3/8" nm clamp

[rick boyd]

Always wondered why a 3/8"NM romex clamp is called 3/8
nothing on it is 3/8 inch
the mounting hole is 7/8"
it fits 1/2" ironpipe thread
the inside hole is 9/16"
and the romex is 1/2" wide and not 3/8 thick

I use them all the time but why are they not called what they are?

 

[al hildenbrand]

This is my "opinion", only.

I have a couple pieces of 1938 NM, 14/2 and 14/3 without ground, cable that I cut out of a remodel job. This is an earlier, though not the earliest, form of NM cable.

The 14/2 will squeeze into a 3/8" NM cable clamp, but the 14/3 is the diameter of my thumb, that is, it is nearly an inch in diameter.

I strongly suspect that the original reference to 3/8" is referring to a trade diameter designation (such as: trade size 1/2" iron pipe which actually has an O.D. greater than 1/2") given to nonmetallic tubing. I'm thinking, in particular, of knob and tube loom.

Loom was sold in a range of diameters.

 

[jrclen]

I asked an old electrician that a long time ago. He told me it was for 3/8 romex. I have no idea if he was right, but it satisfied my young mind at the time. Now that I'm an old electrician, I'll tell you the same thing. Some day you can tell another electrician. :grin:

 

[480sparky]

Could it be from using 3/8" flex conduit?

 

[infinity]

What brand of NM connector are you using that is listed as 3/8" size and fits into a 7/8" hole?

 

[480sparky]

What brand of NM connector are you using that is listed as 3/8" size and fits into a 7/8" hole?

All of them. They're all listed as a 3/8" connector, and the hole for them measures 7/8".

Brideport

 

[jrclen]

Could it be from using 3/8" flex conduit?

Flex in a romex connector? :grin: Have you seen the old 2 wire cloth covered romex? Pretty close to measuring 3/8".

 

[480sparky]

Flex in a romex connector? :grin: Have you seen the old 2 wire cloth covered romex? Pretty close to measuring 3/8".

I mean 3/8" is a standard trade size.

 

[JOHNEO99]

Believe its due to standardizing trade sizes from the code book.

 

[electricman2]

I noticed Bridgeport lists them for up to .380 cord. 3/8" is .375". Maybe that is the connection.

 

[jrclen]

I mean 3/8" is a standard trade size.

Oh I get it. Maybe so.

 

[al hildenbrand]

I have the 1948 American Electricians Handbook, Sixth Edition, open in front of me. On page 1228, in Division 9, #44 is a table titled Properties of Flexible Tubing or Loom. The so-called 3/8" loom has:

• inside diameter of 3/8"
• outside diameter of 21/32"
• 250 feet per coil
• largest wire it can be used for is #12 (that would be R, RW, RH, RHW, etc. - old rubber insulated 1900s wire)
• weight of 75 lbs per 1000 feet

NM, as a cable, wasn't developed for normal use until the late Teens or the 1920s.

Loom and the Knob & Tube method predates 3 phase (Tesla & Westinghouse at Niagra Falls in the 1890s).

This connector, in its early non-die cast form, predates NM, IMO, and would have been referenced to things like 3/8" loom.

Again, quoting from the 6th Ed. American Electricians Handbook:

When metal outlet boxes or switch boxes are used, flexible tubing is required from the last porcelain support, extending into the outlet box at least 1 in. and held in place by an approved fitting such as a universal bushing or a clamp.

While most K&T installers didn't squander the labor to install the NM cable clamp above, in favor of materials requiring less labor, I'll just bet that this lowly 3/8" cable clamp has its creation for K&T or other methods much older than NM.

 

[jrclen]

Al,you might well be correct. Great research.

I still run across quite a bit of knob and tube in my area and I find boxes with internal clamps. I've never seen a romex connector on knob and tube. But then that is just in my area and this might be different in other places and other time frames. Most of the K&T wiring in my area is REA era from the mid 1930's thru the 40's I believe. I'm out in rural farm country.

 

[al hildenbrand]

. . . I find boxes with internal clamps.

