A subpanel helps with organizing a service panel, and to provide greater control over the electricity entering a home or building. There is a feeder wire that is required to go from the main panel to the subpanel. Today, I will explain what size wire is needed for a 100 Amp subpanel.
The size wire you need for a 100 Amp subpanel is a 3 gauge (AWG), 0.2294 in (5.827 mm) diameter copper wire. For a 115 Amp subpanel, you need 2 AWG feeder wire which is thicker than that required for a 100 Amp subpanel, and for an 85 Amp subpanel you need a 1 AWG feeder wire which is thinner.
Each jurisdiction has specific standards and you should check with a qualified electrician before installing the feeder wire for your subpanel. A subpanel can be located in the same building as the main panel but it can also be located some distance away.
Below, I will explain if the distance of the subpanel from the main panel affects what wire you should use and if the wire needs to be a particular color to be building compliant.
Does the Distance of the Subpanel From the Main Panel Matter
Understanding why electricity works is a bit of an undertaking, and there are a lot of considerations. One is how the distance electricity is traveling affects how thick a wire needs to be. Here’s a rundown of how the distance of the subpanel from the main panel affects how thick the feeder wire needs to be.
The distance of the subpanel from the main panel does not affect the size feeder wire you need for a subpanel. The gauge wire you need for a subpanel is embossed on the main circuit breaker you are going to use for a subpanel. It’s typically provided as a range such as #2 to #12, for 2 to 12 gauge.
Typically, somewhere in the middle is best. For example, if it says you should use a 2 gauge to 12 gauge wire, then a 6 gauge is good. But, there are building codes that need to be checked, as these can vary based on your specific jurisdiction.
To find out virtually all jurisdictions will have contact details where they can answer this question for you. For example, in Texas, they provide an email address where you can ask specific questions about whether an electrical setup will be building compliant in Texas.
In general, contact the office of your jurisdiction that administers building compliance certificates, and they’ll be able to get you in touch with the office that can answer this question for you.
Ampacity rating needs to be matched with a thermal rating
There is also a thermal rating for the specific amperage of the circuit breaker that varies based on the thickness of the wire. Luckily, this info is also provided on the circuit breaker. The two options are typically 65°C and 75°C.
Here’s a table that shows how the amperage rating changes based on the thickness of the wire:
| Wire size | 60°C | 75°C |
| #12 (12 gauge) | 20 Amps | 25 Amps |
| #10 (10 gauge) | 30 Amps | 35 Amps |
| #8 (8 gauge) | 40 Amps | 50 Amps |
| #6 (6 gauge) | 55 Amps | 65 Amps |
| #4 (4 gauge) | 70 Amps | 85 Amps |
| #3 (3 gauge) | 85 Amps | 100 Amps |
| #2 (2 gauge) | 95 Amps | 115 Amps |
| #1 (1 gauge) | 110 Amps | 130 Amps |
(source)
As you can see a 3 gauge wire is ideal for 100 Amps, at 75°C, is 3 gauge, this is very common. But, if your insulator is rated at 60°C then the wire will need to be 1 gauge.
However, this is only a guide and does vary based on the specific circuit breaker you are using. To be sure, refer to the specs embossed on the circuit breakers you’re using. It will state what the thermal rating is.
Coloring the wire is also very important
Some wire thicknesses such as 3 gauge only come in one color. However, building codes require that certain wires such as the neutral wire which goes from a main panel to a subpanel must be labeled a certain color. For example, some wires only come in black, but the neutral wire needs to be white in color.
This requires that you wrap white tape around the wire to color it white. The color requirements for wires do vary from jurisdiction to jurisdiction. Therefore, it’s best to get in touch with the electrical building compliance office in your jurisdiction to find out how they should be color coded.
Can You Run a 100 Amp Subpanel off a 100 Amp Main Panel
There are different Amp ratings for main panels and subpanels. For example, a main panel can have a 300 Amp circuit breaker, and a subpanel can have a 100 Amp circuit breaker. When feeding a subpanel from a main panel, here’s whether it’s fine to use run a 100 Amp circuit breaker on a main panel to a 100 Amp circuit breaker on a subpanel.
In general, running a 100 AMP subpanel off a 100 Amp main panel is perfectly fine. The maximum you can send from the 100 Amp main panel to a subpanel is 100 Amps. Therefore, any subpanel that is equal to or less than 100 Amps is fine. Such as a 60 Amp subpanel.
Many electricians have reported that these are the rules for connecting subpanels and main panels. However, this does not constitute advice from a qualified and licensed professional. Before doing any installation work check with a qualified or licensed professional first.
Is #4 THHN Good for 100 Amps
A 100 Amp circuit breaker is a common size and is good for the electrical needs of a standard family home. When connecting a main panel or subpanel, the size wire is important. Here’s whether a 4 gauge or #4 THHN wire is good for a 100 amp circuit breaker.
In general, a #4 THHN thickness feeder wire is good for a 100 Amp main panel or subpanel. The gauge wire you need for a 100 Amp circuit breaker is embossed on the circuit breaker. It’s typically provided as a range such as #2 to #10. Provided the gauge wire is within this range it’s generally fine.
It’s a good idea to get a list of the specs for the circuit breakers, the subpanel, and the main panel. And then check it over with a licensed electrician. Also, for a very good full video guide on installing a subpanel to a mainpanel refer to this video below:
