# What size Cable Clamp for junction box

### Help Support House Repair Talk:

#### Burgy

##### Supporting Member
Hello. I am adding some junction boxes in my basement. The metal boxes have a 1/2" hole and also a 1" hole on opposite sides. For the 1" hole I had thought that a 1" cable clamp would work. I ordered a few and to my surprise, the diameter is just a tad to big. Do you know if there is a cable clamp that will work with a 1" hole?

Thank you.

#### Snoonyb

I always find it interesting, when folks won't visit the store, with products in hand, and find a product to fit; 1" romex clamp connector

#### Burgy

##### Supporting Member
I always find it interesting, when folks won't visit the store, with products in hand, and find a product to fit; 1" romex clamp connector
Thanks but 1" are to big even though the hole measures 1". The 1" clamps are slightly bigger than 1". I had purchased sigma brand and the stores here didn't have any.

#### Snoonyb

Cable clamps are, generally, packaged in a flex pkg. so that, for trial & error, you, could punch the K/O on the box and again, thru trial & error, find a clamp that fits the K/O, but you need to be in the vendors estab.

#### Guzzle

##### Well-Known Member
I read where a Romex twin screw clamp that they call 3/8 fits in a 7/8 hole. This is worse than calling a 1.5 x 3.5 a 2x4.

Fun with a digital micrometer & basement elec. H/W.
Hole size 0.891”, clamp needs min diameter hole of 0.795” & max of 0.923”
Hole size 1.098”, clamp needs min diameter hole of 1.042” & max of 1.318”

0.891" X 32 = 28.5, call it 28/32 = 14/16 = 7/8" give or take.
1.098" x 32 = 35, call 36/32, 1" + 4/32 = 1-1/8"

(0.795” + 0.923”)/2 = 0.895", x 32= 27.4/32, call it 14/16, = 7/8.
(1.042” & 1.318)/2 = 1.18", 0.18 x 32 = 6/32 = 3/16, so 1-3/16.

Last edited:

#### Guzzle

##### Well-Known Member
It seems to be useless to know what they say is a clamp size.

Therefore, I recommend:
Measure the knockout circle in your house box with a ruler
Find a knockout circle of the same diameter in a junction box at the store
Hold a store clamp up to this hole & eyeball whether it fits or not.

You never need to know actual hole & clamp sizes, just compatibility.

#### kok328

##### Well-Known Member
Are you sure it's not a 3/4" KO? Those are more common combination with boxes.

#### Guzzle

##### Well-Known Member
I can't find a link with the mech drawings for these twin screw clamps, nominal dims vs actual dims.

Assuming they round to the nearest 1/8" & assuming all nominal dims,
for my large clamp, the hole for the cable is 3/4" & the mating hole size could be 1-1/4" and
for my small clamp, the hole for the cable is 5/8" & the mating hole size is 7/8".

There's also a main panel KO that measures 1-3/4". IDK what the clamp for this hole measures.

I don't see a pattern yet, except the thread depth is more for the large clamp, prob. because larger cables put more torque on the clamp.

Last edited:

#### Guzzle

##### Well-Known Member
Was at The HD & thought I would check on their
twin screw
cable clamp nomenclature for
"Romex type" cable vs
real life inches.

No help.

This is getting personal for me! Maybe the NSA can crack this code.