Box Fill Calculator

This electrical box fill calculator (or in short, box fill calculator) will help you determine the total box fill volumes you will need to meet so that each of your electrical utility boxes will pass the National Electrical Code®. In this calculator, you will learn:

• The importance of electrical box fill calculation;
• How to calculate box fill volumes yourself;
• How to use our box fill calculator; and
• How to use electrical box fill charts.

If you're ready to start learning, then keep on reading 🙂.

Every three years, the NFPA® updates the National Electrical Code® — a set of rules, regulations, and guidelines when dealing with the electrical aspects of building construction.

When choosing electrical utility boxes, you might ask: "Can't we just decide to get the larger-sized ones to accommodate as many wires and devices as possible?" The quick answer is no. Although there will always be an extra room in our electrical boxes for future renovations or upgrades, it could be expensive to unnecessarily oversize. By knowing how to calculate box fills, we can decide better what electrical box size to choose to be more economical.

In the next section of this text, let us discuss how to perform electrical box fill calculations manually.

Finding out how much volume of an electrical box you need to use is easy. However, it's necessary to do so that we follow the electrical building code to avoid potential danger and fire hazards. The NFPA® presents the use of "volume allowances" in determining the total box fill we would need for a utility box.

Each single volume allowance corresponds to one conducting or grounding wire entering the box. That means if an electrical component has an equivalent of two volume allowances, that component occupies the same volume as two conducting wires entering the box. Each component attached to or entering an electrical box has its equivalent volume allowance. These components that we'll consider in the box fill calculations are the following:

• conductors;
• clamps;
• support fittings;
• devices or equipment; and
• equipment grounding conductor.

Now let us discuss how many volume allowances each of these components will have in an electrical box.

Conductor fill

According to the 2020 NEC®, we should count one volume allowance for each conducting wire entering or passing through an electrical box regardless of their wire sizes. You can use our wire size calculator if you want to learn how certified electricians choose the right wire size for a particular application.

Note that we should count a wire twice if its length is longer than twice the minimum length required for free conductors (which is 12 inches / 30 cm, as specified in Article 300.14: "Length of Free Conductors at Outlets, Junctions, and Switch Points"). We don't need to count any short wires, such as a pigtail wire, where their entire length remains in the box. We can now say that:

and

where:

• $A_\text{w}$ is the conductor fill volume allowance;
• $n_\text{w}$ is the number of conducting wires going inside the box;
• $V_\text{w}$ is the conductor fill volume; and
• $V_\text{largest conductor}$ is the free space we provide for the largest conducting wire entering the box, as specified in Table 314.16(B):

Clamp fill

We should also be aware of whether we plan to use an electrical box with a set of internal cable clamps or external ones. Internal cable clamps take up space, and we count a set of internal clamps as one volume allowance. On the other hand, we don't assign a volume allowance for external cable clamps. In other words:

• If we are going to use an electrical box with an internal clamp, $A_\text{c} = 1$.
• Otherwise, if we are going to use an electrical box without an internal clamp, $A_\text{c} = 0$.

With that in mind, we take the volume we need to consider for the clamp fills to be:

where:

• $A_\text{c}$ is the clamp fill volume allowance; and
• $V_\text{c}$ is the clamp fill volume.

Support fittings fill

When using an electrical box for luminaires or lighting fixtures, we might need to use some studs or hickeys as supports for our fixtures. According to the NEC®, we consider one volume allowance for one or more support fittings like luminaire studs or hickeys in the box.

• If you are going to have one or more luminaire studs or hickey inside the box, $A_\text{s} = 1$.
• If you are going to have no luminaire studs or hickey inside the box, $A_\text{s} = 0$

To find how much space we need to allot for the support fittings, we use this equation:

where:

• $A_\text{s}$ is the support fittings fill volume allowance; and
• $V_\text{s}$ is the support fittings volume.

Device or equipment fill

For the devices or equipment (e.g., switches, outlets, or fixtures) that we need to install on the box, we count a double volume allowance for each of them.

where:

• $A_\text{d}$ is the device fill volume allowance; and
• $n_\text{d}$ is the number of devices.

Like the first three components, we base the volume allowance for devices and equipment on the largest conductor entering our box, as shown in this equation:

where:

• $V_\text{d}$ is the device fill volume.

Equipment grounding conductor fill

For grounding wires, we recently had an update on the 2020 NEC® where we now consider 1–4 grounding wires entering our electrical box as a single volume allowance. Then, we consider an additional 1/4 volume allowance for each additional grounding wire.

