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What Size Conduit for 12/2 Wire? (Read This First!)

Conduits are used to protect electric wires from exposure since exposure might cause them significant damage from weather, or impact. 

Similarly, it is necessary for a conduit to have enough space inside for the wires. This is because, when electricity passes through wires, they heat up and emit a specific amount of heat.

If there is not enough space for the electric wires, to use an easy term, to breathe the excess heat out, the wires might be privy to damage that can range from minuscule to big.  

Now, let us know how many wires can fit in a specific conduit and also learn some interesting facts about conduits and wires.

Conduit size for 12/2 wire

For two 12/2 wires, the size for a conduit is 1 inch. This size is applicable for NM (non-metallic) cable and 1-1/4 inch for a UF (underground feeder) cable. The size usually differs depending on the covering of the cables. While NM cables have paper or thin plastic covering, UF has plastic ones.

A conduit is a tube used to protect electrical wires and direct them to their destination. It is used as a guide and protection for exposed wires.

If the heat inside the conduit cannot dissipate, the vinyl insulation of the wires might melt and cause a fire hazard.

This is the reason, the National Electrical Code (NEC) has specified the limit of wires in a conduit of a specific size. The rating is called Conduit Fill Capacity. These depend on the type and size of both the conduit and the wires.

For normal heat dissipation and easy wire withdrawal, NEC has advised conduit fill for a single wire to not cross 53%. 

There are many types of conduit. Some of them are discussed below:

PVC conduit: 

Polyvinyl Chloride, in short, PVC is a material that is a mix between vinyl and plastic, normally used for creating PVC pipes. Conduits, as explained above, are used for protecting electric wiring from exposure to damage. 

PVC conduits are mainly used outdoors or where the wiring might come in contact with liquid, for instance, underground. 

They don’t react to corroding materials as metals do and have a strong resistance against moisture. They’re comparatively easier to install, as well.

Rigid conduit: 

Rigid Metal Conduit, or, RMC is a thick-walled conduit made of coated steel, aluminum, or stainless steel. They’re used for protecting wires from outward damage, like weather and fire or impacts like blowing up, and like all other conduits, provide support. 

EMT conduit: 

Electric Metallic Tubing or EMT is a metal-based conduit used to support and protect electric wiring from potential damage when they’re used in garages, laundry rooms, bathrooms, etc. 

While RMCs are thick-walled metal conduits, EMTs are made of simple coated steel or just aluminum. 

These particular conduits are used anywhere because they are easily changeable and provide EMI (electromagnetic interference, which is an unwanted noise in an electrical path that disrupts it). EMT conduits can be recycled as well.

Metal conduit: 

Metal conduits, as their name entails, are metal-based conduits. They are noncombustible and give strong protection from fire and blowing up. They don’t burn and emit smoke and carcinogens, unlike plastic conduits. 

Metal conduits include RMC and EMT conduits and they have capabilities that PVC conduits cannot provide, as a result, they are costlier than PVC conduits.

What size conduit for single and two 12/2 outdoor wires?

The fill decides the number of 12/2 wires that might fit in a conduit. Conduit fill is the amount of space covered by the wires. 

Most 12/2 NM cables have a 0.40 inches width, meaning it has an area of 0.126 sq. inches. If you are using a ¾-inch Schedule 80 PVC conduit, which has an area of 0.409 sq. inches, a single 12/2 NM cable will have a 61% fill.

Conduits can manage a 100% fill, but in that case, the wires will be tightly packed and congested, and you might not be able to pull them out due to that. Therefore, it is wise to keep it well below 100%.

Since a single wire occupies almost 61% of the conduit, it’ll be safer to use a PVC conduit of 1 inch than ¾-inch when it comes to two 12/2 outdoor wires.

What determines conduit size for 12/2 wire?

