Electrical Conduit Sizes

spiral of silver colored Electrical Conduits
  • Intermediate

An electrical conduit is an excellent way to protect and route electrical wiring that runs throughout a building or your home. A conduit can be made of various types of materials, such as fiber, metal or plastic, and it may be flexible or rigid in its design. Conduits are pipe or tube-like in appearance and are used to shield electrical wires from impact, water and vapors. Conduit comes in many sizes that vary in diameter based on the application and use of the conduit.

Most Common Sizes of Conduit

In most home construction jobs, conduit sizes of ¾-inch to 1½-inch are the most common sizes of conduits that are used. For most residential construction jobs, these sizes are adequate for protecting wiring that runs throughout the home from the main breaker box to lights, outlets, and switches. These common sizes of conduits are usually installed by a certified electrician in order to meet building code specifications. A ¾-inch to 1½-inch conduit is usually the accepted size range for conduit in most areas of the country.

Other Frequently Used Sizes of Conduit

While a ¾-inch to 1½-inch conduit will usually suffice for most residential jobs, there may be times that larger conduit sizes are required. For instance, when constructing a laundry room - where a washer and dryer will be present - you may need a conduit diameter size of 3 to 4 inches in some cases to properly insulate the electrical wiring and avoid crowding of the wires.

Other times that may call for larger conduit sizes are when installing a central heating and air conditioning system or if you need to install an outdoor heat pump. These types of installations frequently use conduit diameter sizes above 2 inches.

Determining the Conduit Size You Need

To determine the size of the conduit you will need to run for a particular application, you will first need to understand how much space is available inside the conduit. Industry standards suggest that they don't exceed a 40 percent fill rate of the conduit being used. This simply means that the total diameter of all electrical wiring inside the conduit should not exceed 40 percent of the inner diameter size of the conduit pipe or tubing.

However, if you think you may be adding circuits or wiring in the future, you might want to consider a 30 percent fill rate as a maximum for a new installation. Below is a table that shows you the amount of space in the most common conduit diameters. This table can help you plan the size of the conduit you will need based on the amount of fill space available.

Conduit Size

Square Inch Equivalent

30% fill

40% fill

50% fill
















1 ¼





1 ½










2 ½










3 ½










Now that you know the amount of space available in a piece of conduit, you half of the equation needed to determine the diameter size of the conduit you will need. In order to calculate the diameter you will need, you will also need to know the outside diameter (OD) of the wire you will be using. Once you have the OD, you can easily calculate the area of the wire by using the following method:

Multiply the OD by itself and then multiply the product by a factor of 0.7854. This will give you the area of the diameter of the wire in square inches. You can then add the area for each piece of wire that you will be running through the conduit to come up with the total area of all wires.