8-gauge wire is 0.1285 in (3.264 mm) bare solid; typical 19-strand builds run near 0.143 in, and #8 THHN sits roughly around 0.217 in overall.
American Wire Gauge (AWG) sets fixed diameters for bare, round, solid conductors. That’s the base line. Out in the field you’ll often handle stranded conductors with insulation, so the number stamped on the reel rarely matches what calipers read on the jacket. This guide spells out the exact 8 AWG diameter values in inches and millimetres, explains why stranded and insulated builds look larger, and gives quick measurement tips you can rely on when buying lugs, sizing ferrules, or planning conduit runs.
8 Gauge Wire Diameter In Inches And Mm
For a bare, round, solid #8 copper conductor, the standard diameter is 0.1285 in, which is 3.264 mm. That size is set by the AWG system defined in ASTM B258. Cross-sectional area at that diameter is 8.367 mm² (about 16,510 circular mils), and typical copper resistance runs near 0.628 Ω/km at 20 °C. Those are the numbers you’ll use when checking lugs, ferrules, and conduit fill against drawings.
| Property | Value | Notes |
|---|---|---|
| Bare solid diameter | 0.1285 in (3.264 mm) | AWG fixed size per ASTM B258 |
| Cross-sectional area | 8.367 mm² | Area of bare solid conductor |
| Circular mil area | ≈ 16,510 cmil | Used in NEC tables |
| DC resistance (Cu) | ≈ 0.628 Ω/km | At 20 °C, uncoated copper |
| 19-strand conductor OD | ≈ 0.143 in (3.63 mm) | Metal OD before insulation |
| Typical THHN overall OD | ≈ 0.217 in (5.51 mm) | Varies by maker and rating |
Solid Vs Stranded: Why The Numbers Differ
Stranded #8 keeps the same copper area as solid, yet it needs a bit more room because round strands don’t pack perfectly. A common 19-strand copper build measures about 0.143 in over the conductor, then the PVC insulation and nylon jacket add more. On a popular product like Southwire #8 THHN, the sheet lists a nominal overall diameter near 0.217 in, which is the dimension you’ll plug into conduit and bend calculations.
Where The Sizes Come From
AWG uses a geometric step between gauges. The diameter ratio between adjacent sizes is the 39th root of 92 (about 1.12293). Plugging n = 8 into the standard formula dn = 0.005 in × 92(36−n)/39 lands on 0.1285 in, which is the accepted bare-solid size. The same approach appears across many reference tables and matches the conductor data used with the National Electrical Code. That’s why every table you check lands on the same 0.1285-inch figure for #8 across references.
Diameter Of 8-AWG Wire In The US Standards
In the United States, the dimensional baseline for bare, round, solid conductors comes from ASTM B258. Working electricians, designers, and inspectors tend to reach for NFPA 70 (NEC) during planning, since Chapter 9 tables summarise areas, resistances, and typical builds that show up in real installations under typical conditions.
Reading A Data Sheet Without Guesswork
Most #8 copper product pages present three different diameter rows. Here’s how to read them in seconds:
- Diameter over conductor — the metal only, including lay and compaction for stranded builds.
- Insulation thickness — wall thickness per side, usually in mils, defined by the insulation family.
- Approximate overall OD — finished cable outside diameter. This is the number you use for conduit fill, jam ratio, and bend radius.
Some sheets add a tolerance band. ASTM calls for four digits of precision for AWG diameters above 44 AWG and sets rounding rules, so you’ll see small variations from brand to brand that still track the same nominal size.
Measurement Tips That Save Time
When you’re confirming size, measure the right thing and write it down the same way every time. These quick moves keep mistakes off the bid sheet and out of the panel:
- Reading bare metal? A micrometer on the copper gives the AWG diameter. Add no more than a light touch.
- Reading jacketed cable? Measure the overall outside diameter (OD) across the insulation, not the copper.
- Got stranded? If you strip it, don’t compress the bunch; let the strands relax, or the number skews low.
- Comparing to a data sheet? Check “diameter over conductor” vs “approx. OD”; they’re different rows.
- Working in mm? 1 inch equals 25.4 mm, so 0.1285 in is 3.264 mm and 0.217 in is 5.51 mm.
Real-World Uses And Why Diameter Matters
#8 copper shows up in feeders, water-heater circuits, EVSE runs, and small subpanels. None of that works without parts that fit. Lugs and ferrules are sized by the conductor gauge, yet many are machined to fit the stranded metal OD. If the barrel is tight, strands splay; if it’s loose, crimp quality drops. Knowing that a 19-strand #8 conductor sits near 0.143 in helps you pick the right hardware first time.
Conduit Fill And Pulling
Pulling friction depends on jacket OD, not bare diameter. That’s where the 0.217 in figure for common THHN helps. Multiply by the number of conductors, compare to the raceway area, and you’ll see if a trade-size bump smooths the pull. Bend radius rules also reference the finished cable OD, so don’t use the bare 0.1285 in number for layout decisions that involve the jacket.
Threading #8 Into Lugs And Devices
Device markings usually call out a gauge range and a set screw torque. Ring terminals, splices, and ferrules also list a gauge along with stud size or crimp die color. Match the part to stranded #8 when you’re working with THHN, XHHW-2, or USE-2, and match to solid #8 on bare bus work. When in doubt, check the product sheet for the accepted conductor classes.