• For 1–4 grounding conductors entering the electrical box, $A_\text{g} = 1$.
• For 5 or more grounding conductors: $A_\text{g} = 1 + \frac{n_\text{g}-4}{4}$ or $A_\text{g} = n_\text{g} / 4$.

where:

• $A_\text{g}$ is the grounding conductor fill volume allowance; and
• $n_\text{g}$ is the number of grounding conductors.

Unlike the first four components we considered, according to the 2020 NEC®, we should base the grounding wires' volume allowance on the largest equipment grounding conductor or wires. In equation form, we express that as:

where:

• $V_\text{g}$ is the equipment grounding conductor fill volume;
• $A_\text{g}$ is the grounding conductor fill volume allowance; and
• $V_\text{largest ground wire}$ is the free space we provide for the largest grounding wire entering the box, as also specified in Table 314.16(B).

Once we've determined the box fill volumes for each component, we can now take their sum to get the total box fill volume, $V_\text{total}$, as shown in this equation:

If our largest conductor's equals the largest grounding wire size, we can instead take the total volume allowance first and multiply that by the free space required for the largest conductor.

where:

• $V_\text{total}$ is the total box fill;
• $A_\text{total}$ is the total volume allowance; and
• $V_\text{largest conductor}$ is the free space we provide for the largest conducting wire entering the box, as specified in Table 314.16(B).

In those cases where the largest conductor and the largest grounding wire are equal in size, we can use the electrical box fill charts available from various manufacturers to choose the correct size box to meet our requirements quickly.

Before we proceed on how to determine the proper electrical box size using a box fill chart, let us first discuss how to use this box fill calculator.

Let's say that in our electrical plan, we need to feed 6 pieces of 12 AWG wires, 2 pieces of 16 AWG wires, together with 5 pieces of 12 AWG grounding wires into an electrical box. The electrical plan also indicates we need to install 2 convenience outlet devices onto this electrical box. We prefer to use electrical boxes with a set of internal clamps, so we need to consider that too.

1. First, we enter the number of conducting wires that we need to feed into our electrical box (no matter their sizes). Since we're going to feed six 12 AWG and two 16 AWG conducting wires, we enter 8 wires into the calculator.
2. Among the selection, we then choose the wire size of the largest conducting wire that we will feed into our electrical box. In this case, 12 AWG.
3. We're planning to use an electrical box with internal clamps, so we select Yes for the question of "Are you using a box with internal clamps?".
4. On the other hand, we choose No for the question on support fittings.
5. Next, we enter 2 for the number of devices.
6. Then, we enter 5 for the number of grounding wires.
7. And finally, we choose 12 AWG for the largest ground wire size selection.

Our box fill calculator will immediately display that we need 14.25 volume allowances. And since our largest conducting wire is 12 AWG, our tool calculated the total box fill volume required of 32.06 in³ out of that wire size's required volume of 2.25 in³.

That means we need to find an electrical box with at least 32.06 in³ of volume to meet our requirements. We can usually see the capacity of electrical boxes indicated inside them.

Using electrical box fill charts is an easy way to find the box size we need to fit a particularly calculated volume allowance. With that said, we only use box fill charts after determining how many volume allowances you would need. Here is a copy of the 2020 NEC® Table 314.16(A) electrical box fill chart labeled as Metal Boxes for your reference:

As you can see, each electrical box size in this chart can fit the typical 4-inch drywall framing.

To use this electrical box fill chart:

1. We first look for the column of the largest conducting wire entering our box. Let's use our example a while ago and find 12 AWG under the table's Maximum Number of Conductors section. The "maximum number of conductors" is equivalent to the maximum number of volume allowances that will fit the provided box sizes.
2. Below the column for 12 AWG, we need to look for the following number closest to our calculated total volume allowance. In this case, the following number closest to 14.25 would be 18 — which corresponds to a box size of 4 ¹¹/₁₆" × 2 ⅛" square box (120 mm × 54 mm square box) with a volume capacity of 42.0 in³ (or 689 mm³).

Based on our earlier box fill calculation, we can choose this box size since it can accommodate the total box fill volume of 32.06 in³ of the electrical components that we need to fill in this box.

To calculate box fill fast, you need to:

1. Know by heart the designated volume allowances for each electrical component as specified by the National Building Code®.
2. Memorize the free space needed by each of the wire sizes (6 AWG to 18 AWG) for this calculation.
3. Practice counting volume allowances correctly the first time you do.
4. Have a calculator to multiply the total volume allowance by the free space needed by each volume allowance. You can also do mental calculations if you've already developed that skill.

The results in this tool are for informational purposes only and do not intend to replace professional guidance from a certified electrician. Please be aware of your electrical competence, or consult a qualified electrician before proceeding with any electrical installation.