nominal vs actual cable OD?

[gaaah]

Whenever I run into a nominal dimension in any industry there is always a table or formula published the will provide the actual dimension should the need arise. This is true of lumber, pipe, conduit, etc. But electrical cable has me perplexed. I can't find a cable manufacturer out there publishes the actual OD, or any clue how to deduce it. Rather they publish the nominal OD.

Lately I bought some ChainFlex brand 16/4 (a nice German mfgr) flex cable where the nominal ODwas quoted at 0.35. The actual OD measured 0.30. That's nearly 1/16" different, which can make a huge difference when using things like compression cord connectors, some types of strain reliefs, grommets, etc.

I have searched the entire NEC and Handbook and nowhere can I find how they come up with the nominal dim or some sort of rule of thumb.

Really I can't even say with certainty that the nominal OD will always be larger than actual OD. The with the ChainFlex I mentioned it is true, but what about SJOOW, or other types?

So could some cable sage out there enlighten me?

 

[MasterTheNEC]

Whenever I run into a nominal dimension in any industry there is always a table or formula published the will provide the actual dimension should the need arise. This is true of lumber, pipe, conduit, etc. But electrical cable has me perplexed. I can't find a cable manufacturer out there publishes the actual OD, or any clue how to deduce it. Rather they publish the nominal OD.

Lately I bought some ChainFlex brand 16/4 (a nice German mfgr) flex cable where the nominal ODwas quoted at 0.35. The actual OD measured 0.30. That's nearly 1/16" different, which can make a huge difference when using things like compression cord connectors, some types of strain reliefs, grommets, etc.

I have searched the entire NEC and Handbook and nowhere can I find how they come up with the nominal dim or some sort of rule of thumb.

Really I can't even say with certainty that the nominal OD will always be larger than actual OD. The with the ChainFlex I mentioned it is true, but what about SJOOW, or other types?

So could some cable sage out there enlighten me?

@gaaah - Most all of the wire and cable manufacturers (and we are one of them) do publish the O.D. values on our product cut sheets. In terms of your question regarding SJOOW, they are cords and we do not make those so i can't comment on that. However, for cables (e.g Type AC,MC,SE,SER,TC and so on) the Nominal OD or Specific OD is listed on the cut sheets. On those products that only offer nominals is quite frankly due to allowances by the design specifications..which can be + or - tolerances that are permitted. Some of the products get quite specific.....and list Outside Diameter and make no mention of it being nominal, just remember that all manufacturers are afforded the various design tolerances due to production variances.

Hope this was helpful....or even a little at the least....and I won't post links to our specs because some people (not naming any names) would use it as an opportunity to be snarky.

 

[GoldDigger]

For some cables the cross section may not be circular.
Either elliptical or having surface irregularities corresponding to the underlying conductors.
In those cases one measurement of the actual OD in one direction at one specific cross section may not be representative.

 

[winnie]

As a historical artifact, many industries use the same name for different units of measure.

A 'trade size' 1 inch pipe is not 1 inch in diameter.

A 2x4 piece of lumber is not 2 inches by 4 inches.

Frankly a pain in the nether regions.

I have not encountered 'inch' trade sizing for wires and cables. The history there is to use 'gauge', and you will of course find tables/formulas for converting AWG to linear dimensions.

If a cable is specified as having a diameter of 0.35 inches, then that is the size, in inches, with no conversion.

As MasterTheNEC notes, even if the diameter given is an actual value in standard units, it may be subject to quite large tolerances.

-Jon

 

[Julius Right]

The overall diameter of a cable it is a ?virtual? one ?the definition of IEC 60502-as MasterTheNEC and GoldDigger explained it very well .
First of all the strand has to be drawn from a delivery size to the required one. The drawing machine is provided with a number of dies. The die has a diamond die embedded in. Each km of wire passing through erodes and enlarge the hole in the diamond and so each km of wire present a different diameter-thin at the beginning and thick later.
The extruded insulation thickness depends on velocity of extruder screw and the take-up speed and this ratio is not constant. The standard states only the limits not the final size.+/-5% it could be a deviation from the ?rated?.
I know the problem with the hazardous location appurtenances-namely-where the overall diameter of the cable has to be accurately known. The manufacturer can overcome this issue. The gland could be provided with possibility of adaptation.
You may use sealing unilets using sealing material instead fitting a gland[for instance].

 

[Carultch]

This isn't like dimensional lumber or conduit sizes, where everyone speaks in terms of a "trade size" that isn't the actual size, and you can notice the difference with the naked eye. A skilled tradesperson could even notice that difference without a tape measure or ruler.

Nominal OD's of wires are a decent representation of all wires that you will get, of that exact part number. And for most purposes in an electrician's work, these values are good enough for your purposes to assume they are exact. The reason it is nominal, as opposed to actual, is that manufacturing processes have variance. They try to achieve the nominal OD, but of course achieving it exactly within a nanometer is impractical.

One kind of nominal OD you see on the datasheet could be a mean value. So maybe some are a millimeter larger, and some are a millimeter smaller.
Another kind could be a "representative maximum" of perhaps 99% or another high percentage, of all possible manufacturing runs.

For common wire types (THWN-2, USE-2, etc), the NEC has data that indicates the maximum permissible of that wire type to be manufactured, as industry standards. For instance, #10 USE-2 has 0.236" indicated in the NEC, but in practice I find that most USE-2 in this size is 0.2". Good data to use for conduit calculations, when you aren't sure what manufactured wire will actually be installed within your NEC designation.

 

[Tony S]

This is a very old 1944 educational film about cable making. It shows the wire drawing process and laying up of the cores followed by the armouring and serving.

As an apprentice in the 70?s I went to visit the factory, it hadn?t changed much ;-)

https://www.youtube.com/watch?v=0OlDcrb1plk

 

[Julius Right]

This is a very old 1944 educational film about cable making. It shows the wire drawing process and laying up of the cores followed by the armouring and serving.

As an apprentice in the 70?s I went to visit the factory, it hadn?t changed much ;-)

https://www.youtube.com/watch?v=0OlDcrb1plk

Very interesting Tony, indeed! However since no varnish is using, no lead jacket is required, the armor is not the same and no serving .The insulation is extruded-some time triple extrusion-also the overall jacket. The armor is wire made-if any. The are many change in technology but the principles are the same.