How to fill electrical knockout holes with inpunity

Home Inspector with Structure Tech Home Inspections

One of the most common Truth-in-Housing repair items is an unfilled opening in an electrical box or panelboard.  If you walk through the garage or basement of just about any old Minneapolis or Saint Paul home, you'll probably be able a few of these.

Missing knockout plug Opening in panel cover

The national electric code says that unused openings in such equipment need to be closed.  Covering these openings helps to prevent accidental shocks (think of kids), helps to prevent hot sparks from escaping if something goes wrong inside the enclosure, and helps to prevent pests from entering.  It happens.

Mouse in panel

How to fix a missing knockout plug

First, determine the size of the missing knockout hole and subtract 3/8" to figure out what size knockout plug you'll need.  For example, if you have a 7/8" hole, you'll need a 1/2" knockout plug.

Next, go buy some knockout plugs.  They're sold in small quantities at hardware stores and home improvement stores for about a quarter each.

Knockout plugs

Next, kill the power to any circuits going through the box you're working on.  As GI Joe always thought to himself, "being safe is the other half."

Finally, put the plug in the hole.  If you bought the right size, it will be nearly impossible to push the plug in to place with just your hands - you'll need to tap the plug in to place using a hammer.

Knockout plug missing Knockout plug installed

If you want to make the plug super tight and impossible to remove, you could bend the tabs inside the box with a screwdriver.  I'm sure some electricians would say this should always be done.  The photo below left shows the tabs in their normal position, and the photo below right shows the tabs bent out.

Knckout plug inside view Knockout plug inside view with tabs bent

For Truth-in-Housing evaluations, missing knockout plugs are required repair items in Bloomington, Hopkins, Robbinsdale, and South Saint Paul.  They're considered a hazard (H) in Saint Paul, and they're a suggested correction (SC) in Minneapolis.

How to fix a missing twist-out at the front of a panel

First, determine the brand and type of panel.  It will probably say so right on the door.  If it helps, take a photo of the label.

Next, go buy some filler plates at a home improvement store or hardware store.  You'll find them next to the circuit breakers.

Filler Plates

Finally, shut off the power to the panel and pop the appropriate filler in to place.  This can typically be done without even removing the cover.

For Truth-in-Housing evaluations, openings in the front of an electric panel are required repair items in Minneapolis, Bloomington, Hopkins, Robbinsdale, and South Saint Paul.  They're considered a hazard in Saint Paul.

Now go forth and fill your holes with impunity.

Comments (17)

Rob Lyszczarz
RE/MAX Properties Unlimited - Westfield, NJ

Reuben,  That is great information, and a very easy and necessary fix.  By the way, it that a real photo with the mouse in the electrical box (or a staged photo)?

Mar 12, 2013 08:12 PM
Barbara Todaro
RE/MAX Executive Realty - Happily Retired - Franklin, MA
Previously Affiliated with The Todaro Team

Good morning, Reuben.... good information for everyone.... I enjoyed your post except for the creature!!

Mar 12, 2013 08:13 PM
Clint Mckie
Desert Sun Home, commercial Inspections - Carlsbad, NM
Desert Sun Home, Comm. Inspection 1-575-706-5586

Hi Reuben,

I call this type of stuff all the time. I tell them it's an easy fix. Not much too it. Too bad the local pet met his maker. LOL

Did you remove the pet and give it the proper burial?

Have a good day and a very good post. Suggested.

Best, Clint McKie

Mar 12, 2013 10:47 PM
Mike Cooper, Broker VA,WV
Cornerstone Business Group Inc - Winchester, VA
Your Neighborhood Real Estate Sales Pro

We use them a lot, Rueben.  In a pinch, we'll stick a plactic box connection in a hole.  I've even taken two washers with a 1/4-20 X 1/2" machine screw to plug up a hole.  The key is to plug it up.  I bet that mouse was surprised at what he found.  Yeow!

Mar 13, 2013 10:08 AM
Matthew O'Hare
Osterville, MA

Thanks, information like this is always handy in the business.

Mar 13, 2013 10:18 AM
Laura Filip
Laura Filip Broker , Opening doors for All Seasons of Life - Whitesboro, TX
What can we do for you today?

Great post but I will let an electrican do this for me.

Mar 13, 2013 11:22 AM
JoAnn Borelli-Mardesich
Placentia, CA
Thanks for this information. I won't be doing the work myself. I like knowing what to watch out for. Great post!
Mar 13, 2013 12:22 PM
Praful Thakkar
LAER Realty Partners - Andover, MA
Andover, MA: Andover Luxury Homes For Sale

Reuben, am I seeing something special in the third picture......does that happen, too? What is the fix for that? (just kidding...)

Mar 13, 2013 03:22 PM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Rob - that's definitely a real photo of the mouse.

Barbara - I almost put up a warning about that photo :)

Clint - no way :).  We left the panel exactly the way we found it.

Mike - the washer solution sounds just as good as anything.

Matthew - thanks.

Mar 13, 2013 08:00 PM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Laura - there's certainly nothing wrong with letting an electrician handle this.

JoAnn - thanks.

Praful - what you're seeing in the third picture is probably one of the most common problems caused by a missing knockout hole.

Mar 13, 2013 08:02 PM
Jay Markanich
Jay Markanich Real Estate Inspections, LLC - Bristow, VA
Home Inspector - servicing all Northern Virginia

Hammer, schmammer.  Why go to all that trouble when there is chewing gum right in your mouth!?

Some people...

Remember the KISS principle, - Keep It Sweet and Simple!

And chewing gum is sweeet.

And to think I had to say this.  Sheesh.

Mar 13, 2013 08:39 PM
Michael Setunsky
Woodbridge, VA
Your Commercial Real Estate Link to Northern VA

Reuben, these are very good suggestions for everyone, whether or not it is required by code. I would think it would be required for reasons you pointed out.

Mar 13, 2013 08:53 PM
Marc McMaster
RE/MAX Centre Realty - State College, PA
Putting my clients before myself

People think it's just a cosmetic thing...until they see the photo of that mouse in the panel box.  I bet that smelled bad when it happened!

Mar 13, 2013 09:50 PM
William Feela
Realtor, Whispering Pines Realty 651-674-5999 No.

Reuben...Agaijn it is the do it yourselfer that just has no idea and or hates to part with a buck to do it right.

Mar 14, 2013 08:31 AM
Robert Butler
Aspect Inspection - Montreal West Island, QC
Montreal Home Inspector | Aspect Inspection
Over half the inspections I do have his condition. I've considered bringing a selection of the most common with me on inspections. It wouldn't cost more than $ 20.00.
Mar 14, 2013 11:55 AM
Reuben Saltzman
Structure Tech Home Inspections - Minneapolis, MN
Delivering the Unbiased Truth.

Jay - don't forget duct tape.  You can fix anything with that.

Michael -  and it's so simple.

Marc - and if it didn't, it's going to ;)

William - it might also be a case of 'out of site, out of mind.'

Robert - I've thought about doing the same thing.  I need to pick up some of those 1/2" knockouts in bulk.

Mar 14, 2013 07:40 PM
James Quarello
JRV Home Inspection Services, LLC - Wallingford, CT
Connecticut Home Inspector

I haven't found a dead mouse inside a panel in a long while. I have found a few large nest's. Wonder what happened to the mice :)

Mar 16, 2013 11:10 PM