Easiest way to enlarge an electrical knockout?

[HerHusband]

I just bought a new heater for my garage, but all the electrical knockouts are sized for 1/2" conduit. I need a hole for 3/4" conduit to accomodate the size of my power cable.

My first thought was to use a knockout punch, but good golly those things are expensive. I don't want to spend $60 for a one time task like this. I'm also curious if a punch would work for enlarging an existing hole?

Another option is a step-drill, but those are almost as expensive at the home centers. I'm also a little worried about metal shavings falling inside the heater, or overshooting the size I need and making the hole too big.

I haven't had time to check, but I wonder if anyone rents the knockout punches?

Thanks,

Anthony

[Dean Hoffman]

You might find a set of used KO punches on Ebay for a decent price. I knew a guy years ago who used a hole saw. Metal shavings weren't a problem for his work. It wasn't a super neat job either. The saw would wander a bit if I recall correctly.

[The Daring Dufas]

Use a Sharpie marker to mark the diameter of your 3/4" hole around your

1/2" KO. If you have an automatic center punch, use it to pop little pits around the hole a bit more than 1/8" apart. Use your drill with a 1/8" bit and drill through wherever you have the small pits which keep the drill bit from sliding away from where you want to drill through. You can do it without an automatic center punch if you put moderate pressure on the drill and start very slowly until you're sure the bit won't slide. After you've drilled all your closely spaced holes, you can go up another bit size to remove more metal. You can use diagonal wire cutters and pliers to cut out and break off the little tags of metal left around the circumference. You can finish up with a round or half round file if you want to make it prettier. An automatic center punch and half round file can be purchased from Harbor Freight for very little money. It may cost more at Lowe's Depot but you will be surprised at all the uses you will find for an automatic center punch. ^_^

Harbor Freight also sells step drill bits.

TDD

[clare]

Find an old radio hobbyist or an electrician with a chassis punch and buy him a beer.

[bud--]

If you are near a big city I think it is real likely you can rent them.

[HerHusband]

Oren,

I have a Fahrenheat FUH54 space heater, not a water heater. I have full access to both sides of the opening.

I just wonder how well a punch will work only having an extra 1/8" on each side of the existing opening.

Anthony

[bud--]

If you have a good rental place it should have KO punches.

Change the wiring method to something with a 1/2" connector?

[bob haller]

will the box be overfilled? might be easier and more code compliant to just replace the box

[Damning Dumass - Welfare TeaBi]

Yes, the easiest way is a knockout punch. You, however, seem to be looking for the cheapest way. That would be a round file.

[philoÂÂ]

YEP, this is exactly what to do.

[clare]

The cover on the water heater enclosure is removeable. At least on any I've worked on.. And I was not under the impression this was a water heater, but I may have missed it.

[trader4]

just replace the box

Except that from what is described, there is no box to replace. The NO is on a *heater* he's trying to connect. I suspect he has a *cord* that's too big. An option might be to install a box and transition from the cord to 1/2" liquidtight, etc that will fit.

[philoÂÂ]

Good point and now that I think of it the answer by "Caulking-Gunn" might actually be the best...to simply use a transitional fitting.

Half inch hole, 3/4" conduit such things exist and would make it a "no-mod" job.

[John Grabowski]

*I seem to recall a tip from one of the trade magazines. Try clamping a piece of wood behind the hole and use an 1 1/8" hole saw to enlarge it. I haven't tried this. Please post the results if you do.

You might find a rental place that has knockout punches.

[N8N]

Harbor Freight has step drills for relatively cheap. I doubt they're any *good* but as long as they make one hole, it's probably the easiest way to go.

[Stormin Mormon]

I doubt they're any *good* but as long as they make one hole, it's probably the easiest way to go.

Years ago, when I did heating. HF has a three pack of step drills up to 3/4, but that's not big enough. Need one which goes to 7/8, and the knock out size is 13/16.

Two pack:

Either of these two should make 13/16 hole.

[HerHusband]

John,

After considering multiple options, I remembered I had a metal nibbler in my tool chest. I wasn't sure if it would handle the sheet metal on the heater, but it worked fine.

You can see the full process with pictures at:

Thanks everyone!

Anthony Watson Mountain Software

[DerbyDad03]

One thing obvious from a couple of your pictures:

You have too many clamps.

Wait...that's not possible! ;-)

[hrhofmann]

You didn't show a picture of the side of the heater where the old hole was and how you filled that hole in!!

[HerHusband]

I used a standard knockout seal like these:

You can also see the back side of the seal on the right side of the picture in the "Rewiring the new cord" section of my web page:

Anthony Watson