previous
  • Basement Remodeling Tips
    Basement Remodeling Tips
  • Energy-Smart Details
    Energy-Smart Details
  • The Hobbit House and More
    The Hobbit House and More
  • Deck Design & Construction
    Deck Design & Construction
  • 7 Smart Kitchen Solutions
    7 Smart Kitchen Solutions
  • Read FHB on Your iPad
    Read FHB on Your iPad
  • Electrical Articles & Videos
    Electrical Articles & Videos
  • Master Carpenter Videos
    Master Carpenter Videos
  • All about Roofing
    All about Roofing
  • How to Install Housewrap Solo
    How to Install Housewrap Solo
  • Remodeling in Action
    Remodeling in Action
  • Tips & Techniques for Painting
    Tips & Techniques for Painting
  • Buyer's Guide to Insulation
    Buyer's Guide to Insulation
  • 9 Concrete Countertops Ideas
    9 Concrete Countertops Ideas
  • Shorten a Prehung Door
    Shorten a Prehung Door
  • 7 Trim Carpentry Secrets
    7 Trim Carpentry Secrets
  • Magazine Departments
    Magazine Departments
  • 7 Small Bathroom Layouts
    7 Small Bathroom Layouts
  • Clever daily tip in your inbox
    Clever daily tip in your inbox
  • Play the Inspector Game!
    Play the Inspector Game!
  • Video: Build a curved step
    Video: Build a curved step
  • The Passive House Build
    The Passive House Build
  • 12 Remodeling Secrets
    12 Remodeling Secrets
next
Pin It
continued 12next>VIEW ALL

More Wires Need Bigger Boxes

The National Electrical Code limits the number of wires in a box. Here's how to figure the size box you need.

Box-fill calculations are done to make sure there's enough room in an electrical box to accommodate safely the wires, splices, cable clamps (usually none in plastic boxes) and outlets or switches that you intend to put in the box.

The volume in cubic inches is usually marked inside nonmetallic boxes. If it's not, you can measure the inside dimensions of the box and figure it yourself.

Here's how to figure the size box that complies with the NEC.

Step one

Add up the wires and devices, A K A "conductor equivalents" (wires that start and end in the box -- pigtails -- aren't counted)

Each current-carrying wire = 1
All ground wires together = 1
All clamps together = 1
Each receptacle or switch = 2

Step two

Multiply the number of conductor equivalents (total from step one) by their volume factor in cubic inches (listed below)

14-ga. wire takes 2 cu. in. per conductor
12-ga. wire takes 2.25 cu. in. per conductor
10-ga. wire takes 2.5 cu. in. per conductor
If a box contains different gauges of wire, use actual volume factors for the wires and the largest volume factor for ground wires, devices, and clamps.

Example

A PVC box has two 14/2 cables, one 14/3 cable, and one receptacle. The box has no internal clamps. What's the minimum box volume needed?

Step 1. Count the conductors:
2 conductors per 14/2 cable x 2 cables = 4
3 conductors in the 14/3 cable = 3
All grounds together = 1
No internal clamps = 0
Receptacle outlet = 2                 
Total conductor equivalents = 10

Step 2. The volume factor for 14-ga. wire is 2 cu. in., so:

10 conductor equivalents
 x 2 cu. in.                   
 = 20 cu. in. box

Single-gang boxes come in three sizes, 18 cu. in., 20.4 cu. in. and 22.5 cu. in. Although smaller is cheaper, larger may avoid box-fill violations. Single-gang boxes come in three sizes, 18 cu. in., 20.4 cu. in. and 22.5 cu. in. Although smaller is cheaper, larger may avoid box-fill violations.
A 4-in. junction box solves most box-fill problems. Throw on a single- or double-gang plaster ring that comes flush with the drywall, and you've got a box that's hard to overfill. A 4-in. junction box solves most box-fill problems. Throw on a single- or double-gang plaster ring that comes flush with the drywall, and you've got a box that's hard to overfill.

Clifford A. Popejoy is a licensed electrical contractor in Sacramento, California. See his article "Adding an Outside Outlet" in the February/March 2004 issue of Fine Homebuilding (#161).

Photos: Andy Engel

From Fine Homebuilding144 , pp. 79 December 1, 2001
continued 12next>VIEW ALL