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Out of curiosity, what is the equivalent AWG of 1/4 inch copper tubing?

The tubing would be mostly hollow; how much current would it be rated for?

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    \$\begingroup\$ This question belies a number of misunderstandings about the nature of wire, pipe sizing, and ratings, to the point that it's not even an answerable question. \$\endgroup\$
    – Hearth
    Commented Mar 9 at 4:41
  • \$\begingroup\$ At higher frequencies 1/4" OD tubing might be similar to solid 1/4" wire. Also '1/4"' tubing can have 3/8" OD and ID of 0.245" or 0.311", for example. Go figure. \$\endgroup\$ Commented Mar 9 at 5:20
  • \$\begingroup\$ @Hearth They said "tubing", not "pipe" -- tubing is specified OD, while pipe sizes are types, not dimensions. \$\endgroup\$ Commented Mar 9 at 7:51
  • \$\begingroup\$ The questions starts with Out of curiosity, but do you expect answers which will comply with industry/regulatory standards? If so, which industry/regulatory standards and what is the intended voltage? \$\endgroup\$ Commented Mar 9 at 10:03
  • \$\begingroup\$ '1/4" copper tubing' can be sold by pipe size, which is 3/8" OD. Go figure. petersenproducts.com/pipe-specs/copper-tubing-sizes. \$\endgroup\$ Commented Mar 9 at 18:01

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Well, let's see here...

Assume equal temperature rise, long enough wire that heat sunk through the ends doesn't count, and whatever ampacity the nominal size / type corresponds to.

Power dissipation depends on surface area, so, tracks with diameter.

Ampacity depends on allowed power dissipation and cross-sectional area (and resistivity, which we'll also say is the same in both cases; tubing is usually high-purity copper, e.g. 110 alloy).

Tubing must be lower ampacity than wire of the same OD, because there's less material; but higher than wire of the same cross section, because there's more dissipation.

The most common, unspecified "copper tubing" will be this product and similar:
General Purpose Copper Tubing, 1/4" OD, 0.032" Wall Thickness, 8967K88 | McMaster-Carr
It is widely used for water, compressed air, pressurized fluids, etc., and widely stocked in hardware stores, industrial supplies, etc.

which has a cross-section of 14.14 mm2. Referring to Bare Copper Wire and Cable | Southwire Company, this is between 5 and 6 AWG (16.76 and 13.3 mm2 respectively). 2 AWG has an OD of 0.2576 in (close enough), area 33.6 mm2, and is rated 225A at 75°C, outdoors/overhead condition.

To dissipate the same heat, but with 33.6/14.14 times less area, requires square-root the area ratio, or 146A. Round it to 145A for convenience.

In case the alloy differs, C11000 and C12200 copper alloys are typically 85% of annealed high-purity copper, but notice wire isn't usually annealed, but half- to full-hard drawn, which costs it a few points as well; I would expect resistivity within 90% of equivalent wire. Call it 130A if you're feeling cautious. Or lower for a lower operating temperature, poorer air circulation, etc. (Note that the comparable ampacity for plastic-jacketed wire in an installation condition (in walls, conduits, etc.) is more like 75A; see American wire gauge | Wikipedia.)

Interestingly, the ampacity rating given for 5 AWG is also 145A. The reference given on the Wikipedia page ([9] "Table 11: Recommended Current Ratings (Continuous Duty) for electronic equipment and chassis wiring". Reference Data for Engineers: Radio, Electronics, Computer and Communications (7th ed.). pp. 49–16) might very well assume constant current density, or the difference including convection happens to be down in the rounding error so we don't really see much effect here. Similar assumptions or effects may underlie the Southwire reference.

You would have to research these references, and other supporting information, to see where exactly it comes from; up to and including committee discussions that led to major industry/regulatory standards, such as the National Electric Code (NEC).

Real ampacity in an application, of course depends on other factors as well -- voltage drop for example. For fairly ordinary wiring applications, it seems conductor cross section dominates over dissipation area, so the ampacity of tubing is not much more than its equivalent area.

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Depends on the wall thickness - calculate the outer (1/4" diameter) cross-sectional area subtract the "hole" area. The hole diameter will be the OD minus 2 times the wall thickness. Then, look up what AWG the remaining copper area amounts to. Your second-order problem is the resistivity - plumbing tubing is greater than normal wire. This matters if you are aluding to current capacity requirements.

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  • \$\begingroup\$ While the conductivity may be slightly worse for non-electrical-grade copper (mildly suspicious of that), the copper tube will also have far more surface area than a solid copper wire of the same CSA. That will lead to much better cooling. \$\endgroup\$ Commented Mar 9 at 5:24

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