Several points determine the size of a conduit for 12/2 wires. Some of the most crucial determinants of conduits size for 12/2 are mentioned below:

Major Diameter of the cable: 

The major diameter of a cable denotes the diameter of a 12/2 NM cable, which carries 2 conducting wires, as said in its name, along with a ground wire. For Southwire® Romex® SIMpull® 12/2 wire, the major diameter is normally 0.41 inches. 

Area of the cable: 

It is found using the major diameter. This tells us about the area the cable might occupy in a conduit once it is placed in it. 

Since the major diameter is 0.41 inches, the area is 0.132025 sq. inches. This means that a single 12/2 NM wire might occupy 1.32025 sq. inches in a conduit.

Conduit Fill: 

This portrays the space a single 12/2 cable might occupy in a specific conduit in the form of a percentage. 

A conduit fill should always be under 100% no matter what for the ease of moving the wires, and so that they don’t become congested inside the conduit.


Some exceptions come when 12/2 UF wires are used for additional durability and protection from moisture. 

In that case, the area is more due to the increased thickness of the 12/2 cables covered by plastic and not paper, like NM wires. 

While NM wires have a paper or loose-plastic covering, UF has plastic casings for extra protection against moisture, for instance, the underground.

How many 12/2 wires are in 1 conduit? 

You can easily include 2 12/2 NM-b wires in a conduit of 1-inch. This is because most 12/2 NM-b wires have a width of 0.40 inches. As a result, two of those wires will need 1.016 inches of conduit ID.

In the market, most 1-inch trade size conduits have a size larger than 1-inch. As a result, one conduit of 1-inch is enough to contain two of 12/2 NM-b wires.

For two wires, the maximum fill is 31% and for three wires, the conduit fill can reach 40% of the total conduit space. Although there are exceptions, these are standard rules along with some local codes. However, this amount might vary depending on the width and type of the wire.

How to determine conduit size? 

For determining the conduit size, one needs to know the conductor fill of the wires used. Here, we will be using 12/2 NM-b cables from Romex and a ¾-inch Sch 80 PVC conduit.

Finding out the Major Diameter: 

The major diameter of a specific cable is always mentioned in the manufacturer’s documentation. Since we are using 12/2 cables from Romex, their major diameter tends to be 410 mils or 0.41 inches. 

Finding out the Area of the cable: 

As we know, the formula for finding out the area of rounded objects is the multiplication of pi and the square of the object’s radius. So, 

A= πr2

A= π * (0.41/2)2

A= π*0.042025

A= 0.132025 sq. inches

Finding out the Conductor and Total Area: 

The conductor area is the area occupied by the specific wires inside a conduit, and in this case, the area is 0.26405 sq. inches (0.132025*2). 

For a ¾-inch Sch 80 PVC conduit, the area is 0.409 sq. inches, and it is manufactured as per the NEC (national electric code).

Finding out the Conductor Fill of two 12/2 NM-b wires: 

If we’re using the conduit just as a sleeve and not a whole system, we can fill it up to 100%. But doing that will make it impossible to pull out the cable later on. 

As a result, it is advisable to keep the conductor fill below 100%. So, for two 12/2 NM-b cables,

Fill= Conductor Area/ Total Area

Fill= 0.26405/0.409

Fill= 0.6456 = 65%

Therefore, the fill capacity of two 12/2 NM-b wires from Romex will be 65% unless it is a whole conduit system, in that case, the fill will be only 31% of the total area of the conduit (0.127 sq. inches).

Special Places: 

These scenarios include wet or damp places, for which we cannot use NM-b wires but use UF wires. UF wires have a plastic casing, making them occupy more space than NM wires. Only the fill capacity is changed, so it might not be a matter of big concern. 

Final Thoughts 

The size of a conduit is 1 inch for two 12/2 wires. Non-metallic (NM) cable and underground feeder (UF) cable both fit in this size range. The covering of NM cables is paper or thin plastic, but the coating of UF cables is plastic. According to how the wires are covered, the size often varies.