I've worked in the Minneapolis St. Paul area on buildings first built in the late 1870s and on.

Considering only original installations (early 1900s), by far, the most common entry of loom into the old black painted metal boxes, that I've seen, has been to shove the loom over an inch into the box thru undressed holes, holes just a little larger than the loom (but smaller than the standard 1/2" K.O.).

No clamps are used. They may well be present, as part of the original box, but weren't used.

But I have also seen:

• Threaded porcelain bushings through steel enclosure walls (much like the modern chase nipple) with the loom ending on the outside of the porcelain. These have been used on Octagons, Round boxes, square boxes and fuse centers.
• Circular spring wire retaining clips that, when slipped onto the end of the loom, inside the box, snag to the loom and help prevent the loom from pushing back out through the hole in the box
• Porcelain tubes through the wood and asbestos enclosure walls of the old fuse center - the loom stops, outside the enclosure, at the porcelain tube.

The classic two-screw 3/8" connector shows up, from time to time, in the transition from conduit and boxes to legacy K&T. The 3/8" connector will have a single loom in it.

In all of these old installations, what I have always found interesting, is that the modern 2005 NEC 300.20(B) equivalent apparently wasn't a concern for the original electrician to do the wiring "to Code". The steel is never cut in ways to minimize Eddy Currents.

 

[jrclen]

I've not worked with K&T in a city which would be much older than the wiring I deal with here. But I have seen some of the things you describe, such as the porcelain bushings and tubes. I've never seen the spring clips, that I can recall anyway. I do have a small collection of old stuff in the basement. I think these subjects are interesting.

Maybe someone here knows the history of 300.20B. Now that you mention it, I have never seen cuts or notches in boxes to comply with that article.

Thanks for posting this great stuff.

Earlier this year I removed and replaced the last K&T circuit in a remodeled farm house. The entire house had been redone, but there was one K&T lighting circuit from the new load center, to a porcelain switch (metal box) in the hallway, down the basement stairs and over to a porcelain box with a pendant light fixture. To bad, but the customer wanted to keep the old materials.

 

[cripple]

3/8" nm clamp

This is what I found in UL 514B standard, about 8/3 cable fittings. I could not found anywhere in the standard permitting the use of 3/8 cable fitting to be used on anything except armored cable. I know as electricians we or I have use the 3/8 fitting cable fitting on Romex and have seen the box labeled that it can be used with two cables.

5.12 FITTINGS and BUSHINGS for armored cable
5.12.1 An armored cable FITTING of the 3/8 (12) trade size shall secure a No. 14 AWG (2.08 mm2), 2-wire
cable, unless marked in accordance with Clause 7.12.1 to indicate its use with a cable of another size.
5.12.2 An armored cable BUSHING shall be usable for the purpose intended. The BUSHING size for a given
range of armored cable is shown in Table 13.
5.12.3 An armored cable BUSHING shall be of a readily distinguishable bright color such as red, orange, or
yellow.
5.12.4 An armored cable BUSHING shall have dimensions as specified in Table 13. For all sizes of BUSHINGS,
the maximum percentage of the circumference that contains projections shall be 40 percent of the outside
diameter of the BUSHING.

 

[Lxnxjxhx]

Nominal ("named") vs. actual

Nominal ("named") vs. actual

A 2x4 is nominally 2" thk but is actually 1-5/8" or 1-1/2" thk. Maybe in the lumber yard it was closer to 2".
And some "hardwoods" are softer than some softwoods.

 

[POWER_PIG]

good point by cripple,,,,,,,I know mc type connectors are also called 3/8 in cable connectors, Maybe its just the maximum diam. that the connector can accept. Metal clad is considered cable, just as romex is.

 

[K8MHZ]

What I found to be odd is that the size listed that will fit in a hole sized for a 1/2 inch box connector is '3/8' but the size listed that will fit in a hole sized for a 3/4 inch box connector is '3/4'. I have never seen an NM clamp labeled as 1/2 inch.

So, that being said, I like to ask the new people working at Home Depot to help me find one.