Box Fill for Remodel Work - Old Work Box Calculations

Remodel (old work) electrical boxes present unique challenges for box fill calculations. This guide covers special considerations when working with cut-in boxes in existing walls.

What Are Old Work Boxes?

Old work boxes, also called cut-in boxes or remodel boxes, are designed to be installed in existing finished walls without access from behind. They use various mounting mechanisms:

Swing tabs/ears: Flip out behind drywall
Side clamps: Grip drywall edges
Mounting wings: Spring-loaded brackets

Volume Differences

Old work boxes often have smaller volumes than new work boxes of similar face dimensions because:

Mounting mechanisms take up internal space
Designed to fit between studs with minimum depth
Cable clamps are usually internal

Box Type	New Work	Old Work
Single gang	18-22 cu.in.	14-18 cu.in.
Double gang	34-42 cu.in.	30-36 cu.in.

Always check the volume stamped on the specific box you're using.

Key Considerations for Remodel Work

1. Verify Existing Wiring

Before calculating fill, identify what's already in the box:

Count existing conductors
Note wire gauges (could be mixed)
Check for grounds (older homes may lack them)
Identify any abandoned conductors

2. Account for Cable Entry Limitations

Old work boxes have limited knockout positions. Plan how cables will enter to avoid creating fill problems in one area of the box.

3. Internal Clamps Count

Most old work boxes use internal cable clamps—these count toward box fill. Add one conductor volume (largest size) for clamps.

Common Remodel Scenarios

Adding a Receptacle to Existing Circuit

Tapping into an existing outlet to add a new one:

Original box: 2 cables (4 conductors + 2 grounds)
Adding: 1 cable (2 conductors + 1 ground)
Total: 6 conductors + 3 grounds (count as 1)
Plus: clamps, device

A standard 18 cu.in. old work box may not be large enough—verify with our calculator.

Replacing a Switch with Smart Switch

Smart switches often require neutral, adding conductors to an existing switch box that may have only had hot, switched leg, and ground.

Before installation, verify the box can accommodate the added neutral splice.

Converting Junction to Device Box

Adding a device to what was a junction box adds the double volume device fill requirement. Recalculate to ensure compliance.

Solutions When Box is Too Small

Option 1: Replace with Larger Box

Cut a larger opening and install a bigger old work box. This may require drywall repair.

Option 2: Use a Box Extension

Box extension rings add volume to existing boxes. Calculate combined volume of box + extension.

Option 3: Add a Separate Junction Box

Install an accessible junction box nearby to handle splices, reducing conductors in the device box.

Option 4: Use a Surface-Mount Box

Where appearance isn't critical, surface-mount boxes offer more volume and easier installation.

Code Requirements for Remodel Work

All boxes must be accessible [NEC 314.29]
Boxes must be flush with combustible surfaces [NEC 314.20]
Ground continuity must be maintained
Fill calculations apply equally to old and new work

Related Guides

Calculate Your Remodel Box Fill

Use our box fill calculator to ensure your remodel installation meets NEC 314.16 requirements.

Box Fill Calculations for Remodel Work: Old Work Box Guide