Material And Coating Notes
AWG is geometric. That means #8 aluminum and #8 copper share the same nominal bare diameter of 0.1285 in. The difference shows up in resistance and current ratings, not in diameter. Many branch-circuit designs step up aluminum to a larger gauge to carry the same load as copper. Tinned copper keeps the same base diameter as bare copper; the coating adds a thin film that doesn’t change the nominal size for terminations or sizing math.
Temperature Rating And Ampacity Context
Ampacity lives in its own table, but diameter feeds the conversation. The copper area behind #8 sits near 8.37 mm², and NEC 310 rules apply insulation temperature, bundling, and ambient to that base. A common case is THHN/THWN-2 at 90 °C column values, which many shops memorise as 55 A for #8 copper before any adjustments. That current rating doesn’t change the diameter, yet it explains why so many #8 circuits live in conduit, not in jacketed cable.
Comparing #8 AWG To Metric Cable Sizes
Many spec sheets list metric cross-section in square millimetres alongside AWG. For #8 copper, the area is about 8.37 mm². The closest common metric builds in IEC land are 6 mm² and 10 mm². On dimensions, #8 sits well above 6 mm² and a bit below 10 mm². Hardware that fits 10 mm² often fits #8 stranded copper, though you should always check the barrel range on the part drawing. If you’re swapping a machine imported with 10 mm² conductors, expect the job to need fresh ferrules or new gland inserts sized for 10 mm².
Why Your Caliper Reading Can Drift
Field measurements can wander for simple reasons. Ovality from the drawing process makes one axis a hair larger than the other. Temperature shifts change plastic jacket sizes by small amounts. Micrometer pressure can pinch a soft PVC jacket and bias the number low. Dirt on the anvils, a nicked blade on the stripper, or an out-of-square cut can change the reading you write in the notebook. Take two measurements at right angles and use the larger number when you’re near a fill limit.
| Build | Nominal Diameter | Source/Notes |
|---|---|---|
| Bare solid conductor | 0.1285 in (3.264 mm) | Fixed AWG size (ASTM B258) |
| 19-strand conductor (no insulation) | ≈ 0.143 in (3.63 mm) | Typical metal OD before jacket |
| #8 THHN/THWN-2 overall | ≈ 0.217 in (5.51 mm) | Southwire published spec |
Quick Math: From AWG To Diameter
If you like to double-check numbers, the standard formula makes it easy. Use d = 0.005 in × 92(36−n)/39. For n = 8, the exponent is 28/39. 9228/39 equals 25.700…; multiply by 0.005 in to land on 0.1285 in. Convert to metric by dividing by 25.4 to get 3.264 mm. The same approach works for any gauge, including the larger 6 AWG or smaller 10 AWG that you might pair with #8 on a project.
Handy Checks And Edge Cases
Old Stock And Mixed Lots
Legacy reels sometimes carry older jacket recipes, which can shift the overall OD by a millimetre or two. The copper diameter stays the same, so lugs fit, yet conduit fill math feels tighter. When you mix lots, go by the largest OD you measure or the manufacturer’s current sheet.
Compact And Compressed Constructions
Power cables aimed at feeders may use compact or compressed stranding. The copper area still equals #8, while the conductor OD drops a touch compared to standard stranding. Jacket OD can move as well, depending on the insulation family. Read the sheet, since compact builds often publish both “conductor” and “overall” diameters.
Measuring In Tight Spaces
On an installed run, you might not have enough jacket showing for calipers. A wrap-around cloth tape gives a quick circumference; divide by π and you’ve got an OD close enough for planning. If you can only reach bare copper at a termination, measure there and note whether you grabbed a solid or stranded end.
Labels And Notation You’ll See On Reels
Print on the jacket and stickers on the spool carry a lot of shorthand. Here’s what each line tells you about diameter and build:
- #8 AWG or 8 AWG — both mark the same gauge. Some labels use “8 ga.” or “8-gauge” in plain text.
- CU or AL — the metal. The gauge defines diameter; current ratings change with the metal.
- 19 STR — strand count. Nineteen-strand #8 lines up with a conductor diameter near 0.143 in on many sheets.
- THHN/THWN-2, XHHW-2, USE-2 — insulation family. These lines hint at wall thickness and the finished outside diameter printed farther down the page.
- OD — outside diameter. If it says “nominal,” treat it as a typical size with a small band around it.
- cmil — circular mil area. For #8 copper, look for 16,510 cmil, which pairs with the 0.1285 in bare-wire number.
- mm² — metric cross-section. A sheet may show 8.37 mm² next to the AWG mark so crews working in mixed units can line up tools and ferrules.
One more labeling tip: don’t mix British SWG charts with American AWG. SWG 8 sits at about 0.165 in, which is much larger than #8 AWG. If a spec mentions SWG, confirm the system before pulling parts, since the hole size, lug barrel, and bushing ID will all miss the mark when gauges swap across systems.
Short Checklist Before You Buy Or Pull
- Write the bare #8 copper diameter in your notes: 0.1285 in (3.264 mm).
- When you see stranded #8, expect a conductor OD near 0.143 in before insulation.
- For THHN/THWN-2, plan around a finished OD near 0.217 in, unless the sheet says otherwise.
- Match lugs and ferrules to the conductor class you’re using, not just the gauge mark.
- Use jacket OD for conduit fill, jam ratio, bend radius, and pulling setup.
- For code lookups, start with NFPA 70 tables; for the bare-wire math, see ASTM B258.