What size Cable Clamp for junction box

[Burgy]

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]

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]

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.

 

[Guzzle]

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.

 

[user 4883]

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

 

[Guzzle]

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.

 

[Guzzle]

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.

 

[superwooman]

I'm going through the same thing recently, and had visited the hardware store three times to get the right item as none of the in-store helpers know the answer to what size to get for a certain size knockout. Apparently I am one of those who didn't bother to take a sample on my shopping trips. Lesson learned. My bad.
I'm just a regular diy homeowner, and I'm not saying what I shared below is definitive nor authoritative. If some information is incorrect, it's mentioned unintentionally. Posting false information is the last thing I want to do. So, don't grill me please if I'm wrong.
The information I gathered is from multiple trade organizations and agencies.
First, it's important to understand the concept of nominal size, trade size, actual size, OD size and ID size.
Nominal size and trade size are NOT actual sizes. Just like 2x4 is not exactly 2" by 4".
Also try to remember that these standards were started many decades ago, and situations tend to change slightly over time.
I don't exactly understand why the trade uses this way of specifying sizings, but I think the logic is that an electrician doesn't have to dig through the codes to find out what actual size of fittings he/she needs to purchase if he started out with a certain size cable bundle.
It seems that, as far as connectors matching knockout goes, the diameter sizings are dictated by using EMT as the usage case. This is simply because we are focusing on knockout and connector sizes here. The different usage for EMT, NM or MC cabling only means different methods of fixing the cable to the connectors.
A 1/2" trade size conduit means that conduit size is for cabling up to 1/2" diameter. The actual OD of this conduit is larger than 1/2".
There's then a standard dictating what hole size is required for this '1/2" ' conduit to go through. And that is a 7/8" actual size hole.
3/8" and 1/2" seem to be interchangeable. The conduit standard doesn't seem to have a 3/8" size that I can find. However, there seemed to have 3/8" sizing for flexible metal sleeve cabling.
In the same token, a connector marked as 3/4" should be used for a knockout hole with approx 1 1/8" diameter actual size.
So there you go. These are reasons why they are marked this way. Again, I could be wrong, but I believe I'm fairly close. Hope this helps

 

[PJB12]

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?

Hi Burgy,
That hole uses what is called a 3/4" connector. (Cable or conduit box connector or threaded IMC or RMC)
If you measure carefully, you'll find that the larger hole is 1.114" diameter. The smaller is 7/8" diameter.

It is important to use a clamp rated for the size of the cable. Too large is not good.
Should you want to use a smaller clamp that better suits the cable size in the larger hole, you can use a pair of reducing washers and a half inch connector. One washer goes inside the box and one goes outside.

Paul

Here's a handy little chart:

 

[afjes_2016]

This thread is 3 years old and the OP has not been seen on the forum since then.
I guess it is good information for the future though.