A junction box, also known as an electrical box is an enclosed housing space to accommodate electrical connections. But there is a limit on how many wires in a junction box are acceptable.

It depends on two key factors. The size of the box itself, and the size of the cables or conductors. Most non-metallic boxes on the home stores will have a Cubic Inch per junction box printed in them. And there are several box fill guidelines for metal boxes.

This article will detail everything you need to know about the wire capacities of a junction box along with several important matters regarding this topic.

What Is a Junction Box?

A junction box is a typical electrical box that connects two or more circuit cables or wires and is often fixed to the structure of a building or residence. They can be either made of plastic or metal. Their sizes may also vary.

Fig 1- Plastic and Metal Junction Boxes
Fig 1- Plastic and Metal Junction Boxes

The main purpose of the junction box is to protect the connections, which usually contain vulnerable points of contact. Which include wire splices, wire connections, live wires, ground wires, etc.

Can you have too many wires in a junction box? You definitely can. Having too many wires above the accepted limit can lead to various dangerous situations as I will discuss later on in this article.

As stated earlier, junction boxes can either be made up of plastic or metal. Which cable and conduit materials you use will determine your decision.

A stainless steel junction box is typically required to provide grounding for metal-sheathed cable and metal conduit. For other wires, a plastic junction box will suffice. But it may vary.

How do you connect 4 wires to a junction box? Simply install the junction box, and pass the wires through the side of the box. There are openings on both sides for the entry and exit of wires.

How to Calculate the Number of Wires in an Electrical Box?

Stuffing in too many wires within a single junction box may lead to dangerous outcomes. Therefore, it is highly important to know how many wires in a junction box are safe.

But first, let’s clear off a few things.

  • How do you count wires in a junction box? A single wire running through the box counts as one wire.
  • The amount of ground wires does not matter. They are counted as one conductor regardless of their amount.
  • When devices are mounted in the box, the total conductor count must be increased by two for each mounting strap (not to be confused with ground wires).

To put it simply, each current-carrying conductor (hot, neutral) that enters the box, counts as one box fill unit. All the grounding conductors together, count as one box fill unit. The device counts as two box fill units.

Time to take a look at the box fill table mandated by the NEC.

Box Fill Capacity of Junction Boxes

Box DimensionsBox ShapeWire Size (AWG)
(inches) 1614121086
(4 X 1 1/4 )round/octagonal765542
(4 X 1 1/2)round/octagonal876653
(4 X 2 1/8)round/octagonal12109874
(4 X 1 1/4)square10109763
(4 X 1 1/2)square121210874
(4 X 2 1/8)square17171512106
(4 11/16 X 1 1/4)square1412111085
(4 11/16 X 1 1/2)square1614131195
(4 11/16 X 2 1/8)square24211816148
(3 X 2 X 1 1/2)device433321
(3 X 2 X 2)device554432
(3 X 2 X 2 1/4)device654432
(3 X 2 X 2 1/2)device765542
(3 X 2 X 2 3/4)device876542
(3 X 2 X 3 1/2)device1098763
(4 X 2 1/8 X 1 1/2)device554432
(4 X 2 1/8 X 1 7/8)device765542
(4 X 2 1/8 X 2 1/8)device876542

One may use this table to determine the safe wire capacity for junction boxes. For example, how many 12 wires in a junction box?

If it is a square box with the dimension of (4 X 1 1/4) inches, then 9 wires of size 12 can be fit in the box. 12 gauge extension cords also follow the same rule.

Dangers of Overfilling a Junction Box

Overfilling a junction box with too many wires may lead to several issues such as overheating, electrical fires, short circuits, etc. That is why it is so important to follow the box fill capacity of each junction box properly.

Problems Caused by an Overfilled Junction Box:

  1. With more wires, there is more electricity passing through the junction box. With more electricity passing through the box, more heat Is generated within the box. Thus leading to overheating.
  2. The excess heat within the box may affect the wires and connections within it. It can lead to the melting of the plastic covers and connectors. Thus causing a short circuit.
  3. A short circuit can therefore again lead to an electrical fire causing further damage.
  4. It is not only dangerous but also goes against the National Electrical Code.

Therefore, it is of the utmost importance to not overfill a junction box. For the safety of your own circuitry, electrical equipment, and household.

Summary

This article should suffice in informing you about how many wires in a junction box are safe for installation. Make sure to follow the box fill capacity during every junction box installation. If any confusion still exists, you can always consult a professional.