within these conductors and push them to these limits.

The most commonly encountered temperature rating of copper conductors is 167°F (75°C), though this may vary, and the panel or equipment should list this. If multiple temperature ratings are listed across the equipment, use the lowest rating in the system to ensure it is not overwhelmed.

The American Wire Gauge (AWG) system establishes the thickness of the wires involved. The lower the gauge, the thicker the wire is, and the more amps it will support reliably.

At the 167°F temperature rating, a size 3AWG is required to support 100 amps (source). Lower temperature ratings require a lower gauge wire to reduce resistance and heat involved to support the same ampacity. The inverse is true for higher temperature ratings.

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Wire Length

The reason why the thickness and length of the wire are important to the amperage going through it is that it is fundamentally facilitating the movement of electrons. So, the wider and shorter the wire, the easier it is for the electrons to move through it.

Depending on how far away the subpanel is from the main panel, you may have to increase the wire’s width to accommodate a greater traveling distance. It is generally necessary to move to the next, thicker, wire gauge if the cable length exceeds 50 feet (source). This is to prevent any voltage drops that may cause problems.

It is advisable to get an electrician in to ensure that the length and thickness of the wiring are adequate for the load it will accommodate. 

Ambient Temperature

According to the NEC, the ambient temperature surrounding the conductors also affects the wiring requirements’ ampacity. This is to say, ambient temperatures above 86°F will require a lower gauge wire, meaning increased thickness to support the equivalent amperage.

Be aware of the area you are placing your subpanel and the environment it is in. For most of the usual places we find these panels, this should not be a significant factor, but, if there is doubt, either bring in a specialist or shift to a lower gauge wire to be safe.

Another temperature-related factor is that of neighboring hot conductors. The more of these you have bundled together, the more heat transitioned between them, affecting these conductors’ ampacity and maybe requiring a shift to a lower gauge to compensate.

Again, it is important to note the risks are mostly around using a wire gauge that is too high for the required current. Rather, err on the lower gauge side, a thicker wire, as the risks around this are significantly less.

It is essential to place the subpanel in the right area and with the necessary clearance and weather protection. For the most part, this involves a commonsense approach, requiring you to avoid things like water and direct sunlight as much as possible.

An essential factor to remember when dealing with electricity is to ensure that there is some essential grounding available (source). 

This protects users and equipment from electrical surges and faults. Grounding is also important to prevent the panel and its enclosure from conducting current, which can be very dangerous.

Due to the importance of proper grounding and the risks that can occur when done improperly, we highly recommend that you get a licensed electrician to inspect the circuitry involved.

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Consult An Electrician

An article such as this cannot cover all eventualities and codes that apply in all countries and all local municipalities.

For this reason, as well as for the safety of you, your family, and your home, it is best to consult with a local electrician before any type of electrical work.

When electricity is concerned, a little bit of knowledge can be deadly, and it is not worth taking the risk simply to save a little money.

Final Thoughts

It is clear that understanding each panel’s electrical loads and their uses plays a vital role in their installation. You must consider the thickness, length, and temperature involved for wire sizes to install them properly.

For a 100-amp subpanel within the most common environment, the average wire gauge is 3AWG. This adjusts based on the above factors and environmental changes. A lower gauge is generally safer than higher gauges. 

Ultimately, you should consult a professional electrician when installing a subpanel to ensure safety and long-term operation. Any type of electrical work should never be considered a DIY job with little basic knowledge of electrical work.

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